Flare KO drum pressure

[thaiiaht]

I am confused about the pressure on the flare system, and it stitched in my mind for quite long time.

Could you please help to clarify ?

Supposing I am going to design a flare system, and would like to start at the K.O. drum to determine the size of the flare stack. Of course, I will give some pressure drop across the flare tip, and pressure drop on the flare stack itself. Here come the problem.

In order to size the flare stack, you need to know "Allowable pressure drop". Thus I need to know the pressure at the K.O. drum to get the maximum allowable pressure drop for my flare stack.

According to API 521 the recommendation for KO drum design is

"Most knockout drums and seal drums are operating at relatively low pressures. To ensure sound
construction, a minimum design gauge pressure of 345 kPa (50 psi) is suggested for knockout drums in
subsonic flare or other low-pressure applications. A vessel with a design gauge pressure of 345 kPa
(50 psi) should not rupture if a deflagration occurs. Stoichiometric hydrocarbon-air mixtures can produce
peak explosion pressures on the order of seven to eight times the absolute operating pressure. Most
subsonic-flare seal drums operate in the range of gauge pressure from 0 kPa to 34 kPa (0 psi to 5 psi)."

At my governing load, what should be the pressure that you see at the knock out drum ? At operating pressure (34 kPa) or at its design pressure (345 kPa) ?

Thank you in advance for help me out...

 

[Chance17]

I have seen studies that report 24-psig flare header pressure in event of general Refinery Fire.
This is obiviously excessive and is currently being mitigated.

You want the Flare drum pressure to be as low as practical.
Not at its 50-psig MAWP.
The normal (no PSV) Flare pressure is about 2-psig. This is set by the Flare drum water seal height.

If you have the luxury of a new facility design, you should target 5-psig when all credible safety valve combinations are relieving to the Flare.
For existing facilities, it is important to document the maximium pressure for a "global" type event.

 

[thaiiaht]

Thank you very much for your kind answer. This helps me a lot.