The expansion tube issue in heat exchangers

Tube expansion process: The expansion of tubes in heat exchanger tube sheets can be carried out using various processes: 1 – Mechanical tube expansion: using rotating rollers. 2. Hydraulic tube expansion: Pressurized by the liquid inside the tube. 3. An **explosive expansion tube** installed inside a tube. Bulging should be carried out across the entire thickness of the tube sheet. These different processes can be used individually or in combination. In combined use, when mechanical tube expansion is combined with another process, it can be partial tube expansion. When only mechanical tube expansion is performed, it is called full-depth mechanical expansion. Other uses of mechanical tube expansion include: • Stress-relief tube expansion (Mechanically improved expanding): After full-depth mechanical expansion, controlled mechanical expansion is used to relieve the stress in the outer wall of the tube in the transition zone. •Initial expansion bonding (positioning tacking): Partial tube expansion during welding to reduce the gap. This operation is part of the tube/tube sheet welding evaluation. Function of F4413 expansion tubes: Expansion tubes can fulfill the following functions: a) Protecting the welds between the tubes and the tube sheet from the stresses exerted by the tube bundle during operation. Under these conditions, the mechanical strength provided by the tube/tube sheet expansion joint is sufficient to meet the requirements for the mechanical strength of the joint as specified in B3353.3.b.7, C3353.3.a.3, or D3300. b) Seal the gap between the tube and the holes in the secondary side tubes, in order to prevent the fluid on the secondary side from passing through the gap between the tube and tube L, and to minimize the height of this gap. c) Mechanical loads on the absorption components. d) Sealing between the two circuits of the heat exchanger. Functions a and b are mandatory for the welded tube/tube sheet joints. Functions e and d are mandatory for non-welded tube/tube sheet joints. F4414 Preliminary tests: In order to select the brazing process that is most likely to meet the operational requirements, the contractor shall, together with the manufacturer, carry out a preliminary test plan based on its experience and technical capabilities in order to investigate the joint properties of the tubes/tube sheets obtained using the selected process. Tests should be conducted on the specimens to improve the limits of the expansion tube parameters, and the inspection methods for the components used in the manufacturing process should also be examined. The contractor shall prepare and submit documents containing the results of all preparatory tests. He should specify the range of parameters for exploration and the values used to determine the valid range for process evaluation. The document can include the work accomplished during the production of other manufactured items, as well as supplementary specialized tests for the heat exchangers involved. These tasks and tests should be able to explore the sealing and mechanical properties of the joints. The equipment technical specifications should indicate whether the risk of stress corrosion needs to be considered. Under these circumstances, the residual stresses in the inner and outer surfaces should be evaluated. Residual stress can be evaluated using various methods: stress corrosion crack testing in boiling magnesium chloride solution (according to MC1360) ； X-ray diffractometers, and so on. When stress-relief expansion is to be used, the preliminary tests should be able to verify the conditions required to reduce the residual stresses in the surface layer of the transition zone within the numerical range adopted in the process qualification. These tests shall be conducted in accordance with MCl362. F4420 Process evaluation; F4421 Documents to be prepared. Before performing the tube expansion operation, the manufacturer shall develop a tube expansion process that complies with the requirements of the relevant sections, and it shall include at least: ——all parameters for determining the valid range of evaluation as specified in F4422. ——All inspections to be carried out before and after tube expansion. ——The desired result. Valid range of evaluation for F4422: F4422.1 Workshop – The evaluation tests shall be conducted in the same workshop or site where tube expansion is carried out. However, provided that the tube expansion is carried out by personnel with experience in expansion operations, the valid scope of the tube expansion process evaluation can be extended to other workshops or sites of the same manufacturer. F4422.2 Basic factors of the process: Any change in any of the basic factors