Low Temperature Pressure Vessels
Currently, there are no specific standards for low temperature pressure vessels in our country. JB4732 does not differentiate between low temperature and normal temperature boundaries.
★ For low temperature shell-and-tube heat exchangers, see Appendix A of GB151-1999.
★ For low temperature pressure vessels, see Appendix E of GB150.3-2011 (in the old version GB150 it was Appendix C).
● Why attention needs to be paid to low temperature pressure vessels:
At low temperatures, the toughness of materials decreases, leading to brittle fracture at low temperatures. The stress before such brittle failure is far from reaching the yield limit of the material (or yield stress). Failure occurs without obvious signs, so the design, material selection, manufacturing, and inspection of low temperature pressure vessels require varying degrees of enhancement.
● Definition of Low Temperature Pressure Vessels:
Containers made of carbon steel, low alloy steel, duplex stainless steel, and ferritic stainless steel with a design temperature of < -20°C (new standard GB150-2011 defines this as clause 3.1.15; the old standard used ≤ -20°C), and containers made of austenitic stainless steel with a design temperature lower than -196°C.
Relevant Definitions:
● Minimum Design Metal Temperature (MDMT)
According to clause 4.3.4d of GB150.1-2011, when determining the minimum design metal temperature, due consideration should be given to the effect of atmospheric environmental low-temperature conditions on the metal temperature of the vessel during operation. Atmospheric environmental low-temperature conditions refer to the lowest average monthly minimum temperature (the sum of daily minimum temperatures divided by the number of days in that month) recorded over the years.
● Low Temperature Low Stress Conditions
As defined in clause E1.4 of Appendix E of GB150.3-2011: Low temperature low stress conditions refer to situations where the design temperature of the shell or its pressure-bearing components is below -20°C, but the design stress (maximum overall membrane and bending stress actually borne by the vessel component under these design conditions) is less than or equal to one-sixth of the standard normal temperature yield strength of the steel, and not more than 50 MPa. (Note: primary stress refers to the normal stress or shear stress required to balance pressure and other mechanical loads.)
This definition differs from the old standard, distinguishing between design stress and hoop stress, making the use of design stress more rigorous.
The new standard clarifies that when judging the “low temperature low stress condition” of the vessel, in addition to verifying the overall primary membrane stress of the shell component, the primary bending stress-bearing vessel components such as flat heads, tube plates, and flanges should also be examined.
● On the rule of selecting materials under low temperature low stress conditions by adding 50°C (or 40°C) to the design temperature
According to clause E2.2 of Appendix E of GB150.3-2011: When the shell or pressure-bearing component is used under “low temperature low stress conditions,” the material can be selected based on the temperature value after adding 50°C (for equipment not requiring post-weld heat treatment, add 40°C) to the design temperature, but this rule does not apply to:
a) Q235 series steels;
b) Steels with a standard tensile strength lower limit Rm ≥ 540 MPa;
c) Bolting materials.
The new standard specifies that for equipment not requiring post-weld heat treatment, only 40°C should be added, which is the biggest difference from the old standard. Under the same design, material selection, manufacturing, and inspection conditions, post-weld heat treatment of the low-temperature vessel can significantly reduce welding residual stress within the joint area, thereby improving the toughness of the material and joints and reducing the tendency of brittle fracture at low temperatures. Therefore, the new standard stipulates that for equipment not requiring post-weld heat treatment, only 40°C should be added (so PWHT is best; even if determined not to be a low-temperature vessel according to “low temperature low stress conditions,” designers can still design according to the requirements of low-temperature vessels based on specific conditions or engineering experience, just not using low-temperature steels).
● Determination of Design Temperature
Since -20°C is the key indicator for determining whether a container is a low-temperature container, the determination of the design temperature is particularly important.
Principles for determining the design temperature are specified in clause 4.3.4 of GB150.1-2011.
For metal temperatures below 0°C, the design temperature shall not exceed the lowest possible design metal temperature that the component metal may reach.
a) The metal temperature obtained through heat transfer calculations is an average value;
b) Measuring metal temperature on similar containers already in use;
c) Obtaining metal temperature based on internal medium temperature combined with external conditions;
If reliable insulation or cryogenic protection measures are in place, the metal temperature can directly be taken as the medium temperature.
d) For containers placed outdoors or in unheated workshops, the ambient climate conditions must be considered, i.e., MDMT, especially for storage containers.
