While reviewing flare systems we come across some plants having flare systems at two design pressures. One is a Low pressure flare system and other a high pressure flare system. I understand that the LP flare shall have subsonic velocity at flare tip and HP flare shall operate at sonic velocity at tip. This means HP flare shall offer more superimposed back pressure on relief valves discharging into it. Correct me if I am wrong.
The only benefit of HP flare as I could gather was that somekless combustion is possible without additional utilities.


My question is why should there be flare system swith two design pressures? Why cannot we have do away with HP flare system when we have an LP flare system?

Is there any guideline about Which relief streams shall we route to HP flare system and which to LP flare? When should we have a HP flare system?

A complex can have two flare systems based on the back pressures involved. What is important is the set pressures of the PSV or other sources to the flare, rather than combustion efficiency of the flare itself. For instance if a tank essentially operating at atmospheric pressure is connected to the LP flare, it is not prudent to add a PSV whose set pressure and relieving rate is much higher. The back pressure generated in the header when this PSV relieves may be higher than the design pressure of the tank. For course HP flares are the first choice as with higher backpressures, one can reduce the size of the flare headers.