Knock out drum

Install a knock-out drum immediately upstream of the flare seal drum, to remove material condensed in the flare header. 

This specification covers the minimum requirements for the process, mechanical and structural design, materials, fabrication, shop inspection testing and supply for a flare system. The knock-out drum will be by N P Refinery. 

Relieving vapors from various pressure-relief and depressuring valves in the system must be collected in individual flare headers that should be appropriately located near each process area. Subheaders must be interconnected to a main flare header which feeds to a knock- out drum and disposal system. Condensates that are carried over by vapors are separated in the knock-out drum. The vapors that exit the vessel go to the flare stack where they are burned. 

A second trial is required for two main flare headers, one collecting the low-pressure flares (usually 5 to 10 psig) and the other collecting relatively high-pressure flares (usually 15 to 20 psig). The two headers are connected to their individual knock-out drums. The vapor lines from the knock-out drums are combined into a single header connected to the flare stack. 

The maximum simultaneous load in each header must be calculated separately and the pressure drop must also be computed for the entire length of the pipe including the combined length from the knock-out drum to the stack. 

The main flare header connecting the subheaders leading to the knock-out drum. 

The final header connecting the vapor line (or lines) from the knock-out drum leading to the flare stack. 

In the absence of actual line distances, a conservative estimate of the distance between the knock-out drum and the flare may be taken as 500 ft. 

Once the diameter is known, the Reynolds number. Re can be computed and the friction factor f obtained from Figure 21. Assuming a straight length of pipe for L = 5(X) ft, N (line resistance factor) can be calculated. Next Gj, is calculated based on the downstream pressure and G/G j evaluated. From Figure 20 the ratio Pj/Pfl can be obtained. Since Pj is known, Pq can then be calculated. The pressure at the inlet of the knock-out drum is given by Pq -I- 0.5 psi. Table 6 provides typical values of resistance coefficients for various pipe fittings. 

Example The maximum flare load of a system is 1,000,000 Ibs/hr of vapor. The pressure at the base of the flare stack is 2 psig, the average molecular weight of the vapor is 50, at a temperature of 200°F at the combined header to the flare stack. The distance from the drum to the stack is 500 ft. The line consists of two 90° welding elbows and an orifice for a flow controller. The total pressure drop at the knock-out drum is 0.5 psi. Determine the pressure at the inlet of the knock-out drum. 

Example Calculate the dry flare header size, which is connected to the above knock-out drum. Previously noted conditions remain the same. Additional data are ... 

It is wortli noting tluit design calculations for tlie sizing of relief systems (relief valves, headers, scmbbers and knock-out drums, etc.) are conservative in order to protect tlie integrity of vessels and relief systems. Tlie calculations used for risk assessments are tliose which most accurately describe the discliarge rate from tlie luizardous incident being modeled. 

A superheat exchanger is not needed for a centrifugal compressor system as far as gas condition is concerned. It may be a good unit as fer as other aspects of the process are involved. The centrifugal compressor requires only a suction knock-out drum to remove entrained liquid and foreign particles. 

The knock-out drums or separator tanks/pots can be designed using the techniques offered in the chapter on Mechanical Separation, and will not be repeated here. API-RP 521 [13] specifies 20-30 minutes holdup liquid capacity from relief devices plus a vapor space for dropout and a drain volume. 

The primary features of a flare are safety and reliability, while the primary objective of the flare is to prevent the release of any unburned gases. In reviewing existing facilities worldwide, from Russia to South America, onshore and offshore, most installations have admitted either officially or unofficially that on occasion, a liquid release has occurred from the tip of the flare stack. This has occurred even with the installation of a flare header liquid knock out drum. In most cases this has caused no apparent problems although in a few... 

S S Grossel, "Design and Sizing of Knock-out Drums/ Catchtanks for Reactor Emergency Relief Systems", Plant/Operations Prog, 5 (3), 129-135,1986... 

An 1/8-inch thick compressed asbestos gasket failed on a 20-inch (50.8 cm) Class 600 ASNI flange after a regular semi-annual turnaround. About 66 pounds (30 Kg) of hydrogen leaked from the manway flange on a compressor knock-out drum, ignited, and... 

Entrainment. If entrainment is excessive, column diameter or tray spacing are usually increased. It has been recommended (2,67) that entrainment from the tray should not exceed about 0.10 lb liquid entrained per pound of liquid flow. At higher values, significant efficiency reduction occurs (34). Depending on the service, a lower or higher value can be set (4). For instance, if the column overhead stream is compressed and no knock-out drum is present, the entrainment that can be tolerated is smaller. Also, for trays operating at a high liquid-to-vapor ratio, 0.1 lb of liquid entrained per pound of liquid is an excessive quantify of entrained liquid, and a lower limit is set. 

Knock-out drums, used when the hquid content of the incoming stream is low, is a special case of a gas-hquid separator. The drum is placed before a compressor inlet to prevent hquid drops fi om entering and damaging the compressor. In this case, allowing a sufficient residence time for the hquid is not a consideration. 

Table 6.7 Summary of Equations for Sizing Knock-Out Drums...

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