Drums compressor surge

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 complete evaluation of the surge capacity and pulsation frequencies of the system (i.e., suction header and suction surge drum. Figure 13-1 A), compressor, discharge surge drum, and discharge header is necessary before reaching an... 

Figure 13-1 A. Parallel acting compressor cylinders with common suction and discharge surge bottles or drums.

Generally, when placed close to the compressor cylinder, surge drums will minimize acoustic wave amplitudes, but they do not eliminate high-frequency acoustic response (Figures 13-11 and 13-12). 

Figure 13-11. Single compressor cylinder with surge drums.

Figure 13-12. Parallel compressor cylinders with surge drums.

Example 13-1. Surge Drums and Piping for Double-Acting, Parallel Cylinder, Compressor Installation... 

Size the discharge surge drum and associated piping for a single-acting, single-cylinder, motor-driven compressor with the following performance characteristics ... 

This can be used for the pipe between the compressor cylinder and the surge drum. The length should be such that the fundamental and second harmonic are avoided, and these should differ by several cycles, preferably more than five, from the base and harmonics produced by the compressor. 

As the ACFM increases, we run out to the right on the compressor curve, shown in Fig. 28.3. As we move away from the surge point, the polytropic feet of head decreases. As the polytropic feet of head is reduced, the compressor AP comes partially back down to its initial value, until a new equilibrium is established. But because the initial disturbance of the equilibrium—the increased molecular weight— moved us away from surge, the new equilibrium will be established farther away from surge than the initial equilibrium. Not only will the new equilibrium be established farther away from surge, but the pressure in the wet gas drum will also wind up lower than the initial pressure in the drum. 

For a period of one week, unknown to the FCCU operating crew, the molecular weight of the gas from the reflux drum had dropped from 34 to 29. The compressor was slowly pushed back on its curve by the automatic closing of the pressure control valve, which was trying to maintain the 180 psig differential pressure between the reflux drum and the gas plant absorber. When this pressure control valve was shut, the flow of gas to the suction of the compressor was only 10,000 ACFM. This was also the absolute minimum flow that the compressor needed to prevent surge. 

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