Some questions regarding safety valves

There are too many issues and it requires too much expertise; non-professional designers cannot handle it. Moreover, in such issues, the proportion of technical exchanges is **lower than that of skill transmission, so they may not be willing to teach. A layperson sharing their own understanding: 1. The temperature may be determined by the manufacturing process; the temperature of the process medium when it is under release conditions is likely to be the release temperature. For example, in the case of liquid release, it is the temperature of the liquid itself; for liquefied gas release, the release temperature is the temperature at which the vapor pressure exceeds the release pressure. For pure gaseous substances, the release temperature is likely to be the temperature of the gas itself. Treat each case individually. 2. If the regulations do not specify a particular range, then it is sufficient to use a pressure that is lower than the design pressure; adjust it as needed. There is no mandatory requirement, so one can decide according to their own needs – isn’t that convenient? 3. The average molecular weight has nothing to do with whether it is in a gas phase or a liquid phase; the safety valve of the gas-liquid separation tank is installed at a high position, so the liquid does not leak out; any leakage occurs only after vaporization. 4. The vaporization heats of the various components are calculated separately and then added together

HG/T20570 has a chapter dedicated to safety valves

Well, I’ve read this document; it’s described in very general terms

1. The temperature unit is k, and the pressure unit is kPaA. 2. The set pressure is generally taken as the design pressure or MAWP

Thank you. Then how should the discharge temperature be determined?

In API 521, the release temperature of steam is determined using an equation of state based on the release pressure

1. The discharge temperature refers to the temperature at the discharge pressure; for liquids, the operating temperature under those conditions is used. For example, in the case of pressure buildup at the outlet of a gear pump, the discharge temperature is the temperature of the fluid being pumped, and the maximum temperature encountered during normal operation should be used； If it is a gas (steam), the value is calculated based on the equation of state or obtained using software such as ASPEN. 2. The setting pressure of the safety valve, that is, the pressure at which the valve starts to operate, should not be higher than the design pressure. For containers, the design pressure is generally used; for pump outlets or pipes due to thermal expansion, this value can be chosen according to process requirements, with the range being more than 10% above the maximum normal operating pressure and equal to or less than the design pressure. 3. The fire condition refers to the situation where a liquid vaporizes due to heating, resulting in pressure buildup; therefore, the compression ratio and molecular weight of the gas phase should be used. If the compression ratio is not clear, 1 can be chosen directly. 4. The heat of vaporization can take an average value, and values for mixtures can be obtained through simulations such as ASPEN. The discharge volume calculated for safety valves should be estimated on a conservative basis by using a low value for the heat of vaporization, which will result in a higher discharge volume. The standard for calculating safety valves is API520; related standards include ASME VIII, API521, API526, API527, and HG20570

However, the temperature calculated in this way is as follows: if the operating pressure is 10 MPa, the set pressure is 11 MPa, and the operating temperature is 300°C, then the discharge temperature = 11/10 * (300 + 273.15) = 573.15 * 1.1 – 273.15 = 357.315°C. Isn’t this far above the designed temperature of 330°C? Moreover, the design institute specified a temperature of 350°C; it’s not clear how this discharge temperature was determined

Thank you very much for your reply, but I still have a few questions: Regarding the first response, for a safety valve in a fire scenario where 80% of the liquid level inside the equipment is an alcohol-based liquid and the upper 20% is hydrogen-rich gas, what state – liquid or gas – is the discharge temperature in such a case? Regarding the third response: regarding the selection of the compression ratio and molecular weight of the gas phase under fire conditions, does this gas phase refer to the gas at the top originally, or to the gas formed after the liquid vaporizes? As for the fourth response, I think the method you mentioned for calculating the heat of vaporization is quite feasible; there are also methods online that use the Chen method to calculate the heat of vaporization. What do you think of that? Thank you

The safety valve of the gas-liquid separation tank is installed at a high point; no liquid will be released, and if anything is released, it will be in gaseous form. http://www.tc29.net