Page 48 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 39

exit flow from the compressor and direct it into the diffuser which is often
10%–20% narrower than the impeller flowpath at the exit. This reduction in dif-
fuser height is called pinch. Pinch is used to further accelerate the flow as it
enters the diffuser, which improves the stability of the flow in the diffuser at
low flows, and increases compressor range. Once the flow is inside of the dif-
fuser, it may be diffused in a vaneless space, or there could be vanes. In most oil
and gas applications, a vaneless diffuser is employed since they tend to have
wider range than vaned diffusers, however, vaned diffusers tend to offer better
performance at an optimized flow point. After the flow is sufficiently diffused,
it returns through the u bend and starts traveling back along the return channel
where the flow enters the next compressor stage.

Inlet
The inlet of a compressor takes the flow from a pipe flange, and attempts to
distribute it circumferentially equally in the axial direction. This can be accom-
plished with inlet vanes.

Collectors
Collectors take the flow exiting the last stage of a section and route the flow out
through a flange on the casing to the discharge pipe. These generally take two
forms, constant area collectors and volutes.

Volute
Volutes are more common than collectors, as they generally have less losses,
improving the efficiency of the compressor. A representative volute is shown
in Fig. 3.9. The tongue of the volute is generally the “start” of the volute, which

FIG. 3.9 Volute.