Mechanical Design

Introduction

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.

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Figure 5-28. Chart for minimum straight inlet piping. Use this chart and the given factors in conjunction with Figure 5-29. {Courtesy of Elliott Company
Figure 5-31. Straightening vanes. (Courtesy of Elliott Company
Centrifugal Compressor Design Labeled

Figure 5-32. Centrifugal compressor nomenclature. (Courtesy ofA-C Compressor Corporation)

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The internal passages are formed by a set of diaphragms. Figure 5-32 depicts a typical multistage barrel compressor. Refer to this figure to locale the relative position of the various parts that are described in the following section.

Figure 5-32. Centrifugal compressor nomenclature. (Courtesy ofA-C Compressor Corporation)

Casings

All centrifugal compressor casings were initially of cast construction, and this method is still used on many casings today, particularly in the smaller sizes. In the past 15 years, some manufacturers have fabricated steel casings, generally converting their line of steel casings beginning with the larger frame sizes. The reason for this was economics; while the fabricated casings cost more to make on a per-unit weight basis, the net cost was less. Two factors were responsible. Quality of large steel castings was hard to control, with much time spent repairing the casing after inspection. Secondly, fabrication techniques and costs have improved significantly. There also came a side benefit of flexibility, once the manu facturer discovered he was no longer tied to a set of patterns or was bound by the time consumption and cost of pattern changes.

Casing materials are, in most cases, cast iron, nodular iron, or cast steel. Fabricated casings are generally made of carbon or alloy steel. Cas ings are, on occasion, made of austenitic stainless steel or one of the high nickel alloys. For low temperature inlet conditions, a low nickel alloy may be used. API Standard 617 [12] includes material guidelines in its appendix. The standard also mandates steel for all flammable and toxic gases, for air or nonflammable gas at pressures in excess of 400 psig, arid for air or nonflammable gas with operating temperatures anywhere in the

The casing construction and materials covered to this point have generally applied to all kinds of compressors, including both horizontally split and vertically split. The vertically split, multistage barrel compressor is somewhat different. It is generally constructed of steel or steel alloy. It may be cast, fabricated, or, for very high pressure service, it may be forged. It should always be used when the gas contains hydrogen at or above a partial pressure of 200 psig. It may also be required in those services where the overall pressure is too high for the horizontally split compressor. This occurs when the horizontally split joint deforms too much at the operating pressure to maintain a gas-tight seal.

The stationary members located inside a multistage casing are referred to as diaphragms. The function of the diaphragm is to act as a diffuser for the impeller and a channel to redirect the gas into the following stage. The diaphragm also acts as the carrier for the impeller eye seal and the interstage shaft seal. Diaphragms are either cast or fabricated. Most cast diaphragms are made of iron. Fabricated diaphragms are steel or composite steel and cast iron, with straightener or guide vanes of cast iron. Diaphragms are normally not highly stressed, with some exceptions. On compressors with out-in streams, if the differential pressure is relatively high from the outlet to the return nozzle, then the differential is taken across the diaphragm at the two nozzles. This diaphragm should be made of steel. The diaphragms are split, located with matching grooves in the upper and lower half casing and pinned to the upper half for maintenance ease. The diaphragms are hand-fitted to center them to the rotating element. It is important for the horizontal joint to match well, to keep the joint leakage to a minimum. On barrel compressors, the diaphragm assembly makes up an inner barrel (see Figure 5-33). The assembly and rotor are removed from the barrel casing as a unit using a special fixture. The diaphragm assembly is split to permit the removal of the rotor, and the diaphragms are generally constructed in the same way as those of the

Figure 5-33. Inner barrel assembly. (Courtesy ofA-C Compressor Corporation

Casing Connections

Casing inlet and outlet nozzles are normally flanged. General preference, in process service, is for all casing connections to be flanged or machined and studded. On steel-cased machines, this normally is not a problem. On the smaller, refrigeration compressors that are highly standardized, constructed of cast iron, and originally designed for other than process service, connections will generally have flanged inlet and outlet nozzles. However, most of the auxiliary connections on these machines will be screwed. It is desirable to use standard flanges throughout the connections on the casing. However, for space reasons, on rare occasions, a nonstandard flange arrangement may become necessary. It is quite important to have the equipment vendor furnish all nonstandard mating flanges and associated hardware.

Forces and moments which the compressor can accept without causing misalignment to the machine are to be specified by the vendor. Many factors go into this determination, and as one may guess, the limits arc determined quite arbitrarily in most cases. With all the many configurations a compressor can take, a single set of rules cannot fit all. Despite this. NEMA SM-23[13] for mechanical drive steam turbines is used as a basis. API 617 has adapted the NEMA nozzle criteria to centrifugal compressors. This works on larger steel-cased multistage compressors, but is not good for the overhung style. Moreover, the user or piping designers want a higher number to simplify piping design, while the manufacturer wants a small number to assure good alignment and fewer customer complaints. From a user's point of view, where long-term reliability is a must, the vote must go to the manufacturer. Experience shows that the lower the piping loads on the nozzles, the easier coupling alignment can be maintained. This seems reasonable since most compressors are equipped with plates called wobble feet to provide flexibility for thermal growth. The feet will flex from pipe loads as well as from the temperature. The piping loads tend not to align themselves as well with the shaft as the temperature gradients. Even when guides and keys are used, as is customary on the larger machines, they may bind despite the fact that they are stout enough to carry the load.

Impellers

Impeller construction was covered in the performance section and need not be repeated here. The impeller is the most highly stressed compressor component, and generally becomes the limiting item when it comes to establishing the rotating element performance limit. Impellers are made of low alloy steel for most compressors in process service, either chrome-moly or chrome-moly-nickel. Because of the high strength-to-weight ratio, many of the high head, integrally geared units use aluminum. Austenitic stainless, monel, and titanium are some of the other materials used for impellers in certain special applications, generally with corrosive gases involved. Stress levels must be adjusted for the materials involved. Some of the precipitation hardening steels in the 12 chrome alloy have been used and found to provide a good alternate material with moderately good corrosion resistance and very good physical properties.

Impeller construction for the cover-disk style impeller historically has been by built-up construction and welding. The traditional method uses die formed blades (see Figure 5-34). More recently, with the increased use of 5-axis milling, blades have been milled integrally with the hub disk. This alternate construction method is somewhat more costly because of the machining but produces a more accurate and repeatable gas path, which offsets the added expense (see Figure 5-35). Cover disks are welded to the blades to complete the milled impeller. Physical prop-

Centrifugal Compressor Figure
Figure 5-34. A fabricated centrifugal compressor impeller. (Courtesy ofA-C Compressor Corporation)
Figure 5-35, A centrifugal compressor impeller during manufacture. The blading was milled with a five-axis milling machine. The blading is integral with the back plate. (Courtesy of Dresser-Rand\

erties are derived by heat treating and stress relieving. Some small sizes are cast. In the semi-open construction, casting is quite common, though fabricated impellers are used. Fully open impellers, which are not as common, can be either fabricated or cast. Impeller shaft attachment for multistage applications is by shrinking the hub to the shaft either with or without a key, depending on the vendor philosophy. There are numerous other methods used, each peculiar to the individual vendor.

Although not universal, on most multistage compressors, the impellers are axially located by shaft sleeves. The sleeves form a part of the interstage seal and are shrunk onto the shaft with a shrink level less than the impeller.

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