Calculation of the discharge area for the discharge device
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Safety valves and rupture discs are subject to GB/T 150.1-2011 when calculating the discharge area of discharge devices. The discharge pressure (absolute pressure) pf of the relief device is given by pf = design pressure p + overpressure limit Δp. Here, should the design pressure p refer to the design pressure of the pressure vessel, or to the setting pressure of the safety valve/design burst pressure? When calculating the discharge area of the SW6 discharge device, the design pressure p used is the design pressure of the pressure vessel. However, when the set pressure/design burst pressure of the safety valve is less than the design pressure of the pressure vessel, the design pressure p is taken as the set pressure/design burst pressure of the safety valve; as a result, the calculated value for the discharge area will be higher than that calculated based on the design pressure of the pressure vessel, making it a more conservative approach. Additionally, according to Annex 5 of Edition 99 of the standards, when calculating the discharge capacity of a safety valve, the discharge pressure (absolute pressure) pd of the safety valve is equal to 1.1 times the set pressure ps of the safety valve plus 0.1. In summary, when calculating the discharge pressure, if the set pressure/design burst pressure of the safety valve is less than the design pressure of the pressure vessel, is it correct to use the set pressure/design burst pressure of the safety valve as the design pressure in order to calculate the discharge area?
In my opinion, the discharge pressure of the safety valve – which is determined by adding the design pressure of the container to the allowable overpressure limit – already takes into account sufficient safety margins; there is no need to consider it based on a set pressure or the design burst pressure.
This post was last edited by AtlantisH on 2018-11-26 at 14:06. Under normal circumstances, the design pressure of a pressure vessel equals the set pressure of the safety valve; this is how we determine the design pressure in the absence of any special circumstances. Furthermore, the design pressure of the pressure vessel must not be lower than the set pressure of the safety valve. It is recommended to take a look at Appendix B, section B.3 \"General Provisions\" in 150.1, and you will understand. In fact, that pressure refers to the highest pressure that can be reached at the inlet of the safety valve.
You must be wrong; the pressure of the relief device definitely cannot be higher than the design pressure of the equipment. The discharge pressure of the relief device is generally 1.1 to 1.2 times the operating pressure. If the discharge pressure of the relief device is lower than the design pressure of the equipment, what is its purpose?
The last edit to this post was made by Dongfang1979 on 2018-11-26 at 18:43. The formula for calculating the discharge area in Appendix B of GB/T150.1-2011 is essentially the same as that in API 520; API 520 defines the discharge pressure as the set pressure plus the overpressure limit plus atmospheric pressure. Therefore, I believe that when calculating the discharge area of the SW6 discharge device, using the design pressure of the pressure vessel as the value for p is not sufficiently conservative.
B.3.2.3 When there are multiple relief devices, the operating pressure of one of them shall not be greater than the design pressure, while the pressures of the others can be increased to 1.05 times the design pressure. If the design pressure of the pressure vessel is used, it is not conservative enough.
It can be improved when designing multiple discharge devices
Since GB/T150.1 specifies that the relief pressure consists of the design pressure and the overpressure limit, the values used in SW6 software are appropriate; there is no need to be more conservative in this regard.