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BP - Heat Exchanger Tube End Fitting | Leak | Welding
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GS 118-8 HEAT EXCHANGER TUBE END FIXING
GS 118-8
HEAT EXCHANGER TUBE END FIXING
(Replaces BP Engineering Std 191)
This specification gives BP's general requirements for the expansion and tube end welding of ferrous and non-ferrous tubes in heat exchangers. It incorporates a BP Chemicals standard and makes detailed reference to several British Standards on tube end welding.
AMENDMENTS Amd. Date Pages Description ___________________________________________________________________
FOREWORD ..................................................................................................................... iii 1. INTRODUCTION........................................................................................................... 1 1.1 Scope ................................................................................................................ 1 1.2 Definitions and References......................................................................................... 1 2. GENERAL REQUIREMENTS...................................................................................... 1 2.1 Types of Tube End Joint............................................................................................ 1 2.2 Quality Assurance...................................................................................................... 3 3. PREPARATION OF TUBES AND TUBE SHEETS..................................................... 3 4. TUBE END WELDING.................................................................................................. 3 4.1 Welding Processes..................................................................................................... 3 4.2 Joint Details............................................................................................................... 4 4.3 Metallurgical Considerations...................................................................................... 4 4.4 Welding Procedure Specification (WPS).................................................................... 5 4.5 Welding Procedure Qualification Test........................................................................ 5 4.6 Welder Qualifications ................................................................................................ 7 4.7 Tube Location For Welding ....................................................................................... 7 4.8 Preheat ................................................................................................................ 8 4.9 Post Weld Heat Treatment (PWHT) .......................................................................... 9 4.10 Welding ................................................................................................................ 9 4.11 Quality Control During Welding .............................................................................. 9 4.12 Production Control Test Blocks..............................................................................10 4.13 Cleaning and Inspection ..........................................................................................10 5. TUBE EXPANSION ......................................................................................................11 5.1 General ...............................................................................................................11 5.2 Roller Expansion ......................................................................................................11 5.3 Hydroswaging ..........................................................................................................11 5.4 Wall Thinning ...........................................................................................................12 5.5 Expansion After Welding..........................................................................................12 5.6 Expansion Procedure Test Block ..............................................................................12 5.7 Expansion Check ......................................................................................................13 6. LEAK DETECTION .....................................................................................................13 6.1 Leak Detection of Welded or Welded and Expanded Tube Ends ...............................13 6.2 Leak Detection of Expanded Only Tube Ends...........................................................14 7. REPAIRS........................................................................................................................14 8. PRESSURE TESTING ..................................................................................................14 9. DRAINING AND DEWATERING ...............................................................................15 10. INSPECTION...............................................................................................................16 SUMMARY OF INSPECTION ACTIVITIES ...............................................................16
APPENDIX A.....................................................................................................................17 DEFINITIONS AND ABBREVIATIONS .....................................................................17 APPENDIX B.....................................................................................................................18 LIST OF REFERENCED DOCUMENTS......................................................................18 APPENDIX C.....................................................................................................................19 TYPICAL JOINT DETAILS..........................................................................................19 C.1 PLAIN FILLET WELD ...........................................................................................19 C.2 RECESSED TUBE..................................................................................................20 C.3 GROOVE WELDS..................................................................................................21 C.3.1 GROOVE PLUS FILLET.....................................................................................21 C.3.2 GROOVE .............................................................................................................21 C.4 CASTELLATED WELD PREPARATION..............................................................22 C.5 BACK FACE TUBE SHEET WELDING................................................................23 C.6 DESIGN TO AVOID HOT HYDROGEN SULPHIDE CORROSION ....................24 APPENDIX D.....................................................................................................................25 WELD PROCEDURE AND WELDER QUALIFICATION TEST BLOCKS ................25 FIGURE D.1 TEST SPECIMEN FOR SQUARE PITCH...............................................25 FIGURE D.2 TEST SPECIMEN FOR TRIANGULAR PITCH .....................................25
FOREWORD Introduction to BP Group Recommended Practices and Specifications for Engineering The Introductory Volume contains a series of documents that provide an introduction to the BP Group Recommended Practices and Specifications for Engineering (RPSEs). In particular, the 'General Foreword' sets out the philosophy of the RPSEs. Other documents in the Introductory Volume provide general guidance on using the RPSEs and background information to Engineering Standards in BP. There are also recommendations for specific definitions and requirements. Value of this Guidance for Specification Reliable tube end joints are essential in shell and tube heat exchangers and air coolers. The annual cost to operators of poor tube end joints is substantial. Satisfactory service performance should be obtained providing appropriate design, fabrication and inspections are specified. BP's recommendation on this are contained in this document Application This Guidance for Specification is intended to guide the purchaser in the use or creation of a fit-for-purpose specification for enquiry or purchasing activity. This Specification supersedes BP Standard 191 (which was largely based on EEMUA 143). It incorporates a BP Chemicals standard and makes detailed reference to several recently issued British Standards on tube end welding Text in italics is Commentary. Commentary provides background information which supports the requirements of the Specification, and may discuss alternative options. It also gives guidance on the implementation of any 'Specification' or 'Approval' actions; specific actions are indicated by an asterisk (*) preceding a paragraph number. This document may refer to certain local, national or international regulations but the responsibility to ensure compliance with legislation and any other statutory requirements lies with the user. The user should adapt or supplement this document to ensure compliance for the specific application. Specification Ready for Application A Specification (BP Spec 118-8) is available which may be suitable for enquiry or purchasing without modification. It is derived from this BP Group Guidance for Specification by retaining the technical body unaltered but omitting all commentary, omitting the data page and inserting a modified Foreword.