● Material Selection for Low Temperature Containers
Generally, commonly used ferritic steels for pressure vessels will experience a sharp drop in toughness when the temperature drops to a certain point, becoming very brittle, often referred to as the brittle transition temperature. If a pressure vessel operates below the transition temperature, the presence of higher localized stresses caused by defects, residual stresses, stress concentrations, etc., could lead to brittle fracture without obvious plastic deformation, resulting in catastrophic accidents. For low-temperature pressure vessels, suitable materials must first be chosen, which should have good toughness at the operating temperature. Fine-grained low-alloy steels can be used down to -30°C to -70°C, quenched and tempered high-strength steels down to -20°C to -50°C, 3.5% nickel steels down to -100°C, and 9% nickel steels down to -196°C. Below -196°C, austenitic stainless steels and aluminum alloys can be selected.
Clause 3.7.2 of GB150.2-2011 stipulates that austenitic stainless steels with a service temperature not lower than -196°C can be exempted from impact testing; for temperatures below -196°C to -253°C, the impact testing requirements should be specified in the design documents, with no specific regulations provided.
Clause 3.5 of GB150.2-2011 requires that low-temperature steel plates and forgings used at temperatures below -20°C should be manufactured using ladle refining processes.
Clause 4.1.7 of GB150.2-2011 specifies the requirement for drop-weight testing for detecting the non-brittle transition temperature (NDT) for steel plates used at design temperatures < -40°C.
Clause 4.1.8 of GB150.2-2011 specifies that the ultrasonic testing quality grade for steel plates > 20 mm used in low-temperature applications should be upgraded from level III to level II.
Table 4 of GB150.2-2011 specifies the lower temperature limits for steel plates, including those used in low-temperature applications.
Clause 5.1.1 of GB150.2-2011 stipulates that carbon steel and low-alloy steel pipes used at temperatures below -20°C must be delivered in a normalized state, and cannot be substituted with hot-rolled products whose final rolling temperature meets the normalized temperature range.
Clause 5.1.2 of GB150.2-2011 stipulates that the steel used for pipes designed for temperatures below -40°C should be manufactured using ladle refining processes.
Clause 6.1.2 of GB150.2-2011 stipulates that low-temperature steel forgings used at temperatures below -20°C should be forged from ladle-refined steel (as per NB/T47009).
Clause 6.1.3 of GB150.2-2011 stipulates that low-temperature steel forgings thicker than 200 mm used at temperatures below -20°C should be selected at levels III or IV.
Clause 6.2.3 of GB150.2-2011 specifies the usage temperatures for high-alloy forgings:
a) Ferritic type S11306 steel forgings at 0°C;
b) Austenitic-ferritic type steel forgings at -20°C;
c) Austenitic types as per clause 3.7.2 of GB150.2-2011.
Clause 7.1.4 of GB150.2-2011 stipulates usage restrictions for low-alloy steel studs used at temperatures below -20°C.
Clause 7.2.3 of GB150.2-2011 stipulates usage temperature restrictions for high-alloy studs.
To avoid excessively high localized stresses in low-temperature pressure vessels, the design should avoid excessive stress concentrations and additional stresses; during manufacture, strict inspections should be performed to prevent dangerous defects in the vessel. For excessive residual stresses caused by welding, post-weld heat treatment should be carried out to eliminate residual stresses.
Material Standards for Low Temperature Pressure Vessels
Plate Standards:
Plates: GB3531-2008 “Low Alloy Steel Plates for Low Temperature Pressure Vessels”
Four grades (the standard lists three grades; Grade 1 amendment added 15MnNiNbDR):
16MnDR: t ≤ 60 mm, -40°C, 47 J
60 mm < t ≤ 120 mm, -30°C, 47 J
15MnNiDR: t ≤ 60 mm, -45°C, 60 J
15MnNiNbDR: 10 mm – 60 mm, -50°C, 60 J
09MnNiDR: t ≤ 36 mm, -70°C, 47 J
36 mm < t ≤ 120 mm, -70°C, 60 J
Plates: GB19189-2011 “Quenched and Tempered High-Strength Steel Plates for Pressure Vessels”
Four grades:
07MnMoVR: 10 mm ≤ t ≤ 60 mm, -20°C, 80 J
12MnNiVR: 10 mm ≤ t ≤ 60 mm, -20°C, 80 J
07MnNiVDR: 10 mm ≤ t ≤ 60 mm, -40°C, 80 J
07MnNiMoDR: 10 mm ≤ t ≤ 50 mm, -50°C, 80 J
Plates: GB24511-2009 “Steel Plates and Strips for Pressure Equipment”
Seventeen grades, including eleven austenitic stainless steels (-196°C), three duplex steels (-20°C), and three ferritic stainless steels (0°C).
