triple-offset butterfly valve aug2018 english

Triple-Offset Butterfly Valve How to achieve Leakage Class VI by Metal-to-Metal Seat?

S&T S&T International Co.

Stanley Park (박시우 대표, 기계기술사)

President, P.E.S&T International Co.Mobile : +82(0)10 3840 0721Tel/ Fax : +82(0)2 400 8039E-mail : [email protected]

Triple Offset Butterfly Valve Triple Offset Geometry Leakage Class VI (TSO)

▶ References- Valve Handbook, Philip L. Skousen- ISA Handbook of Control Valve- Control Valve Handbook, Y. Kitamura

S&T International Co.

According to my experience, when strictly enforcing regulations, it is understood that when the fluid is a gas, there is virtually no

control valve maker capable of achieving Leakage Class VI while operating in a metal-to-metal seat during commercial operation.

Furthermore, it is stipulated in the international standard ANSI/FCI 70-2, which is the basis of all purchase specifications, that the

Class VI applies to Soft Seat, and in the case of Metal Seat, Class V or less. This is done only three times with low-pressure air or

nitrogen gas in the valve when conducting a leak test in the manufacturing process. However, in commercial operation, the tested

valve may not be able to realize the Leakage Class VI if the valve frequently opens and closes with a different fluid.

Although a hard-faced material is applied to the seat, if it contains solid materials in the fluid, it may cause catastrophic damage if

the material is clashed the surface of seat at high velocity at very low opening of the valve whenever the valve frequently opens

and closes. At some point, a Class VI may not be possible. To achieve Class VI, the disc must be closed with a stronger torque than

the lower Leakage Class. At this time, precise triple off-set design and production is a prerequisite for eliminating the deformation

of the disc and ensuring that the torque is evenly distributed throughout the entire 360 degree of the seat.

In order to realize a strong torque, a hydraulic(or electro-hydraulic) actuator is applied rather than a pneumatic type in case of an

extra-large size or high pressure. When operated with pneumatic type, the difference in density between air and hydraulic oil is

about 900 times, using as the medium to transfer kinetic energy. As a result, the actuator size becomes excessively larger than that

of the hydraulic type, resulting in difficulty at installation and maintenance..

Another consideration for designers is that when this valve is installed in the pipeline connected to the pump or compressor, the

vibration is transmitted and the eccentricity of the drive part of the valve may occur. Once eccentricity occurs, it is possible to leak

between the seats even if the disc is completely closed. If a Leak occurs, the Leakage Class shall be determined by evaluating

whether the leaked flow rate affects human safety, operating costs, and the production loss of the plant.

This article would like to help engineers apply triple offset butterfly valves according to the process and operation of each plant,

and also please let us discuss to modify properly this article should you have any questions or different opinions.

August, 2018

Preface

Triple-Offset Butterfly Valve Design


Eccentric butterfly valve is designed to enable high performance throttling control using disc eccentrically mounted at the valve body. Butterfly valve has the disadvantage of not being able to seal well even though it has the advantage of being able to control the flow rate taking up less space, and having a light weight at large size even. Triple Offset Butterfly Valve is a valve that structurally improves this sealing problem. Swing Check Valve is the typical valve using this eccentric force.

Triple offset improves operability and tightness by forming three types of offsets.1st Offset: The shaft is offset behind the seat axis to allow complete sealing contact around the entire seat.2nd Offset: The shaft centerline is offset from pipe and valve which provides interference free opening and closing valve.3rd Offset(Triple Offset): The Cone axis is offset from the shaft centerline to eliminate friction during closing and opening and achieve

uniform compressive sealing around the entire seat.This structure allows butterfly valves to implement Tight Shut-Off (TSO).Triple-Offset also serves as a mechanical stopper to prevent over-stroke, especially when the disc exceeds the seat

Triple-Offset Geometry

Principle of Off-set Valve

By locating the center of gravity close to the head of golf club, it is like having a stronger force to hit the golf ball even if you swing the golf club lightly.