Feedback and Further Information Users are invited to feed back any comments and to detail experiences in the application of BP RPSEs, to assist in the process of their continuous improvement. For feedback and further information, please contact Standards Group, BP International or the Custodian. See Quarterly Status List for contacts.
INTRODUCTION 1.1 Scope This Specification details BP's general requirements for the expansion and tube end welding of ferrous and non-ferrous tubes within the following size ranges: nominal diameter 15 mm (0.5in) to 40 mm (1.5in) wall thickness 1.6 mm (0.064in) to 4 mm (0.160in) tubesheet thickness 15 mm (0.5 in) and above
Wall thicknesses down to 1.25mm are sometimes used with zirconium and nickel alloy tubes; in these cases, weld joint details shall be approved by BP. While this Specification is independent of any particular design code it should be noted that BS 5500 Appendix T provides useful information on the design, fabrication and testing of tube to tubesheet welds.
The requirements given apply to both shell and tube exchangers and air coolers. 1.2 Definitions and References Definitions and abbreviations used in this document are given in Appendix A. Referenced documents are listed in Appendix B. 2. GENERAL REQUIREMENTS 2.1 Types of Tube End Joint The following combinations of tube expansion and tube end welding may be adopted depending on service conditions:Expanded only Strength welded only Expanded and seal welded Strength welded and lightly expanded Strength welded and expanded Back face welded A strength weld is defined as a weld in which the minimum throat thickness is not less than the tube wall thickness (t). A weld having a smaller throat thickness than this is considered to be a seal weld and its function is solely to seal between the tube and the tubesheet.
In BS 5500 para. 3.9.6 and ASME VIII Division 1 Appendix A, strength factors are assigned to the above types of tube end joint. This is to check during design, the strength of the joint for the axial loads which may be applied in service.
The combination of welding and expansion required on each exchanger shall be specified by, or approved by, BP. The vendor shall ensure that an adequate tubesheet ligament is provided for the specified fabrication technique.
For many applications, tube expansion into grooves in the tubesheet without welding is satisfactory and economic. In deciding whether tube end welding is necessary, an assessment is required of the likelihood of leakage and the possible consequences. With properly applied strength welds, tube expansion is frequently unnecessary as it does not significantly contribute to the mechanical strength of the tube end joints. However, for certain service duties, it is necessary to provide intimate contact between the outside diameter of the tubes and the bore of the tubesheet holes. This contact may be accomplished by light expansion of the tubes after both welding and successful leak testing, but before final pressure testing. A light expansion avoids the build-up of corrosion products in the annular gap, but does not guarantee that crevice corrosion will not occur. If service conditions preclude any crevice between the tubes and the tubesheet, back face welding must be used. (Appendix C, Figure C.5). It is used where there is a high heat flux as the tubes enter the tubesheet and therefore concern that the tubes might crack. It is also used where the shell side fluid is corrosive such that no crevice may be permitted. It is relatively expensive. Where the additional security provided by strength welds in combination with tube expansion into grooves is considered necessary, the sequence of operations and the technique employed for tube location is important. Weld cracking may occur with expansion after welding. Porosity can occur in the welds if the tubes are fully expanded prior to welding because, if the tubes and tubesheet are not clean, the expanding operation may increase the amount of dirt at the root of the weld.
In air coolers, access for tube end welding can only be through the header plug plate and it is very limited. Tube end welding is usually GTAW with the torch and wire coming through separate holes in the plug-plate, and the operator controlling through a third. It may be done manually or automatically. The principles on which the tube end preparation is selected are very similar to those for a shell and tube exchanger, but the lack of access for welding and inspection greatly increases the complexity of the work. Failure of tube to tubesheet attachments can be extremely costly. Selection of the optimum materials for tubes and tubesheet and specification of the correct combination of expansion and welding are both essential to ensure maximum integrity and service reliability.
Quality Assurance Tube bundle fabrication should take place in a controlled environment employing manufacturing procedures administered by a Quality Assurance programme based on ISO 9001 or an agreed equivalent standard.
