heat exchangers

WELCOME ALL 3rd BATCH GELs TO ‘TECHNICAL

FOUNDATION COURSE’ & WISH THEM ALL THE

VERY BEST IN ALL THEIR ATTEMPTS

HEAT EXCHANGERS

A Heat Exchanger is a mechanical equipment in which heat is exchanged

between two fluids.

Heat exchange equipment is vital to the operation of the Refinery. Its purpose is to ensure effective heat transfer as per design, functional requirements – resulting in

• LARGE HEAT / UTILITY SAVINGS

• UN-INTERRUPTED OPERATION

• PREVENT UNSCHEDULED OUTAGES

• SAFETY, RELIABILITY

Most commonly used Heat Exchangers in Refinery, Petrochemicals are :

• Shell & Tube Heat Exchangers

• Air Fin Coolers (Fin Fan Coolers)

• Plate Type Heat Exchangers – Utilities

• Speciality types – Packinox type

• Finned / Studded tubes in Convection banks

• Heaters, Boilers Etc.

CONSTRUCTION BASIS

• STATIONARY HEAD TYPES (A, B, C, D)

• SHELL TYPES (E, F, G, H, J, K)

• REAR HEAD TYPES (L, M, N, P, S, T, U)

Example:a) A : Removable Channel & Cover

E : One Pass Shell

S : Floating Head with Backing Device

b) B : Bonnet integral Cover

E : One Pass Shell

M : Fixed Tube Sheet

c) A : Removable Channel Cover

E : One Pass Shell

P : Outside Packed Floating Head

d) C : Integral with Tube Sheet, Removable Cover

F : Two Pass Shell with Longitudinal Baffle

U : U – Tube Bundle

e) A : Removable Channel Cover

K : Kettle Type Reboiler

T : Pull-through Floating Device

f) A : Removable Channel Cover

J : Divided Flow

W : Welded Tube Sheet

Types according to TEMA

• Class ‘R’ – Used for severe requirement of Petroleum and related processing applications.

• Class ‘C’ – Used for moderate requirements of commercial and process applications.

• Class ‘B’ – Used for chemical process service.

VARIOUS PARTS

• Shell, Appurtenances (Tube Bundle)

• Stationary Head, Shell, Shell Cover, Channel Cover, Tube Sheet, Floating Head, Nozzles, Vents, Drains, Expansion Joints Etc.

• For details, please Ref: Table N-2, Fig.N-2 in TEMA Standards.

DESIGN

• THERMAL DESIGN

• MECHANICAL DESIGN

• ASME Section VIII Division 1 / TEMA ‘R’

• Other Sections:

Section II A, B, C : Material, Welding Rod, Consumables

Section V : NDT

Section IX : Welder Qualifications

• Other Unit / Process / Licensor specific requirements

• ASME Sec VIII Div 1 ( UG, UW, Mandatory, Non-mandatory appendices)

• ASME Sec II A – Ferrous Material Spec’s

• ASME Sec II C – Welding Consumables

• ASME Sec V – NDT, specific articles

TEMA

Sec 1 : Size numbering, Type designation, Nomenclature of Heat Exchanger components.

Sec 2 : Recommended Fabrication Tolerances, Clearances.

Sec 3 : General Fabrication, Performance Information, Name Plate, Specification Sheets Etc.

Sec 4 : Installation, Operation and Maintenance.

Sec 5 : General Requirements of Shell, Tube Sheet, Covers, Baffles, Support Plates, Gaskets, Channel Cover, End Flanges Etc.

Sec 6 : Vibration Damage Patterns, Failure Regions, Dimensionless Numbers, Natural Frequencies, Axial Tube Stress, Effective Tube Mass Damping, Acoustic Vibration, Design Considerations for Vibration Aspects.

Sec 7 : Deals with Various Thermal Relations like Fluid Temperature, Mean Metal Temperatures of Shell and Tubes.

Sec 8 : Physical Properties of Fluids viz: Density, Specific Heat, Thermal Conductivity, Viscosity, Critical Properties Etc.

Sec 9 : General Information on Dimensions, Pressure-Temperature Ratings, Conversion Factors Etc.

Sec 10 : Deals with Vessel Supports, Lifting Lugs, Wind and Seismic Design, Plugging of Tubes, Gaskets, Tube Sheet, Nozzles, Covers Etc.

