Centrifugal compressors pressure

Motor-driven, multistage reciprocating compressors have reportedly been the most popular choice for aeroderivatives. Motor-driven, oil-fiooded screw compressors are also used in some cases. High horsepower, multistage centrifugal compressors, similar to those used at many pipeline compressor stations, may be required for the newer heavy-duty units if the distribution pipeline pressure is insufficient (see Pipelines). Gas turbines have more stringent fuel-gas specifications in terms of cleanliness than do gas-fired boilers. Thus oil- and water-knockout systems, coalescing filters, and fine-mesh filters are used. 

On a typical pipeline, compressor stations are located at 81—161-km intervals and may contain up to 15 compressors. These stations may use either gas-turbine, reciprocating-engine, and/or motor-driven centrifugal compressors capable of boosting pipeline pressure and keeping gas moving at an average speed of about 24 km/h. Gas-turbine-driven units are the most popular. 

To be able to decide which type of compressor would best fit the job, we should first divide the compressors into three main categories positive displacement, centrifugaf and axial flow In general terms, positive displacement compressors are used for high pressure and low flow characteristics centrifugal compressors are used for medium to high pressure dehvery and medium flow and axial flow compressors are low pressure and high flow... 

Compressor Configuration To propeny design a centrifugal compressor, one must know the operating conditions—the type of gas, its pressure, temperature, and molecular weight. One must also know the corrosive properties of the gas so that proper metallurgical selection can be made. Gas fluctuations due to process instabilities must be pinpointed so that the compressor can operate without surging. 

High-Pressure Compressors There is a definite trend in the chemical industry toward the use of high-pressure compressors with discharge pressures of from 34.5 to 172 MPa (5000 to 25,000 IbFin ) and wim capacities from 8.5 X 10 to 42.5 X 10 mVh (5000 to 25,000 ftVmin). These require special design, and a complete knowledge of the characteristics of the gas is necessary. In most cases, these types of applications use the barrel-type centrifugal compressor. 

The performance of a centrifugal compressor (discharge to suction-pressure ratio vs. the flow rate) for different speeds is shown in Fig. 11-87. Lines of constant efficiencies show the maximum efficiency. Unstable operation sequence, called surging, occurs when compressors fails to operate in the range left of the surge envelope. It is characterized by noise and wide fluctuations of load on the compressor and the motor. The period of the cycle is usually 2 to 5 s, depending upon the size of the installation. 

The expanders also remove energy from the gas, using that energy to drive a centrifugal compressor for pipeline recompression. As gas expands through the expander s inlet nozzle, pressure drops from 90 bar (1,300 psig) to 55 bar (800 psig). Temperature drops as well, below the dewpoint, and the liquids formed can be separated from the main gas stream. 

Compressor performance can be represented in various ways. The commonly accepted practice is to plot the speed lines as a function of the pressure delivered and the flow. Figure 3-9 is a performance map for a centrifugal compressor. The constant speed lines shown in Figure 3-9 are constant aero-dynamic speed lines, not constant mechanical speed lines. 

Centrifugal compressors in general are used for higher-pressure ratios and lower-flow rates compared to lower-stage pressure ratios and higher-flow rates in axial compressors. Figure 6-3 is a map for centrifugal compressors that shows the effect of specific speed (A ) and specific diameter (Z) ) on their... 

Figure 6-12. Estimated effeot of inlet prewhirl. (Rodgers, C. and Shapiro, L., Design Considerations for High-Pressure-Ratio Centrifugal Compressors, ASME Paper No. 73-GT-31.)...

Klassen, H.A., Effect of Inducer Inlet and Diffuser Throat Areas on Performance of a Low-Pressure Ratio Sweptback Centrifugal Compressor, NASA TM X-3148, Lewis Research Center, January 1975. 

Calibrated pressure drops from the inlet flange to the eye of the firststage impeller in centrifugal compressors, when such data is available from the manufacturer. 

Centrifugal compressors range in volumetric size from approximately 1,(X)0 to 150,000 cfm. In single-wheel configuration, pressures vary considerably. A common low pressure compressor may only be capable of 10 to 12 psi discharge pressure. In higher-head models, pressure ratios of 3 are available, which on air is a 30-psi discharge pressure when the inlet is at atmospheric conditions. 

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