involved in the process evaluation requires a re-evaluation or an expanded evaluation. The basic factors are determined according to each type of process. All quantifiable factors should have tolerances to determine the valid range. F4423 Evaluation Test This section applies to all tube expansion processes other than stress-relief expansion (whose testing is specified in F4463). F4423.1 Number and type of test specimens for evaluation tests: The evaluation tests require at least 10 specimens (1 specimen = 1 tube). The specimen should be made into a porous block, with holes drilled and cleaned in the same manner as used during manufacturing. The sample thickness should be representative of the thickness of the heat exchanger tube sheet. The specimen is neither welded nor pre-expanded (even though these operations are required in actual production), in order not to affect the properties of the expansion joint. F4423.2 Tests to be performed and required results The following tests shall be carried out on all specimens: F4423.2.1 Heat exchangers with pipe/tube sheet welded joints a) Visual inspection A visual inspection of the inside of the pipes shall be conducted to check for any damage. b) Dimension inspection: Record the shape of the inside of the tubes throughout the entire thickness of the tube sheet. In particular, check the starting position of the transition zone at the tube sheet outlet. ——Normally, this position should be recessed from the tube sheet outlet, and this distance should not exceed 6 mm. ——In the special case of full-depth expansion of carbon steel tube sheets, the transition zone can start up to 2 mm outside the tube sheet outlet. c) Gap sealing test: The sealing performance of the gap shall be checked by a hydrostatic test using deionized water to which 5% organic red has been added. The minimum pressure applied to the gap on the secondary side of the tube sheet should be 1.5 times the maximum operating pressure of the fluid on the secondary side of the heat exchanger, with a maximum limit of 50 bar. Using an indicator on the primary side of the sample, the penetration rate of the fluid should be equal to or less than 40 mm/min. d) Mechanical strength testing: Each specimen shall undergo a tensile test to measure the pulling force of the tube. The applied load and the displacement of the tensile test head should be recorded. The requirements are as follows: ——The force required to pull out the tube should be twice the basic force, which is determined by the following formula: F≥PπD2 / 2 Where: F = the force required to pull out the tube; P = the maximum pressure difference that occurs under the second type of operating conditions. D = inner diameter of the pipe – The stress determined based on the nominal cross-section shall be greater than half of the minimum yield strength specified for the pipe at 20°C. F / S0＞0.5RP0.2 S0 is the nominal cross-sectional area of the tube. F4423.2.2 Heat exchangers with pipe/tube sheet connections that are not welded: The requirements of F4423.2.1a) apply for visual inspection. b) The requirements of Dimension inspection F4423.2.1b) apply. c) Joint sealing inspection: The sealing of the gaps shall be checked by conducting a hydrostatic test using deionized water to which 5% organic red has been added. The minimum pressure for the gap on the secondary side of the tube sheet should be as follows: ——When the maximum operating pressure is equal to or less than 1.0 MPa: 2 MPa. ——When the maximum operating pressure is greater than 1.0 MPa: 2 times the maximum operating pressure. The seal integrity should be checked using an indicator on the primary side of the sample. d) Mechanical strength testing: Each specimen shall undergo a tensile test to measure the pulling force (F) required to pull the tube apart ； The applied load and the displacement of the pull head should be recorded. The force required to pull out the pipe should be the smaller of the following two values: —— The pulling force determined as a function of the maximum operating pressure (P) of the heat exchanger: When P≤0.5 MPa, F＞800 daN; when 0.5＜P≤1 MPa, F＞1000 daN; when 1＜P≤2 MPa, F＞1500 daN; when 2＜P≤3 MPa, F＞2000 daN; when P＞3 MPa, F＞2500 daN. —— The force equivalent to the minimum yield strength specified for the pipe material at 20°C. F4424 Evaluation Record: The test results of the inspection shall be included in the evaluation record. The validity period for the F4425 assessment: The qualification for the swelging process is valid for 3 years from the date it is approved. The evaluation period can be extended for the same duration starting from the date when the process was last used in the workshop. Beyond this deadline, the manufacturer may apply to the contractor for an