TEMA Basis

• Not to exceed Inside Diameters of 100 inches (2450 mm)

• Product of Nominal Dia (inches) and Design Pressure (psi) not to exceed 1,00,000 (17.5 x 106 kpa)

• Not to exceed Design Pressure of 3000 psi (2068 kpa)

INTENT OF THESE PARAMETERS IS TO LIMIT THE MAXIMUM WALL THICKNESS TO

APPROXIMATELY 3 inches (76 mm) AND MAXIMUM STUD DIAMETER TO 4 inches (102 mm)

MATERIAL

Shell : CS / SS / Non-ferrous types

Tubes : CS / AS / SS / Other special types

Tube Sheet : Forgings / PlatesNozzle Pipes : CS / AS / SSNozzles : CS / AS / SSGaskets : CAF, Spiral Wound, IJA,

Ring Joint typeFasteners : CS / AS / SSName Plate : SS

ORDERING INFORMATION

• PO / PR / Datasheet / Drawings / Other relevant Specifications

• QAP / ITP, Stage, Final Inspections

• Shall be as per Heat Exchanger Specification Sheet

• Performance / Construction details, MOC

• CDD

FABRICATION (Key Steps)• Material Identification (Pressure parts)• Forming• Welding• Review of WPS / PQR / Welder Qualification• NDT• Stage and Final Inspections• Mock-up Assembly• Skeleton Assembly• Tube Bundle• Insertion of Tube Bundle into Shell• Testing• Painting• Documentation• Packing / Protection / Despatch

Various Testing / Examinations Used

• Visual Examination• Dye Penetrant Examination• Magnetic Particle Examination• Radiographic Examination• Ultrasonic Examination• Check Tests (Chemical, Mechanical, Special Tests)• Tests for determining pullout load ‘Fr’ value• Pneumatic Tests• Hydraulic Tests• Special Tests (IGC, NACE related, Hardness, Eddy

Current Testing, Helium Leak test Etc.)• Pre-shipment checks

INSTALLATION

• Shell, Tube Bundle

• Site Checks

• Alignment to upstream downstream tube side / shell side piping

• Validation tests

• Passivation and Insulation

Maintenance Activities

• Identify leaks• Repair, rectify, plug leaks• Validation tests• Cleaning as per requirements• Replacement of tube bundle, retubing• Attend leaks from gasket joints, piping, fittings• Offer statutory tests as per schedule (IBR Etc.)

Problem Areas

• Performance related (CTS will monitor)

• Corrosion, Erosion related

• Leaks, reliability related

• Other issues (vibration Etc.)

TUBE TO TUBE SHEET JOINT

Purpose of Tube to Tubesheet joint

• To join tubes and tubesheet and keep the tubes structurally stable and support the skeleton assembly under design conditions.

• To prevent intermixing of shell and tube sheet fluids.

• To take care of Longitudinal, Compressive, Mechanical and Thermal axial loads coming on tubes.

Selection of Tube to Tubesheet joint

Tube to Tube sheet joints are selected based on the effective tube longitudinal, compressive stress, and loads caused by differential thermal expansion between shell and tubes.

Tube to tube sheet joint loads are calculated as per Clause 7.25, section 5 of “TEMA”.

Types of Tube to Tubesheet joints

1. Expanded

2. Seal Welded

3. Strength Welded

DEFINITION

Expanded tube joint is the tube to tube sheet joint achieved by mechanical or explosive expansion of the tube into the tube hole in the tubesheet.

Welded tube joint is a tube to tubesheet joint where the tube is welded to the tube sheet. Welded tube joint can be either strength weld or seal weld.

As per ASME Sec VIII Div 1 UW-20

Strength weld is one in which the design strength of the weld is greater than or equal to the maximum allowable axial tube strength.

A strength weld shall be designed to transfer all of the longitudinal, mechanical and thermal axial loads in either direction from the tube to the tubesheet as well as provide tube joint leak tightness.

Tube

Tube sheet

TYPICAL SKETCH SHOWING STRENGTH WELD

As per ASME Sec VIII Div 1 UW-20

Seal weld is one which has not been designed to be a strength weld. A seal weld is used to supplement an expanded tube joint to ensure tube joint leak tightness.

It is recommended to use for following cases where

1. intermixing of shell and tube side fluid causes safety hazards.

2. Lethal fluids are used.

3. Hydrogen service with partial pressure greater than 6.8 bar

Tube

Tube sheet

TYPICAL SKETCH SHOWING SEAL WELD

ASME SEC VIII DIV 1 - NON MANDATORY APPENDICES - APPENDIX A

BASIS FOR ESTABLISHING ALLOWABLE LOADS FOR TUBE TO TUBESHEET JOINT

CONSIDERATION

Effect of different co-efficients of expansion of tube and tube sheet material.

Joint integrity at service conditions.

SHEAR LOAD TEST

The value of Fr is calculated by carrying out shear load test on the joint.

In this T-T/s joint shall be loaded until mechanical failure of the joint or tube occurs. Essential requirement is that the load should be transferred axially.

Min of three specimens shall constitute a test. Any retest shall include a min of nine additional specimens from the tube from which the original specimens are taken. All previous test data shall be rejected.

Fr (test) = 0.8 * L (test) / (At * St)

Where

fr = test efficiency

L (test) = lowest axial load at which failure of the tube specimen occurs in lb

St = min tensile strength of tube material in psi

At = nom cross sectional area of the tube wall in Sq in.