Plates: “9% Ni Steel Plates for Low Temperature Pressure Vessels” (GB24510-2009)
This standard applies to 9% Ni steel plates not exceeding 50 mm thick for the manufacture of low-temperature pressure vessels such as LNG storage tanks and LNG ships.
Appendix A of GB150.2-2011 adds three types of low-temperature steel plates:
15MnNiNbDR as per GB3531;
08Ni3DR, 6 mm – 100 mm, -100°C, 47 J (this material has no special standard)
06Ni9DR, 6 mm – 40 mm, -196°C, 100 J (more stringent than GB24510-2009)
In addition:
JB/T4734 “Aluminum Welded Vessels” is a comprehensive standard for aluminum pressure vessels (including design, material selection, manufacturing, and inspection). It covers both pressurized and atmospheric vessels, as well as fully aluminum and aluminum-lined welded vessels. Design pressure ≤ 8 MPa, lower temperature limit is -269°C.
JB/T4755 “Copper Pressure Vessels”, the focus of the standard is materials and manufacturing. Since the structural form and strength calculation of copper pressure vessels are similar to those of steel vessels, relevant parts are all referenced from GB150. Its welding procedure qualification and product weld trial plate sections cite relevant regulations and standards, only supplementing the special requirements for copper materials. This standard applies to vessels with a design pressure ≤ 35 MPa, and the design temperature is determined by the allowable use temperature of copper materials and their composite steel plates. Typically, there are no special requirements for copper materials and welded joints when the operating temperature is not lower than -198°C; when the operating temperature is below -198°C (generally not lower than -268°C), it should still maintain good elongation after tensile fracture.
JB/T4756 “Nickel and Nickel Alloy Pressure Vessels”, the focus of the standard is materials and manufacturing. Since the structural form and strength calculation of nickel and nickel alloy pressure vessels are similar to those of steel vessels, relevant parts are all referenced from GB150. Its welding procedure qualification and product weld trial plate sections cite relevant regulations and standards, only supplementing the special requirements for nickel materials. This standard applies to vessels with a design pressure ≤ 35 MPa, and the design temperature is determined by the allowable use temperature of copper materials and their composite steel plates. Typically, there are no special requirements for nickel and nickel alloy materials and welded joints when the operating temperature is not lower than -198°C; when the operating temperature is below -198°C (generally not lower than -268°C), it should still maintain good elongation after tensile fracture.
Forging Standards:
NB/T47009 “Low Alloy Steel Forgings for Low-Temperature Pressure Equipment” (replacing JB/T4727)
Six grades:
16MnD: t ≤ 100 mm, -45°C, 47 J
100 mm < t ≤ 300 mm, -40°C, 47 J
20MnMoD: t ≤ 300 mm, -40°C, 47 J
300 mm < t ≤ 700 mm, -30°C, 47 J
08MnNiMoVD: t ≤ 300 mm, -40°C, 60 J
10Ni3MoVD: t ≤ 300 mm, -50°C, 80 J
09MnNiD: t ≤ 300 mm, -70°C, 60 J
08Ni3D: t ≤ 300 mm, -100°C, 47 J
NB/T47010 “Forgings for Stainless Steel and Heat-Resistant Steel Used in Pressure Equipment” (replacing JB/T4728)
Sixteen grades in total.