PREPARATION OF TUBES AND TUBE SHEETS Tube holes shall be normal to the tubesheet surface, parallel, circular, free from burrs and shall have a smooth internal surface. The periphery of the holes on the tube bundle side shall be chamfered or radiused to 1.5 mm (.06 in) approximately. The diametral limits of the tube holes shall not exceed those defined by TEMA. For light tube expansion no grooves are required. For full expansion grooves shall be machined to suit the intended expansion technique (see section 5.0 of this Specification). Immediately prior to assembly, the tubes and the tubesheet shall be cleaned with a chloride and sulphur free non-residue forming solvent.
Care shall be taken to ensure that all cleaning agents employed are fully compatible with the materials of construction. On titanium and zirconium, methanol shall not be used because of the possibility of stress corrosion cracking.
The face of the tubesheet, the holes and the tubes shall be free from dirt, grease, scale and other foreign matter when they are assembled. To avoid possible damage during assembly or entrapment of contaminants, baffle and support plate holes should be free from burrs and cleaned/degreased as above prior to the commencement of tube threading. The ends of tubes which are to be welded shall similarly be cleaned and degreased, both inside and out, for a length equal to the tubesheet thickness plus 50 mm (2 in). 4. TUBE END WELDING 4.1 Welding Processes The requirements of this Specification are based on the use of either manual or automatic welding techniques. While the SMAW, GMAW and GTAW processes may be manually applied, automated variants of the latter two processes, and particularly the GTAW process, are frequently employed for tube to tubesheet welding. With GTAW the power source shall employ a high frequency starting unit or an
alternative programmed arc initiation device. A current decay device shall also be incorporated.
Numerous leaks have occurred in service even on very mild duties with welds made by the SMAW process. Consequently this process is not recommended for tubes below 40 mm (1.5 in) inside diameter. Automatic welding is capable of producing large numbers of consistent and high quality welds. However, it is important that the joint set-up should be controlled within tight tolerance limits in order to fully realise these benefits.
Joint Details The selection of the joint detail is influenced by a number of factors including the intended service, the design requirements and the available welding technique. Typical joint details are shown in Appendix C. Alternative forms of preparation which meet the requirements of this Specification may be proposed for consideration by BP.
Metallurgical Considerations Welding Consumables and Filler Wires Welding consumable or filler wire compositions should be selected to be compatible with both the tube and tube plate material.
While this is easily achieved when the tubes and tubesheet are specified in the same alloy, often these components are specified in different materials. In this situation, care must be exercised to ensure full compatibility and the avoidance of fabrication problems, such as weld metal cracking, or service problems, such as enhanced corrosive attack.
When undertaking automatic welding it may be appropriate to introduce filler material to the weld by means of pre-placed filler wire rings or inserts. 4.3.2 Austenitic Stainless Steel Weld Metal Austenitic stainless steel weld metal shall contain 3-8% ferrite. 4.3.3 Autogenous Welding When autogenous welding (i.e. without filler wire) is proposed, sufficient welding trials shall be performed in advance of the welding procedure qualification to demonstrate that a high integrity weld having an acceptable combination of mechanical properties and weldment microstructures can be produced.
22% and 25% Duplex Stainless Steels Particular care is required when 22%Cr or 25%Cr duplex stainless steels are selected for either the tubes or the tubesheet. The mechanical properties and corrosion resistance of these steels depends critically on the microstructural balance between austenite and ferrite. A balance of nominally 50/50 austenite/ferrite is generally considered necessary to impart the optimum combination of properties. However, the weld thermal cycle can significantly influence the microstructural balance, e.g. slow cooling in a thin wall tube can result in relatively high levels of austenite while rapid cooling in a heavy section tubesheet can lead to relatively high levels of ferrite. Thus at an early stage in the design it is recommended that welding trials should be undertaken to ensure that adequate microstructural control can be maintained with the proposed fabrication technique. Ferrite levels of 35-65% are generally considered acceptable in both heat affected zones and weld metal microstructures. It should also be noted that prolonged thermal cycling / slow cooling can lead to the precipitation of intermetallic phases in these alloys. Such precipitation can lead to a marked reduction in corrosion resistance.
Welding Procedure Specification (WPS) The WPS shall be compiled by the manufacturer and submitted to BP for approval before the procedure qualification tests are performed. Additionally, the WPS should include details of the repair welding method. The ASME IX P and Group numbers and the ASME IX F numbers shall apply for parent and filler materials and may be used to determine the extent of qualification (as an alternative to BS 4870 Part 3 Table 2). The weld procedure shall be in accordance with BS 4870 Part 3 or equivalent. At the discretion of BP, previously qualified and authenticated welding procedures may be acceptable. Where such qualifications are available they should be submitted for review at the same time as the WPS. If the manufacturer intends to employ any special techniques during welding, such as the use of tapered ceramic plugs to prevent weld spillage, these techniques shall be clearly detailed in the WPS and incorporated into the welding procedure qualification test.
