Gas characteristic coefficient calculated for the safety valve
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The gas property coefficients given in API520 are unitized; why then are the calculation formulas in HG20570 identical to those in API520? HG20570 is a national standard unit; why is it using the same formula as the European standard units defined by API? Different formulas are used in ASPEN’s help files.
The gas property coefficient, formally known as the imperial gas property coefficient, is denoted as C in Table B.4 of Appendix B to GB150.1; this value is obtained by consulting a table. HG20570.2-1995 is marked as X; it is based on API520, and all formulas start with 520. As for the ASME algorithm, whose full name is Gas Property Parameters in the SI system, the ASME code for it is C; the value can be obtained by referring to Figure 11-1M in ASME8, or using the formula 0.03948.
Great, thanks for the answer. Also, does the original poster know why GB150 does not mention the discharge rate of gas storage tanks in case of a fire, that is, the calculation formula for non-wetted surfaces?
For gas fire conditions, refer to HG20570.02-1995, Part 9: Release rates of containers without wetted surfaces under external fire conditions; the formula there is quite clear
It is indeed included in HG20570. I just attended a training session on safety valves, and the instructor mentioned that the discharge volume of gas storage tanks in case of a fire is not taken into account, as expansion occurs at the inlet and outlet. Therefore, GB150 specifies that the discharge volume is simply the intake volume calculated based on the inlet gas flow rate. This consideration seems a bit lacking; moreover, what about the situation where both the inlet and outlet valves are closed? Why isn’t that mentioned?
Section 9 of HG/T 20570: Safety valves for gas storage containers. 9.0.1 In the event of an external fire, containers without a wetted surface will fail within a short period of time due to the softening of the metal material. Installing a safety valve will not provide independent protection for such containers from damage; it can only be effective for a short period of time (before the metal softens). Therefore, other methods such as external insulation, water spraying, or automatic/manual pressure relief systems (with control valves) must be employed.