Flare stacks knockout drum

Flare Systems. There is a good chance that the operating company will not have anyone experienced in flare system design. For feasibility cost estimates, rough estimates can be made by comparison with existing plants or a vendor can be contacted for budget cost estimates for the flare stacks and associated knockout drum, burner tip, igniter, and molecular seal. 

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. 

Figure 6-7. Nomogram for flare stack diameter, knockout drum diameter and length of flame.

A knockout drum in a flare system is used to prevent hazards associated with burning liquid droplets escaping from the flare stack. Accordingly, the drum must be of sufficient diameter to produce the desired liquid-vapor separation. 

The flare header, which collects the vapors from the safety valves for safe discharge to the knockout drum and the flare stack, is sized for the largest vapor load caused by a single failure. This vapor load is obtained from a tabulation of relief loads from safety valves connected to the flare system. The loads which may occur simultaneously as a result of fire, cooling water failure, etc., are summed up. From these summations the largest load is determined. 

In start-up it is desirable to purge the headers with fuel gas to remove the air prior to igniting the pilots at the flare tip. The purge time should be at least V/cfh hrs, where V is the total volume of headers (cubic feet), including knockout drum and flare stack, and cfh is the flow rate of fuel gas (cubic feet per hour). 

Branan (2005) has given a simplified method for quick estimation of manifold pressure drops. Flare knockout drum sizing needs to be checked for proper vapour-liquid separation and hence prevention of carryover of liquid to the flare stack. Steam injection to the flare stack also needs to be reviewed to prevent potential black smoke and noise. Relief headers ... 

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