uop flanges

EDS-2004/FL-1

Flanges

Training Services

The purpose of this presentation is to discuss the common types of flanges and gaskets used in refineries and petrochemical plants. Included is the means used to select an appropriate standard flange for a given service. The means of designing a flange for circumstances where a standard flange cannot be used are briefly introduced. The presentation closes with a discussion of the common causes of and corrections for leakage problems.

EDS-2004/FL-2

Purpose

Introduce the common types and uses of flanges, outline the methods to select or design a flange for a given application, describe some of the common reasons for flange leakage, and outline methods for correcting leakage.

The presentation will cover the applicable Standards encountered in the determination of a flange class, materials in terms of product form, specified dimensions of a flange, flanged connection components with their associated tolerances, flange design basis criteria, and some common causes of flange leakage and proposed solutions.

EDS-2004/FL-3

Outline

IntroductionStandardsMaterialsFlange SelectionFlange FacingsGasketsFinishesFlange DesignLeakage Causes and Correction

The flexibility of a flanged connection comes with a price. The joint is subject to leakage. Providing a seal while maintaining flexibility is the goal of the system design. Fluid molecule size is a factor in determining the difficulty of sealing a joint. It is more difficult to achieve adequate joint tightness to prevent leakage of material with a small molecule size. For example, hydrogen services are common in hydrocarbon processing plants, but are very difficult to seal due to the small size of the hydrogen molecule. Because of leakage concerns (either initially or during service), the use of flanged joints is limited to locations where the ability to easily disassemble the joint is important. Other joints are welded.

The contained fluid may be a vapor or a liquid. The design considerations are nearly the same.

EDS-2004/FL-4

Flanged Joint

A flanged joint connects piping or equipment by means of bolting, allowing “easy” disassembly and assembly (e.g., valves, instruments, manways)The joint provides a seal against the contained fluid at design conditions– Fluid molecule size plays a role (e.g., water is much

bigger than hydrogen)A flanged joint is composed of three components --flanges, gasket, and boltsPerformance is influenced by another factor, assembly of the jointBecause of their cost and the potential for leaks, use flanges only when absolutely necessary

The gasket provides the seal against the contained fluid. Seating of the gasket involves compressing the gasket so it flows and fills the flange face surface imperfections without being crushed. The resulting seal must be maintained throughout the operating cycle when internal pressure is attempting to separate the flange faces and find a leak path between the gasket and the flange. In elevated temperature services, differential thermal expansion of the joint components and the time dependent effects of creep conspire to create leaks by reducing the forces holding the flanges together. Proper assembly techniques must be observed and alignment of the flange faces must be maintained throughout the operating cycle.

Connection by means of bolting allows a joint to be easily and quickly taken apart to accommodate access for installation, removal, maintenance, inspection, or disassembly of a vessel, piping system, or piece of equipment. Examples include vessel manways (for access), valves or expansion joints (for removal or replacement), filters or strainers (to allow removal of the internal element(s)), and unloading nozzles (to allow removal of the vessel contents such as catalyst).

EDS-2004/FL-5

Flanged Joint(continued)

The gasket provides the seal, bolts provide the forces necessary to seat the gasket and hold the joint together, flanges provide the surfaces for the gasket to seal against and carry the applied forces around the gasket.The joint allows for “easy” disassembly and reassembly of piping or removal of components (e.g., valves and instruments).Because of their cost and the potential for leaks, use flanges only when absolutely necessary.

EDS-2004/FL-6PPF-R00-01

Gasket

Flange

Stud Boltswith Hex Nuts

Gasket – Compressible material providing the seal

Bolts – Provide the force to compress the gasket and form the seal

Flange – Transmits the force from the bolts to the gasket (must not distort or deflect)

A joint consists of two flanges, one on each side of a gasket, and bolts used to squeeze the assembly together. The squeezing force comes from tightening the bolts, which applies force to the opposing flanges. The flanges must remain, essentially, ridged in order to transmit the forces evenly to the gasket surface(s). The gasket is then compressed to provide and maintain the seal.

A flanged joint is composed of these three separate and independent, although interrelated, components. Proper controls must be exercised in the selection and application of all three elements to attain a joint which has, and maintains, an acceptable leak tightness.

Stud bolts and hex nuts are, by far, the most common bolting configuration. For cast iron flanges, which tend to be brittle, machine bolts are often used because they will fail (break) before the flange

The flanged system can be uniquely designed for a specific application or selected from industry recognized and accepted standardized code compliant designs.

The existence of localized stresses, stress concentrations, and discontinuity stresses of a relatively high order in all pressure equipment is well known. The code accounts for localized stresses by using compensating factors in the design formulas for stress.

EDS-2004/FL-7

Flanges

Flanges may be uniquely designed or selected from standardized designs in a recognized document giving dimensions and pressure –temperature capacitiesDesign methods or standards referenced must be in accordance with the governing code

Standardized design avoids the costs and delays associated with customized design and fabrication. Standardized flanges should be used whenever possible. Unique designs are reserved for instances where standard designs are inadequate, e.g. limiting clearance constraints or sizes and design conditions outside of the scope of the available standards. Many flange fabricators have their own “standard” flanges. Taylor Forge’s large diameter Class 175 and Class 350 flanges are examples. The fabricator has the necessary dies and equipment to efficiently produce these flanges, which are not covered by industry standards. These flanges must be checked to insure that they are adequate for the intended service, but significant cost and delivery time savings can be realized if they can be used. Similarly, flanges with industry standard dimensions, but non standard materials, may be suitable. Again, the design must be reviewed for adequacy but, if acceptable, the flange can be easily made with existing dies, etc.

Code compliance assures consistent safe engineering practice.

EDS-2004/FL-8

Standard Flanges

Flanges from accepted standards have a proven record of widespread safe, reliable useThey are economical because standardized dimensions allow vendors to “tool up” for efficient productionEveryone uses the same basis and benefits from economies of scaleUsers may obtain flanges easily and quickly, and need only stock a limited number of varieties

ASME (American Society of Mechanical Engineers) Section VIII, Division 1, Pressure Vessels, governs the design and fabrication of the majority of all refinery equipment.

ASME B31.3, Process Piping, is the document to which the majority of the piping in a petroleum refinery or chemical plant is designed and fabricated.

Flanges must either comply with one of the industry standards incorporated into the applicable Code, or be uniquely designed in accordance with that Code. In most cases, the applicable Code references the design rules/methods in ASME Section VIII, Division 1, Appendix 2 when a special design is necessary.

ASME B16.5 is referenced for flange selection by both Section VIII, Division 1 and B31.3.

Class 2500 flanges are for use in very high pressure service and have an upper size limit of 12". The concept of flange classes will be discussed later.

EDS-2004/FL-9

Referenced Standards

ASME B16.5, “Pipe Flanges and Flanged Fittings”– Covers flanges from nominal pipe size ½ to 24 inches

(12 inches for Class 2500)– Most commonly used standard for refinery and

petrochemical plant flangesASME B16.42, “Ductile Iron Pipe Flanges and Flanged Fittings Classes 150 and 300”– Used for many ASME pumps (usually in non-

hazardous service)

The following standards are referenced by the Pressure Vessel Code (ASME Section VIII) and the Process Piping Code (ASME B31.3)

The scope of ASME B16.47 begins where B16.5 ends. B16.47 covers a more limited range of materials than B16.5 (i.e., no high alloys such as Inconel). Two styles are included. Series A flanges are similar to flanges built to MSS-44, a standard often applied to pipelines. They are designed for connection to relatively thin piping and are limited to a maximum temperature of 450°F. Series A flanges may also be found on valve bodies intended for process services. Series B flanges follow the basis used for B16.5 (and the old API 605) and are used for refinery/petrochemical services. Series B is generally more compact than Series A. It is important to note that Series A and Series B flanges are incompatible, i.e., they cannot be bolted together.

The edition of each referenced document that is specified (i.e., accepted) by the governing Code must be used. The referenced edition may not be the most recent edition.

B16.20 is the standard covering the majority of gasketing used with B16.5 flanges. These are the gaskets commonly found in refineries and petrochemical plants. B16.20 includes gasket dimensions, materials, marking requirements, etc.

B46.1 is referenced in B16.5 and B16.47 and defines the flange face surface finish specifications and tolerances.

EDS-2004/FL-10

Referenced Standards(continued)

ASME B16.47, “ Large Diameter Steel Flanges”– Covers flanges from 26 to 60 inches, in Classes 150

through 900– Range of included materials is more limited than in

B16.5 (e.g., few nonferritic materials such as Inconel)ASME B16.20, “Metallic Gaskets for Pipe Flanges -Ring - Joint, Spiral - Wound, and Jacketed”– Unlike its predecessor, API 601, it does not specify

default materials.ASME B46.1, “Surface Texture (Surface Roughness, Waviness, and Lay)”

Both of these are obsolete and are not maintained. These have been replaced by B16.20 and B16.47 respectively.

EDS-2004/FL-11

Obsolete Reference Standards

API 601, “Metallic Gaskets For Raised - Face Pipe Flanges and Flanged Connections (Double - Jacketed Corrugated and Spiral Wound)” – replaced by ASME B16.20.

API 605, “Large Diameter Carbon Steel Flanges” – replaced by ASME B16.47.

Subject areas of B16.5 include acceptable flange materials, pressure-temperature ratings for flange class determination, tables of standardized dimensions (based upon flange class), tolerances associated with different components of a flange, required code compliant marking, and the testing required for flanges.

EDS-2004/FL-12

Scope of ASME B16.5

MaterialsPressure - Temperature RatingsDimensionsTolerancesMarkingTesting

Materials tables have been grouped to provide compatible flanged joint ratings for materials likely to be used together.

Groups frequently have more than one material covered in a respective group.

With ratings based on the material in the group with the lowest allowable stress, ratings for some materials within a group are conservative.

Material type is also dependent on product form: forging, casting, or plate.

EDS-2004/FL-13

Materials for B16.5

Permissible flange materials are listedin Table 1A, bolting materials in Table 1BFlange materials are organized into groups of materials with similar compositions and mechanical propertiesRatings are based upon the material in each group with the lowest allowable stress

Bolts must have adequate strength to be able to both seat the gasket and maintain a seal for the specific application throughout the operating cycle.

Low strength bolts are used in the lower Class flange applications, which are generally lower temperature and pressure operating conditions.

Stainless steel bolts are avoided because of low yield strength and high thermal expansion.

Stud bolts, or bolts without heads and threaded over their full length, are used because they are less expensive and can be safer. A nut is used on each end. Two nuts on each end, tack welding of the nuts, or “spiking” (damaging of the bolt thread just outside of the nut) of the thread may be used to prevent backing off or loosening of the nuts when vibration is a concern. Bolt engagement must be enough so that at least two threads show outside of the nut to ensure full engagement. Otherwise, the bolt may project any distance beyond the nut and either nut may be engaged first, either before or after the bolt is inserted through the bolt hole from either direction. Either nut may be tightened to stress the bolt This provides more flexibility in assembly than a headed bolt. Headed bolts are normally used only with studding flanges (see slide 38) where use of a nut on both ends of the bolt is impossible. Even there, a stud bolt with one nut could be used.

EDS-2004/FL-14

Materials for B16.5(continued)

Bolt materials are divided into three groups based upon strength – High strength (e.g., A193 B7 or B16) may always

be used– Intermediate strength (e.g., A193 B8 Class 2)

may be used, provided it has the ability to maintain a sealed joint

– Low strength (e.g., A307 and A193 B8 Class 1) is limited to Class 150 and Class 300 flanges and certain gaskets

Stud bolts with 2 nuts are typically used

Forged flanges are generally considered to be a higher quality product. Cast material can have inclusions or imperfections that can affect material properties, even leading to brittle (sudden) failure. They are permitted as part of valves and other complex components because casting is the accepted (and cost efficient) method of producing these components, including integrated flanges. Rather than weld a separate flange to the component, the flanges are included in the casting. Welding would be more costly, would impose the potential problems found at welds, may require heat treatment, and would often increase the flange face to flange face length of the component,

EDS-2004/FL-15


Flanges may be either forged or castForged flanges are preferred due to a lower likelihood of flaws or brittle materialCast flanges are usually provided as an integral part of cast valves and other componentsIn forged and cast construction, the grain tends to be non-directional or circumferential, limiting the potential for large cross grain stresses

Although plate material is listed in ASME B16.5, there are severe limitations placed upon its use. It is rarely acceptable to make a flange out of plate material.

EDS-2004/FL-16


Only blind and certain reducing flanges (those without hubs) may be made from plateOne reason is that a flat plate closely approximates a blind flange’s shape (e.g., there is no raised hub)Another reason is that the directional nature of the grain in these flanges is not a serious additional concern because there will be cross grain bending stresses in blind flanges regardless of how the grain is oriented

Classes provide an easy method to identify categories of flanges. The dimensions of each size of flange within each class are standardized.

The design pressure and temperature for the flange form the required rating. The rating is used to determine the applicable flange class.

Flange class and flange rating are not the same. An example of a flange class is the designation “Class 300”. An example of a rating is 250psi at 300°F. A rating of Class 300 is meaningless.

EDS-2004/FL-17

Flange Classes

Flanges are organized into classes for identificationClasses used by B16.5 are:

Class 150Class 300Class 400Class 600Class 900Class 1500Class 2500

Class 150 flanges are typically used in low temperature and pressure service where their light weight construction is suitable. They are not suitable for cyclic services or where large imposed loads are present. They are inexpensive and very commonly used in refineries and petrochemical plants.