extension of the evaluation validity period. When applying, documents showing that the manufacturer has completed the procedures for manufacturing similar items shall be provided as support. F4430 Manufacturing F4431 Overview: To account for the effects of changes in tube length and the operations carried out after the heat exchanger is manufactured, such as welding heat treatment, stress relief, and correction of part deformation, the manufacturing outline should include the sequence of expansion joints. For F4432, the technical requirements should be drafted before manufacturing the document ； Specify: ——The type and nature of the device being used ； ——Process and its parameters ； ——Inspections to be carried out before, during, and after expansion joining. F4433 Cleanliness before swelling: Before inserting the tubes, the tube holes in the tube sheet should be cleaned using appropriate methods to remove all traces of grease, oxides, adhered impurities, or dust. Appropriate protective measures should be provided to maintain the cleanliness of the workpiece between cleaning and threading operations. F4434 Cleanliness after crimping: The cleanliness inside the tube should be restored after crimping. F4440 Inspection: Equipment inspection for F444l. The set values, condition, and proper functioning of the equipment should be checked at the start of manufacturing, at the beginning of each shift, at appropriate intervals as determined by experience, and whenever any notable deviations occur. During the execution of procedure F4442, inspections should be carried out using appropriate systems (marks, symbols on drawings) ； Plugs, stops, positioning tools, etc., are used to ensure that all pipes are expanded in accordance with the applicable procedures. Specific inspections for each process should be specified. In the event of an accident, all pipes that may be damaged should be inspected using endoscopy. F4443 Product witness specimens: Product witness specimens are required only for non-welded tube/tube sheet joints — Number of product witness specimens: 15, or the greater of 0.5‰ of the number of heat exchanger expansion joint operations performed. Note: For the same type of heat exchangers with no less than 1000 tube holes, and in cases where the tube expansion operation is completed within the same month, only one sample per group of 3 heat exchangers will be taken as a representative sample. ——The requirements of F4423.1 shall be complied with to ensure that the witness samples represent the product. The witness fitting expansion should be carried out simultaneously with the product expansion operation. The expansion joining of witness pieces shall be marked in accordance with the corresponding expansion joining of heat exchangers. This mark should also indicate the number of the expansion jointing equipment used in each area. ——Inspection and testing: The inspection and testing of witness materials should be carried out as soon as possible, and the results should be recorded in the test report. ——The required results: The tests to be conducted and the required results are the same as those of the process qualification tests. F4444 Final inspection: The starting position of the secondary side transition zone in all tubes of the heat exchanger shall be inspected. The applicable criteria are as described in F4423.2.1.b). The seal integrity of the tube-to-tube sheet joints welded in the heat exchanger should be checked. F4445 Rework: After checking the crimping operation, if the relevant operation records are included in the inspection report, rework can be carried out after technical analysis. Basic factors for evaluating F4450 full-depth mechanical expansion tubes and F4451 – Equipment ’ • Roller material grade ； •Roller geometry dimensions ； •Angle of the roller groove ； •Slope of the spindle’s conical portion ； •Spindle rotation speed. ——Addition or removal of lubricant — Pipe material grade — Sheet metal material grade. Other grade materials that can be used during the process qualification, whose chemical composition is comparable to that of the specified product material grade and whose room-temperature yield strength deviation does not exceed 20%, may be employed. Other grade materials —— Dimensional properties • Nominal outer diameter of the tube • Nominal diameter of the tube hole • Nominal thickness of the tube sheet • Expansion length L ； Here, L represents the expansion length for evaluating the test. When L < 150 mm, the effective length for evaluation is (1–1.5)L; when L > 150 mm, all lengths greater than 150 mm are considered valid. — The ratio of the drilling center pitch to the diameter of the drilling area. — The drilling process for tube holes. — The expansion torque. — The