Help: Relationship between design pressure and safety valve setting pressure
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For example, if the design pressure of a container is 0.6 MPa, but its operating pressure is 0.2 MPa, does it mean that the safety valve only needs to have a setting equal to or less than the design pressure? Can I set the setting value of the safety valve at 0.6 MPa, or should it be set according to a value of 1.05–1.1 times the operating pressure?
This post was last edited by sdwfsgyykai on 2019-4-7 08:01. It’s best to set the allowable working pressure according to your method. The safety valve is configured according to your process design, and the operating pressure of the container is set based on your desired pressure range. If the operating pressure level of the pressure vessel is higher than your intended operating pressure, then the safety factor will also be higher. It is something different from the safety valve that is installed.
Under normal circumstances, as you said, it follows Appendix B, value 150.1: the setting pressure is determined based on the operating pressure, with the design pressure being slightly higher than the setting pressure. But in practice, it is usually the customers and sales teams who have already set the design requirements. Let me share my approach for your reference. It must be ensured that the set pressure of the safety valve × 1.03 (or ±0.015 MPa, whichever is greater) is less than the design pressure (maximum allowable operating pressure), and as close as possible to this design pressure ； It corresponds to the requirements of 150.1 B.4.6 regarding the pressure deviation of safety valves.
A factor of 1.05 to 1.1 should suffice; this mainly depends on your work stress level.
Set the safety valve setting pressure at a working pressure of 1.05–1.1
This should be considered by the process system; generally, the design pressure is chosen more often. We also need to take into account start-up, shutdown, purging, or emergency conditions. The difference between your operating pressure and the design pressure is a bit large; using a value that is only slightly higher than the operating pressure may not be sufficient
If the device is to undergo a gas-tightness test, the setting pressure of the safety valve should be higher than the design pressure.
Of course, it should be set at 1.05~1.1 times the working pressure of the container.
For those that do not require a gas-tightness test, the setting pressure of a regular safety valve is generally taken as 1.05 to 1.10 times the operating pressure, preferably as close as possible to the design pressure； When a device needs to undergo a gas-tightness test, the maximum allowable operating pressure of the device should be specified. The setting pressure of the safety valve must not be higher than this maximum allowable operating pressure; essentially, it has nothing to do with the operating pressure.
The sealing pressure of a standard safety valve is approximately 90% of the set pressure; therefore, it is essential to ensure that the operating pressure of the equipment does not exceed this sealing pressure. Moreover, if there is only one overpressure relief device, the set pressure of the safety valve should not be higher than the design pressure or the maximum allowable operating pressure of the container.