Welding Procedure Qualification Test The welding procedure qualification test shall be performed and evaluated in accordance with the requirements of BS 4870, Part 3 or equivalent. Brief details of the test blocks are also given in Appendix D
of this Specification. A summary of the testing requirements is given in the following table.
Visual Examination Liquid Penetrant Examination Macroscopic Examination Hardness Survey Weld Strength Test Radiography
SEAL WELD x x x When specified
STRENGTH WELD x x x When specified When specified When specified
BS 4870 Part 3 has been chosen because it is up to date and directly applicable to tube end welding. ASME IX does not set out a specific testing regime for tube end welds and is thus not considered appropriate.
The following additional requirements shall apply:(i) When qualification is undertaken for a specific fabrication, the materials used for the procedure test shall be of the same grade and specification as the production materials. Exceptionally, BP may require contract materials to be used for the procedure test. If the tubes are to be expanded following the completion of welding BP may require that a sample representing the full thickness of the tubesheet is employed for procedure qualification. (Ref para 5.6. of this Specification). When the test plate is welded in the vertical position, the 'top' of the block shall be identified by hard stamping. Where specific maximum hardness levels are required these shall be specified by BP. Where a detailed microstructural assessment of weld metal and HAZ is required, it will be necessary to prepare metallographic specimens for micro examination. This will require specimen preparation to a 1 micron diamond finish. BP may also require micro examination to assess any welding defects, such as cracking. When either 22%Cr or 25%Cr duplex stainless steels are used for the tubes or the tubesheet reference shall be made to Appendix C of BP Group GS 118-7, which details the special requirements associated with the fabrication of these steels.
Particular attention is drawn to paragraph C3.1 which requires each specific duplex alloy to be separately qualified. The metallurgical qualification and the hardness determinations detailed in paragraphs C3.3 and C3.5 shall form an integral part of the procedure qualification. Any need for corrosion testing to paragraph C3.7 as part of the procedure qualification shall be specifically identified by BP. (vii) When either titanium or zirconium are used for the construction of the heat exchanger reference shall be made to the requirements detailed in Appendix D of BP Group GS 118-7.
Welder Qualifications Welders and automatic welding operators shall be qualified in accordance with BS 4871, Part 3 or equivalent, except that specific qualification is required for each grade of duplex stainless steel, titanium and zirconium. Welders and automatic welding operators shall weld an agreed quality control test piece at regular intervals during the production welding of titanium or zirconium, see Appendix D, paragraph D5 of BP Group GS 118-7.
Tube Location For Welding Accurate fit-up and intimate contact between the tube and tubesheet is essential for the achievement of consistently high quality joints. This is particularly the case with automatic welding. Fit-up may be assisted by light expansion of the tube ends. This may be achieved by the use of taper expanders or specially designed punches.
Any expansion prior to welding must be carefully controlled since if the tubes are too tightly expanded gases can only escape through the joint gap and this may cause weld metal porosity.
Preheat For guidance, pre-heat temperatures are proposed for the materials listed in the following table:Material Carbon steels with > 0.25% C Alloy steels with up to 2%Cr Alloy steels with 2%-6%Cr Pre-heat temperature °C 50-100°C 100°C min 200°C min
Other alloys do not in general require preheating.
A wide range of carbon and carbon manganese steels, low alloy steels, austenitic and duplex stainless steels nickel alloys and other non ferrous materials are used in heat exchanger applications. Many of these materials may be welded without the need for preheating.
Any specific need for the application of preheat shall be established as part of the welding procedure qualification test. Welding should not take place when either condensed moisture is present on the components or the ambient temperature is below 5°C.
Although the parent materials selected for a given application would perhaps require preheating when welded with a nominally matching composition filler, changing the filler wire to an austenitic stainless steel or nickel based material may allow either the preheat temperature to be reduced or the preheat to be removed. The use of an automatic rather than manual welding process may also allow reduction or removal of the preheat.
When preheating is applied and it is necessary to interrupt the welding the assembly shall be insulated and allowed to cool slowly. Before welding is resumed, the assembly shall be brought back to the required preheat temperature. On completion of welding the assembly shall again be allowed to cool slowly as above. Electrical pre-heating shall be used whenever possible. Fixed gas burners of suitable design giving a soft diffused flame may be used for preheating and maintaining the preheat, provided that an adequate degree of control can be demonstrated.
