Disengaging drums

The reactants dissolve and immediately begin to react to form further dichloroethane. The reaction is essentially complete at a point only two-thirds up the rising leg. As the liquid continues to rise, boiling begins, and finally, the vapor-liquid mixture enters the disengagement drum. A very slight excess of ethylene ensures essentially 100 percent conversion of chlorine. 

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. 

With the flare tip and flare seal pressure drop and flare elevation fixed, the flare stack, headers and laterials are sized for the largest release, while not exceeding the maximum allowable operating pressure on the associated blowdown drums and water disengaging drums. These maximum allowable operating pressures are in turn determined by ... 

Routing of Flare Header through Process Areas - Flare headers in process areas should be routed to avoid locations of particularly high fire risk, such as over pumps, near furnaces, etc. The headers and subheaders should also be laid out and provided with isolating CSO valves and spectacle blinds, unless prohibited by local codes, such that it is not necessary for flare lines to remain in service in units which are shut down separately. Blowdown and water disengaging drums should be spaced from process areas. 

Vessels Containing Light Ends - Discharges should be sent to either a water disengaging drum, a sour water disengaging drum or a spent caustic disengaging drum, and accompanied by appropriate treatment methods. 

Disengaging drums are provided to remove hydrocarbon liquid and vapor contaminants from aqueous plant effluent streams, to permit them to be safely discharged to the sewer. Criteria for the routing of effluent streams to disengaging drums were described earlier. 

A typical water disengaging drum system is illustrated in Figure 5. The... 

A design pressure of 545 kPa, gage is normally specified for water disengaging drums. The water outlet system is designed to seal the drum and prevent entrainment of hydrocarbon or air into the sewer. Figure 5 indicates the normal layout incorporating a single loop seal. 

Aqueous plant effluent and drawoff streams such as steam condensate, sour water, or spent caustic soda solution may require disposal to a disengaging drum. 

The design of these drums generally follows the same basis as that for water disengaging drums, except that a pump (with spare) is required to transfer the aqueous liquid imder level control to the appropriate receiving facilities. 

In some cases, it is possible to combine the functions of blowdown and disengaging drums in one vessel. However, PR devices discharging liquid hydrocarbons lighter than pentane should not be connected into the drum if there is a possibility that such liquids could accumulate and be released to the sewer through the seal leg. Also, the drum vent should be sized to prevent pressure buildup due to vaporization. In these applications, the design criteria for both services must be met and special attention should be paid to potential hazards and problems which may be introduced, such as ... 

This chapter discusses some of the criteria for selecting, designing and spacing elevated, burning-pit, and multijet flares. The design of safety valve and flare headers was covered in an earlier chapter, as well as discussions concerning associated blowdown drums, water disengaging drums, etc. 

A flare seal drum may also be used as a sour water disengaging drum, if economically advantageous. In such cases, special care should be given to ensure that the drum is adequately sized to simultaneously meet all design features required for both functions. Also a separate source of makeup water must still be provided to ensure continuity of the seal. 

If HjS is continuously present in the flare gas or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to a sour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normal loop seal arrangement. Two pumps are provided one motor driven for normal use, and the other having a steam turbine drive with low pressure cut-in. The seal drum level is controlled by LIC with high and low alarm lights plus an independent high level alarm. 

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