hi, sorry for my poor memory. API 521 suggests the minimum design pressure of knockout drum should be 0.345MPa. My thinking is that the flare stack should be not less than this value.
Normally, the rate of pressure increase rise of hydrocarbon is roughly 7~8 times of deflagration, and 12~13 times for H2. So if the operating pressure of flare system is 0.5bar, so 0.5* 8 = 4 bar. Then 7 bar design pressure is reasonable.
There is also code from API, it has recommended valve. If...