Post Weld Heat Treatment (PWHT) The application of PWHT to tubesheet assemblies requires particularly careful control and support to ensure even heating and avoid distortion of the tubes. Therefore it can be beneficial to consider measures that will avoid PWHT. For example, the use of a clad tubesheet may allow fabrication without the need for PWHT of the final assembly. In the absence of suitable clad material, the application of weld overlay to the tubesheet may be considered, allowing PWHT of the tubesheet prior to drilling and tube end welding. Tube selection should also take into account the need to avoid PWHT. When PWHT is unavoidable, procedures detailing tube bundle support, thermocouple locations, heating and cooling rates, and soak times shall be submitted for the approval of BP. The avoidance of PWHT during fabrication also considerably enhances the ability to repair the tube end welds on-site.
Welding The tubes shall be welded to the tubesheet using the qualified and approved procedure. All tubes shall be welded individually. Procedures such as 'figure 8' welding and other complex welding patterns are not recommended. The tube joints shall be welded in such a manner as to minimise distortion of the tube sheet. Unless otherwise agreed with BP, where manual multi-run welds are used, no second run shall be deposited until the first run has been completed, cleaned as necessary and the weld visually examined, (Ref para 4.13. of this Specification). An intermediate low pressure air test or dye penetrant test may be required by BP, (Ref para 6.1. of this Specification).
An intermediate low pressure air test or dye penetrant test after the first weld pass ensures that any defects that may give rise to leakage are detected at an early stage in manufacture. It also ensures that no attempt is made to make a two pass weld in only one run.
Quality Control During Welding The manufacturing and quality control procedures shall ensure that all welding is adequately monitored. Equipment checks shall take place prior to the start of each shift of production welding and at regular intervals during the course of production. The objective of these checks is to ensure that all welding is performed in accordance with the qualified and approved procedure.
Production Control Test Blocks When specified by BP, a sample tube end weld shall be made at the commencement of each shift employing a test block identical to that employed for the welding procedure qualification test (see Appendix D of this Specification). This weld shall be visually examined before production welding starts and, if found unsatisfactory, the cause shall be established and the test repeated prior to the commencement of production. The production control test blocks shall be sectioned and checked at agreed intervals to ensure that the specified requirements in terms of weld throat thickness, penetration, profile, ductility and hardness are met. If the results of these tests are unsatisfactory, production welding shall cease. The cause shall be established and any sub-standard production welds rectified to the satisfaction of BP.
Production control test blocks should be specified for all critical heat exchangers and when unfamiliar materials or automatic welding techniques are being used.
Cleaning and Inspection All cleaning and inspection activities shall be undertaken in accordance with documented procedures submitted in advance by the vendor for approval by BP. All NDT procedures shall be submitted to BP for review prior to the commencement of welding. After welding, the face of the tubesheet, the welds and the tube bore to a distance of at least 25 mm beyond the fusion line should be cleaned and examined visually for surface defects. Defects such as weld spatter, surface breaking porosity, slag deposits, lack of fusion and cracks are unacceptable and shall be rectified in accordance with Section 7 of this Specification. Any over-run or spillage of weld metal into the tube bore which will be detrimental to subsequent expansion or exceeds 5% of the bore diameter at any one location shall be carefully removed. Where a more searching examination is required, dye penetrant testing may be specified by BP. Radiographic examination shall be required for backface welds, although alternative NDT techniques may be proposed for consideration by BP.
When critical heat exchangers are being fabricated, it is essential that tube end welding and inspection progress together to an agreed programme, e.g. each shift of tube end welding should be subject to inspection before further welds are made. This approach will ensure that any short comings in weld quality are identified at an early stage and that the situation can be rectified before it escalates. For backface welding the progressive assembly of the unit will dictate the welding and NDT sequence.
All inspection personnel shall have relevant experience which must be documented in the manufacturer's quality system. NDT operatives shall possess the relevant level of PCN qualification. 5. TUBE EXPANSION 5.1 General The expanded zone shall lie at least 19 mm from the weld root (if the tubes are welded) and at least 3 mm from the back of the tube sheet. A 50 mm length expansion is normally sufficient. Tube expansion shall be carefully controlled to avoid expansion beyond the tubesheet. The equipment used for tube expansion should be either of the mandrel and parallel roller type, or the hydroswage type. The vendor shall provide a procedure for the strength expansion of tubes for review by BP prior to commencement of fabrication. 5.2 Roller Expansion Roller expanders should incorporate limiting controls to give a predetermined amount of tube wall thinning, i.e. controlled torque equipment should be employed. The tube expander rolls should have radiused ends.
Two 3 mm (0.125 in) wide x 1.5mm (0.064 in) deep grooves are normally used for roller expansion.
Hydroswaging A special testing programme is necessary for each material combination to ensure that hydroswaging is fully effective. Particular attention is drawn to the need to allow sufficient time for the metal to flow into the expansion grooves.
The groove detail for hydroswaging is different from that used in roller expansion. Grooves 5 mm (0.200 in)wide x 0.8 mm (0.032 in) deep are used for hydroswaging.