Use of Class 400 may be acceptable for systems that do not contain valves or other components not available in the classification. Consider, however, the need to stock a few Class 400 flanges for these lines. It is often cost efficient to use Class 600 even for these instances to avoid the need to have rarely used spare flanges, gaskets, and bolts in the warehouse. As a practical matter, these spares will probably not be found if and when they are needed.

EDS-2004/FL-18

Flange Classes(continued)

Class 150 flanges are lightly built and are often avoided, especially when imposed loads (e.g., from piping) or cyclic loads are presentClass 150 flanges are not used above a design temperature of 700ºF because they may tend to deform or creep, possibly opening and leakingClass 400 (and sometimes Class 900) flanges are usually avoided because valves and fittings are not commonly available for them

Ratings are the maximum allowable non-shock working gage pressure at the design temperature for the applicable material. The appropriate flange class is the class with a pressure rating at the design temperature that is greater than the design pressure.

Note that B16.5 and B16.47 each have their own set of rating tables. Although these tables are normally identical, there may be differences - especially just after one document’s tables have been revised. It is possible for a different flange class to be required for the same design conditions because of this difference in the selection tables.

EDS-2004/FL-19

Flange Classes(continued)

A rating table is provided for each material groupFor a given material group, temperature, and non-shock pressure, the table indicates the appropriate flange classEach size flange in each class is built to a standard set of dimensions

Gasket seating is the application of sufficient force to deform the gasket, causing it to flow into and fill imperfections in the flange surface. The characteristics of the gasket and the flange, i.e., hardness and roughness, must be matched to efficiently produce a seal and not deform the flange.

During operation, the forces on the gasket are reduced due to the effects of the internal pressure. Enough force must remain to prevent the internal fluid from flowing between the gasket and the flange, i.e., leaking.

As previously mentioned, deformation of the flange itself will affect the ability to produce and maintain a seal. The flange must remain stiff and undistorted.

EDS-2004/FL-20

B16.5 Rating Considerations

Ability to withstand stresses necessary to seat the gasket– Special attention is required for some Class 150

and Class 300 flanges with spiral wound gasketsAdequate thickness to sustain the stresses due to pressure and other loadings necessary to maintain a fluid sealDistortion due to loadings is transmitted through the piping or bolting

Material group determination includes consideration of the chemical composition and the product form.

Pressure can be considerably different at different locations in a vessel considering liquid head and pressure drops. Temperature can also vary throughout a piece of equipment .

Use internal pressure (or the external pressure applied as an internal pressure). External pressure is not a concern because it tends to increase gasket seating forces (and/or reduce the bolt force) and does does not “pry” the flange open as much as internal pressure.

If determination of the required flange class considers a reduced design temperature for uninsulated flanges, as permitted by the Piping Code (B31.3), the affected flanges must be clearly identified. Future insulation of these flanges is restricted or prohibited.

EDS-2004/FL-21

Use of B16.5 Rating Tables

Determine the applicable group for the material used (Table 1A)Determine the design temperature and pressure (including hydrostatic head)that apply

The allowance of a hydrotest maximum permitted pressure of 1.5 times the 100oF (ambient) rating is based on the fact that this is a short-term loading condition, and the material properties are in the elastic range.

Excluding the effects of hydrostatic head, hydrotest is not intended to govern the design of a flange. Determination of the test pressure involves the same allowable stress ratio (1.5) used to determine the required flange rating for hydrotest. Hydrostatic head may occasionally result in hydrotesting governing the required class.

Section 2.5 of ASME B16.5 states;

“Flanged joints and flanged fittings may be subjected to system hydrostatic tests at a pressure not to exceed 1.5 times the 100°F rating rounded off to the next higher 25 psig. Testing at any higher pressure is the responsibility of the user, subject to the requirements of the applicable code or regulation .”

Section 8.3 requires flanged fittings to be tested at a minimum pressure of 1.5 times the pressure rating at 100°F, rounded up to the next multiple of 25 psig.

EDS-2004/FL-22

Use of B16.5 Rating Tables(continued)

Enter Table 2 and determine a flange class with a pressure rating equal to or greater than the design pressure for the applicable material and temperatureCheck the flange for hydrotest conditions (including hydrostatic head) with a maximum permitted pressure of 1.5 times the 100ºF rating rounded up to next multiple of 25 psi

Be sure to consider both flanges used in the system - the metallurgy's may differ. One example is a thermowell connection. The thermowell assembly, including the flange, is usually made as one piece using stainless steel. The flange may be paired with a low chrome or carbon steel flange on the vessel. The required flange class for both metallurgy's must be checked, and the greater class used. This may be a different class than is necessary for the remainder of the vessel!

EDS-2004/FL-23


If the flanges are made of different materials, both must be checked– The highest resulting flange Class governs

both flangesInterpolation is permitted between the listed temperatures

The “pound” designation has a historical origin based on the empirical testing method development that equated a class with a certain allowable pressure at a standardized reference temperature.

EDS-2004/FL-24


Flanges used to be designated by “Pound” rather than “Class” (e.g., 600 Pound).Previously, this referred to the pressure capacity of a carbon steel flange at 850ºF (500ºF for 150 Pound).This is no longer true; therefore, the word Class is used and, except as noted below, the number is only an identifier.

The allowable stresses used are not exactly as listed in other Codes (e.g., B31.3 or Section VIII).

EDS-2004/FL-25


Rating table pressures for Class 300 and aboveare based upon the formula:

PT = PR x SI / 8750 ≤ PCwhere: PT = rated pressure (psi)

PR = Class (e.g., 300 for Class 300)SI = material allowable stress at

temperature (psi), determined from the rules in Annex D of B16.5

PC = ceiling pressure per Annex D of B16.5

EDS-2004/FL-26


Rating table pressures for Class 150 comply with the formula on the previous slide, except use 115 for PR and limit PT to 320 - 0.3T (T = temperature in ºF)B16.5 ratings originated with experience and were essentially empiricalRecently (1996), the ratings have been revised to agree more closely with the formulas

For each respective material, as the temperature increases, the allowable stress decreases and, accordingly, the allowable pressure (pressure-temperature rating) decreases. This trend of a decreasing allowable stress with an increasing temperature is the same for both materials. However, the magnitude of the decrease is different and dependent on the respective material composition and mechanical properties. Different materials have different capacities at the same temperature. Note that, below the creep range, the allowable pressure for stainless steel flanges declines more rapidly than for low alloy (or even carbon steel) flanges. This is because stainless steel is softer and more ductile than most other materials, hence, it may be more easily deformed and leak.

EDS-2004/FL-27

Class 300Temperature - Pressure Ratings

Temp 321 S.S.(ºF) 2 ¼ Cr - 1 Mo (psi) (psi)

100 750 720200 750 645300 730 595400 705 550500 665 515

Material

EDS-2004/FL-28PPF-R01-02

Class 300

This figure illustrates the decline in allowable pressure with temperature in a graphical form.

Stainless steel ratings are generally lower than for many other materials.

A182-F11 Class 2; Low Chrome (1-1/4 Cr - 1/2 Mo) Forging material -- Use ASME B16.5 material group No. 1.9 to select the required flange class.

Allowable stresses at temperature are taken from the governing Code, in this case, the ASME Boiler & Pressure Vessel Code, Section II, Part D - Properties; Table 1A, Maximum Allowable Stress Values S for Ferrous Materials. Allowable stress values are used for the check of hydrotest conditions.

EDS-2004/FL-29

Example

MaterialSA182 - F11 class 2 (1¼ Cr - ½ Mo)

Design PressurePD = 400 psig

Design Temperature1000ºF

Allowable Stress (per the Pressure Vessel Code)

@ 1000ºF SH = 6,300 psi @ ATM. SC = 20,000 psi

Use Class 600 Flange

The purpose of the hydrotest is to confirm the adequacy of the piece of equipment for the service conditions by performing the test at an inflated (elevated) pressure and ambient conditions. The objective is to get to a similar relative stress level (i.e., stress vs allowable stress) as that seen during operation (which has a higher design temperature and lower design pressure), in a controlled, safe, test environment at the low temperature ambient conditions. At the higher design temperature, the material has a lower allowable stress. At the ambient test conditions, the material has a higher allowable stress which is used to adjust the pressure to approach the relative stress level present at operating conditions.

EDS-2004/FL-30

Check for Hydrostatic Test Pressure

psi, psi < , = P

, ) . ( P

SS ) P. ( P

T

T

H

CDT

25026501

3006000,2040031

31

∴

×=

×=

Allowable Pressure @ Ambient

1,500 * 1.5

EDS-2004/FL-31

B

A

C

D

Flange Material:A B C , SA182-F11 class 2 (1¼ Cr - ½ Mo) D , SA182-F316 (16 Cr-12 Ni-2 Mo)

Design Pressure: PD = 850 psiDesign Temperature: T = 850°F

Allowable Stress (per the Pressure Vessel Code):A182-F11 @ 850°F : SH = 18,700 psi

@ ATM. : SC = 20,000 psi

A182-F316 @ 850°F : SH = 11,600 psi@ ATM. : SC = 20,000 psi

For A & B, Use Class 600For C & D, Use Class 900

PPF-R00-03

A182-F11 Class 2; Low Chrome (1-1/4 Cr – ½ Mo) Forging material -- Use ASME B16.5 material group No. 1.9 for selection of the required flange class.

Allowable stresses at temperature are taken from the ASME Boiler & Pressure Vessel Code, Section II, Part D - Properties; Table 1A, Maximum Allowable Stress Values S for Ferrous Materials.

A182-F316; Stainless Steel Forging material -- Use ASME B16.5 material group No. 2.2 for selection of the required flange class.

The class of the low chrome flange C is increased to match the Class 900 required for the mating stainless steel flange D. By itself, the pressure-temperature rating of the low chrome flange C for Class 600 would be acceptable. In this case, the higher resulting flange class of the mating flanges governs both flanges. This is necessary so that the bolt size and locations for the two flanges match, and the sealing surfaces and gasket requirements are compatible.

EDS-2004/FL-32

Check for Hydrostatic Test Pressure

H

CDT S

SP=P × ).( 31

psipsiP

x=P

T

T

, < , =

,,0 ).(

25021821

70018002085031

SA 182-F11

EDS-2004/FL-33

Check for Hydrostatic Test Pressure(continued)

Allowable Pressure @ Ambient

2,160 * 1.5

SA 182 - F 316

psi 250,3psi 905,1

600,11000,20850 )3.1(

<=

×=

T

T

P

P

This classification covers types of construction where the flange is integral (i.e., part of the same piece) with the neck or vessel wall, butt-welded to the neck or vessel wall, or attached to the neck or vessel wall by any other type of welded joint that is considered to be the equivalent of an integral structure. In welded construction, the neck or vessel wall is considered to act as a hub. The whole system acts as one.

EDS-2004/FL-34

Flange Types

Integral Flanges (flange is part of or buttwelded to the piping or vessel so the system acts as an integral structure)– Welding neck– Long welding neck– Integrally reinforced– Ring– Studding– Specialty joints such as exchanger closures

EDS-2004/FL-35

Welding Neck

Weld Neck RingPPF-R00-04

Integral Flanges

Welding neck is the most common type of an integral flange. They are designed so that the outside diameter at the junction to the pipe matches the outside diameter of the joining pipe. The inside diameter must be specified. They are used where joining to a pipe.

Ring weld is a ring butt-welded onto the end of a pipe. This is typically used only in low pressure service.

EDS-2004/FL-36PPF-R00-05

Long Welding Neck

Integral Flanges(continued)

Long welding neck flanges are usually dimensioned by their inside diameter. The outside diameter varies with their wall thickness. They are one piece nozzles that provide much of the required reinforcement for the vessel opening in their neck. Because their dimensions do not match those of standard pipes, they are not suitable for welding to a pipe. They are normally welded to the vessel and used where control of the inside diameter is important. Two examples are manways and small nozzles through which instruments are inserted. They can use a constant ID and a varying OD because they do not attach to a pipe, hence they do not need to match a pipe’s OD.

EDS-2004/FL-37PPF-R01-06

Integrally Reinforced

Allow room for the nuts on the bolts (bolts are removed through the mating flange)


These flanges have a thick nozzle neck. The reinforcement for the vessel opening is integral to the flange. The reinforcement is provided by thickening the nozzle neck, sometimes to a greater dimension than the outside diameter of the flange itself! The thick hub makes them unsuitable for connection to piping. Therefore, they are dimensioned by their inside diameter for the same reason as a long welding neck flange, and welded directly to the vessel.

Integral reinforcement is preferred over built-up pad reinforcement for several reasons:

• The reinforcement is concentrated near the nozzle/shell junction, where local stresses are at their maximum

• The reinforcement is one piece, stresses and forces do not transfer from piece to piece across welds

• There are far fewer welds to make. Welds are costly, are prone to flaws, create residual thermal stresses, create heat affected zones with related metallurgical concerns, and can cause warpage of the joined pieces.

• There is no flange to pipe weld. This weld would be in a high stress zone caused by the flange rotation and local stresses , as the bolts are tightened.

On the other hand, integral nozzles are machined from a large forging, itself an expensive undertaking

EDS-2004/FL-38PPF-R00-94

Flared Nozzle

R

Weld

Often a “flare” is provided at the base to move the vessel/nozzle weld away from the stress concentration zone at the junction of the shell and nozzle. This places the stress concentrations due to geometry and stresses due to welding at different locations. The detail also allows a contoured, controlled junction geometry to be more easily provided because the transition is part of the forging, not the weld. Additionally, the “flared” detail provides the needed clearance to adequately radiograph the welded joint.

