Compressor casing

The only a-ahoy of commercial importance is Ti—5A1—2.5Sn. This ahoy is weldable, has good elevated temperature stabhity and good oxidation resistance to about 600°C, and is used for forgings and sheet-metal parts, such as aircraft engine compressor cases because of its weldabhity. The ELI version of this ahoy is used in the cryogenic area of rocket engines. However, the ahoy is difficult to produce and has not been designed into more recent aerospace systems. 

Lowering the inlet velocities and providing proper inlet silencers can reduce air noise. Considerable work by NASA on compressor casings has greatly reduced noise. 

Straight-lobe compressor casings, also called housings or cylinders by different manufacturers, are furnished in cast iron by all vendors. There is an optional aluminum construction available for special applications. Inlet and outlet are suitable for a 125 pound standard ANSI flanged connection. 

The balance chamber leakoff line, while recommended to be held within the confines of the compressor casing may well turn out to require some user piping. There are some situations where the desire for keeping open space around the compressor for maintenance may require compromise on the part of the user. The balance of the connections on the axial are for lube oil and other auxiliary equipment not different from that found on other compressors. 

The low speed gear shaft and the housing must be designed to permit installation of a stub shaft for a torsiograph unit if an operational problem occurs. API 613 gives the details of the shaft end requirements for attaching a torsiograph. This should be done on all synchronous motor compressors and on multiple driver or multiple compressor case trains. 

Compressor casing, cylinder and knockout bottle drains Closed drain header -- -... 

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.)...

No leakage of process gas to ambient. Leakage toward gas stream is trapped in baffle to prevent contamination of gas in compressor casing. This liquid may be recycled if not contaminated. Out-leakage of liquid is recycled. 

From Table 12-9B select the nominal speed for the size compressor casing (frame) established in Step 2. 

Tables 12-9A and 12-9B are a guide to a specific compressor case s capabilities. This is not a standard for each manufacturer. On the contrary, each is considerably different. Figure 12-73 is also useful as a guide to inlet suction condition capacities for various case sizes. These case sizes have no relation to the cases in Table 12-9.

In actual design, the manufacturer uses wheel capacity data to properly select the sequence of wheels required to develop the head in each compressor case. Each wheel has its own efficiency at the rated speed (usually 70-75%). 

Actually, because the alternate scheme is based on 75% efficiency, if the values are corrected for the differences in efficiency of the two individual compressor cases used, the results will be close enough for engineering application. As they stand, the bhp values cannot be resolved to a more accurate basis without specific data on case selection, efficiency, and losses. 

From Figure 12-75 the approximate wheel diameter for (first compressor) case size no. 3 is 24 in. dia. 

The horsepower is established by the manufacturer by testing the various types and models. In general, the horsepower requirements will be a combination of the power to pump the liquid inside the compressor casing plus the power to compress the gas or vapor. If a recirculating seal liquid system is used, the recirculating pump horsepower is not reported as a part of the compressor requirements. 

There has been a reluctance to provide water spray on compressors or turbines that typically tend to run "hot," with respect to the risk that a sudden deluge of cold water will result in severe thermal shock and cause cracking of the compressor case. However, studies indicate that this hypothesis is not valid (EPRI, 1985). These studies support the use of water spray protection where the fire hazard so warrants. Water mist systems are now being approved for gas turbines and compressors. 

The mechanical pressure rating of the compressor case or the refrigerant condenser shell. 

The gas enters the compressor s rotor through the large wheel shown in Fig. 28.2. The purpose of this wheel is to increase the velocity or kinetic energy of the gas. After the high-velocity gas escapes from the vanes in the wheel, the gas enters the stationary elements fixed to the inner wall of the compressor case. This is called the stator. Inside the stator the velocity or kinetic energy of the gas is converted to polytropic feet of head, or potential energy. 

Regardless of the particular compressor case for the hp you finally derive, Fig. 8.9 is applicable to all cases for both reciprocating and centrifugal-type compressors. See Table 8.28 for compressor module associated cost factors. 

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