What are the special requirements for cryogenic pressure vessels

Structural design
The structural design of cryogenic pressure vessels should be considered sufficiently flexible, with the following main requirements.
① The structure should be as simple as possible to reduce the constraints between the welded parts.
② The structure should be designed to avoid excessive temperature gradients.
③ Sharp changes in the cross-section should be avoided as far as possible to reduce local stress concentrations and the inner end of the inserted receiver should be polished to a rounded angle to make a smooth transition.
④ The connecting welds of accessories should not be discontinuous or spot welded.
⑤ The saddle seat, ear seat, legs (except for spherical tanks) or skirt seat of the vessel should be set with a pad or connection plate to avoid welding directly with the vessel shell as far as possible, and the pad or connection plate should be considered according to the low temperature material.
(vi) joint reinforcement should be used as far as possible as a whole reinforcement or thick-walled tube reinforcement, if the reinforcement plate is used, the weld seam should be rounded and smooth transition.
(vii) For vessels that cannot be heat-treated as a whole, if the parts to be welded with them need to be stress relieved, the parts should be considered to be able to be heat-treated separately.

Tube openings
Low temperature pressure vessels should try to avoid the main weld and its nearby area, such as must be opened in the weld area, should meet the requirements of the relevant standards.
The receiver on the low temperature pressure vessel should meet the following requirements.
① with the shell welded pipe section, the wall thickness should be not less than 5mm, of which the diameter DN ≤ 50mm receiver, it is appropriate to use thick-walled pipe, its extension using ordinary wall thickness seamless steel pipe.
② turn should be simmered or pressed elbow, shall not use straight pipe collocation welding (shrimp waist).
③ for the insertion of the receiver, the sharp corner of the end of the tube inside the shell wall should be turned or polished into a rounded corner of R ≥ 3mm.
④ longitudinal welds and ring welds between pipe sections where the receiver is rolled should be fully welded through the structure.
⑤ For flammable or toxic dangerous media of extreme or high hazard, or when the pressure is ≥1.6MPa, the T-shaped receiver should be a seamless extruded tee or a structure with thickened pipe openings for welding.

Flanges
Butt-welded flanges shall be used for flanges meeting all of the following conditions.
① design pressure ≥ 1.60MPa, and the flange of a container containing flammable or toxic for extreme, highly hazardous media, or a receiver flange with a large applied load.
② design pressure ≥ 2.50MPa container flanges and receiver flanges.
Butt welding flange should use seamless forging or rolling process production, not allowed to use thick steel plate cutting and made; allowed to use steel or steel plate bending, welding and made, but need to be post-welding heat treatment. Such as the use of steel plate bending, the steel plate should be cut into strips along the direction of rolling, bending should make the surface of the steel plate parallel to the centre line of the flange, but also must be ultrasonic testing of the steel plate.

Fasteners
The main requirements are as follows.
① low-temperature pressure vessel flange with bolts, studs and other fasteners shall not be used in general ferritic commodity fasteners supporting nuts, allowing the use of general commodity nuts, but the use of temperature should not be lower than -40 ℃.
② recommend the use of core rod diameter is not greater than 0.9 times the thread root diameter and no thread in the middle of the elastic bolt, stud.
③ design temperature of not less than -100 ℃ ferritic steel containers, should be used ferritic steel fasteners (bolts, studs, nuts, washers), the design temperature of less than -100 ℃ austenitic steel containers, should be used austenitic steel fasteners.
④ in line with GB 3098.6 "Fastener mechanical properties - stainless steel bolts, screws and studs" in the A2 grade austenitic steel commodity fasteners can be used not less than -196 ℃ low-temperature pressure vessels.
⑤ for reducing stress conditions, when the adjusted impact test temperature is equal to or higher than -20 ℃, can use general ferritic commodity fasteners.

Sealing gaskets
Gaskets for low temperature pressure vessels are commonly made of metal (including semi-metallic gaskets) and non-metallic materials, with the following conditions and requirements.
① Metal materials for sealing gaskets for use at temperatures below -40°C should be austenitic stainless steel, copper, aluminium and other metal materials with no obvious transformation characteristics at low temperatures, including metal bands for wound gaskets, metal clad gasket shells and hollow or solid metal pads.
② Non-metallic material gaskets should be made of materials with good elasticity at low temperatures, such as asbestos, flexible (expanded) graphite, PTFE, etc. The conditions of use are as follows.
Temperature not lower than -40℃, and pressure not higher than 2.5MPa flange sealing gaskets are allowed to use high quality asbestos rubber sheet, non-asbestos rubber sheet, flexible (expanded) graphite sheet, polyethylene sheet, etc.; temperature not lower than -120℃, and pressure not higher than 1.6MPa flange gaskets are allowed to use high quality asbestos rubber sheet soaked in paraffin.

Welding
The main requirements are as follows.
① For Class A, B and C welds a full weld-through construction should be used. For Class D welds, in addition to the flange and the container wall welding, small diameter receiver (DN ≤ 50 mm) and thicker head or cover welding, with internal threads of the pipe joint and the container wall connection can be in accordance with the relevant provisions of HG 20582, should also be used in full weld through the structure.
② low-temperature pressure vessel welding should be assessed before welding process, assessment should focus on the weld and heat-affected zone of low-temperature Charpy (V-notch) impact test, its qualified indicators should be determined by the requirements of the parent material, shall not be lower than the performance of the parent material.
③ welding process should be strictly controlled welding line energy in the process evaluation of the line energy range, it is appropriate to choose a smaller welding line energy, multi-channel welding.
The butt weld must be welded through and the residual height of the weld should be minimised and should not be greater than 10% of the thickness of the weld and not greater than 3mm. The fillet weld should be rounded and not allowed to bulge outwards. The surface of the weld shall not be allowed to have defects such as cracks, porosity and edge biting, and there shall be no sharp change in shape, and all shall be smoothly transitioned.
⑤ The arc shall not be induced in non-welding parts, and the arc shall be induced by means of an arc guide plate or in the bevel.
(6) Welding accessories or tooling fixtures, ties, etc. must use the same welding material and welding process as the shell material and be welded by qualified official welders, and the length of the welding channel must not be less than 50 mm.
(7) Due to mechanical processing, welding or assembly caused by the container surface damage, such as scratches, welding scars, arc pits and other defects, should be repaired grinding. After repairing the wall thickness shall not be less than the calculated thickness of the container plus the corrosion margin, the grinding depth shall not be greater than 5% of the nominal thickness of the container, and not more than 2mm.
⑧ Not allowed to use discontinuous or spot welding connection of the weld.
Source: Pressure Vessel Design and Research
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