Although a clearly better detail than a conventional integral nozzle, flared nozzles are more costly than conventional integral nozzles. The differential becomes greatest when the flared portion extends beyond the flanged portion. This requires an increase in the initial forging size and additional machining to arrive at the required geometry.

UOP uses the flared detail in high temperature services (i.e., the creep range of the material) where differing stresses and creep rates may redistribute (and concentrate) stresses to the weld. Significant cracking has been detected in services such as catalytic reforming. Flared nozzles are also used in high pressure, heavy wall (greater than 4 inches (100 mm)) services. Here the stresses are high and it’s difficult to ensure the proper material properties throughout the thickness. The ability to radiograph the joint is also important. Cyclic or fatigue services are another instance where UOP uses a flared nozzle detail.

EDS-2004/FL-39PPF-R01-07


Studding

These flanges set into a vessel head or shell where clearance is critical. The design allows for compact installation applications. They are dimensioned by their inside diameter.

This type of flange allows a low projection from the mating vessel shell. Often this design includes the nozzle reinforcement as part of the flange. When determining the flange dimensions to provide sufficient opening reinforcement, the effect of the bolt holes must be considered because they project into the area providing the reinforcement.

A danger with this type of nozzle is that the bolt engagement is within the flange and is not visible. Use of a short, or wrong diameter, bolt is not visually apparent. A flange failure due to insufficient thread engagement is possible. When stud bolts are used with a through bolted flange, a short bolt is easily seen. A small diameter stud bolt may be seen by comparison with the bolt hole. If the bolt size is still adequate, nuts of the proper size may be used and the bolt will be fully engaged and will work. The thread size in a studded outlet cannot be adjusted to fit a bolt of the wrong size (e.g., only the tip of the threads may be engaged).

UOP uses studded outlets only where there is a severe space or clearance problem in a low pressure service (e.g., CCR’s).

Loose flanges cover types of flanged construction where there is no direct attachment between the flange and the pipe, neck, or vessel wall and where the means of attachment between the flange and pipe, neck, or vessel wall is not considered to be equivalent to an integral structure. In these cases, the pipe, neck, and/or vessel wall has less influence upon the response of the flange to applied and pressure loadings than in an integral design.

EDS-2004/FL-40

Flange Types

Loose Flanges (no direct attachment between the flange and piping or vessel, does not act as one integral structure)– Slip-on– Lap joint

Blind Flanges (closures)– Flat, solid discs used to close an opening (e.g., a

manway cover). Generally thick because they are flat, spanning the opening like a beam, developing through thickness bending moments

EDS-2004/FL-41

Loose Flanges

PPF-R01-08

Slip-on

Lap Joint

Vent

A lap joint flange rests on the lap or stub of the piping. Slip-ons slide onto the end of a pipe and are welded to the pipe, usually after the flanges are bolted together. Slip-ons provide some ability to adjust the axial location of the flange. They are limited to low pressure, non-hydrogen, services with no applied moments or forces.

Both types of flange may be rotated in place so that the bolt holes of the mating flanges line up. This is not possible with welded in place welding neck flanges (unless the butt welds are made insitu).

EDS-2004/FL-42

InexpensiveUsed with a full face gasket in low temperature/pressure servicesEasily sealed (i.e., containment of large molecules), non-dangerous, non-cyclic services that require low bolt loads and sealing pressuresASME non-process pumps and utilities such as cooling water are examples

PPF-R00-09

Types of Flange FacingFlat Face

This style of flange may be used in refineries for low pressure utility lines and low pressure/temperature process services where ASME pumps are acceptable. This is especially true when the pumps are cast iron, which tends to be brittle. Flat face flanges and full face gaskets are used to reduce the imposed bending moment stresses by moving the bolt force and resultant gasket force closer together. Tightening of the bolts is likely to deflect the flange surfaces towards each other. This causes a non-uniform application of load to the gasket, possibly interfering with the ability to seat and/or seal the gasket. If the deflected flanges contact each other, a portion of the bolt forces will transfer directly from one flange to another without further seating/sealing the gasket.

The large gasket surface area, coupled with the limited force available from the bolts, means that the gasket seating/sealing stress that can be achieved is limited. Therefore, these flanges are not applicable for gasket systems requiring “high” seating or sealing forces.

EDS-2004/FL-43

Mating flanges are different - one with a 3/16 inch recess, the other with a ¼ inch raised portionThe gasket is confined on both edges and partially protected from the internal environmentThe projecting sealing surface is subject to damageReduced bolt load when compared to raised face, due to the smaller gasket area

PPF-R00-10

Types of Flange FacingTongue and Groove

The gasket is contained in the mating flange’s groove. A potential detriment to this type of flange facing is that the small protruding portion on one of the flanges can be subject to damage during handling, reducing or destroying the ability to subsequently effect a seal.

EDS-2004/FL-44

Similar to tongue and groove except the gasket is confined only against blowoutThe sealing surface is not protected from the internal environmentProjecting portion is larger and less likely to be damaged than on the tongue and groove style

PPF-R00-11

Types of Flange FacingMale-Female

Blowout is the internal pressure force that directionally acts to push the gasket radially outward. Although the projecting portion of the flange is much sturdier than on the tongue and groove type, it can still be damaged. Even a radial scratch will affect sealing.

EDS-2004/FL-45

Allows use of a different metallurgy for the flange than for the pipingIs not weldedAvoid in cyclic or services other than low pressureCompensates for some misalignmentLimit to low temperature services to avoid differential thermal expansion problemsSealing may be more difficult because the flange and sealing surface (stub) are independent

PPF-R00-12

Stub end fitting

Types of Flange FacingLap Joint

Because the flange rests on the lap (stub end) of the piping, it does not see the internal service fluid. This potentially allows different materials to be used for the flange and pipe/stub. The flange may be a lower grade of material (e.g., low chrome) that is acceptable for the temperature and pressure, but not the internal atmosphere. A higher grade piping material (e.g., Inconel) is required because it is directly exposed to the corrosive environment. Very significant cost savings can be realized in these cases if this style of flange is acceptable.

Lap joint flanges are limited to low pressure, non-cyclic services where there are no external loads. These flanges are subject to severe distortion because the end of the hub is not restrained.

Distortion or flange rotation under imposed loads or during gasket seating is likely and can reduce the seating and sealing pressures on the gasket, leading to leakage. Differential thermal expansion between the flange and the pipe can also pose a big problem. The differential expansion may be due to differing coefficients of thermal expansion between the two materials, and/or differing temperatures because the flange and stub are not directly connected, thereby limiting heat transfer.

EDS-2004/FL-46

Allows adjustment of the flange position(s) in situMust be double welded and ventedNot used above Class 150 (except for manways and some reducing flanges) or 500°FNot used in hydrogen atmospheres and cyclic services

PPF-R02-13

Types of Flange FacingSlip-On

Vent

The enclosed space between the flange and the pipe must be vented to relieve any trapped vapor. The vapor may come from gasses released during welding, or may gather from diffusion of the contained materials through the pipe or weld into the open space between welds. There, it can combine into molecules too large to get out. Hydrogen is a prime example of this phenomenon. If not vented, the trapped gas can crack the welds. Hydrogen accumulation may also cause blistering and hydrogen embrittlement.

The system depends upon a couple of fillet welds to hold it together and prevent leakage between the flange and the pipe. Thermal stresses, piping movements, and the forces/moments developed when the flange is bolted together can damage, even break, one or both of the welds, permitting leakage, and/or rotation of the flange, reducing the gasket sealing forces also leading to leaks.

UOP restricts the use of slip-on flanges to Class 150 in low temperature (<500°F), non-cyclic, non-hydrogen, services that are not subject to severe corrosion or external loads and do not require postweld heat treatment due to process or material characteristics. This is because of their poor fatigue life, potentially large distortions and internal stresses, and susceptibility to thermal cracking at the fillet weld attachments due, in part, to the stress concentrations at the notch at the edges of the fillet welds. Externally applied loads are also a concern because a pair of fillet welds does not provide the smooth stress flow or dependability of a butt weld. The fillet welds are also small, limited by the pipe and flange hub thicknesses, and subject to stress concentrations, undercuts, and cracking at the inside base. This area cannot be inspected, backgouged, or ground. Inspection of the fillet welds is difficult at best. Cracking or internal corrosion cannot be seen.

EDS-2004/FL-47

The gasket is confined within grooves provided in the mating flange facesHistorically used for high pressure, high temperature services, or aggressive operating conditionsExpensive

PPF-R02-14

Types of Flange FacingRing Joint

Most commonly used in very severe or aggressive service. The confined gasket will not be displaced by high internal pressures, and tends to be self sealing. A higher internal pressure forces the gasket more tightly against the outer portion of the groove, increasing the seal at that point. Because of their infrequent, specialty use (requiring stocking of a few flanges and gaskets not generally used in the plant), the drawbacks and other points noted on the following slides, and experience that shows other flange and gasket styles (notably raised face with spiral wound gaskets) are as safe and reliable, ring joints are being specified and used less frequently. Still, some refiners mandate their use because of their self sealing characteristics and their intrinsic resistance to blow out.

Ring joint flanges are expensive. Both flanges must be machined to create the grooves in the respective flange. The grooves in mating flanges must align with each other. The height of the raised portion of the flange must be such that the groove does not project into the portion of the flange relied upon to carry the imposed stresses.

Unlike male and female and tongue and groove flanges, both mating flanges are identical.

The seating surfaces for the gasket are small, resulting in very high seating pressures (high unit load per surface area). The gasket seals along a line on either side of the groove. An added advantage is that the outside sealing surface is isolated from the possibly damaging effects of the internal atmosphere (e.g., corrosion). The recessed groves are also less likely to be damaged when the flange is exposed or handled, although foreign material (e.g., dirt, moisture) can gather in them and must be thoroughly removed before the flange is closed. Cleaning is necessary for any flange; it is even more critical for ring joints. Because of the small, narrow, sealing surface, debris or damage (e.g., scratches) on the surface will have an even greater effect. The gaskets are typically a soft metal material (e.g., iron) suitable for the internal atmosphere.

Good resulting seal with applications for hydrogen service and high pressure service.

EDS-2004/FL-48

Ring Joint Flanges

The seal is provided by pressure against the sides of the groove (the gasket does not contact the bottom of the groove)Two narrow sealing surfaces are formed, one on each side of the groove, with very high imposed stressesOne sealing surface is protected from the internal atmosphere and possible corrosionThe sealing surfaces are small and sheltered, protecting them from damage (e.g., when the flange is open(ed) during a turnaround)

The gasket behaves like a self actuated gasket. In other words, higher contained pressures increase the gasket seal by pressing the gasket against the side of the groove.

During regular service, flanges tend to distort, even on a small scale, which can be a maintenance concern causing difficulty replacing the gasketing.

EDS-2004/FL-49

Ring Joint Flanges(continued)

Internal pressure tends to increase the sealing pressure on the outside of the gasketDue to flange distortion during operation, the groove, and the gasket contained in it, may become non circular– The gasket cannot then be (easily) replaced with a

new oneGrooves have flat bottoms– Flat is used to insure the gasket touches only where

desired– Groove dimensions are standardized in B16.5 and

identified by a groove number

The junction of the groove side and bottom is a discontinuity which results in stress concentrations. The junction is susceptible to cracking, with cracks observed to extend deep into the flange body. UOP specifies a larger radius for the corners of the groove than is required by B16.5 to reduce the stress intensification at this location.

Different mating flange materials thermally expand at different rates, offsetting the grooves and setting up a shearing of the gasketing. The limitation on flange size (10-inch) keeps the magnitude of the differential movement within acceptable bounds.

Because of the very small width of the sealing surface (little more than a wide line), and the fact that ring joints are most frequently used for elevated pressure systems, even a small scratch can allow leakage around the gasket.

As noted earlier, the flange must be thick enough that the groove depth does not infringe upon the portion of the flange considered to be carrying the stresses. This may require a thicker flange than necessary for other types of flange facings.

Another potential concern is that small amounts of process fluid may remain in the groove, a potentially dangerous situation when the flange is opened.

EDS-2004/FL-50

Ring Joint Flanges(continued)

A generous radius (1/8 inch or 3mm) is provided at the intersection of the flat bottom and the sloped sides to prevent initiation of a crackWhere differential expansion occurs, do not use for flanges over 10 inch NPSVery sensitive to scratches in the sealing area of the groovesGrooves must be machined into the flange – an additional costGrooves of mating flanges must match each other and the gasket

EDS-2004/FL-51

The sealing surface is raised 0.06 inch (1mm) for Class 150 and Class 300 flanges and 0.25 inch (6 mm) for other flangesRaised faces help prevent contact of the flange edges due to rotation from high bolt loadsThe most common type of facing in refineriesThe sealing surface is exposed and susceptible to damage when opened during a turnaround

PPF-R00-15

Types of Flange FacingRaised Face

Raised face flanges are the most common type of refinery/petrochemical flange. The protruding portion of the flange is the raised face. There are standard dimensions to the raised faces and the associated tolerances are specified in B16.5.

The raised face concept arose from the rotation problem discussed for flat faced flanges. Using a raised, or protruding, sealing surface moved the flange extremities, where the bolt forces are applied, further apart. If the flange did deform a bit, the increased separation kept the flanges from coming into contact. The raised face also adds a bit more metal to the flange, increasing its bending stiffness.

