Capacity Control

Probably the most widely used capacity control for the centrifugal compressor is speed control. The capacity curve when used with speed control covers a wide range. While electric variable speed motors offer a continuation to the speed control practice, there are some other alternatives available. Suction throttling has been widely used and offers a reasonable control range for a relatively low cost.

A more efficient control available on some centrifugals is the movable inlet guide vane. The movable inlet guide vane adds pre-whirl to the gas stream entering the impeller, which, in turn, reduces the axial componenl of the absolute velocity, which controls the capacity to the impeller as discussed earlier. By modifying the inlet whirl component, the capacity is reduced with little loss in efficiency. It is obvious that this method would be most effective on the single-stage compressor (see Figure 5-55). It can be used on the multistage compressor, however, it can only be installed in front of the first impeller. If the compressor has more than a few stages or is more complex in arrangement the idea is not practical.

The guide vanes in a single-stage are generally pie shaped and center pivoted. They are located directly in the flow path immediately in front of the impeller. The shanks of the vanes extend through the inlet housing and connect to an external linkage. The linkage is connected to a power operator to supply the motive power to position the vanes. Control for the vanes can be by a remote manual station or connected to an automatic control as the final element.

In the multistage compressor, the vanes are rectangular and located in a radial position ahead of the first impeller, with a linkage connecting the vanes to a power positioner. From that point, the control is affected in the same manner as the single-stage.

The largest problem with the use of movable inlet guide vanes is the danger of the vanes or the mechanism sticking. Obviously, the vanes are not suitable for dirty or fouling gas service. The vane bearings and linkage should be buffered with clean, dry gas and exercised regularly. While this does take extra effort, the vanes will work well and give efficient control.

Figure 5-55. Single-stage centrifugal with movable inlet guide vanes. (Courtesy of A-C Compressor Corporation

Maintenance

At the risk of misleading the reader, this section will just touch a few points concerning maintenance. One frequently asked question is, what is critical and what needs special consideration when performing maintenance on the centrifugal compressor? While there are many areas that must be carefully reviewed, the clearances should be restored as close as practical to the new machine values. Probably the single most important consideration, therefore, concerns concentricity. Interstage seal clearances should be concentric to the rotating element. It is better to allow larger than desired clearances in the machine than to leave seals in an eccentric condition. Leakages approach 2M times the concentric values for eccentric seals, with the same average clearance.

While the axial position of multistage impellers to their diffusers is not critical, they should line up reasonably well. Impellers are not extremely sensitive to leading-edge dings and minor damage, but anything, such as erosion on the exit tips, that tends to decrease the effective diameter of the impeller is more serious. Front shroud clearance on open impellers should be maintained close to the design values to minimize capacity loss.

Bearings normally have a specified clearance range. Allowing clearances to exceed the specified maximum clearance may encourage the onset of rotor dynamics problems. Dams in dam type bearings are very critical. The edge of the relief must be square and sharp, not rounded. The clearance of this bearing is also quite sensitive and must remain inside the specified limits for stability.

Care must be taken in the assembly of the buffered shaft end seals, particularly in the area of the secondary o-ring seals. A cut or damaged ring can allow more oil to be bypassed than from a damaged main seal.

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