Fr for various joints is provided in Table A-2 of Appendix A of ASME SEC VIII DIV I.

Tube holes in Tube sheets ( RCB 7.43)

•Tube hole affects the mechanical strength and leak tightness of an expanded tube to tubesheet joint

In general

•A rough tube hole provides more mechanical strength than a smooth tube hole. This is influenced by a complex relationship of modulus of elasticity, yield strength and hardness of the materials being used.

•A smooth tube hole does not provide the mechanical strength that a rough tube hole does, but it can provide a pressure tight joint at a lower level of wall reduction

Contd..

Very light wall tubes requires a smoother tube hole finish than heavier wall tubes

Significant longitudinal scratches can provide leak paths through an expanded tube to tubesheet joint and should therefore be removed.

Tube wall reduction

Tube wall reduction depends on number of factors. Some of these are

• Tube hole finish (1/32 “ to 1/64” over the OD of tubes)

• Presence or absence of tube hole serrations

• Tube hole size and tolerance.(RCB 7.41 table)

• Tube sheet ligament width and its relation to tube dia and thickness

• Tube wall thickness

Contd..

• Tube hardness and change in hardness during cold working.

• Tube OD Tolerance

•Type of expander used

• Type of torque control and final tube thickness control.

• Length of expanded joint.

• Compatibility of tube and tube sheet materials

• Groove width should be 1/8” and groove depth should be 1/32”.

Length of Expansion (RCB 7.511)

Not less than two inches or tubesheet thickness minus 1/8” whichever is smaller. In no case shall the expanded portion extend beyond the shell side face of the tubesheet.

Expansion of tube joints to be done after carrying out required PWHT.

Testing of T-T/s joint

Following tests are carried out during/after tubing

1. Check tube ID before and after expansion.

2. Carry out DP if the joint is welded

3. Check by Pneumatic test / helium as per requirement

4. Check by hydrotest at test pressure

MOCK UP TESTS

1. DP Checks in case of welded

2. Pneumatic

3. Hydrotest

4. Pull out test

5. Determination of fr value

6. WPS / PQR / Welder competence

7. Quality of consumables

PETROKEMYA RECOMMENDATION

Conditions Tube-to-tubesheet jointsDesign P, Design T

bar G deg C

< 41 < 350 Use expansion with grooves.> 41 < 350 Use expansion with seal weld.any > 350 Use strength weld and contact expansion ( without grooves ), > 50 - Expansion shall be performed for the entire thickness of the tubesheet. Caution shall be taken not to expand tubes beyond backface of tubesheets.

In any of the following conditions, TTS joints shall be seal welded as a minimum :

* Where mixing of shell and tube side fluids could possibly cause problems such as explosion or contamination.* Where exchanger contains lethal substances.

* Cyclic service.* H2 service where hydrogen partial pressure exceeds 6.8 bar.

Contd..

Reduction ratio of tube thickness shall be …..

* 5-10% for CS, LAS and high alloy tube material.* 4-8% for copper alloy tube materials.

Notes :* Expansion of TTS joints shall be performed after any heat treatment in which the temperature of the joint exceeds 200oC.* All strength welds shall have a minimum of two layers.

Contd..

List of exchangers upgraded as per Petrokemya recommendations

ME-AY281-S13

ME-AY-281-S14

ME-AY281-S03/04/05

ME-AY281-S20

ME-RF413-S04

For other exchangers joint upgradation will be carried out while procuring new bundles. QAP has been modified accordingly.

FORWARD PATH

• Selection of vendors

• Incorporating additional inspection requirements as part of PR spec.

• Review/approval of QAP/ITP for each P.O. With adequate intervention points for TPI (Hold, Witness,Review)

• Witnessing of final Hydrotests / Review of TPI reports at vendor shop.

• Upgradation of existing joints during opportunity shutdown / new supplies.

• Supervision/ procedure compliance during retubing / repair in CES Workshop

Inspection related issues

• Maintain history (of failures, reliability issues)• Recommend repair, replacement based on

inspection findings• Recommend insurance spares• Resolve corrosion, other degradation issues

through TRIPOD• Fulfill statutory requirements (as applicable) –IBR

cases

Some Manufacturers

• L & T – Mumbai

• Godrej & Boyce – Mumbai

• ISGEC – Yamuna Nagar

• BHPV - Vizag

Pre-order requirements

• Vendor survey

• Enlistment

• Monitoring performance

• Appraise purchase department

• Review technical bids

Cost of Heat Exchangers depend on

• Size / type of Heat Exchanger• Material of construction• Type of tube to tube sheet joints• Inspection, NDT requirements based on severity

of service• Imported / indigenous• Location of Manufacturer vs. user• Government / statutory regulations (IBR Etc.)• Quantity, lead time

THANK YOU