Flanges with raised faces are extensively used because of their simplicity of design and they are adequate for average and most severe services. Gasket replacement is simple because the gasket rests on the flat, raised surface. It does not need to fit into a groove, which may become distorted, or otherwise align with any features of the flanges. Another useful feature is that the two flanges are identical, they are not a matched pair. This eases the warehousing problem.

In a few instances, a groove has been machined into one of the flanges into which a thicker gasket was placed. The idea is to provide gasket confinement or retention, as well as a somewhat self actuating performance. This has been viewed as a safety enhancement. Unlike tongue and groove or male/female flanges, there is no projecting portion of the flange to be damaged by handling. On the other hand, the flanges are no longer identical, the gasket has to be thicker, and it cannot have inner or outer stiffening rings. The thicker gasket is more compressible and more difficult to seal. All in all, this design has not achieved wide usage. The gasket styles normally used provide blow out resistance, eliminating the groove’s perceived benefit.

Because of the added cost and the need to stock non standard components in the plant warehouse, specialty flanges are used only where they provide a significant advantage. Two of the most common uses are where space and access are limited, requiring either a smaller flange or a different joining (bolting) method than is used for the typical flange, and for systems requiring rapid assembly and disassembly.

EDS-2004/FL-52

Specialty Types

Generally developed to make the joint smaller and lighterNormally proprietary and, therefore, more expensiveMay be less tolerant of mis-alignment and applied forcesComponents are matched and may not be interchangeable

There are many other types of specialty flanges.

EDS-2004/FL-53

Specialty Types(continued)

Examples are:– Dur O Lok

• Uses a coupling style system with a self energized gasket

– Graylock• Clamping style system using two clamps

with the bolts oriented parallel to the piping diameter

EDS-2004/FL-54

DUR O LOK® CouplingsChanging the Load Path Reduces Size

and Eliminates Leaks

PPF-R01-16

10-1/2" Dia.

Large offset in loadpath leads to leakage

under stress, temperaturechanges and bolt

relaxation.

Load path through a bolted flange.

Small offset in loadpath and multi-groovedcouplers insure positive

gasket seal and consistentassembly dimensionsunder all conditions.

Load path through DUR O LOK coupling.

4-3/4" Dia.

Three Inch Class 300 Joint

A full encirclement multiple-groove design distributes the holding force around the entire pipe perimeter without excessive stress concentration. The load path through the flange is nearly straight, minimizing the bending imposed on the system. The offset bolting used for standard flanges creates large internal bending loads.

The compact design reduces cross-sectional area by 60% compared to flanges -- twice as many lines can fit in same rack space

Weight is as little as one-tenth that of a standard flange, saving material, transportation, and support costs.

Boltless, threadless design means pipelines can run close to walls or to each other because wrench swing room is not a factor. Assembly and disassembly is faster and easier.

Good for low pressure drop services and catalyst transfer (there is no inner “lip” to create catalyst fines). Also good to reduce the turbulence caused by the internal ledges of conventional flanges.

The components are supplied as a matched set and may not be interchangeable.

With Dur-O-Lok flanges, the piping must be assembled with a near perfect fit because it is not possible to use the bolts to pull or bend the system into alignment. They are also not suitable where external loads and moments are imposed.

Dur-O-Loks are currently available through 20-inch NPS. Larger flanges are possible, but the manufacturer is not set up to make larger flanges.

EDS-2004/FL-55

DUR O LOK® Couplings

PPF-R00-17

Lock Device &Assembly Prover

Seal Cavity

TaperedRetaining Ring

Split Coupler

Hub

The smooth interior surface with no lips, edge, or burrs prevents damage to catalyst during transport, prevents leaks in liquid service.

The interlocking grooves transmit forces evenly across the seal cavity to provide dimensional stability.

EDS-2004/FL-56

DUR O LOK® Couplings

PPF-R00-18

Seal Cavity

Lock Screw Groove

Split Coupler

Hub

Split Coupler

Multiple Matched Grooves

Self energized SealHub

TaperedRetaining Ring

This Dur-O-Lok Coupling has a simple, 7-part (counting the locking device) assembly process. A standard ASME flange has bolts, each with a nut on each end, two flanges, and a gasket. The Dur-O-Lock assembly process is easier than a standard flange - there is no concern with the order or amount of bolt tightening.

The self energizing seal uses the internal pressure to add to the sealing force. O-rings are used for low temperatures (up to about 450°F) and a proprietary design is used for more elevated temperatures.

EDS-2004/FL-57

Grayloc Assembly

PPF-R04-19

Seal Ring(Install Priorto Start-Up) Grayloc Clamp Half (Typical)

Bolting Centerline 4-Places (Typical)

"O" RingValve

GraylocHub

Studbolt (Typical)

Studbolt Nut

Grayloc Hubbeveled for

attachment topiping

Temporarily Installed Gasket Plate.Do not remove until welding of hubsto piping is completed and seal ringsare ready to be installed. (Save for

future use.)

The hubs are clamped together and then bolted. This design uses far fewer bolts than a standard flange. The bolts are oriented perpendicular to the pipe axis. The standard flange bolt orientation is parallel to the pipe axis. As with the Dur-O-Lok, this is an easy system to assemble and disassemble, and it requires less space than a traditional flange. There must, however, be bolt removal clearance perpendicular to the pipe, and the two sides of the connection cannot be pulled into alignment by the bolting process.

Gaskets are the component of a flanged joint that provides the seal against leakage. Compression of the gasket is introduced by bolt-up of the flange, causing the gasket to “flow” and fill the irregularities in the flange surface. This is called seating the gasket. The force must not be too high, however, because it can crush the gasket. The compressive force is applied perpendicular to the face of the gasket. The gasketing flow characteristics and the amount of force/stress needed to seat the gasket are dependent on the gasketing material.

EDS-2004/FL-58

Gaskets

Gaskets create a static seal between two members of an assembly and maintain the seal during operating conditions that may fluctuateThe seal is provided by gasket material flowing into imperfections in the mating surfacesThe force to effect the seal is provided by the bolting compressing the gasket

Different types and styles of gaskets have unique flow characteristics. The force to seat the gasket is the stress in pounds per square inch required to compress or deform and flow the gasket material. Sufficient force must also be maintained during operation to seal against internal pressure. The magnitude of this net force is quantified by a ratio termed “M” as described above. The stress on the gasket will be reduced during operation because the internal pressure tends to try to force the flanges apart. Resisting this force uses some of the force imposed by the bolts, leaving less available to compress the gasket.

EDS-2004/FL-59

Gaskets(continued)

Sufficient initial force is necessary to deform the gasket into the imperfections, thus “seating” the gasketFor gasket design, the necessary compressive stress depends upon the gasket style and materials and is called “Y”Sufficient force must be present during operation to maintain the seal against the internal pressure, preventing leakageFor gasket design, the required ratio of gasket compressive stress to internal pressure depends upon the gasket style and materials and is called “M”

Asbestos was used for years in nearly all services. Asbestos has excellent resistance to nearly every conceivable refinery atmosphere and flowing stream content and is suitable for use at all temperatures encountered in these plants. However, in the past 10 or 15 years, health concerns related to the inhalation of asbestos fibers have reduced its use.

Graphite is now a commonly used filler material. It is inert in a reducing atmosphere up to 3000°F; however it has a limitation of 850°F in an oxidizing atmosphere.

Teflon is also resistant to the great majority to atmospheres in refineries, but it is only good for low temperature service, up to about 450°F.

Many other proprietary materials are available, most of which are tailored for a specific application or set of circumstances. These materials are a complex combination of components and binders and are not suitable for services outside of those for which they were developed. If used in other services, they may fail very rapidly.

EDS-2004/FL-60

Gaskets(continued)

“Y” and “M” have no theoretical basis but are empirical, developed from experience.Gasket material must be suitable for the temperatures, pressures, and environment to which it is exposed.Gasket filler material was asbestos, now it’s generally graphite or, sometimes, Teflon or another non-asbestos, compressible material. Teflon is generally used in acid services below 400 ºF.

Spiral wound gaskets are the most commonly used. Ring joints can be seen to require a large seating force (“Y”) and must also maintain a large compressive stress vs contained stress ratio. Because they are most often applied to high pressure services, the large “M” value (5.5 - 6.5) means the sealing stress must also be very high. This can may make it more difficult to maintain a seal during operation when using a ring joint gasket than with other types of gasket. This is not usually a concern because the required seating stresses (“Y”) are very high to begin with and the sealing area is small, resulting in continuously high stresses.

Neither “M” nor “Y”values have theoretical standing, and those now in use are based on practical experience and some formal experimentation.

EDS-2004/FL-61

Representative “M” and “Y” Values

“M” “Y” (psi)Spiral Wound 2.5 - 3 10,000Corrugated & Jacketed 2.5 - 3.5 2900 - 6500Flat & Jacketed 3.25 - 3.75 5500 - 9000Ring Joint 5.5 - 6.5 18,000 - 26,000Flat FaceAsbestos 2 - 3.5 1600 - 6500Elastomers 1 - 2.75 400 - 3700Metal 4 - 6.5 8800 - 26,000

EDS-2004/FL-62

Metallic

Nonmetallic

Corrugated Metal

Covers most of the face to minimize flange rotation and possible contactLow seating stresses due to the large gasket areaUsed only in limited, non-hazardous servicesNonmetallic gaskets are made of a suitable, compressible material held together with a binder. Asbestos was common, most are now proprietary materials PPF-R00-20

Types of GasketsFlat Face

Flat ring gaskets are widely used wherever service conditions permit because of the ease with which they may be cut from flat sheets and installed. With a large seating surface, the seating load is spread over a large area and, as a result, it is difficult to obtain a high sealing pressure and “solid” seal. Therefore, these gaskets are used for low pressure services and services where small leakage is not a hazard (e.g., water and air). They are also used where piping joins to an ASME pump (used only in low pressure/temperature services) with a flat face flange.

EDS-2004/FL-63

DoubleJacketed

Jacketed

CorrugatedDouble Jacketed

This was the standard refinery gasketStill common for large diameters and applications with more thancircumferential sealing surfacesSoft compressible filler material is contained within a thin “wrapper”Corrugations provide the seal, and a labyrinth leak path

PPF-R00-21

Types of GasketsJacketed and Filled

The outer “wrapper” on a jacketed gasket makes the gasket more rugged by protecting the soft inner material from damage. It also provides some resistance to blow out.

Corrugated jackets are more rugged than flat jackets because the deformations add strength to the system. They also can provide a better seal because the sealing force is concentrated at the peaks of the corrugations; it is not spread over the gross surface of the gasket. The “labyrinth” path referred to means that it is a complex path with a lot of twists and turns, difficult to pass through. If the fluid finds its way past one corrugation, there is another waiting.

Jacketed gaskets, especially the corrugated style, require less seating force than spiral wound gaskets.

This type of gasket is sometimes used in large diameter, often low pressure applications, where a spiral wound gasket may tend to unravel or “spring” apart during handling. The problem is particularly acute for vertical sealing surfaces. When the spiral wound gasket is removed from its backing cardboard, it’s quite unstable. For a horizontal sealing surface, the gasket can lie atop one of the flanges. Regeneration Tower body flanges and large diameter access openings on FCC Reactor and Regenerators are examples of where they are used. Jacketed gaskets are also used for specialty exchanger gaskets, such as channel gaskets that also seal at the baffle between passes. It would be difficult, if not impossible, to wind a spiral wound gasket to include the diametrical piece.

EDS-2004/FL-64

A serrated metallic filler sandwiched between two layers of sealing materialUpon bolt-up the sealing material deforms into and is held by the serrationsSeal stress is concentrated at the peaks of the serrations, while the valleys prevent the sealing material from flowingMetallic filler can be reusedBecoming more popular, especially in Europe PPF-R00-22

Types of GasketsKammprofile

The serrated filler is fabricated of solid metal and has concentric grooves machined into the faces. This greatly reduces the gasket contact area on initial tightening, thereby reducing the total required bolt load.

The gasket material flows into the serrations of the metallic filler which holds the gasket in place. Blow-out of this style gasket is nearly impossible, and no special flanges (e.g., with grooves) are necessary. The assembly is mostly metal, hence it is rugged and requires less careful handling than other gaskets. No grooves or projections on the flange are necessary. The soft gasket material must be changed each time the flange is opened.

EDS-2004/FL-65

Gaskets for which an increase in the pressure to be contained increases the sealing pressure.Examples are O-rings, K-rings, and to a lesser degree, gaskets for ring joint flanges.Gasket material must be soft and deformable. Acceptable materials have low limiting temperatures (200-400°F).

O-Ring

Pressure

Seal

Seal

Pressure

Seal

Seal

PPF-R00-23

K-Ring

Types of GasketsSelf Actuated

This the type of gasket used in the Dur-O-Lok coupling.

The seal with this style of gasket is based on the internal pressure. Internal pressure deforms the gasket and seals it against the outside of the groove. The greater the pressure, the greater the provided seal.

Self actuated gaskets must be changed every time the flange is opened because they are soft and depend upon the ability to plastically deform. After use, they cannot be reliably removed and then re-deformed into another, slightly different, configuration to seal the flange again.