Wall Thinning The amount of tube wall thinning for strength expansion should normally be 5-7% of the original tube wall thickness. The machine settings to achieve this thinning shall be determined and checked during procedure testing by measurements as follows:Diameter of tube hole: D Mean outside diameter of tube: Inside diameter of tube after expansion: Inside diameter of tube before expansion: Tube wall thinning = (T − t ) − ( D − d ) 2
Measurements may be made by external and internal micrometer but bore measurements may also be made by a Go-No-go plug gauge.
Where an expansion of 5-7% is not advisable, because of the material type or joint configuration, a suitable percentage expansion shall be agreed with BP.
Because of minute amounts of out-of-roundness in the tubes and variation in thickness, a range for the percentage wall thinning is given rather than a single value. The range is more easily achieved with hydroswaging than roller expansion. Theoretical studies have been made of the strength of tube to tubesheet attachments and one by Jawad, Clarkin and Schuessler is referenced in Appendix B.
Expansion After Welding If the tubes are to be expanded after welding, the bores shall be inspected for weld spillage as detailed in para 4.13 of this Specification. It is permissible to dress the bores lightly in the weld area to avoid jamming of the rollers during subsequent expansion, but specific attention shall be given to ensure the minimum removal of metal from the bores of the tubes.
Expansion Procedure Test Block Expansion shall be performed in accordance with a documented procedure approved by BP. When specified by BP a test block consisting of nine holes, 3x3, for square pitch arrangement or seven holes, 2,3,2, for triangular pitch arrangement shall be used to demonstrate the control and effectiveness of the expansion technique. Pressure or leak testing together with strength testing and sectioning may be used to prove the test block.
When the tubes are to be expanded and welded the welding procedure qualification test block and the expansion test block may be combined. 5.7 Expansion Check During production, BP may require a check to be made of the expansion on selected tubes and the results recorded. This is for heat exchangers on critical applications.
Measurement by Go-No-go gauge is a quick way of checking that all tubes have been expanded by the correct amount.
LEAK DETECTION 6.1 Leak Detection of Welded or Welded and Expanded Tube Ends Prior to the leak test, a dye penetrant check of all tube end welds shall be made.
When specified by BP, the final hydrostatic test shall be preceded by a low pressure air test or by a gas leak test. No liquid shall be applied to the shell side of the tube sheet prior to any gas leak test. Where manual multi-run tube-to-tubesheet welds are used for critical duties, the leak test should be carried out on completion of the first run. By agreement with BP, liquid penetrant testing may be substituted for low pressure air testing or gas leak testing. 6.1.1 Air Testing The assembly should be tested for leaks by applying a pressure of 0.5 bar (ga) (7.25 psig). While the shell is under pressure, a soap detergent shall be used to indicate the escape of air from leaks.
The above pressure has been found to be the optimum for leak testing. Higher pressures should not be used because the air jet at a leak may blow the soapy water away making detection difficult.
6.1.2 *
Gas Leak Testing When specified by BP, a tracer gas leak test shall be used instead of the air test. The tracer gas is usually helium, but other gases may be used subject to BP approval.
Leak Investigation All suspect weld locations shall be marked for repair.
In the event that more than 5% of the tube to tubesheet welds are found to be defective a full investigation into the cause of the high incidence of defects shall be conducted. Unless otherwise authorised by BP, the whole of the tubesheet and all tubes shall be re-prepared and re-welded at the vendor's cost. 6.2 * Leak Detection of Expanded Only Tube Ends Where specified by BP, the assembly shall be leak tested in accordance with the requirements of para. 6.1.1 and, if necessary, leaks shall be investigated and rectified as required in para. 6.1.3, before the final pressure test. REPAIRS Prior to any repairs being undertaken the face of the tubesheet, the welds and the internal surfaces of the tubes shall be thoroughly cleaned to a length of about 25 mm (1.0 in.) by a suitable method. Any grease that may be present shall be removed either by the use of a chloride and sulphur free non-residue forming solvent or by steam jets. The repair of leaks detected by hydrostatic testing may be complicated during rewelding by the boiling of entrapped water behind the weld which can cause weld metal porosity. Therefore, the heat exchanger shall be drained and, if necessary, dried by hot air before any repair welding is carried out. Any leaks discovered shall be repaired to the original procedures taking care not to over expand the tubes. Testing should be repeated until all faults are remedied. Defective welds shall be completely removed to sound metal and repaired using the qualified WPS.
Care shall be taken not to over-expand the tubes as this can lead to tube failure.
