The Knockout Drum

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. 

A practical formula for obtaining the required diameter of horizontal knockout drums (based on 400 microns particle size) is 

This expression can be transformed into a suitable equation for the drum diameter. 

Assuming a liquid density, pi = 40 Ib./cu. ft., and a pressure in the knockout drum, P = 19 psia, and neglecting Pc with respect to Pf, we obtain 

The equation for D is applicable to single-flow knockout drums. Split-flow drums, where the vapors enter at the middle of the drum and leave at both ends, have twice the capacity of single-flow drums. The diameter of split-flow drums is therefore 0.7 times the diameter obtained by the equation for D. 

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Cyclone Separator with Integral Catch Tank This type of containment system, depicted in Fig. 26-19, is similar to the ore-mentioned type, except that the knockout drum and catch tank are combined in one vessel shell. This design is used when the vapor rate is quite high so that the knockout drum diameter is large. 

Cyclone Separator with Integral Catch Tank (See Fig. 26-19.) The diameter of the knockout drum is calculated by the criteria given in the preceding section and Fig. 26-18. Since the liquid is also to be retained in the vessel, extend the shell height below the normal bottom tangent line to increase the total volume by an amount equal to the volume of the hquid carried over. 

A chemical reaction could continue in the knockout drum/catch tank. 

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. 

A gas stream having the composition given in Table 6.2.1 flows into a conpressor suction. Size the knockout drum to prevent Uquid from entering the compressor. The gas enters the drum at 105 F (40.6 "C) and 150 psig (10.3 bar). 

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. 

A fired reboiler control S3rstem using the Wobbe index is shown in Fig. 17.3. The density meter should be installed directly in the fuel gas hne, downstream of the knockout drum and any points where fiiel gas streams are added, and where it would be unaffected by vibrations (370). 

During operation, a small amount of acid may drip into the catch-pot where it can be periodically drained to the acid-storage drum. Acid consumption usually amounts to less than a pound (75 per cent acid) per 140 gal of polymer product. Loss in acid arises mainly by carry-over into the knockout drum which contains limestone for neutralization of... 

I feel sure, dear reader, that you are objecting to my use of such terms as "slight" and "severe," as these are qualitative terms. Well, we do not know enough about entrainment to quantify it. Also, we have neglected such obviously important factors in the knockout drum, shown in Fig. 33.1, as ... 

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