EDS-2004/FL-66

Ring joint gaskets are either oval or octagonal (oval is preferred because it works in the current and old groove shapes)Made of a variety of hard metallic materials to avoid distortionMust be compatible with the internal atmosphere and softer than the flange materialMust be no rougher than 63 microinch Ra Hard to handle

PPF-R00-24

Types of GasketsRing Joint

The rings are fabricated of solid metal, usually soft iron, soft steel, Monel, 4-6 percent chromium, or stainless steels. Oval rings will work in the current octagonal grooves and the older oval grooves. Octagonal gaskets only work in octagonal grooves.

The alloy steel rings must be heat treated to soften them. The gasket must be softer than the flange material to insure that it is the gasket that flows and fills imperfections, i.e., the gasket distorts upon use. The gasket can be replaced, although they are frequently reused. One reason for reuse is that if the flange or groove distorts during operation, the existing gasket (which also distorted) will continue to provide a seal. A new gasket may not function as well because it may not fit well into the distorted grooves. The second reason is that the gasket is made of soft metal that is not crushed or strained well beyond its yield point as other gasket materials and styles are. The gasket can, therefore, function as well on reuse as the initial use.

The gaskets are wedged into the grooves in the flanges. They fit against the sides of the groove and do not touch the bottom.

An internal ceramic rope is frequently provided to shield the gasket from the worst of the internal atmosphere.

EDS-2004/FL-67

Composed of alternating layers of compressible filler material and metal rings wound in a spiral, forming a labyrinth leak path sealed at the peaksMost common type of gasket in refinery service

PPF-R00-25

Types of GasketsSpiral Wound

The upper picture has an outer ring. The lower picture has an inner and an outer ring.

The sealing portion of the gasket is formed by layers of thin metal windings, as shown in the figures, alternating with the compressible gasket material.

Spiral wound construction creates a very difficult labyrinth leak path which gives good sealing characteristics. The spiral metal windings greatly enhance the resistance of the gasket to blow-out, to the point where it essentially never occurs. They act to resist any inner pressure via circumferential tensile stress in the same way a pressure vessel shell acts.

Spiral wound gaskets are forgiving to flange surface finish variation and fit-up.

The metal windings stiffen the gasket (hence the higher required seating stresses) and make it less susceptible to crushing than gaskets where the entire bolt load is carried by the compressible gasket material.

There are few, if any, welds in the gasket assembly.

EDS-2004/FL-68

Spiral Wound Gaskets

Commonly used in refineries due to excellent sealing performance through a wide range of temperatures, pressures, liquid or gaseous atmospheres (including hydrogen), and flange finishesLayers provide many sealing surfaces and a labyrinth path in the direction of leakageForgiving to seal stress variations (e.g., overstress) and less prone to relax over time

These gaskets are now specified in accordance with ASME B16.20. Up until the early 1990’s, API 601 governed gasket design and characteristics. Most API 601 requirements have been incorporated into ASME B16.20, so little was lost by the change. One notable difference is that API 601 specified default materials. If nothing else was called for, they were required. B16.20 does not have default materials so the designer must be careful to call out the filler, winding, and ring material for every gasket.

Type 304 stainless steel was the default winding material specified by API 601. It is still the most common material and is recommended for general use. Type 316L, or stabilized (Type 321 or 347) stainless steel may be required where sensitization, and intergranular stress corrosion cracking, is a concern.

Graphite windings may be unsuitable for oxidizing atmosphere services over 800°F because the graphite “dissolves”. For spiral wound gaskets, consider using another material, e.g., ceramic or mica, for the first few windings, where oxygen exposure is possible. The remaining windings may then be graphite.

Specialty filler materials are often tailored for a specific type of application. They may not work well, and may even be outright dangerous, in another service. They are often a blend of a number of components, and sensitive to variations in the proportions or quality of those materials.

EDS-2004/FL-69

Spiral Wound Gaskets(continued)

Asbestos used to be the “universal” filler materialFiller materials are now commonly a non asbestos material– Graphite and sometimes Teflon (for low

temperature applications) are the most common– Specialty materials are available for particular

conditionsFiller material must be specified for each gasket (there is no default)Windings are commonly 304 Stainless Steel– Must be compatible with the internal atmosphere

Stability refers to handling for installation.

The outer ring aligns the gasket by fitting against the inside edge of the flange bolts.

As noted in the jacketed gasket discussion, handling of large spiral wound gaskets can be difficult - they can “spring” apart and wobble around. Jacketed gaskets are sometimes specified for large diameter, low pressure services.

EDS-2004/FL-70


Gaskets have an outer ring for stabilityBy using the flange bolts as a guide, the outer ring is used to center the gasketThe outer ring also helps resist blowout and acts as a limit stop, preventing crushing of the outer windings from overboltingThe outer ring is normally carbon steel, protected against corrosion

As describer on slide 63, large diameter spiral wound gaskets are difficult to handle, especially in a vertical orientation. An inner ring helps keep them from unraveling. Use of a jacketed gasket is also often considered.

The inner ring also assists in maintaining the integrity of the ID of the gasket as the flanges rotate about the outer edge of the gasket and tend to push the gasket inward. It also provides additional gasket strength to resist blowout. For some filler materials (e.g., Teflon), an inner ring may be required by the governing standard (ASME B16.5).

Inner rings are also advisable when the mating flanges are different metallurgy's with differing coefficients of thermal expansion. As the system heats and cools, the differing movements may put the gasket into shear.

ASME B16.5 requires inner rings for some gaskets in high pressure services (≥ 24 inch Class 900, ≥ 12 inch Class 1500, and ≥ 4 inch Class 2500). UOP’s criteria is to provide an inner ring for all Class 900 and higher flanges.

EDS-2004/FL-71


An inner ring is provided for additional (handling) stability for large gasketsAn inner ring is also used to protect the flange surface from corrosion due to the internal atmosphereUse the same material as for the windingsAn inner ring is frequently used for Class 900 and higher flanges to resist inner deflection and possible gasket buckling due to the high bolt loads present– The bolt load tends to squeeze the outer portion of

the gasket and push the gasket inward

EDS-2004/FL-72


Graphite (and Teflon) filler materials act as incompressible fluids as the flanges press upon the gasketAs the gasket is squeezed, the filler material presses outward into the windings and ringThis can cause bucking of the windings and a failure of any gasket Asbestos is a system of compressible fibers and does not create radial forces as it is compressedAll winding and ring materials must be specified (is no default)

When the bolts for high pressure flanges are tightened, the large forces present may slightly deform the flanges and create a inward component of the force acting on the gasket. As noted on the slide, an incompressible filler material, such as graphite or Teflon, can lead to a similar problem. Asbestos, on the other hand, is a fibrous, compressible, material. Compression of the gasket does not create radial forces on the windings or ring.

The gasket inner ring needs to be checked to insure that this force will not damage the gasket (i.e., buckle the inner ring). Outer rings, and even the windings, may also be subject to buckling. This is the reason that, in high pressure service, the outer ring often has a wavy (non-planar) appearance.

The inner ring and winding material must be compatible with the internal atmosphere of the vessel or piping. As noted previously for the filler material, ASME B16.20 does not specify default materials for the ring(s) and windings. The designer must be careful to call them out. Often the outer ring is made of a low alloy material (carbon steel if the temperature is below 1000°F) because it is not exposed to the internal atmosphere.

Gaskets have standardized dimensions and tolerances specified in ASME B16.20 in accordance with Classes compatible with ASME B16.5 (and B16.47).

Seating of the gasket yields the gasket materials. Parts may even be crushed. When opened, the gasket does not return to its original dimensions or regain its original elasticity. Therefore, it cannot be reliably reseated and must be replaced.

EDS-2004/FL-73


DurableWorks with a wide range of fairly rough flange finishesEasily replaced because it rests upon a flat surface and is less affected by flange distortionsGaskets must be replaced each time the flange is opened

The finish is the smoothness of the metallic surface of the flange face that mates against the gasket. This is the sealing surface. The facing is both the sealing surface finish and the geometry (e.g., ring joint grooves, tongue and groove, raised face).

If the metallic surface is too rough, the peaks and valleys of that surface will be too large for the gasket to deform or flow and fill for seating.

If the metallic surface is too smooth, the gasket deformations will be too large and they will not match the surface deformations of the flange face. The gasket cannot “bite” into the flange surface and may slide or be deformed inward by the bolt load. A surface that is too smooth can be as bad, or worse, i.e., difficult to seal, than one that is too rough. Spiral wound gaskets have been shown to be susceptible to this problem.

B16.5 requires 45-55 grooves per inch so that a leak would have to pass at least 45-55 grooves for a 1-inch wide gasket. The finish is produced by cutting small concentric or spiral grooves, producing a series of peaks and valleys opposing a radial leak. A lapping or back-and-forth method of finish production is not acceptable because it produces some radial grooves, enhancing leakage.

EDS-2004/FL-74

Flange Surface Finish

Surface finish is critical to the proper performance of the flanged joint– Too rough or too smooth and the gasket will

not sealFinish is provided by a cutting tool producing serrated concentric or spiral grooves– Provide 45 - 55 grooves per inch

Surface finish is measured and designatedby “Ra”, an arithmetic average of the surface roughness

EDS-2004/FL-75

Determination of Ra

Centerline – The surface profile defines equal areas above and below this line.

Ra – Arithmetic average of the absolute values of the surface profile deviations from the centerline.

Ra ydL

Ra

LL= ∫

=+ + + + +

(approx) Y Y Y Y Y YN

1 2 3 4 5 N

10

. . .

PPF-R00-34

Ra is defined as Roughness Average and is intended to be a means of quantifying the surface finish. Mathematically the area above and below the centerline (average level of deformation) is equal. Ra is the absolute value of the surface profile deviation from this centerline. In other words, it measures (characterizes) the magnitude of the deviation from the centerline.

Ra is defined in metric terms (micrometers), but is normally represented in micro-inches, e.g., 125 microinch Ra.

ASME B16.5 and UOP allow a surface finish 250 Microinch Ra maximum for spiral wound gasketing. This is based upon vendor recommendations, successful operating experience, and experience that ASME’s older 500 Ra is too rough to obtain a good seal.

A minimum roughness value must also be specified; smoother is not always better. The gasket must deform to fill imperfections in the flange surface, but if the flange is too smooth, the gasket roughness becomes a problem. For spiral wound gaskets ASME B16.5 and UOP use 125 Microinch Ra , minimum.

Ring joints use a harder, usually metallic, material (e.g., soft iron) that is harder to deform and, therefore, requires a smoother finish.

EDS-2004/FL-76

Flange Surface Finish(continued)

There is an optimal surface finish for use with each gasket type -– 125 - 250 microinch Ra for spiral wound gaskets– 63 - 80 Ra for metal jacketed– 125 - 250 Ra for Kammprofile– 63 Ra for ring joint– 500 - 750 Ra for flat non-metallic gaskets

EDS-2004/FL-77

Flange Surface Finish(continued)

Surface finish is evaluated by visual comparison and feel to a standard comparitor of finishes.Mechanical means are not used because they may not distinguish between a uniformly rough surface and a smoother surface with a few large discrepancies (and it’s prohibited by ASME B16.5). Both may have the same calculated Ra but perform differently.Protect flange finishes from corrosion, oxidation, and damage during handling, PWHT, storage, etc. Cover with oil or petroleum jelly and a wood, plastic, or metal cover.Flanges may be refinished in the field.

Use of visual comparison for surface finish evaluation is an ASME B16.5 compliance requirement. Mechanical means are not accepted.

Refinishing must use a spiral or concentric groove pattern corresponding with the requirements in the applicable Standard (e.g., ASME B16.5). Lapping or any other means of finishing that may produce radial scratches or an uneven surface is unacceptable.

EDS-2004/FL-78

Flange Markings Required by B16.5

Manufacturers name and/or trademarkASTM material identification (specification, grade, and melt identification for forged or cast flanges)Flange Class (e.g., Class 300)Designation of governing standard, e.g. ASME B16.5

The size of a flange or flanged fitting is specified by its nominal pipe size. Use of nominal indicates that the stated size or dimension is only for designation, not measurement. The actual dimension may or may not be the nominal size and is subject to established tolerances.

A ring joint number defines a particular set of dimensions for the groove (width, depth, radius).

EDS-2004/FL-79

Flange Markings Required by B16.5(continued)

Nominal pipe size (NPS) of flange (e.g., 6 inch)

Ring joint flanges shall be marked with the letter “R” and the ring groove number per B16.5

All markings are to be on the outer rimof the flange for visibility while in operation

The “R” designation denotes the common, familiar, style of ring joint gasket. This is the gasket to use with ring joint flanges complying with ASME B16.5. These are the flanges used in refineries and petrochemical plants.

The “RX” designation is used for special nonsymmetrical, vented, gaskets for use with API 6B flanges, classes 720, 960, 2000, 2900, 3000, and 5000.

The “BX” designation denotes a symmetrical gasket with a vent for use with API 6BX flanges, classes 2000 - 10,000. These gaskets are boxier than the “R” style.

API 6B and 6BX are both old, out of print, unused publications for line pipe. “RX” and “BX” gaskets will probably disappear eventually. Note that these gaskets will not fit, and cannot be used with, B16.5 ring joint flanges.

EDS-2004/FL-80

Gasket Markings Required by B16.20

Ring Joint Gaskets– Manufacturer’s name or trademark– Gasket number prefixed by R, Rx, or Bx– Gasket material identification– Designation of ASME B16.20

EDS-2004/FL-81

Gasket Markings Required by B16.20(continued)

Spiral Wound Gaskets– Manufacturer’s name or trademark– Flange size (NPS)– Pressure Class– Winding material identification (may be

omitted for 304 windings)– Filler material identification

These markings are to be prominently displayed on the gasket. The markings are usually on the outer ring, and are not visible on an installed gasket.

