NOT ALL MULTI-COMPARTMENT COMPRESSORS ARE BACK-TO-BACK MACHINES

There is currently an ongoing discussion in the compression industry about the advantages and technical differences of tandem compressors with multiple compressor cases versus back-to-back or other type of multi-compartment single-case compressors.

Process requirements for centrifugal compressors often result in compression in several sections. There are various reasons for that:

• High compression ratios

• Process gas streams at different pressure levels

• Process gas off-takes

• Requirement for gas treatment at intermediate pressure levels

• Requirement to operate sections either in series or in parallel, to increase the operating range

• Intercooling to avoid high discharge temperatures, or to reduce the power requirement

• Process conditions require different operating speeds for the compressors.

While these requirements can be met with separate compressor bodies (multicase), it is often desirable and sometimes cost effective to combine the duty in a single compressor body. In this case, the body may contain several compression sections.

In general, these sections are set up to operate in series, but there are also compressors where the sections can operate in series or in parallel and allow for side-streams. From the outside, this becomes apparent as a compressor with more than two nozzles.

Frequently, these machines are referred to as back-to-back compressors. However, there are actually different ways of accomplishing multiple section machines: The impeller inlets in two sections can either point into the same direction, or in opposite directions. The former are often referred to as compound machines, while the latter are, indeed, back-to-back machines.

These configurations show differences in thrust load, leakage, behavior in series and parallel operation, nozzle design, emergency shutdown behavior, and impeller interchangeability.

In back-to-back machines the thrust loads of the opposing impellers can almost cancel each other out. Straight through machines will require a balance piston for this purpose.

Since the rotor lateral movement is largest in the center, a back-to-back machine may incur significant leakages across the separation seal between the sections when the sections are operated in series, because the division seal sees about half the overall machine pressure ratio. On the other hand, a compound machine would see about half the overall pressure ratio across the division wall seal, when the sections are operated in parallel.

Dealing with axial thrust

Axial thrust is typically the result of different pressures on the front and back of the impeller, resulting in a rather large, net force towards the inlet side of the compressor. In a back-to-back machine, this axial thrust can be almost completely balanced due to fact that about half of the impellers face in opposite direction from the other half.

A compound machine achieves thrust balance using a balance piston on the shaft. Thrust balance has to be checked carefully, especially if the machines are to be operated in series and parallel mode, or if the intermediate side-streamshows large fluctuations during operation.

In particular in back-to-back machines, axial thrust load considerations can reduce the flexibility of the machine compared to two independent compressor bodies. In situations where an emergency shutdown must be initiated, these machines often require a hot gas recycle valve to avoid thrust bearing damage if the sections unload at different rates.

While multi-compartment machines, such as back-to-back or compound, may appear as an elegant and initial cost effective solution to complex flow problems, careful consideration of all operating conditions must be undertaken to avoid unforeseen risks in off-design states. There are clearly some applications where a conventional tandem multicase solution can provide advantages over multi-compartment solutions.

With a new year upon us, we’d like to hear from you about any myths you’d like us to bust. What popular misconceptions are there out there, you think should be exposed? What “everybody knows . . .” turns out to be false and you’d like us to point it out. Send your thoughts to the magazine editor and we’ll take up the best ones in future columns.

Authors

Klaus Brun is theMachinery Program Director at Southwest Research Institute in San Antonio, Texas. He is also currently the Chair of the Board of Directors of the ASME International Gas Turbine Institute.

Rainer Kurz is the manager of systems analysis for Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and past chair of the IGTI Oil & Gas applications committee.