specified gap determined during the preliminary tests. F4452 Manufacturing: When the tubes are pre-welded to the tube sheet, expansion should start from the welded side of the tube sheet. The expansion (roller) stroke should ensure sufficient overlap between the two strokes (at least 3 mm). The marking system for pipe holes should be able to identify pipes that have been connected to defective drilled holes through special expansion joining, or pipes that have undergone re-expansion joining. F4453 Inspection: F4453.1 Inspection during manufacturing – The tube expansion torque should be checked at least once at the start of each shift. F4453.2 Final Inspection: During the final inspection, it must be ensured that no steps are overlooked. F4460 stress-relief expansion tubes reduce the residual stress on the outer wall of the transition zone where the tube is expanded. F4461 Overview: After tube expansion is completed, if the risk of stress corrosion on the outer surface of the tube must be considered (see F4414), stress-relief expansion can be performed on the expansion transition zone. F4462 Process Description: This process causes slight radial deformation in the tube expansion transition zone and the adjacent unexpanded sections. This operation is carried out using a mechanical tube expander. To effectively relieve stress, the amount of deformation must be sufficient, but the tube must not come into contact with the hole; this can be expressed by the following formula: Minimum Δdia. < Δdia. < J, where Δdia. = dia.1 – dia.0, J = minimum gap between the tube and the hole, dia.0 = inner diameter of the tube before stress relief, and dia.1 = inner diameter of the tube after stress relief, measured in the middle of the stress-relief area. The deformation zone of the stress-relief expansion tube should extend at least 10 mm. F4463 Process Evaluation F4463.1 Basic Factors — Initial tube expansion process (including all its basic factors: — Stress relief process — Equipment *Diameter and geometry of the rollers *Number of rollers *Material grade of the rollers *Angle of the roller grooves *Slope of the core welding cone. ——Addition and removal of lubricant — Pipe material grade — Nominal outer diameter of pipe — Nominal wall thickness of pipe — Change in △dia/F4463.2 Evaluation tests are conducted on specimens whose dimensions allow for at least 2 expansion steps. At least 20 specimens should be made using the fully expanded tube process that has been evaluated. These specimens were stress-relieved under the same conditions as those used during manufacturing. Use appropriate equipment to measure the change in tube diameter ΔD. In particular, the height at the expansion joint where measurement is taken should be determined carefully. If the allowable range can be covered by several values different from the display parameters (such as spindle rotation speed, spindle displacement value, etc.), at least 10 specimens shall be manufactured for each value determined by the parameters. F4463.3 requires that the resulting radial expansion value not exceed the allowable valid range. It should be verified that the pipe is free of damage. The prepared report should specify the initial dimensions of the specimen, individual △D records, and the results of visual inspection. F4464 Inspection: F4464.1 Inspections during manufacturing. Whether tube expansion is performed directly on the heat exchanger or on the sample using tools, the △D value should be checked at least once per shift and whenever the tools are changed. The change in △D should be checked to ensure that it remains within the valid range specified by the process qualification, and there should be no scratches in the transition zone. F4464.2 Final inspection: The final marking procedure shall verify that all pipes have had their stresses removed. F4470 Hydraulic tube expansion; F4471 Key factors of the process – Equipment • The trade name of the tube expander determines the processing method. ——Types of expansion fluid — Lubrication at the expansion tool joint — Process parameters: • Expansion pressure • Duration • Sequential number for each expansion cycle — Material grade of the tube sheet. During the process evaluation, it is possible to use material grades that are comparable in terms of their specified chemical composition, with a deviation in room-temperature yield strength of no more than 20%. ——Tube material grade — Size characteristics • Nominal outer diameter of the tube • Nominal wall thickness of the tube • Nominal diameter of the tube bore • The expansion length is between 0.75L and 1.25L. L is the value used to evaluate the expansion length of the test specimen: it is the ratio of the center-to-center spacing of the tubes to the diameter