Knockout drum liquids

Liquid loads are considered from all safety valves that discharge as a result of a single contingency, plus in each case an allowance for knockout drum liquids (fuel gas knockout drums, absorber overhead knockout drums, and compressor suction and interstage knockout drums) equal to the inventory of all drums which discharge to the blowdown drum, at their LHA point. 

Types of Equipment The three most commonly used types of equipment for handling emergency relief device effluents are blowdown drums (also called knockout drums or catch tanks), cyclone vapor-liquid separators, and quench tanks (also called passive scruh-hers). These are described as follows. 

Cyclone Separator with Separate Catch Tank This type of blowdown system, shown in Fig. 26-17 and 26-18, is frequently used in chemical plants where plot pan space is hmited. The cyclone performs the vapor-liquid separation, while the catch tank accumulates the hquid from the cyclone. This arrangement allows location of the cyclone knockout drum close to the reactor so that the length of the relief device discharge hne can be minimized. The cyclone nas internals, vital to its proper operation, which will be discussed in the following sections. 

The author states that this equation is vahd for the design of knockout drums which can separate liquid droplets of 400 [Lm and larger. 

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. 

At the central platform, water and hydrocarbon liquids are first removed in knockout drums. Then saturated natural gas, free of any liquid droplets, enters the twin expanders. The gas is cooled below its dewpoint, allowing heavy hydrocarbon components and water vapor to condense in the discharge stream. Turboexpanders were chosen for two main reasons They are more compact than competing methods of controlling the dewpoint and their operating costs are typically lower than those of many alternatives. 

Fuel systems can cause many problems, and fuel nozzles are especially susceptible to trouble. A gaseous fuel system consists of fuel filters, regulators, and gauges. Fuel is injected at a pressure of about 60 psi (4 Bar) above the compressor discharge pressure for which a gas compression system is needed. Knockout drums or centrifuges are recommended, and should be implemented to ensure no liquid carry-overs in the gaseous system. 

The fuel. skid. This could contain a gas compressor if the fuel gas pressure is low and a knockout drum for any liquid contamination that the gas may have. The requirement of fuel gas pressure is that it should be operated at a minimum of 50-70 psi (3.5-4.83 Bar) above the compressor discharge pressure. The compressor and its motor drive fall under the drive level hierarchy. In the case of liquid fuels, the skid may also contain a fuel treatment plant, which would have centrifuges, electrostatic precipitators, fuel additive pumps, and other equipment. These could be directly controlled by the D-CS system, which would then report its readiness to the gas turbine controller. 

A vessel handling large amounts of liquid or a large liquid surge volume will usually be horizontal. Also, where water must be separated from hydrocarbon liquid, the vessel will be horizontal. A vessel with small surge volume such as a compressor knockout drum will usually be vertical. 

A PR valve is not required for protection against fire on any vessel which normally contains little or no liquid, since failure of the shell from overheating would occur even if a PR valve where provided. Examples are fuel gas knockout drums and compressor suction knockout drums. (Note Some local codes require pressure relief valve protection for "dry drum" situations.)... 

Consideration of All Releases into the System - All releases tied into the closed system must be considered. In addition to PR valve discharges, these may include fuel gas compressor and absorber knockout drum drainage, vapors vented from water disengaging drums, feed diversion streams, closed drainage from equipment, vapor blowdowns and liquid pulldowns. 

Liquid hydrocarbons accumulated in non-condensible blowdown drums, originating from safety valves, closed drain headers, knockout drum drainage, etc. Facilities are normally provided at the drum for weathering volatile liquids and cooling hot liquids before disposal. 

Figure 12-38. Tangential inlet knockout drum with separate liquid catchtank.

J. Knockout drums (vessels) usually operate with only a small amount of liquid. Therefore, the wetted surface would be in proportion, but to maximum design liquid level. 

Knockout drums ahead of compressors should hold no less than 10 times the liquid volume passing through per minute. 

Knockout drums are sometimes called catch tanks or blowdown drums. As illustrated in Figure 8-12, this horizontal knockout drum system serves as a vapor-liquid separator as well as... 

Determine the maximum vapor velocity in a horizontal knockout drum to dropout liquid particles with particle diameters of 300 pm, where... 

Any stream containing liquid or solid or both should not be vented directly to the environment, which means essentially any relief vent stream since it is unusual that the stream would be fully as a gas. The liquid and/or solid should be collected and contained for later treatment. Typical collection and separation methods include knockout drums and cyclones. If the remaining gas stream contains toxic or flammable substances, it should also be treated or flared. 

No generation of appreciable liquid waste as would be the case for a quench pool or an emergency scrubber, but knockout drum required upstream of the flare to remove liquids and any condensate. 

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. 

Normal vertical knockout drums are designed for a K value of about 0.20 to 0.25. If we are installing a KO drum ahead of a reciprocating compressor—and they really hate liquids in their feed—a K value of 0.14 might be selected. If we really do not care very much about entrainment, a K value of 0.4 might be selected. An example of this would be venting waste gas to the flare from a sour-water stripper reflux drum. 

A light, frothy foam is naturally more susceptible to entrainment than is a clear, settled liquid. Knockout drums handling foam, therefore, should be designed for a lower K value. Perhaps the K value that can be tolerated might be reduced by 25 to 35 percent, in the anticipation of such foam. 

Compressors are also served by high-liquid-level trips in their upstream knockout drums. These high-liquid-level trips work in the same way as the low-level boiler trips discussed above, except that the mercuroid switch is activated by a rising, rather than a falling, liquid level so as to protect the compressor from a slug of liquid. 

Bottoms of T-3 proceeds to the top of stripper T-4. Vapor overhead from T-4 is recycled to the middle of T-3. The bottoms product (containing 99.5% oxygen) is sent partly to liquid storage and the remainder to precooler E-l where it is vaporized. Then it is compressed to 150 psig in a two-stage compressor JJ-2 and sent to the battery limits. Compressor JJ-2 has inter- and aftercoolers and knockout drums for condensate. 

Compressor feed liquid knockout drum which is made large enough to hold 10-20 min of liquid flow, with a minimum volume of 10 min worth of gas flow rate. 

Figure 18.10. Dimensions of standard liquid knockout drums with tangential inlets.

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