Pipe Standards:
GB9948-2006 Seamless Steel Pipes for Petroleum Refining
Among them, No. 10 steel pipes can be used at -20°C with an outer diameter ≥ 70 mm and wall thickness ≥ 6.5 mm (31 J, clause 5.1.4 of GB150.2-2011);
GB6479 Seamless Steel Pipes for High-Pressure Ammonia Equipment
Among them, No. 20 steel pipes can be used at -20°C with an outer diameter ≥ 70 mm and wall thickness ≥ 6.5 mm (31 J, clause 5.1.5 of GB150.2-2011); 16Mn steel pipes can be used at -40°C with a wall thickness ≤ 40 mm (34 J, additional impurity content control, P ≤ 0.025%, S ≤ 0.012%);
Appendix A of GB150.2-2011 adds two types of low-alloy steel pipes:
09MnD: t ≤ 8 mm, -50°C, 47 J
09MnNiD: t ≤ 8 mm, -70°C, 47 J
Stainless Steel Pipe Standards Referenced by GB150:
GB 13296-2007 Seamless Stainless Steel Pipes for Boilers and Heat Exchangers
GBT 14976-2012 Seamless Stainless Steel Pipes for Fluid Conveyance (not suitable for heat exchanger tubes)
GBT 21833-2008 Seamless Stainless Steel Pipes of Austenitic-Ferritic Duplex Type (when used for heat exchanger tubes, cold-drawn or cold-rolled pipes must be used, with high dimensional accuracy)
GBT 12771-2008 Welded Stainless Steel Pipes for Fluid Conveyance (not suitable for heat exchanger tubes, usage restrictions see clause 5.2.4 of GB150.2)
GBT 24593-2009 Welded Stainless Steel Pipes for Boilers and Heat Exchangers (usage restrictions see clause 5.2.5 of GB150.2)
GBT 21832-2008 Welded Stainless Steel Pipes of Austenitic-Ferritic Duplex Type (not suitable for heat exchanger tubes, usage restrictions see clauses 5.2.6, 5.2.7, 5.2.8 of GB150.2)
● General Requirements for Materials Used in Low Temperature Containers
a) All pressure-bearing components must be produced using ladle refining processes;
b) Comply with relevant low-temperature material standards;
c) Some materials cannot be used, such as Q235 series steels and steels with a standard tensile strength lower limit Rm ≥ 540 MPa;
d) Welding materials also have requirements: they should match the properties of the base metal, especially in terms of toughness. Welding rods should be retested batch-wise for moisture content of the coating or hydrogen content of the deposited metal.
In addition,
Clause 6.1.3 of GB150.2-2011 stipulates that low-temperature steel forgings with a nominal thickness greater than 200 mm used at temperatures below -20°C should be selected at levels III or IV.
Clause 4.1.8 of GB150.2-2011 stipulates that the ultrasonic testing quality grade for steel plates > 20 mm used in low-temperature applications should be level II.
● Structural Design of Low Temperature Containers
a) The structure should be as simple as possible, reducing constraints;
b) Avoid creating large temperature gradients;
c) Minimize sudden changes in structural shape to reduce stress changes;
d) The connection between branch pipes and shells should be smoothly transitional, with rounding at the inner end of branch pipes;
e) Supports or legs of the container should not be directly welded to pressure-bearing components; spacer plates should be used;
f) Branch pipe reinforcement should preferably be integral or thick-walled pipe reinforcement (suggested by experience).
These requirements remain unchanged in both the old and new standards, being general principles without detailed specifications.
● Design of Welding Joints for Low Temperature Containers
a) Full penetration is required for Class A and Class B joints;
b) When Class B joints cannot be double-sided welded, single-side butt joints with non-removable backing plates after welding can be used;
c) There are specific temperature and pressure usage limitations for flat flanges of Class C joints; flat flanges are not recommended, but rather butt-welded flanges;
d) All welding of heads to cylinders must be full penetration;
e) All welding of branch pipes to shells must be full penetration;
f) There should be no lack of fusion in the fillet welds between reinforcement rings and shells, and they should be continuously welded;
g) Short welding or spot welding of Class E welding joints should be avoided except for structural requirements;
h) Cross welding should not be used in shell welding (generally cross welding is not recommended for ordinary vessels).
● Manufacturing, Inspection, and Acceptance of Low Temperature Containers
a) Welding procedure qualifications should be carried out according to NB/T47014 or supported by qualified welding procedures (general requirements);
b) Control welding line energy, multi-pass welding is preferred (clause 7.2.5 of GB150.4);
c) No lack of fusion, lack of penetration, cracks, porosity, undercutting, etc., should be present, and excess height should be minimized.
d) Hard stamping marks are not allowed on the pressure-bearing components of low-temperature containers (clause 5.3.3 and 7.2.6 of GB150.4)
e) Clause 7.2.3 of GB150.4 stipulates that the welding joints of austenitic stainless steel with a design temperature below -100°C but not lower than -196°C need to undergo impact testing (31 J).
f) Clause 8.2.2.1 of GB150.4 specifies the thickness requiring PWHT: 16MnDR: > 25 mm; 09MnNiDR, 15MnNiDR: > 20 mm (design temperature ≥ -45°C), any thickness (design temperature < -45°C); 08Ni3DR, any thickness (it is suggested that PWHT should be done as much as possible for low-temperature containers; in the old standard C4.4, PWHT was required for all low-temperature steel plates thicker than 16 mm).
g) Product weld trial plates should be fabricated for each vessel (clause 9.1.1.1 of GB150.4);
h) Clause 10.3.1 of GB150.4 specifies the conditions requiring 100% RT or UT: design temperature below -40°C, or thickness of welding joints greater than 25 mm for low-temperature containers.
i) Local RT or UT for low-temperature containers should not be less than 50%, and not less than 250 mm.
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