Zero OffsetDouble Offset (1, 2)Triple Offset (3)


Butterfly valve used as control valve is designed with almost all eccentric disc. Eccentric butterfly valve is usually specified as standard material such as Carbon Steel, Stainless Steel, and Alloy Steel. Mounting actuator and position Indicator results in much more precise control than concentric butterfly Valve. Compared to globe valve, butterfly valve weighs 40% to 80% and is much smaller and lighter, less than half of globe valve. Furthermore, since the weight of the butterfly valve control element is not an important factor in calculating the actuator force, a relatively small actuator can be applied to the butterfly valve. Hence, butterfly valve is preferred where space is subject to weight constraints. Since the butterfly valve operates in a rotary motion, the frictional force is much smaller than the globe valve, so smaller actuator is used. The disadvantage is that the operation is unstable (possibility of chattering) when the disc is placed near the seat, and the rangeability is not as good as 20:1.Chattering: The vibration that occurs when the valve opens and closes at low opening degrees.In the butterfly valve, the flow velocity increases within 5% of opening/closing of the valve, causing a pressure difference, which causes the disk to strike the seat, resulting in high noise and vibration. That is, when the valve is completely closed, the valve outlet is at 0bar.g, so the disc receives the fluid pressure just before the closing, and the higher the inlet pressure, the faster the fluid velocity is by the square power. The larger kinetic energy becomes, the greater possibility of occurrence. When the valve is opened a little, the differential pressure (ΔP) rises rapidly and it works in the closing direction again. If this operation continues, chattering occurs. Triple offset is an effective valve to prevent chattering. The below figure demonstrates the chattering phenomenon occurring in the pressure reducing valve. Chattering can occur easily if valve closing speed is slow.

Chattering PhenomenonCv

Opening

Valve Characteristic Curve

The fluid pressure acting on the butterfly disk generates a considerable side load and uses a large diameter shaft as a solution to this. In addition, dead band and hysteresis are increased due to the linear movement of the actuator and the mechanical conversion of the rotational motion required for the control element. Butterfly valves are sized to at least 2 inches (DN 50) or larger due to rotational design limitations.When the disk is fixed so that a slight difference occurs vertically and horizontally, it is called an eccentric cam disk.The seat has soft seat, polymer seat, and metal seat. Soft seated valves, normally TFE-based materials, are designed for either Class VI or true "Bubble Tight", but they are limited to a maximum of 205°C. Eccentric butterfly control valves are available in various resilient metal seats and are applicable to fluids with foreign materials, viscous fluids, high temperature and high pressure, and can be used to 570°C in Leakage Class V or VI depending on pressure (There is a difference in each manufacturer).

One of the most popular soft seat designs is the Seat with Poisson Effect.

Poisson Effect (double effect of compression load on an object): When compressed, the object is shortened in the compression loaddirection and spreads laterally.

Thus, a compressed intervertebral disc will have a shorter vertical dimension, but will expand laterally. For each type of material thereis a specific rate of axial strain for transverse strain. This is called the Poisson ratio.

It depends on the material, and steel is 0.3 and aluminum alloy is 0.33


[Fig. 1] Butterfly metal seat assisted by process pressure (Poisson effect)(Courtesy of The Company, Valve Division)

[Fig. 2] Combined metal and soft seat used for fire-safe(Courtesy of The Duriron Company, Valve Division)

Outer SleeveInner SpringCoil Spring

When the disc seat is seating, the pressure of the soft seat is deformed by this pressure because considerable pressure is applied toO-ring or elastomer. When the disc is seated on the seat as shown in the figure 3, the deformation occurs when the surface of the softmaterial is pressurized, and when the upstream pressure is higher than the downstream pressure, the sealing force becomes largerdue to the Poisson effect. Polymer-encapsulated O-ring is very flexible and the Poisson effect takes place [Fig. 1 and Fig. 3].

Poisson Effect with Pressure Downstream Poisson Effect with Pressure Upstream

[Fig. 3] Poisson effect on a Butterfly valve seal for both upstream and downstream pressures.(Courtesy of Valtek International)

There is a Jam-Lever or Toggle effect associated with the Poisson effect. It uses Hinged Elastomer, which is designed to be thinner atthe middle position than at outer diameter or inner diameter.

Toggle effect: The effect of transmitting pressure sideways while rotating.

It switches from one set value to another. When there are only two states, it means that the switch is changed to one value by pressingthe switch once, and once again by pressing the switch.

Jam-Lever Effect: If the lever like belt buckle is pressed, it is pushed down and is locked.


Seat is forced into gap between bodyand disc causing valve to seal

Seat is forced into gap between bodyand retainer causing valve to seal

This design allows the outer diameter of the seat to bend when the fluid pressure is applied, as shown in the figure, to seal the

metal surface [Fig. 2]. The resilient seat design applies the mechanical pre-load effect and allow the seat inner diameter to

slightly interfere with the outer diameter of the disc. As the disk approaches the seat to close, it makes contact with the seat. Asthe disk moves further into the seat, the seat physically deforms because of the pressure applied by the disk, causing thepolymer to seat against metal surfaces. In some cases, a manufacturer may use both the mechanical pre-load and Poisson effectsto achieve the correct shutoff [Fig. 4 and Fig. 5].