It is critical that the proper pressure class of gasket, one matching the flange class, be used. Use of the wrong gasket can result in leaks and even catastrophic failure because the gasket does not fit properly. The gasket may be over or under compressed or warped, creased, and even folded. The outer ring may be too large for the bolt circle, resulting in gasket distortion as it is squeezed into place. Too small of an outer ring may result in the gasket not being properly centered on the sealing surface, especially when the sealing surface is vertical (because the gasket outer ring may rest upon the lower bolts). The gasket may not be strong enough for the seating forces or internal pressure.

Before starting up a unit, check to insure that the proper gasket is in each flanged joint. Plants have been started up with temporary rubber or cardboard gaskets in place, or even with no gasket at all!

EDS-2004/FL-82

Gasket Markings Required by B16.20(continued)

Spiral Wound Gaskets (continued)– Centering and inner ring material

identification (may be omitted for carbon steel outer and 304 inner rings)

– Flange identification if other than B16.5 (e.g., B16.47)

– Designation of ASME B16.20

EDS-2004/FL-83PPF-R02-26

In the illustrated case, the gasket was manufactured by Lammons. It is for a 2-inch pipe flange and is suitable for both Class 300 and Class 600 flanges. The windings (and inner ring, if it had one) are made of type 347 stainless steel. The filler material is flexible graphite and the gasket complies with ASME B16.20

Standard flanges conforming to ASME B16.5 and B16.47 may be freely used without any design calculations at the pressure-temperature ratings assigned. Listed gaskets and bolting materials must also be used in accordance with any guidelines in B16.5 and B16.47.

A flange may also be designed from scratch for custom applications. This allows wide flexibility in the choice of dimensions for the flange. Flanges for situations not covered by the recognized flange standards may be addressed, as may situations require different dimensions or ratings than used for the standard flanges. An example is a flange for a location with limited clearances. All designed flanges must comply with the rules of the governing Code.

EDS-2004/FL-84

Flange Design

Codes encourage use of standardized flanges

Codes often recognize standards in addition to B16.5

In situations where no suitable flange exists in an accepted standard, the flange may be designed

The rules in the ASME Boiler and Pressures Vessel Code, Section VIII, Division 1, Appendix 2 apply to most, if not all, situations. The provisions of Appendix S, including the Rigidity Index, should also be applied. Other Codes, specifications, and practices (e.g., ASME B31.3, the Piping Code) refer to the Appendix 2 rules.

Gasket seating is the stress required to deform the gasket to fill the mating surface imperfections and form a seal. It is achieved with direct force by bolting the flanges together with no internal pressure present. The full force in the bolts transfers directly to the gasket, supplying an applied stress.

Maintenance of a seal includes consideration of the internal design pressure and any other forces operating on the flange. The bolts provide the force necessary to maintain a seal against these conditions, using a seated gasket. The gasket stress generally decreases as internal pressure is applied because the internal pressure works to separate the flanges. Resistance to this force uses some of the force present in the bolts. The remainder of the bolt force is available to provide a sealing stress on the gasket. Increased internal pressure results in both a greater pressure trying to bypass the gasket (leak) and less force sealing the gasket (by tending to separate the flanges, counteracting some of the bolt force), a double whammy!

EDS-2004/FL-85

Flange Design(continued)

Design rules and methods are given in Appendix 2 of ASME Section VIII, Division 1

Method is required by the Pressure Vessel and Process Piping Codes

Design rules analyze two situations– Gasket seating– Maintenance of a seal against the internal

design pressure

Taylor Forge is one manufacturer that offers custom specialty size and class flanges (e.g., Class 175 and Class 350 large diameter flanges) designed in accordance with the requirements of the ASME Code. They are not included in B16.5 or B16.47. If a unique flange is needed, use of a specialty flange available from a supplier will save cost because the supplier has the necessary dies and manufacturing equipment available. The specialty flange must be checked in accordance with the Code rules. It cannot be used without a check even if the vendor says that it complies with the Code.

EDS-2004/FL-86


Design rules are actually evaluation methods

Design begins by selecting a gasketing system and flange dimensions, then evaluating them by comparing stress levels to allowable stresses for the materials at temperature

Flange dimensions may be from any available flange (e.g., Taylor Forge specialty classes) or may be uniquely developed

The design rules that are currently in Section VIII, Division 1, Appendix 2 are stress based. The stresses within the flange are determined and compared to allowable stresses. Although it has proven to be a successful approach, this is actually an indirect and somewhat flawed method. This is because leaks develop as a result of gaps, which are themselves a result of deformations or strain. Strain is often related to stress -- at least while in the elastic region -- deformations may not be directly related to stress. The evaluation method may ensure the flange does not yield and that a reasonable load is applied to the gasket but it does not, for example, ensure that the force is uniformly applied to the gasket.

Other methods are being developed that directly analyze the potential for leakage (considering the contained fluid, including the size of its molecules), the real problem being targeted. These design methods will likely become a part of the Code shortly, but they are not included yet. The design method covered in this presentation is the Code’s stress based approach.

EDS-2004/FL-87


Flanges in most accepted standards have been designed in accordance with the ASME Code method– B16.5 flanges are an exception

Other design methods which evaluate flanges for their tendency to leak based upon the design conditions and the contained fluid are available, but not yet incorporated into Codes

Internal pressure acting across the inside diameter is commonly termed the Hydrostatic End Force (HD) and comes to the flange from the closed end of the piping system or vessel to which it is welded. The end force reaches the flange through the hub and pulls on the ring portion of the flange.

For a gasket covering the entire raised face, e.g., a sheet gasket, the internal pressure acting upon the exposed face (HT) would be zero. As a conservative allowance leakage, hence pressure, is assumed to be possible as far as the average gasket diameter. Spiral wound gaskets do not cover the full surface of the flange face, hence the force acts over the exposed surface inside of the gasket as well as half of the gasket width.

EDS-2004/FL-88

Forces Acting on Flanges

Internal pressure acting across the inside diameter acts to open the joint

This force is applied axially through the pipe or nozzle wall

Internal pressure acts upon the exposed flange face inside of the gasket, tendingto open the joint

Bolting drawing the flanges together creates the sealing force on the gasket.

The bolts are the primary source of force to seal the gasket and keep the joint closed. Sometimes other factors, such as an axial compressive force or a bending moment at the flange, also contribute, at least over part of the circumference.

The bolt force is defined as W.

In order to calculate the moments and resulting stresses acting on a flange, the forces are multiplied by the appropriate lever arms which are measured from the point of force application to the bolt circle.

The initial compression force applied to a joint must be sufficient to initially seat the gasket and flow the gasket into the imperfections on the gasket seating surfaces. It must also be great enough to compensate for the total hydrostatic end force and any applied forces and moments present during operating conditions while maintaining enough residual load on the gasket/flange interface to maintain the seal.

EDS-2004/FL-89

Forces Acting on Flanges(continued)

The sealing force on the gasket acts over most of the gasket surface

The reciprocal force tends to open the joint, but relaxes if the joint does tend to open

Bolt forces act at the centerline of the bolt circle– This force tends to close the joint

An external bending moment acts to compress one side of the gasket and unload the other side, which has a direct effect on sealing of the gasket against the internal fluid.

For gasket seating, there is no internal pressure because the unit is not yet operating.

For seating of the gasket, the stress analysis involves a comparison of calculated values against ambient condition allowable stress values. Some ASME B16.5 Class 150 and Class 300 flanges, particularly those just before the number of bolts changes, may be found to be overstressed for gasket seating of spiral would gaskets, although experience shows that they work in service.

EDS-2004/FL-90


Applied axial or bending loads– May tend to open or close the joint

For gasket seating, consider only the seating force necessary (based upon “Y” for the gasket) and (to allow for overtightening) the average of the resulting bolt load and the bolt load using the allowable bolt stress

Use ambient allowable stresses

The required sealing force on the gasket (HG) must be large enough to squeeze the gasket together and maintain the seal against the internal pressure trying to leak around the gasket during operation.

The gasket factor “M” relates the gasket stress required to maintain a seal at design pressure to the design pressure. It is generally on the order of magnitude of 3. An “M” factor of 3 means that a sealing stress over the gasket surface of at least three times the design pressure is necessary to maintain a seal, e.g., 1500 psi for an internal pressure of 500 psi.

See slide 61 for representative “Y” and “M” values for various gasket styles.

The calculated stresses in the flange for operation are compared against the allowable stresses at the coincident design temperature.

EDS-2004/FL-91


For operation, consider all of the forces and allowable stresses of each component at their design temperature

Sealing force on the gasket is based upon the internal pressure and “M” for the gasket

EDS-2004/FL-92PPF-R01-27

ForcesHG = Compressive force on

the gasketHT = Pressure force on the

flange faceHD = Hydrostatic end forceW = Tightening bolt force

Forces Experienced by a Flangein Operation

This figure illustrates the forces acting upon a flange.

HT is determined from the internal pressure and is considered to act up to the point of the resultant gasket reaction force HG. This will be a point beyond the inner diameter of the gasket.

The bolt cross sectional area determines the magnitude of the available cold boltup seating force and the operating bolt load available to maintain a seal. The total bolt area is determined by the number and size of the bolts and the bolt area at the root of the threads because the bolt is threaded the full length, exposing the threaded portion to the full bolt load.

EDS-2004/FL-93

Follow the procedure of ASME Section VIII, Division 1, Appendix 2For convenience some flange and gasket manufacturers (e.g., Taylor Forge) provide worksheetsCheck required bolt area (must be less than the available bolt cross sectional area at the root of the threads), bolt spacing (maximum per Code to prevent flange deflection and possible loss of seal between bolts), and stresses in the flange

Design Procedure

Most of the stresses within the flange are uniform across the cross section, i.e., membrane stresses. There is no redundancy or ability to redistribute these stresses if they become excessive; therefore, the Code allowable stresses are applied.

For a bending stress, the maximum exists on the inside and outside surfaces of the hub, and decreases to zero at a point half way between. If a slight overstress in the hub causes yielding, the load redistributes more to the ring portion of the flange. The ring is also subjected primarily to bending and is thus able to absorb the additional load so that a new equilibrium within safe limits is established. Therefore, a higher allowable, 1.5 X the Code value, is permissible in these cases.

EDS-2004/FL-94

Design Procedure(continued)

Most allowable stresses are as listed in the CodeLongitudinal hub stress may reach 1.5 x Code allowable because it is a bending stress (maximum at a point, decreasing to 0 at the neutral axis)If slight yielding occurs, the load redistributes safely

EDS-2004/FL-95

PositionsG = Average gasket diameterB = Flange I.D. (bore)hG = Distance from bolt hole to the gasket force, HGhT = Distance from bolt hole to the position of HThD = Distance from bolt hole to the position of HD PPF-R02-28

Critical Dimensions in a Flange

Standard practice is to dimension, and determine the internal bending moments, relative to the position of the center line of the bolt holes.

Bolting tends to load the outer portion of the gasket more than the inner portion of the gasket because the flange body is not infinitely stiff and does rotate very slightly. This can have a tendency to expose an inner portion of the flange face to the internal atmosphere over which HT acts. Conservatively this increases HT..

This commonly used approach results in a very conservative design. It is intended for routine design investigations into the effect of loadings other than internal pressure on flanges. The internal design pressure is added to the calculated equivalent pressure from the other loads. Determination of other loads can come from a flexibility analysis of the piping system, for example.

The applied loads may be axial or shear forces and torsion or “prying” moments. The axial force and the resultant “prying” moment (determined by a vector sum of the y and z moments) are considered to affect the flange seal and are converted to an equivalent pressure. Shear forces and torsional moments apply shear stresses to the bolts but do not (directly) affect the flange’s ability to maintain a seal.

EDS-2004/FL-96

External LoadingsEquivalent Pressure Method

A conservative method of evaluating the suitability of a flanged joint for axial and bending loads imposed by the piping

Loads are converted into an “equivalent” internal pressure, which is added to the actual internal pressure

The flange is then evaluated for the total pressure (internal pressure + equivalent pressure)

EDS-2004/FL-97

External Forces on a Flange

PPF-R00-92

Thermal Growth

F

M

V

V = Vertical shear from, for example, thermal growth acting parallel to the flange face (mostly affects the bolt shear)

M = The resultant bending moment from, for example, thermal growth or weight acting perpendicular to the plane of the flange face. It tends to open the flange on one side and compress or close it on the other.

T = Torsional moment from, for example, thermal growth acting in the plane of the flange face.

F = Axial force due, for example, to thermal growth of the horizontal run of a piping system acting perpendicular to the flange face. It can be a tensile or compressive force on the flange.

The equivalent pressure formula converts the axial load into a force per bolt and determines the maximum bolt force caused by the bending moment and assumes it’s presence on all of the bolts. The internal pressure that would cause these same bolt forces is then found, and called the equivalent pressure.

One reason this is conservative is that is considers the maximum tensile bolt force caused by the moment to be present on all of the bolts. It also assumes only the bolts resist the moment when, in fact, the moment places the gasket into compression over about half of the circumference.

EDS-2004/FL-98

External LoadingsEquivalent Pressure Method (continued)

Imposed loads are converted into an equivalent pressure via the following formula:

PEQUIV = 16 M / π G3 + 4 F / π G2

where: M = Bending momentF = Axial force

G = Diameter at gasket reaction

The equivalent pressure method illustrated is used for looking up the Class of a flange to be used for anticipated conditions of internal pressure and externally imposed loads.