of the drilling area, as well as the drilling process and the specified gap determined during preliminary tests. According to F4472, the expansion pressure should be checked at least once at the start of each shift. F4480 Hydraulic-mechanical hybrid expansion tube expansion – Hybrid expansion tube expansion is typically a composite process that combines two or more expansion tube expansion techniques described in F4412. In the hydraulic-mechanical hybrid expansion method, the tube is first expanded hydraulically, and shortly after that, local mechanical expansion is carried out on the secondary side. If the basic hydraulic expansion process has been evaluated in accordance with F4420 and F4470, the manufacturer may submit to the contractor special supplementary documents regarding the evaluation of the hybrid expansion process. The main factor in the F4481 process – hydraulic tube expansion equipment: • The trade name of the tube expander specifies the processing method. ——Types of expansion fluid. ——Lubrication of the expander joint. ——Process: • Tube expansion pressure ； •Holding time •Sequential number of the additional tube expansion cycles. ——Tube sheet material grade. During the process qualification, grade materials with chemical compositions that are comparable to those of the specified product material grades, and whose room-temperature yield strength deviation does not exceed 20%, can be used. ——Tube material grade. ——Dimensional features: •Nominal outer diameter of the tube ； •Nominal thickness of the pipe ； •Nominal diameter of the pipe hole ； •The expansion tube length is between 0.75L and 1.25L. L is the expansion length of the test specimen used for evaluation ； •Ratio of tube center pitch to drilling area diameter ； •Drilling process ； •The specified gap determined during the preliminary testing phase. ——Tube expansion equipment: • Roller material grade ； •Geometry of the roller ； •Angle of the roller groove ； •Slope of the weld cone portion of the mandrel ； •Spindle rotation speed. ——Addition or removal of lubricant —— Butt joint length: Effective between L1 and 1.5L1 ； Here, L1 represents the expansion length of the test specimen in the evaluation test. ——Bending tube torque. F4482 Manufacturing: Local tube expansion is performed from at least two steps away from the welds on the tube/tube sheet. The steps of tube expansion should be adjusted to ensure overlap between them (minimum 3 mm). F4483 Check F4483.1 Basic hydraulic tube expansion; see F4472. The requirements of F4453.1 apply to F4483.2 local tube expansion. Additionally, at least 1% of the tubes should be sampled to check the locations where local expansion is applied. F4490 Explosive expansion tube: The explosive expansion tube is formed by expanding the inner tube of the detonation tube. *The tube explosion generates sufficient pressure to deform the tube and the holes in the tube sheet; as a result, after the tube sheet rebounds, a interfacial pressure is maintained between the outer wall of the tube and the tube holes. Main factors of the F449l process — Equipment: •**Type and name ； •Loading per unit length ； •Type of buffer device used. ——Tube material grade — Plate material grade: In the evaluation process, it is possible to use grade materials whose chemical composition is comparable to that of the product’s material grade, with a deviation in room-temperature yield strength of no more than 20%. ——Dimensional characteristics: • Nominal diameter of the tube holes ； •Nominal outer diameter of pipe ； •Nominal thickness of the pipe ； •Ratio of tube center pitch to diameter of the drilling area ； •The specified gap determined during the preliminary testing phase. Expansion tube length: When L＜150mm: valid in the range of (0.75～1.25)L ； When L≥150mm: A length of ≥150mm is valid. Here: L is the expansion length of the test specimen used for evaluation. ——*Position of the tube relative to the secondary side of the tube sheet — Ignition sequence — Drilling process. Manufacturing per F4492. Under no circumstances should tube expansion be carried out at both ends of the same tube simultaneously. Under no circumstances is it allowed to re-expand the tube if it has not exploded. This is the most up-to-date and detailed expansion tube processing procedure, covering all aspects of manufacturing and inspection. Request for an improved version. Last edited by vivo1314 on 2009-3-19 18:38.]

Thank you for the reply from the second floor; thank you for your effort!

The answer on the second floor is too comprehensive. The original poster should first check the relevant national standards and manuals; asking questions about the parts that are not understood after that is the best way to communicate. What a foolish opinion!