[Fig. 4] Jam Lever or Toggle effect on the Butterfly Seal(Courtesy of Valtek international)

(a) (b) (c) (d)


Pressure“jams” leveredge of seatinto disc

Pressureflattens seatforcing it to“toggle”into disc

[Fig. 5] Butterfly seal using both mechanical pre-loading and the Poisson effect(a) Basic seal design. (b) Pre-loading effect on the seat caused by disc seating(with minimal pressure effects). (c and d) Poisson effect on the seat

caused by increased upstream or downstream pressures. (Courtesy of Flowseal. A unit of the Crane Valve Group)

Metal seats are typically applied to high temperatures (above 205°C). Metal seats are integral to the seat-retainer with a gasket placed where a soft seat is normally inserted [Fig. 6]. In some designs, both a soft and metal seat can be used in tandem, allowing the metal seat to be a backup in case of failure of the soft seat [Fig. 7]. When butterfly valves are specified for fire-safe applications, the tandem seat is installed.

[Fig. 6] Butterfly Metal Seat [Fig. 7] Butterfly Soft and Metal Seat [Fig. 8] Typical Metal-to-Metal Seat(Courtesy of Valtek international)


The typical laminated structure of a seat seal disc uses a double alternating layer of compressible and elastic materials such as flexible graphite or other gasket seats and a solid metal. This provides flexing and resilience required to seal the rigid valve seat when the valve is closed with an offset stem. This composite structure is a very good advantage, but alternating layers of solid and soft materials are not easy to create the complex geometry required for a fully machined seat sealing disk surface to achieve leak-proof sealing. The torque applied to the valve's offset shaft compresses it in the radial direction of the valve seat, and flexing, elasticity, and surface treatment are very important to prevent valve leakage. By maintaining the surface of the metal layer without projections on the surface of the seal and ensuring uniform elasticity, the load is evenly applied to the circumference of the seat seal, achieving the tight shut-off with minimum shaft torque and less actuation. A wide range of metals, such as Stainless Steel, Duplex, Super-Duplex Alloy, Inconel, Monel, Hastelloy. It is not affected even if another layer is damaged since each layer performs independent sealing function [Fig. 8].

As the disc approaches the seat, The overall pressure and fluid velocity are affected by the direction of flow, and the disc acts eitheron the front or the back, resulting in poor stability. Up to 5% of the final stroke can not be used when the valve close. When the diskcomes into contact with the seat, some deformation occurs and the resilient elastomer or soft strip is molded on the disk seatingsurface. The opened disc is pushed against the fluid force on one side and pulled on the other side, resulting in a nearly equilibriumstate. However, since both sides of the disc are not identical (one side is connected to the axis and the other side is flatter), thedirection of flow tends to push the disc open or pull it close. In a concentric disc-seat, a part of the disc always remains in contact withthe seat. When fully opened and closed, the entire seating surface is fully in contact. Because the concentric disk seat is always incontact and there is a tendency to wear easily in automatic control applications, the wear takes place. Leak eventually occurs at twopoints when closed.

Users prefer eccentric cam disc seat that is always in contact with the seating surfaces and have no leaks. This valve ensures that thedisc is fully in contact with the seat when the disc is closed, but when the valve opens, it is lifted from the seat and sufficiently awayfrom the seating surface to avoid contact. The butterfly valve has a different sliding motion between the closing member and the seat,and so has a different seating mechanism. Such repetitive wiping can result in a loss of firmness of the seat after prolonged use. Thetorque required to move the disc away from the seat can be twice that of the PTFE seat, and as a result the throttling performance isgenerally only possible at an angle of 15° or higher.

● Attachments - Advanced manufacturer’s Triple-Offset Butterfly Valve

► VELAN Torqseal®

► ORTON-IMI MV Series

► VANESSA®-EMERSON 30,000 series

► CURTISS-WRIGHT PermaSeat SP

Triple-Offset Butterfly Valve Operation


TM

The Laminated Disc SealTorque seating during closing of the valve provides uniform forces around

the entire circumference of the valve seat. The self-adjusting, resilient seal

flexes and energizes, assuming the shape of the seat. The compression

forces equally distributed around the perimeter provides a tight bi-directional

shut off. The resiliency of the seal allows the valve body and disc to contract

to expand, without the risk of jamming due to temperature fluctuation.