PRESSURE TESTING The final acceptance pressure test shall be conducted in accordance with the applicable design code. 2% by volume of an approved wetting agent or detergent shall be added to the test water. When austenitic stainless steels are being tested the chloride content of the water shall not exceed 30 ppm. Other test media may be specified by BP in special cases where water may be unsuitable because of complexity of design, or for duty with
process fluids whose admixture with water is undesirable, e.g. SO2 or LPG. Such specifications will include the procedures to be used for freeing the exchanger of the test medium prior to despatch from the supplier's works. After maintaining the specified pressure for a minimum period of 30 minutes the welds and bores of the tubes shall be examined for leaks. The location of all leaks shall be marked on the tubesheet and recorded on a tubesheet drawing. All leaks shall be repaired as described in Section 7 and the unit subject to a repeat pressure test. 9. * DRAINING AND DEWATERING The vessel shall be drained thoroughly after testing to avoid corrosion or microbial attack. Where specified by BP, a dewatering fluid approved by BP should be used. Any passivation treatments shall be specified by BP. If the heat exchanger is required to be completely dry and when specified by BP, the assembly should be heated by an appropriate method to a temperature that causes no damage to the unit, but is sufficiently high to remove all water, particularly from the interspaces between the tubes and the tube sheet.
INSPECTION A summary of the inspection activities specified in this Specification is given in the following table:-
SECTION 4.4 4.5 & App C 5.1 5.6 & App C 4.6 & App C 3.0 3.0 3.0 4.12 4&5 5.1 5.7 4.11 6.1 6.1.1 7. 8. 9.
INSPECTION ACTIVITY Approve tube end WPS Inspect welding procedure qualification test blocks. Witness/approve testing and results. Approve tube expansion procedure Inspect expansion procedure test blocks witness/approve testing and result Inspect welder/welding operator qualification test blocks witness/approve testing and results Inspect machining of tube sheet holes and grooves prior to assembly Inspect cleanliness of tubes and tube sheet prior to assembly Inspect cleanliness of tubes and tube sheets prior to welding Approve daily welding test blocks Inspect during tube end welding and expansion for compliance with procedures Examine expanded tubes for damage and over expansion Review report on % expansion - check against test block results Visually inspect tube end welds Witness liquid penetrant examination of root and final passes Witness leak testing after expansion and tube end welding Inspect repairs to this standard Witness final pressure test Confirm that heat exchanger has been dewatered/dried as specified
APPLICABILITY TEW TEW EXP EXP TEW * * TEW * TEW & EXP EXP EXP TEW * *
SUMMARY OF INSPECTION ACTIVITIES TEW - Applies only to tube end welding EXP - Applies only to expansion * - Applies where specified
APPENDIX A DEFINITIONS AND ABBREVIATIONS Definitions Standardised definitions may be found in the BP Group RPSEs Introductory Volume. Abbreviations GMAW GTAW HAZ NDT PCN PQR PWHT SMAW TEMA WPS Gas Metal Arc Welding Gas Tungsten Arc Welding Heat Affected Zone Non-destructive Testing Personnel Certification in Non-destructive testing Procedure Qualification Record Post Weld Heat Treatment Shielded Metal Arc Welding Tubular Exchanger Manufacturers Association Welding Procedure Specification
APPENDIX B LIST OF REFERENCED DOCUMENTS A reference invokes the latest published issue or amendment unless stated otherwise. Referenced standards may be replaced by equivalent standards that are internationally or otherwise recognised provided that it can be shown to the satisfaction of the purchaser's professional engineer that they meet or exceed the requirements of the referenced standards. ASME VIII:1995 ASME IX ASME Boiler and Pressure Vessel Code - Section VIII Division 1 ASME Boiler and Pressure Vessel Code - Section IX Welding and Brazing qualifications Unfired fusion welded pressure vessels Approval testing of welding procedures Part 3: Arc welding of tube to tube-plate joints in metallic materials Approval testing of welders working to approved welding procedures Part 3: Arc welding of tube to tube-plate joints in metallic materials Standards of Tubular Exchanger Manufacturers Association Quality systems - Model for quality assurance in design/development, production, installation and servicing. Fabrication of Pipework to ANSI B31.3 Part 3: Austenitic and Duplex steel pipework, Cupro-nickel and Nickel based alloy pipework.
BS 5500:1997 BS 4870:1985
BS 4871:1985
ASME Pressure Vessels and Piping Conference, Chicago 1986: Evaluation of tube-totubesheet junctions, by Jawad, Clarkin and Schuessler, PVP-Vol.105.
