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GB150 VS ASME Ⅷ-1 on impact test judgment

Attention:     Issuing time:2020-09-14 11:22
GB150 VS ASME Ⅷ-1 on impact test judgment
1. The difference between GB150 and ASMEⅧ-1 on impact test judgment
my country's pressure vessel standard GB150 and ASME Ⅷ-1 code differ in the judging conditions for whether to perform an impact test. GB150.1-2011 stipulates that: vessels made of carbon steel, low alloy steel, duplex stainless steel and ferritic stainless steel whose design temperature is lower than -20℃ are cryogenic vessels. The materials used for cryogenic containers must be subjected to Charpy V-notch impact test. The test temperature should not be higher than the design temperature. The impact energy should meet the value required by the standard at this temperature. At this time, the only exemption condition for the impact test is: "Stress condition", the specific concept of "low temperature and low stress" is specified in Appendix E of GB150.3-2011. However, "low temperature and low stress condition" is not applicable to materials with the lower limit of tensile strength of steel standard Rm≥540MPa and bolt materials .
For vessels whose design temperature is within the range of -20℃~0℃, because our country's plates and forgings only do 0℃ impact test before leaving the factory, the requirement of -20℃ impact test should be put forward when purchasing materials to meet the requirement of -20℃. For toughness requirements at a temperature of ℃~0℃, its impact energy index shall not be lower than the standard stipulation of 0℃ impact energy value. GB150.2-2011 stipulates: For Q245R, Q345R, 13MnNiMoR steel plates, 20, 16Mn, 20MnMo forgings, if it is necessary to specify the -20℃ impact test, it should be indicated in the design document.
In addition, different material thicknesses exhibit different toughness. Therefore, GB150.2-2011 stipulates that steel plates with a shell thickness greater than 60mm should be subjected to a V-notch impact test.
The ASMEⅧ-1 code from 1989 to the current version cancels the concept of cryogenic vessels. Whether to perform an impact test is determined by the material group, control thickness, and minimum design metal temperature. The test temperature should not be higher than the minimum design metal temperature, and the impact energy meets the standard Regulations are sufficient.

2. Compared with GB150, the ASMEⅧ-1 code has the following characteristics:
(1) There is no fixed temperature value to judge whether to do an impact test;
(2) An impact test is not necessarily required for a design temperature below -20℃, and an impact test is not necessarily exempted for a design temperature higher than -20℃ or even above 0℃;
(3) For the same group of materials, the same control thickness, different design temperatures, or the same design temperature, different control thicknesses, the impact test is also different;
(4) The impact test required in a certain clause may be exempted in other clauses. That is to say, the judgment of "impact test should be carried out" is not necessarily the final result, but the judgment of "impact test exemption" is final in conclusion.

3. Steps of impact exemption in ASMEⅧ-1 code
In the ASME Ⅷ-1 code, the exemption conditions are usually used to determine whether a material is subjected to an impact test. Carbon steel, low-alloy steel pressure vessel shells, heads, nozzles, manholes, reinforcement plates, flanges, tube sheets, flat cover plates, as well as permanently retained welding gaskets and counter vessels welded to pressure components The conditions for the "impact test exemption" for accessories and other parts that are essential for the structural integrity of the company are summarized as follows:
3.1. The first step: judge by UG-20(f)
(1) The design temperature range is -29℃≤T≤345℃; (Note: Due to the influence of the atmospheric temperature in the low temperature season, the operating temperature occasionally lower than -29℃ is allowed);
(2) The container has no thermal shock, mechanical shock or cyclic load;
(3) After the vessel is manufactured, perform the hydraulic test in accordance with UG-99(b) or UG-99(C). Generally, the vessel is subjected to hydraulic test, unless the structure and other reasons use UG-100 for pneumatic test;
(4) For P-No.1.1 or P-No.1.2 group materials:
①According to UCS-66, the material belongs to curve A, and its control thickness does not exceed 13mm. Common curve A materials include: SA-283, SA-285-C, SA-36, SA-515-70, etc.;
②According to UCS-66, it belongs to curve B, C or D material, and its control thickness does not exceed 25mm. Common curve B, C, D materials are: SA-516-70, SA-105, SA-106-B, etc.
If the above requirements are met at the same time, the material can be exempted from the impact test. Otherwise, as long as one of them is not met, it will be transferred to the second step for judgment.
3.2. The second step: the conditions for directly obtaining the impact test exemption
(1) The minimum design metal temperature is higher than 50℃, and the impact test is not required for any thickness of any group of materials;
(2) The design metal temperature is not lower than -48℃, and the thickness is not more than 2.5mm, no impact test is required;
(3) UCS-66 curve A materials are used to make metal retention pads, and the thickness is not greater than 6mm. If the minimum design metal temperature is not less than -29°C, no impact test is required;
(4) Choose the following standard flanges, if the minimum design metal temperature is not lower than -29℃, the impact test can be avoided;
① ASME B16.5 ferritic steel flange;
② ASME B16.47 ferritic steel flange;
③ SA-216 Gr WCB split loose flange, the grade is not more than 150 pounds or 300 pounds.
(5) When the minimum design metal temperature is lower than -48℃ and not lower than -105℃, when the ratio S=trE/(tn-c)≤0.35, the impact test is exempted;
(6) For pipes with DN≤100mm and P-No.1 group materials, the minimum design metal temperature is not lower than -105℃, as long as the thickness of the material is not greater than the thickness value corresponding to the lowest yield strength listed in the table below. No impact test.
Minimum yield strength (MPa) Thickness(mm)
140~240 6
250~310 3.2
≥320 2.5
(7) When ASME Ⅷ-1 does not require forced heat treatment and heat treatment of P-No.1 materials, the minimum design temperature can be reduced by 17℃, even if the temperature is lower than -48℃.
(8) For the minimum design metal temperature of bolts and nuts exempt from impact test, please refer to the provisions of ASME Ⅷ-1 UCS-66 General Note (c).
3.3. The third step: judge according to UCS-66(a)
When judging according to this article, you must accurately calculate the "control thickness" of the component. When the intersection point of the minimum design metal temperature and the control thickness of the material falls on or above the corresponding material curve, the impact test can be avoided. If the intersection point falls below the corresponding curve, the fourth step is required.
3.4. The fourth step: judge according to UCS-66(b)
According to UCS-66(a), if the component is not exempt from impact, if the ratio S=trE/(tn-c)<1 and the minimum design metal temperature of the component is not lower than -48℃, the judgment can be continued according to this article. And calculate the MDMT reduction value. When the temperature obtained after the reduction is lower than the required MDMT, the component material is exempt from the impact test.
The MDMT reduction value can be found according to the chart in UCS-66(b) according to the ratio S. When the reduction value does not exceed 22℃, it can be calculated according to the following formula: MDMT reduction value = (1- S)×56℃, for unbearable The overall primary film tensile stress components, such as flat heads, flat covers, tube sheets and flanges (including bolts and nuts), the ratio S is the ratio of the maximum design pressure to the maximum allowable pressure.
Through the above four-step screening, if the impact test is not exempt, there is no choice. The material used in the container can only be subjected to the impact test under MDMT, and the impact energy meets the requirements of the standard.
4. Conclusion
Because the impact test is a large expense for the pressure vessel manufacturer, the designer should make every effort to avoid it. The impact test can be avoided by increasing the control thickness of the component or selecting materials with better toughness and higher grades. However, at this time, the economics of the equipment should be considered. In order to avoid the impact test, whether the increase in these costs is worth it.



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