The illustrated method is simple to implement but is very conservative.

EDS-2004/FL-99

External LoadingsEquivalent Pressure Method (continued)

Shear and torsion on the bolts must be considered separatelyThe method is very conservative, especially for bending loads

Flange faces that are not parallel will impose a moment on the flanged connection when it is bolted up. This moment will act to open up the joint on one side.

Residual forces and moments are developed as a result of flange mis-alignment or poor fit-up and can overstress the bolts or cause flange rotation, resulting in flange leakage.

Gaskets that have been folded, creased, twisted, etc., cannot be seated to form a seal. They may even function like a wedge, thicker on one side than the other, forcing the flanges apart.

ASME B31.3 requires that each flange be aligned to within 1mm per 200mm (1/16 inch per foot) across any diameter. Many believe this is too large, and call for an alignment tolerance of half this value, i.e. 1/2 mm per 200 mm (1/32 inch per foot). In some cases, e.g., flexible small diameter piping, a larger margin may be acceptable.

EDS-2004/FL-100

Flange Assembly

Ensure flange faces are parallelAvoid pulling or pushing the flange into assembly position– The resulting piping forces will affect the joint’s

performanceBe sure the gasket is properly placed and centered– The gasket must not be creased, twisted, bunched,

etc.

Uniform bolt tightening assures uniform seating of the gasket and accurate application of the calculated design loads in the flange.

Bolt tensioning devices achieve an accurate and pre-determined bolt loading in a single operation. The most common form of bolt tensioning essentially provides a hydraulic load which acts directly upon the stud bolt. The hydraulic load is transmitted to the bolt by the “puller”. This force stretches the bolt, and the extension, or strain, is retained by engaging the nut. Since strain and stress are proportional in the elastic range, the applied extension results in a known applied bolt load equal to the force applied by the “puller”.

The hydraulic load, commonly called the applied load, is retained by the bolt when the hydraulic tensioner is released. The magnitude of the load is determined to provide the necessary gasket seating and sealing stresses.

Bolt tension indicating washers are another reasonably accurate method of insuring a known bolt force. These washers are bowed up in the center. They are designed to flatten under a known force. When the bolt is tightened, the washer flattens when this force is reached. Use of the proper washer then indicates when the required bolt force is present.

Other methods of bolt tightening develop much less predictable bolt tensions. Use of a torque wrench, or measurement of the applied torque, is especially unreliable. The angle of the threads, friction along the threads, type of lubricant, and friction between the nut and outer surface of the flange are all variable factors that resist the applied torque without resulting in any load being applied to the gasket. Use of a torque based method for providing bolt tension typically results in a 30 percent (or greater) variation in bolt stress around the flange. Bolt tensioning methods normally result in a variation of about 10 percent.

EDS-2004/FL-101

Flange Assembly(continued)

Do not overtighten the bolts– Bolts may yield, the gasket may be crushed, or the

flange may be distorted (e.g., the outer portion squeezed together, causing a rotation tending to unload some of the gasket)

Tighten the bolts uniformly– Bolt tensioning devices are the most reliable method– Torquing is common, but unreliable

The objective is to close the flange at a uniform and constant rate, bringing all portions of the flange around the full circumference together at the same rate. Uneven compression of the gasket around the circumference must be avoided. Uneven gasket compression from tightening one side before the other cannot be effectively recovered.

Use of a flange cement may even glue the gasket to both flanges, meaning the gasket must be torn apart to separate the flanges. Gasket remains may remain stuck to the flanges. The glue can also damage, perhaps etch, the flange sealing surface, promoting current or future leakage.

Bolt lubricants are often recommended. Lubricants reduce the unknowns related to friction when bolts are tensioned via torquing. The type and amount of lubricant are chosen so that the nut does not “back off” or loosen.

Bolts tend to “cross talk”, or be affected by what happens to other bolts. Each time a new bolt is tightened it will reduce the strain, and stress, on the previously tightened bolts. This is why the bolts are tightened in stages and why after each stage the gap between the flanges is measured and evened by adjusting individual bolt tightnesses before proceeding with the next round of tightening. After three stages, further adjust bolts in a circumferential pattern to even the flanges.

Thick, large diameter flanges with large diameter bolts may also need a disassembly procedure. Usually this is to back-off to 50 - 70% of the bolt load, even the flange gap, drop to 20 - 30% of the bolt load, even the gap, then remove the nut.

EDS-2004/FL-102

Flange Assembly(continued)

The order of bolt tightening is very important– Tightening in circumferential order will not achieve a

uniform seal (recent evidence suggests this may not be as bad as previously thought)

– Tighten bolts in a diametrically opposed pattern– Do not fully tighten the bolts in one step– Proceed through the tightening sequence several times

(usually three) to achieve the desired bolt force– After each step, adjust to an even gap between the flanges– Refer to ASME PCC-1 2000, “Guidelines for Pressure

Boundary Bolted Flange joints Assembly”

Avoid use of flange “cements” on the gasket -they tend to damage the flange surface and are difficult to remove without causing further damage to the sealing surface

EDS-2004/FL-103

Sequential Order1-23-45-67-8

9-1011-1213-1415-1617-1819-20

Rotational Order1

135

1793

157

1911

2146

18104

168

2012

PPF-R00-29

This is a graphical representation of the recommended sequential and rotational order of bolt tightening for uniform sealing of a flanged joint with 20 bolts. It can be seen that two diametrically opposed bolts are tightened followed by two more on a diameter approximately 90o from the previous set. This procedure continues until each bolt has been tightened once, then is repeated. This approach is applicable to any similar situation of bolt tightening, e.g., automobile tires. Usually three cycles are used to fully tighten the bolt. Do not tighten any bolts fully on the first two cycles because the flange system is still loose enough that flange rotation and unrecoverable uneven gasket compression can (will) result.

B16.5 (Annex F) requires the bolt threads to be engaged to the surface of the nut. B16.5 also says that the points on the bolt are not part of the bolt’s overall length. B31.3 permits an underengagement of one thread. Standard practice is for at least two threads of the bolt to be exposed beyond the nut. This ensures full engagement and the ability to fully develop the bolt’s strength. More extension is wasteful, may create clearance problems, and exposes more of the bolt to rust or damage, complicating disassembly. Under engagement may reduce the bolt’s capacity below that required. If below the surface of the nut, a depression is formed that can collect dirt, water, oil, etc. This can complicate disassembly and contribute to bolt corrosion (rust).

EDS-2004/FL-104

Sequential Order1-23-45-67-8

9-1011-1213-1415-1617-1819-2021-2223-24

Rotational Order19

175

13213

11197

1523

210186

14224

12208

1624

PPF-R00-30

This figure illustrates the sequence for tightening a flange with 24 bolts.

Radial scratches or corrosion are one of the most common causes of leakage. The scratch provides a direct, preferential leak path under the gasket. Scratches are often too deep and/or too narrow for the gasket to effectively seal.

Take precautions to prevent damage and protect flanges during any Post Weld Heat Treatment. Corrosion, oxidation, etc., may interfere with the ability to seat the gasket and its subsequent ability to maintain a seal against pressure.

Flanges out of parallel or containing a wavy surface finish (not flat) can cause a non uniform seating of the gasket.

EDS-2004/FL-105

Leakage Causes

Damage to the flange surface (especially radial scratches or corrosion)Imposed loads (e.g., flange misalignment, i.e. mating flanges are not parallel, thermal movements, weight)Flange faces are not flat– Are distorted or contain raised or depressed areas

Flange rotation about the outer ring of a gasket will cause a decrease or unloading of the gasket seating/sealing stress at the ID of the gasket. Stresses on the outer portion of the gasket will increase, possibly to the point they crush the gasket. The gasket will be substantially thinner at the OD than on the ID.

Ensuring the nuts are the proper size for the bolt, and that the bolt is properly engaged in the nut are critical for proper gasket performance, achievement of a seal, and safety. Special care must be taken to ensure proper bolt engagement in studding flanges.

One example graphically illustrates the consequences of improper engagement. The bolts on a studding flange outlet of a high pressure hydrogen compressor were not properly tightened. The bolts were also a size too small. The latter fact meant that only the tips of each thread, not the full depth, was engaged. Because it was a studding flange, neither the bolt nor the engagement was visible. Proper tightening would have ensured engagement of all the threads and revealed any bolt weakness during boltup because the full bolt stress would have been applied. The problem would have been revealed by a pressure test too (either hydro of pneumatic, probably the latter for a compressor), but none was performed. As it was, the bolts held at first, but the thread tips sheared off and the connection failed as startup proceeded and the bolt stress increased. Hydrogen was then released under pressure and ignited. There were several fatalities in the ensuing explosion.

EDS-2004/FL-106

Leakage Causes(continued)

Flange “cupping” or rotation due to bolt loadsImproper surface finish– Too rough and the gasket may not seal– Too smooth and the gasket may slide and/or not seal)

Inadequate thread engagement between the bolts and nutsVibration may cause the nuts to “back off” or loosen slightly, reducing the bolt force and, therefore, the sealing force on the gasket

EDS-2004/FL-107

Flange Prior to Boltup

Weld

Weld

Highly StressedAreas

Possible FlangeContact

Distorted Flange (Exaggerated) after Boltup

Gasket Load Movesto Outside

PPF-R00-93

Bolt Force

Gasket Maybe Forced Inward

Flange Distortion Due to Boltup

If the flanges come into contact, some of the bolt force will be transferred at the contact point rather than through the gasket, reducing the sealing stress on the gasket.

Flange distortion reduces the stress on the inner part of the gasket and increases the stress on the outer part. This shift can result in a net inward radial force on the gasket, possibly extruding the gasket inward, and can also result in crushing of the outer part of the gasket due to high local stresses. For these and other reasons, leakage may result.

Flange distortion produces highly stressed areas in the flange, including the welded joint to the pipe (or vessel). Thermal residual stresses, differing material properties in the weld’s heat affected zone, and weld flaws may already exist here. Cracking potential is increased.

One cause of differential expansion is flanges and bolts at different temperatures during startup/shutdown and other transient thermal conditions. The bolts are typically slower to respond to internal temperature changes. They are generally cooler than the flange because they are on the outer extremity of the flange and exposed to atmospheric cooling conditions. The poor heat transfer mechanism between the flange and bolts also slows heating of the bolts. The greater thermal expansion of the flange increases the bolt strain, perhaps into the inelastic range. The bolts later “catch up” to the flange temperature, further elongating and possibly loosening, leading to leakage. When the system cools, inelastic strain is not recovered, the bolt loosens, and the flange leaks. Steady state conditions can result in the same phenomena when the flange is warmer than the bolts, often the case in an uninsulated system. If the bolts are in the creep range, creep relaxation over time is another problem. On cooldown, cooling of the bolts more rapidly than the flange can create excessive bolt strain. Since the bolts are more exposed to cooling and are also a much smaller mass than the flange, hence containing much less heat, more rapid bolt cooling is not at all unlikely.

Another cause of differential thermal expansion is bolts and flanges of different metallurgy's. If the bolts expand more (e.g., austenitic stainless steel bolts in a low chrome flange), they obviously loosen. If the flange expands more, the resultant bolt strain may yield the bolt. Inelastic creep elongation is also a possibility. In either case, the result is a loss of strain at shutdown because the bolt is inelastically distorted and does not return to its original length at shutdown, thereby loosening. Leaks develop during shutdown. Creep can result in a loss of bolt strain over time, resulting in leakage development after a period of successful service, and/or at shutdown. Another problem is the high gasket loads caused by the flange growth. The load can be enough to crush the gasket.

If the flanges are made of different materials (e.g., austenitic stainless steel paired with low chrome steel), it can be very difficult to insure that one or the other of the above problems won’t occur. Low chrome bolts in such a “mixed” flange are usually best. They expand less than the flanges, tending to tighten (and possible yield). Austenitic stainless steel bolts will expand more than the flanges, thereby loosening.

EDS-2004/FL-108

Inadequate bolt tightness– May be due to:

• An improper bolt tightening sequence• Inelastic relaxation (e.g., creep) or yielding

of the bolt• Yielding the bolt or differential expansion

between the bolt and flange (e.g., the flange expands more and yields the bolt or the bolt expands more and loosens)


EDS-2004/FL-109


Insulating an existing flange increases the bolt temperature and thermal expansion relative to the flange, possibly tending to loosen the bolt.Insulating the assembly may increase the temperature to the point where the flange is no longer rated for the temperature (i.e. piping where credit was taken for the lower flange and bolt temperatures of uninsulated flanges).Uninsulated flanges are exposed to uneven cooling from weather conditions.

Insulating existing flanges to reduce process heat loss can be very detrimental to the flange bolting and can cause the the bolt temperature to increase beyond the material limits of the Code. Even if that is not a concern, elevating the temperature of the bolt relative to the flange when compared to the uninsulated condition, will cause the bolt to elongate relative the the flange thickness, reducing the net applied bolt force, potentially leading to leakage.

In some cases, flange and bolt temperatures lower than the process temperature, in accordance with B31.3 procedures, were used to select the flange class and the bolt and flange materials. Insulating the flange will increase the flange and bolt temperatures and may result in conditions beyond the ratings of the flange Class, in addition to potential concerns with the adequacy of the materials used.

All of the above conditions, and others, can result in a loss of the bolt force, which translates directly into a loss of the gasket sealing stress and leakage.

