Application Notes

As with the reciprocating compressor, care must be exercised when liquids are present in the gas stream. Unlike the reciprocator, the centrifugal is somewhat more forgiving if the liquid is in the form of mist. Small droplets can pass through the machine without problem if the duration is short. The problems with liquid have both short- and long-term characteristics. Short term, the biggest problem with liquid is the ingesting of slugs of water. The compressor is in danger of severe mechanical damage if suddenly deluged with a great quantity of liquid. For this reason, compressors taking suction from vessels containing either liquid and vapor or vapors near saturation should have suction drums to trap any potential liquids. The suction drum is also a good idea where the possibility exists that condensation could take place in the suction lines, forming a slug on its way to the compressor. The long-term problem is with mist or small droplets. With time, any liquid will start an erosion of the moving parts, particularly the impeller vanes. As the tips of the vanes erode, the effective diameter of the impeller is reduced. The foregoing equations showed that the head-producing capability of the impeller is a function of the tip speed squared.

Interestingly enough, centrifugals can be washed "on stream" to counteract the effects of fouling. In some cases, where fouling is continuous and severe, a liquid wash may be used continuously. Care must be used and a certain amount of trial-and-error steps taken to ensure the proper quantity: enough to do the job and not enough to cause significant erosion. In the same manner, when a compatible liquid is available, liquid can be injected into the machine to provide auxiliary cooling. In all liquid injection applications, it is important not to inject the liquid so that it impinges on any of the surfaces. It is better to use tangential sprays to the degree practical to have the liquid flash in the gas stream.

One question that arises quite often is the orientation of inlet piping and its influence on compressor performance. The flow into the impeller has been assumed axial or radial, depending on the impeller geometry, which means there is no pre- or antirotation and it is free from random flow distortions. While centrifugals are somewhat more forgiving than other machines like axials, there are limits. If the flow has rotation or distortion as it enters the impeller, the compressor performance will be influenced in a negative manner. Correct piping practices at the com pressor inlet will help ensure the proper performance of the compressor. Figure 5-28 includes a set of curves that may be used as guidelines to establish a minimum length of straight pipe to use ahead of the inlet. The base case is shown in Figure 5-29 and consists of an elbow turned in the plane of the rotor. While the sketches are shown as multistage compressors, they may be used for axial entry single-stage compressors by obtaining the multiplier for the base case and taking the final result and multiplying by 1.25. The higher multiplier accounts for the more sensitive nature of the axial inlet. These sketches and pipe lengths are conservative, but should a vendor recommend a longer length, the vendor's recommendation should receive the first consideration. When there are problems achieving some of the minimum lengths, varied elbows and straighteners can be used. Figures 5-30 and 5-31 offer suggestions for those not experienced in these areas. Again, these are methods that have been used, but are not the only, or necessarily the best, solutions for any and all applications.

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