Determination of the set pressure and discharge temperature of the safety valve
Thread Content
I have two questions for everyone: 1. When determining the set pressure of a safety valve, is it determined based on the design pressure of the equipment? Is it sufficient as long as the set pressure of the safety valve is less than or equal to the design pressure of the equipment? Are there any principles for this? 2. In the case of a fire, when calculating the discharge volume of gas containers, which discharge temperature should be used for the safety valve? The design pressure of the equipment? Is it the wall temperature of the equipment (593°C for carbon steel), or is it the calculated discharge temperature? Are there any principles for this?
You’d better take a look at API520 and 521; these two standards are like magic swords that can solve all your questions
This post was last edited by W8694218 on 2016-12-13 08:55. 1. In API520, the setting pressure of safety valves is determined based on MAWP (Maximum Allowable Working Pressure), which is greater than or equal to the design pressure of the equipment. The determination of the set pressure for safety valves in Appendix B of GB150 is consistent with what you mentioned. In addition, according to HG/T 20570.2, the setting pressure of the safety valve must also be determined based on its location. 2. In the case of a fire, equipment can be divided into two categories: those with liquid wetting the equipment walls and those without such liquid wetting. There are two types of discharge amounts to be calculated: the discharge amount when saturated liquid vaporizes to reach the discharge pressure (resulting in an increase in the amount of gas within the control volume), and the discharge amount when the liquid vaporizes due to heating and expansion to reach the discharge pressure (with no change in the amount of gas within the control volume). The discharge temperatures corresponding to the two cases can be calculated. Under fire conditions, the discharge pressure of the safety valve is 121% of the set pressure; by applying the gas equation of state, it is possible to determine the discharge temperatures for both conditions. Additionally, the 593°C you mentioned is the maximum temperature allowed for the equipment’s wall temperature; it means that the equipment will fail if this temperature is exceeded. Therefore, it can be considered the upper limit of the discharge temperature. The above are all personal opinions for reference only.
But I see that the discharge temperature specified in API521 is calculated based on an ideal gas, right?
Please share your opinions on the discharge temperature of safety valves under fire conditions, and provide the most authoritative results.