Technical requirements for the procurement of pressure vessel heads

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Appendix H of the standard GB/T25198-2023 \"Pressure Vessel Heads\" specifies all the requirements that must be stated in the technical documents for ordering head assemblies. These requirements include, but are not limited to: a) head specifications (name, type code, diameter, nominal thickness, minimum forming thickness), which can be indicated in the form of head design codes； For example: Standard elliptical head EHA2000×20 (min18.4). b) When thinning due to head forming processes is taken into account, the starting thickness δs for head processing should be specified in the ordering technical documents ； The feeding thickness δs of the head is determined by the head manufacturer; the purchaser only needs to provide the nominal thickness and the minimum thickness. c) When there are special requirements for the lap welds of the head, they should be specified in the ordering technical documents ； To avoid openings in the head or pre-welded gussets covering the joint welds of the head, for heads that are assembled first and then shaped, it is required that there be one or at most two seams in the head ； It specifies the location of the lap welds; generally, it is stipulated that no lap welds are allowed within a certain diameter range from the center. For melon-segment welded heads, the number of melon segments in the head is determined based on the positions of the nozzles and pre-welded components on the head. d) The reference for the alignment of the end cap and the cylinder body is the outer circumferential length or the inner diameter, which shall be specified in the ordering technical documents ； Pressure vessels are generally based on an inner flat surface, hence the standard for joining the head to the cylinder body is defined as the inner diameter. For small-diameter EHB-type heads, corresponding requirements also need to be established by converting the inner diameter of the cylinder into the inner diameter of the head. e) If the height of the straight edges differs from the requirements in 5.1.2, it shall be specified in the ordering technical documents ； Clause 5.1.2 stipulates that for straight-edge hemispherical, elliptical, disc-shaped, flat-bottomed, and flanged conical end caps, when the end cap diameter D is ≤ 2,000 mm, the straight-edge height h is 25 mm ； When the head diameter D > 2000 mm, the straight edge height h is 40 mm. If the height of the straight edge h is not specified in the head specifications, use the default value given in the standards ； If it differs from the specified value, the height of the straight edge h must be given in the head specifications. f) Head cover material (including in-service condition) and re-inspection requirements ； The procurement technical requirements must specify the material of the head to be ordered as well as the corresponding heat treatment condition specified in the drawings. If re-inspection of materials is required as specified in the diagram or technical agreement, the specific re-inspection items must also be stated in the procurement technical requirements. g) Groove type of the end cap (refer to Appendix J); each pressure vessel manufacturer shall determine the groove type suitable for welding at its own facility, based on its manufacturing equipment and capabilities. h) Standards for manufacturing, inspection, and acceptance ； If there are no special requirements for the manufacturing of the end caps, it is sufficient to state in the procurement technical specifications that their manufacturing, inspection, and acceptance shall comply with GB/T25198-2023 ； If there are any other special requirements, they should also be specified in the procurement technical requirements. i) The design drawings of the corresponding head should be provided (if necessary) ； For standard sizes, simply provide the corresponding code as required by the standards. For heads of non-standard sizes, a head drawing must be provided. j) Requirements for heat treatment ； For heads whose cold forming strain does not exceed the requirements specified for the corresponding material, it is necessary to specify that stress-relief heat treatment be carried out after forming ； For heads whose cold forming strain exceeds the requirements for the corresponding material, it should be specified that heat treatment to restore their properties is necessary after forming. For heads that are hot-formed and disrupt the heat-treated condition of the raw material, it should be required that they be heat-treated in accordance with the heat-treated condition of the raw material after forming. For heads made from normalized material, when hot forming is used, although the temperature inside the furnace may exceed the normalization temperature of the material, the temperature drops rapidly after it is taken out of the furnace. By the time the head has been fully formed under pressure, its surface temperature is usually only around 600 degrees; hardly any head manufacturing plant is able to achieve the normalization temperature. Therefore, the 2023 version of the head standard eliminates the requirement that, for