On the selection of the overpressure ratio for safety valves in fire conditions
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Dear experts! There is a current issue that requires everyone's help. According to API520, the overpressure ratio under fire conditions is 21%. And based on this, the discharge area of the safety valve is calculated. However, some design institutes reduce this ratio to 10% or 16% to calculate the discharge area of the safety valve. I don’t know which ratio to choose. Thank you!
Lower it to 10% or 16% to calculate the discharge area of the safety valve, as a more conservative approach. No problem.
This is related to the manufacturing processes and materials of various **devices. In the API standards, the allowable overpressure for equipment under fire conditions is 21%; we usually adopt a value of 10%–15% as it is a more conservative approach. This is because, during pressure testing, pressure vessels (pipes) experience pressures that exceed the design pressure, and this degree of overpressure is considered acceptable
Thank you for the reply. But isn’t it safer to use 21%?
When the design pressure (maximum operating pressure) of the equipment or pipeline is fixed, the smaller the overpressure ratio, the more conservative it is. For example: if the design pressure (maximum allowable operating pressure) of the equipment is 5.0 MPaG, and the setting pressure of the safety valve is also 5.0 MPaG, then when an overpressure of 10% occurs, the discharge pressure after the safety valve activates will be 5.0 * (1 + 0.1) = 5.5 MPaG ； When the allowable overpressure is 21%, the relief pressure of the safety valve is 5*(1+0.21)=6.05 MPaG. Since the design pressure of the equipment is only 5.0 MPaG, a release pressure of 6.05 MPaG clearly has a greater impact on the equipment compared to 5.5 MPaG.
If that’s the case, why is there still a requirement for an overpressure ratio? Can’t we just use 16% or 10%? Moreover, the design pressure should be determined after considering all overpressure conditions, right?
1) The safety valve begins to operate once it reaches the set pressure, and the valve disc starts to rise. The fire condition mentioned by the poster, which allows a pressure excess of 21%, means that the valve remains fully open under a 21% pressure excess to achieve the rated discharge capacity. 2) Under fire conditions, the pressure rises rapidly; the increase from 100% to 121% occurs very quickly. 3) Why do safety valves for fire use allow a pressure excess of 21%, while those for non-fire applications only allow 10%? Has the original poster thought about this question? Since it is a safety valve designed for fires, it is not desirable for it to \"trip frequently\".