Maintaining Flare Header Positive Pressures

It is certainly required to maintain a positive pressure throughout the relief valve effluent piping system to preclude the entry of air, which, at least theoretically, could cause an explosion. 

On many occasions in refineries, 1 have measured such pressures, which vary between perhaps 6 to 20 inches of water. There are two ways of maintaining such a positive pressure. The smart way and the dumb way. 

I m sure you have seen the flame of a flare surging. That is not typically due to a relief valve opening periodically. It is due to periodically blowing the water seal in the flare seal pot (often located integral with 

At the request of one VP of refining in India, who found the flare flame surges objectionable, I reduced the depth of the water seal, and the intensity of the flame surges were thus reduced. But, of course, not the amoimt of hydrocarbons consumed. 

Maintaining a water seal of about 10 inches of water in the flare seal drum is the smart way of preventing air intrusion into the relief valve collection system. 

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Liquid seal drums Emergency vent streams are usually passed through a liquid seal, commonly water, before going to the flare stack. The liquid seal drum is usually located downstream of the knockout drum, and some vendors designs include them in the base of the flare stack. A liquid seal drum is used to maintain a positive pressure in the vent header system and upstream system. It also reduces the possibility of flame flashbacks, caused when air is inadvertently introduced into the flare system and the flame front pulls down into the stack it also acts as a mechanical damper on any explosive shock wave in the flare stack. Figure 23-58 is a schematic of a typical flare stack liquid seal drum, designed per API RP 521 criteria. 

It is not uncommon for my clients to maintain a positive pressure in the flare system by continuously purging the most distant ends of the flare header piping with fuel gas, natural gas, or even propane. This is an expensive alternate to the water seal method 1 described above. It eliminates the need for the water seal and also the annoying sight of the pulsating flare flame. But to me it seems like a wasteful and, hence, dumb alternate. 

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