Brief Specification Torqseal®

● Material: WCB(427°C), Stainless Steel(427°C) and more

● Bi-Directional, Bubble-tight Shut-off

● Size: 3” to 80” and larger

● Connection: Double Flanged, Lug, Wafer, Buttweld

● ASME Class:150# - 900#

● Temperature Range: -196°C to 600°C

CoCr Alloy(Cobalt-Chrome Alloy) Deposit System

(1) Seat is hard-faced with CoCr Alloy as standard(2) The gasket is Spiral Wound SS/Graphite for Zero

Leakage

Part Name1. Bolts

2. Washers

3. Disc Retainer

4. Seal Ring(Duplex)

5. Spiral Wound Gasket

6. Disc and reference Pin

Vanessa’s cone-to-cone concept requires metal-to-metal seating in order to

handle more severe applications. We use Stellite@ grade 21 seats and a

resilient seal ring in Duplex as a minimum with high mechanical resistance.

Stellite@ grade 21 is a cobalt alloy that coats the body seat to provide hardness,

resistance to fluid abrasion and wear, while ensuring high corrosion resistance.

Due to its unique design, the metal seat ring has elastic properties which allow

the radial compression to be uniformly distributed around the seating.

Brief Specification 30,000 Series

● Material: Carbon Steel, High Temperature Alloy Steel, Austenitic Stainless

Steel, Duplex and Super-duplex Stainless Steel, and more

● *Zero Leakage, Bi-Directional

● Size: 3” to 112” and larger

● Connection: Double Flanged, Lug, Wafer, Butweld

● Pressure Class: ASME150# - 1500#

● Temperature Range: -254°C to 815°C

* Zero leakage: no visible leakage when

tested at high pressure with water and low

pressure with air according to existing

international standards.


Stellite® Grade 21 Overlay

®TM

Valve discs include a resilient, laminated metal seal ring, composed of alternating layers of corrosion-resistant alloy and composite graphite.Each individual lamination provides an independent seal, unaffected by damage to the others. While improving long-term sealing properties, the resilience of the laminated design also allows lower operating torques. (For high temperature and more severe applications, a hardened, non-laminated seal ring is available). Assembled on the disc, the seal ring is engineered with tolerance to allow radial compression and flexing when torque is applied to close the valve.Consequently, the valve seals tightly even when pressurized from the reverse direction. In addition, the resiliency of the seal ring compensates for differences in expansion rates between body and trim.

Brief Specification MV Series● Resilient metal-to-metal seal.

● Zero leakage, Tight shut-off and Bi-directional

● Pressure Range: ANSI Class 150 to Class 2500

● Temperature Range: -196ºC to 815ºC

● Size: • 2” - 160” Class 150 • 2” - 80” Class 300

• 3” - 60” Class 600 • 6” - 48” Class 900

• Class 1500 and 2500 on special request

● Material: WCB, Stainless Steel, Bz-Al, Duplex, Titanium, Alloy 20,

Inconel, Incoloy, Hastelloy


Seal ring seated Seal ring prior to seating


The PermaSeat SP triple offset geometry is created by offsetting the shaft in two axes, in combination with a tilting cone ellipsoidal segment. This completely removes contact with the seat and seal during the full 90 degree rotation. The laminated seal uses the seat ring as the stopping point, eliminating separate mechanical stops.There is no need for critical settings for disc-to-seat contact to achieve required shut-off. This is particularly beneficial in situations where actuators require several accessories in the control scheme.When closed, the seat is bi-directionally tight, with zero leakage in both directions. The seat sealing load is torque induced, thus when the disc movement reverses to open, the operating torque quickly reduces. Since there is no rubbing between the seat ring and seal, galling does notoccur allowing the same material to be used for both the seal and seat ring.

Brief Specification PermaSeat SP● Actuator : Pneumatic, Electric, Electro-Hydraulic

● ANSI Rating : 150# - 2500#

● Size : 3” – 84”

● Service : Leakage Class VI Shut-off and Bi-directional

● Temperature: -196°C to 699°C● Resilient Metal Seat

● Available Material: 300 Series Stainless, Duplex, Carbon Steel, 6% MolyBronze, Monel, Incoloy, Hastelloy B and C, Titanium, Zirconium

triple-offset butterfly valve aug2018 english

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