APPENDIX C TYPICAL JOINT DETAILS C.1 PLAIN FILLET WELD This joint detail, involves minimal machining, but requires considerable skill on the part of the welder to avoid burn through when tube wall thickness is below 2.5 mm. It is recommended for seal welding only. In instances where welds overlap it is recommended that the proposed technique is proven to give adequate penetration at the overlap.
t = 1.6 mm min for GTAW T = 2.5 mm min for SMAW T= 2.5 mm min for GMAW Weld size L = t min, but not less than 3 mm. The minimum distance between tubes = 2.5t or 8 mm, whichever is the lesser When service conditions are onerous, preference should be given to Fig.C.3A or C.3B FIGURE C.1 PLAIN FILLET WELD
RECESSED TUBE This joint detail involves minimal machining. It is recommended for seal welding. Where the tube wall thickness is a minimum of 3 mm and access is not restricted the recessed tube joint detail is applicable to the SMAW process. The GTAW process is applicable to this joint detail down to a tube wall thickness of 1.6 mm.
For GTAW, and F = 0.7t min. t = 2.5 mm max. The tube may be flush or up to 1.5 mm max below tube surface. For GMAW and SMAW, and F = 0.7t min. t = 3 mm min. .
FIGURE C.2 RECESSED TUBE
GROOVE WELDS The use of the groove enables a weld of adequate throat thickness to be achieved without having excessively large fillets. Additionally, the use of two layers of weld metal reduces the risk of leaks from pores or inclusions provided weld stop/start positions are not coincident. Grooves for adjacent welds shall not overlap and this preparation is only suitable where tube spacings are wide enough for this requirement to be met.
C.3.1 GROOVE PLUS FILLET
C.3.2 GROOVE This joint detail is intended for the manual and automatic GTAW process with filler additions in two layers. It is applicable for tubes having wall thickness down to 1.6 mm where the tube end needs to be capped. Welders need to possess a high level of skill to avoid melting of the tube wall. The double layer of weld metal reduces the risk of leaks, provided the weld stop/start positions are not coincident. The joint is only applicable where the grooves do not overlap.
W 2= 1.5t
W1= 1.5t B=t W1 t
FIGURE C.3.1 GROOVE PLUS FILLET
R mm 3-5 5 6.5 8 t mm 1.6 - 2 2 - 3.3 4.1 4.9
FIGURE C.3.2 GROOVE
Welding Process GTAW GTAW, SMAW, GMAW GTAW,SMAW,GMAW GTAW,SMAW,GMAW
FIGURE C.3 GROOVE WELDS
CASTELLATED WELD PREPARATION With tubes of a wall thickness of 2 mm or less, this joint detail is adopted where there is a serious risk of tube wall burn through. It is intended for the manual and automatic GTAW processes with or without filler additions. However, with manual welding it can be difficult to control the penetration in order to achieve a strength weld and the detail is not recommended where the weld is expected to corrode in service. The pitch of the tubes should be such that a projection can be formed round each hole, but the intersection of grooves is unimportant.
D = 1t to 2t
t = 2mm or less
Notes: 1) 2) 3) 4) GTAW process only It is advisable to examine the tubesheet surface for laminations before machining. Set-up of tube shall be flush with castellation. These details are recommended for use when it is required to minimise the deformation of the tubesheet due to welding, e.g. clad tubesheet.
FIGURE C.4 CASTELLATED WELD PREPARATION
BACK FACE TUBE SHEET WELDING This technique is used where it is essential to eliminate the crevice at the back of the tube sheet. Specialised automatic GTAW welding equipment is necessary and may generally be used with the tubesheet in the vertical or horizontal position. The welded joints may be radiographed provided assembly, welding and NDT are carefully sequenced.
TUBE END & RECESS SEAT TO BE SQUARE
10 + 0.40
FIGURE C.5 Note 1 The joint shown in Fig. C.5 is suitable for duties where no crevice is permissible between tube and tubesheet and requires the use of a special automatic internal bore welding head. Note 2 During welding, the back of the tubesheet in the vicinity of the joint shall be protected by an inert gas shield. FIGURE C.5 BACK FACE TUBE END WELDS
DESIGN TO AVOID HOT HYDROGEN SULPHIDE CORROSION The joint details shown are used at the hot (front) tubesheet on condensers and waste heat boilers on sulphur units at temperatures of approximately 420°C. Each combines strength with good heat transfer thereby minimising corrosion from hot hydrogen sulphide.
1mm 30 0
30 0 5 max 8
Note: Welding may be by SMAW, GTAW or GMAW.
FIGURE C.6 DESIGN TO AVOID HOT HYDROGEN SULPHIDE CORROSION
APPENDIX D WELD PROCEDURE AND WELDER QUALIFICATION TEST BLOCKS
TUBE SPACING WELD DETAIL TO BE THAT USED ON ACTUAL HEAT EXCHANGER
SECTION OF TUBES & TUBE SHEET OF SAME MATERIAL SIZE & THICKNESS TO BE USED ON ACTUAL HEAT EXCHANGER
FIGURE D.1 TEST SPECIMEN FOR SQUARE PITCH
FIGURE D.2 TEST SPECIMEN FOR TRIANGULAR PITCH
Note: Full details are given in BS 4870 Part 3 and BS 4871 Part 3.
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