Centrifugal compressors casing

Centrifugal compressor cases are classified as horizontally split or vertically split (Figures 12-40A-C). The 5th Edition, April, 1988, API Standard 617 refers to the horizontally split style as Axial Split and vertically split units as Radial Split The first terminology is more standard than that of the API-617. Figure 12-40D illustrates one flow arrangement for back-to-back internal flow, with the following advantages ... 

Figure 12-40A. Centrifugal compressor case types. These usually apply to multistage units rather than single-stage units. See Figures 12-39A and 12-39B. [Note API standards 617, April 1988, Par. 2.2.8.] (Used by permission Bui. 423, 1992. Dresser-Rand Company.)...

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. 

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. 

Small standby power turbines less than 2-MW. The smaller size of these turbines in many cases have centrifugal compressors driven by radial inflow turbines, the larger units in this range are usually axial... 

Industrial Centrifugal Compressor Classification Based on Casing Design... 

The centrifugal compressor is composed of a casing containing a rotating element, rotor, which is supported by a set of bearings. For most multistage compressors, shaft end seals are located in-board of the bearings. 

For centrifugal compressors, nozzle orientation must be specified. If the compressor is on an elevated foundation, the down-nozzle orientation makes maintenance much easier, because major piping does not have to be removed to lift the casing upper half. Figure 11-1 shows an unusual case where all the nozzles are in the upper casing half. 

Figure 11-1. This steam turbine-driven centrifugal compressor, shown just prior to shipping, has all nozzles in the upper half of the casing. Note the non-vertical orientation of the inlet nozzle. Courtesy of A-C Compressor Corporatior ...

Volume 1 explains that pumps ean be classified as either positive-displacement or kinetie. The same is true for compressors. In a positive displacement compressor the gas is transported from low pressure to high pressure in a device that reduces its volume and thus inereases its pressure. The most common type of positive displacement eompressors are reeiprocating and rotary (serew or vane) just as was the ease for pumps. Kinetic compressors impart a veloeity head to the gas, which is then converted to a pressure head in accordance with Bernoulli s Law as the gas is slowed down to the velocity in the discharge line. Just as was the case with pumps, centrifugal compressors are the only form of kinetic compressor commonly used. 

As pointed out in Volume 1, Chapter 3, a compressor may have any number of stages. Each stage normally contains a suction scrubber to separate any liquids that carry over or condense in the gas line prior to the compressor cylinder (or case for centrifugal compressors). When gas... 

Selection of construction materials for rotors and casings is more limited than for centrifugal compressors. 

In a centrifugal compressor, air or gas at atmospheric pressure enters the eye of the impeller. As the impeller rotates, the gas is accelerated by the rotating element within the confined space that is created by the volute of the compressor s casing. The gas is compressed as more gas is forced into the volute by the impeller blades. The pressure of the gas increases as it is pushed through the reduced free space within the volute. 

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