Uneven cooling of uninsulated, hot, flanges can cause them to warp and leak. For example, rain may cool the top of a flange in a horizontal line while the bottom remains hot. One way to address this is to provide a rain or weather shield over the flange. It must allow sufficient air circulation and be far enough from the flange to avoid any insulating effect. The flange may have been designed considering the uninsulated temperature Another problem is that if part of the flange becomes “insulated”, and part is not, uneven temperatures and possible warping are present.

Some leakage may be due to the loading/unloading characteristics of the gasket material. Gaskets behave generally inelasticly and follow different stress strain curves when loading and unloading. This may result in leakage when unloading, for example. Depending on the elasticity or cold flow properties of the gasket, its ability to respond and sustain a load may be impaired. Selection of a different gasket material with preferable cold flow properties and improved load carrying capacity would be warranted.

Leaks have been traced to the presence of shipping or temporary gaskets, cardboard gaskets, and even no gasket at all. Carefully check all gaskets before testing or startup. In one instance, use of a gasket intended for a flange several classes lower resulted in a major flange failure, a fire, and a fatality.

A superheated steam condenser (steam on the shell side) is an example of a case where the temperature may vary around a body flange. The bottom of the exchanger contains liquid water and is much cooler than the top, which contains superheated steam. The temperature differential, and resultant varying thermal expansion around the flange, can lead to leaks. There are many ways to address this problem, from flow changes within the exchanger to reduce the temperature differences to some of the measures discussed later to reduce the bolt strain differentials.

EDS-2004/FL-110


Rapid startup causing the flange to heat and expand before the bolts– May inelastically deform the bolts– When the bolts then heat and expand, they will, in

effect, loosenImproper gasket type or materialDamage to the gasket, e.g. crushed, extruded or buckledUse of a gasket intended for a different flange classDifferent temperatures around the flange circumference

EDS-2004/FL-111

Behavior of Boltin an Unyielding Flange

PPF-R01-31

Creep is the increase in strain with time under constant loading conditions or the decrease in stress under constant displacement.

With a constant bolt displacement imposed by an unyielding flange, creep relaxation, or a reduction in stress with time, will occur if the bolt temperature is in the creep range. The figure illustrates this rather dramatically. In the example, bolt creep relieves 2/3 or more of the initial elastic extension, resulting in a decline in the hot bolt stress to 1/3 or less of the original value. This is one reason to avoid insulating flanges. Insulating them may move the bolt temperature into the creep range.

High temperature bolt creep relaxation causes insufficient gasket load and flange leakage.

Attempting to correct a mis-alignment with flange bolts can overstress the bolts causing yielding of the bolting and subsequently loosening of the joint during operation and leakage. Do not try to correct the problem with the flange bolts, consider a spacer instead.

Flange refinishing can be performed to obtain the proper surface conditions. Refinishing is best done under controlled conditions in a shop, but can be performed in place in the field. Care must be taken not to remove too much material, reducing the flange thickness or raised face height. Some of the benefit of the raised face may be lost. It may also be more difficult to “make up” the connection because the flange sealing surfaces will be a little further apart. A thickness reduction may result in elevated stress levels or rotation also. All of these points are concerns only when a relatively large amount of material is removed. They are not a problem for a normal refinishing operation.

A more common refinishing concern is the method of refinishing used. A back-and-forth method (“lapping”) or moving a circular sander type of device over the surface will be unsuccessful because it will create radial grooves or scratches. These are ideal leak paths. Refinishing must use a spiral or concentric finishing operation, similar to the original finishing method, that produces the proper number of grooves over the sealing surface.

EDS-2004/FL-112

If flanges are mis-aligned, separated, or rotated, consider a spacer between the flanges, with a gasket on each sideIf the surface finish is damaged or incorrect, consider refinishing the flange– Take care to retain the required flange dimensions after

refinishing– In some cases, consider a different gasket that will perform

with the existing finish– Use a spiral or concentric finish, do not lap

Replace the gasketEliminate forces on the flange (e.g., thermal movements, weight, misalignment)

Leak Correction

EDS-2004/FL-113

Solution– Do not try to correct

the problem with the flange bolts – they can be overstressed

– Do use spacers with a gasket on each side to correct the problem

PPF-R01-32

Flanges Badly Cockedor Separated Too Far

This figure illustrates the use of spacers for flange misalignment. Use of a spacer may require longer bolts.

Use of a space for misalignment (the bottom case on the slide) may allow proper gasketing, but is limited by the need for the flange bolts to pass through the holes in both flanges. There may also be a pressure drop created at the misalignment.

Hot bolting is a dangerous operation, with safety the primary concern. By definition, there is something wrong with the joint and it is not sealing well. There is probably a leak of something dangerous and probably hot. Some things, like a hydrogen fire, cannot be seen, yet are lethal. The act of tightening the bolts is also a danger. As described before, they must be tightened in the proper order and to the proper tension, but use of a bolt tensioner is not practical. The only plus to hot bolting is that it may stop the leak and allow continued operation to a scheduled shutdown.

As alternatives to double nutting, nut loosening may be prevented by tack welding the nuts into place or “spiking” (damaging) the bolt threads to prevent movement of the nut. Neither of these methods requires use of a longer bolt, but complicates removal of the nuts if the flange is opened. Most likely new bolts and nuts will be necessary.

EDS-2004/FL-114

Leak Correction(continued)

Tighten bolts while on stream (hot bolting)– Used if the flange leaks after a period of successful

operation at elevated temperature without a shutdown, indicating the bolt has elongated due to creep

– May be used if the bolt has been inelastically strained (yielded), e.g., elongated during startup

– Used if the bolts thermally expand more than the flange– Dangerous, especially if the flange is leaking– Bolt replacement will be required at shutdown

If bolts loosen due to vibration, consider the use of double nutting– Will require longer bolts and more clearance to

accommodate the two nuts– Can also weld the nut or spike the threads

As these points indicate, many of the leakage problems seen in the field are due to differential growth (thermal or creep) of the bolt length between the nuts and the flange thickness for the same distance. Greater bolt growth and they loosen. Greater flange growth and the bolt is overstrained. Different relative bolt and flange conditions during startup and shutdown than are present during steady state operation can cause bolt damage that affects long-term performance.

EDS-2004/FL-115

Leakage Correction

Leakage only at shutdown may indicate the bolt was inelastically elongated during operation by greater thermal expansion of the flange, or possibly creep of the bolts– Bolts should be replaced

Leakage, and the need for hot bolting, after achieving an elevated operating temperature may mean the bolts loosened due to greater thermal expansion than the flange

Further complicating the problem are the different temperatures (hence thermal growth) seen by the flange and the bolts and the potentially different materials (with different expansion coefficients) used for the two components. For example, stainless steel bolts will expand much more than a low chrome or carbon steel flange.

During large start-up temperature changes, the flange heats first and the thermal expansion is greater than that of the cooler bolt, potentially resulting in permanent bolt stretch. When the bolt later heats and expands, the growth is additive to any permanent bolt stretch, causing a lower gasket pressure and possible flange leakage. Hot bolting may retighten the bolt and restore the seal.

Hot bolting is helpful if there is additional elastic strain capacity, i.e., more strain results in more force or if the bolt has thermally expanded and loosened. A bolt inelastically strained during startup (e.g., due to uneven heating of the flange system) will behave elastically when the temperatures equalize, though permanent elongation (strain) is present. If inelastic strain is present due to greater steady state expansion of the flange than the bolt, the bolt is still in the inelastic range and hot bolting will further strain the bolt but will not increase the stress or force. Continued “tightening” of the bolt will not improve the seal, but will strain the bolt, eventually to failure.

EDS-2004/FL-116

Leakage Correction(continued)

If the flange heated first and yielded the bolts, then the bolts expanded and loosened, hot bolting may correct a leak present at or near startup

Hot bolting is not effective in other cases of bolt yielding because the additional strain (due to tightening) will not result in a significant bolt force increase

EDS-2004/FL-117


Hot bolting may be avoided by using proprietary products such as Belleville Washers, or spring washers, to provide a constant bolt stress over wide temperature ranges, elongation of the bolt relative to the flange from thermal expansion or creep, and vibration exposure

Belleville washers look like an umbrella and act like a spring between the nut and the flange. They maintain bolt tension and seal pressures when the bolts creep at higher temperature or are stretched by differential expansion between the flange and the bolt. They are effective when the bolt elongates slightly more than the flange. A small increase in bolt length can greatly decrease the bolt strain and, therefore, the force. The washers keep the strain in the bolt. They do not help for the reverse situation unless the situation is temporary and permanently stretches the bolt resulting, in effect, in an elongated bolt. Of course, the washer must not be crushed during the time the flange expands more than the bolt.

The washers give an effective spring force that compensates for different expansion rates in a joint to maintain a high tension load in the bolts. The spring effect allows load compensation to account for deflection of the bolts.

Strain is a dimensionless value defined as the change in length (deformation) divided by length. Using extra long bolts and a sleeve greatly increases the overall length. The change in length due to thermal expansion is nearly the same as for a shorter bolt because the uninsulated extended portion of the bolt is much cooler than the portion between the flanges and expands little. Therefore, the numerator (change in length) increases a small amount and the denominator increases appreciably resulting in a reduced total strain change. In the elastic range, strain is directly proportional to stress and, as a result, the bolt stress change due to thermal expansion within the flange is also reduced. The bolt stress necessary to seat and seal the flange, provided by tightening the bolt, must still be provided. This will require a greater overall bolt extension to achieve the necessary strain.

For a reasonable case where the thermal growth of the sleeved bolt is 50% greater than the non-sleeved bolt, but the length between the nuts is 4 times as long:

Strain change (normal bolt length) = thermal change in length between flangesbolt length between flanges

<is greater than>Strain change (with a sleeve) = (thermal change in length between flanges) X 1.5

bolt length between flanges X 4

The effects of creep may also be modified. There must be sufficient stress in the bolt to seat and seal the gasket. If part of the bolt is in the creep range, creep will still occur. One benefit is that differential thermal expansion will add less strain and stress to a long bolt. This lower stress will reduce the effects of creep compared to what would otherwise occur. Another benefit is that creep occurs only over the portion of the bolt hot enough to be in the creep range. The extended part of the bolt is probably not hot enough. For the reasons described above, the creep will relieve less strain than for a non-extended bolt.

EDS-2004/FL-118


Use of a sleeve and extra long bolts reduces the relative strain differential due to thermal expansion variances or creep– The bolt extension is cooler and elongates less than the

portion between the flanges– The total strain change averaged over the entire bolt

length is, therefore, less– A lower change in strain means a lower stress change

• This reduces the chance of exceeding the bolt’s yield strength and reduces the creep rate (if in the creep range)

EDS-2004/FL-119

Solution– Consider the use of a sleeve

around the bolts to increase the effective bolt length

– or consider the use of conical spring washers to eliminate force losses over wide temperature ranges

PPF-R01-33

Joint Must Compensate for Wide Temperature Variations

This slide illustrates the topics discussed on the previous two slides.

Deflection and load characteristics can be changed by stacking the conical spring washers in series, parallel or series - parallel.

Lip seal joints consist of two similar mating metal gaskets commonly termed welded membrane gaskets. Each gasket is individually welded on the ID to its respective mating flange and, upon flange joint assembly, a second welding operation joins the gaskets on their outer diameter, thus providing a fully welded joint.

The same welded joint principle is achieved and specified in British Standard BS5500, 3.8.5 Ungasketed seal welded flanges. This construction involves an integral nubbin portion of the flange to be used as the welded connection in place of the gasket discussed above. The flange is specified with a tapered outer ring section to allow access to the nubbin portion and to accommodate welding the flanges together.

Both of the above approaches involve welding inside the bolt circle to eliminate potential leakage through the bolt holes. The fact that the welding occurs inside the bolt circle is a detriment for assembly and disassembly because it is more difficult to access the required weld to initially place it or grind it out. Sometimes provisions are made for future seal welding if it proves to be necessary, but the flange is initially assembled in the normal manner, permitting disassembly.

Spacer blocks at the flange ring OD act as physical limit stops to prevent excessive rotation from high bolt loads which can unload most of the gasket and potentially extrude it inward. Spacer blocks are viable only for low pressure services where the bolt force diverted to the contact point between the flanges and the spacer is not a concern. Enough force remains to create a gasket seal without overstressing the bolt. High pressure services cannot afford to lose this force acting on the gasket - leakage may result. Fortunately, flanges in high pressure service are usually thick and stiff enough to prevent excessive flange distortion. Stiffening the flange or replacement with a thicker flange may be a solution in other instances where a spacer block cannot be used.

EDS-2004/FL-120

Seal the joint by welding the flanges together inside of the bolt circle– Use lip seals to avoid welding directly to the

flange material and to allow subsequent cutting open and rewelding

Spacer blocks between flanges on their outer periphery– Blocks prevent excessive flange rotation


EDS-2004/FL-121

Lip Seal System

Seal the joint by welding the flanges together inside of the bolt circle, e.g., lip seals

Above is an example of a lip seal. Other configurations are also possible.

EDS-2004/FL-122


Loosening the bolts may stop a leak if the leak is due to flange rotation unloading a portion of the gasket

A lower bolt load may reduce the amount of rotation and improve contact with the gasket

Loosening the bolts is only useful if overtightening of the bolts caused a flange distortion and that is the only cause of the leak. The bolts and flange must also still be elastic. Otherwise, loosening will increase leakage, a very dangerous occurrence. Even if some loosening helps, reducing the bolt load below that required for the gasket to affect a seal will restart leakage.

uop flanges

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