steel heads supplied and used in the normalized heat-treated state, if the finishing temperature of hot forming is not lower than the minimum normalization temperature of the material, no further normalization treatment is necessary after hot forming. k) Requirements for welding test pieces and base metal heat treatment test pieces ； In accordance with Clause 6.9 of the head standard, if welded test pieces with joint welds or base metal heat-treated test pieces are required, this shall be specified in the procurement technical requirements. i) Requirements for non-destructive testing ； In addition to specifying the non-destructive testing requirements for the drawings, surface testing of the inner and outer surfaces at the ends of the head as well as at the transition areas where the corners meet is also required after the head has been formed, in order to prevent surface microcracks caused by cold working hardening during the head-forming process. m) Requirements for corrosion resistance testing (if necessary) ； If the drawing specifies requirements for conducting intergranular corrosion tests on the material, these should be included in the procurement technical specifications. n) Requirements for ferrite content measurement ； For austenitic stainless steel pressure vessels designed for temperatures below -40°C and subjected to strain hardening treatment, after the head is formed, a ferrite meter should be used to measure the ferrite content along two mutually perpendicular parent lines on the inner and outer walls; the testing areas should include at least the top, the corner transition zones, and the areas near the ports on the straight sides. For spliced end caps, the ferrite content of their weld joints should also be measured. The measured ferrite content should not exceed 15%. o) Category of pressure vessels ； Indicate the corresponding category based on whether the pressure vessel is of Category I, Category II, Category III, or outside any category as shown in the diagram. p) For heads manufactured in segments, the delivery condition (as a whole, in segments, pre-assembled, etc.) as well as the technical requirements for acceptance shall be specified ； For heads manufactured in segments, the delivery condition should be specified based on the transportation conditions. For heads delivered in segments, it is generally necessary for the head manufacturer to pre-assemble the segments and assign them numbers to facilitate subsequent assembly. q) For flat-ended heads, other requirements for manufacturing, inspection, and acceptance (such as flatness) shall be specified ； For flat-bottomed end caps used as tube sheets in heat exchangers, corresponding flatness requirements should be specified to prevent the flatness from exceeding limits, thereby ensuring that the requirements regarding the extent to which the heat exchange tubes protrude from the tube sheet are met. r) Marking requirements (6.4, Chapter 8) ； 6.4 All materials used for manufacturing heads shall have identification marks for inspection and tracking. During the manufacturing process, if the original identification mark is cut off or the material is divided into several pieces, the marking should be transferred before cutting or moving. The method of marking shall be determined by the end cap manufacturer and must meet the following requirements: a) When not agreed upon by both the supplier and the buyer, hard-printed markings shall not be used ； b) The mark shall not affect the properties of the head material. 8. Product marking 8.1 When the head leaves the factory, it shall be marked with the product identifier in a visible location on the head (usually at the end of the head, about 10 mm away from the end). Methods for marking can include labels, nameplates, spray coding, and marks indicating corrosion of the head elements. Any special requirements regarding marking methods and locations must be specified in the order’s technical conditions. 8.2 The mark shall include at least the following: a) name of the manufacturing unit ； b) Head product marking ； c) Product number ； d) Date of manufacture. s) Requirements for coating and transport packaging ； 9. Coating and Transport Packaging 9.1 The coating of the end caps as well as their transport packaging shall comply with the relevant provisions of NB/T10558, and shall also meet the requirements specified in the design documents and ordering technical conditions. 9.2 For various types of finished end caps that cannot be transported as a whole, it is permitted to cut them at appropriate locations after fabrication for separate transportation; after being rewelded and assembled, their quality shall meet all the relevant requirements specified in this document. t) Other ordering requirements (such as polishing, pickling, etc.). Before packaging and shipping, the inner and outer surfaces should be subjected to acid washing and passivation treatment (for stainless steel or non-ferrous metals) or sandblasting treatment (for carbon steel or low-alloy steel). There should be no oxide scale or other defects that affect the appearance quality on the surface, nor shall there be any pits with a depth greater than 0.5 mm on the inner and outer surfaces.