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Centrifugal Compressor Testing

By Joe Honeywell

 

Introduction

Every compressor, whether it is new or has been in service for many years, will need to be tested for aerodynamic performance and verification of mechanical operation. The testing may be conducted in the manufacturer’s facilities under strict controlled conditions or in the field at actual operating conditions. For new machines, the compressor testing may be limited to only a mechanical running test or a performance test. If a compressor’s final location is at a remote site or if it handles a dangerous gas, this may warrant a complex series of tests. Alternately, compressors that have a proven design or duplicates an existing machine may only need testing after installation. Another possibility is if a machine has been modified during a plant shutdown and is placed back into operation. The machine may require performance testing to prove the modifications were successful. Even compressors that have been in operation for an extended period of time will require testing to verify the efficiency and normal operation. In any event, every compressor will need to be tested at various times in its life-cycle.

Compressor testing is not free; it comes at a cost of time, material, personnel, support equipment and money. Machines of proven design and service may only require a simple test to verify performance. More complicated compressor systems may warrant a more complex testing program to verify all aspects of the design. Whether the testing program is simple or complex, a thorough understanding of the cost and benefits of the testing program is needed before proceeding.

There are many types of compressor tests used to verify mechanical and aerodynamic performance. This article will review the applicable centrifugal compressor codes and the various types of mandatory and optional tests. The primary focus is on aerodynamic performance testing; however, other types of tests are discussed. It will provide insight into the criteria for compressor performance verification and acceptance.

The first section of this article will review the various codes and standards that apply to testing of a centrifugal compressor. It will give a brief description of each one, whether it is mandatory or optional, and how it applies to a centrifugal compressor.

The next section will describe the principle performance relationships and parameters that define and limit a compressor’s performance. The guiding thermodynamic equations and the various parameters that define a machine’s performance are described, along with an explanation of how they affect a compressor. It will also give recommendations for selecting a test gas and the methodology for correlating physical and thermodynamic properties of the gas to the specified gas at operating conditions. Many example problems are included in this section that help guide the reader through the various topics and mathematics.

The following section will examine actual testing requirements. Typical test loop arrangements, data gathering and evaluation of test results are explained. An example problem is given that incorporates all the elements of a test procedure, including data collection, calculation methods, performance analysis and reporting the results.

The final section is a summary of the article. It reviews the various topics given in this article. It also explains the benefits of a performance testing program throughout the life-cycle of a centrifugal compressor. Two appendices are included that recap the terms, “USC units” and “SI units”, equations that used throughout this article.

This article is intended for readers with a basic understanding of conventional type centrifugal compressors. At a minimum the reader should be familiar with the classic definitions of head, flow and power. The article builds on this knowledge, along with the use of other compressor performance parameters, to provide the reader with a more advanced understanding of aerodynamic testing. It also provides a basic understanding of the technical terms that are part of industry’s standard methodology of evaluating compressor performance. In an attempt to simplify the process, the scope of this article has been limited to a single-section, straight-through type compressor. The same basic principles of this article, however, will also apply to more complex compressor configurations. Hopefully, readers will find the simplified topics easier to understand and be able to apply them to more complex applications.

The article is also intended to aid readers responsible for testing of centrifugal compressors in both shop and field applications. The thermodynamic principles are similar but the test facility and operating conditions can be vastly different. Knowledge of shop or field conditions is important because it may impose different limits on the testing protocol and may have an influence on the results. The topics of this article will help the reader understand the differences between the two types of tests, why performance changes can occur, factors that contribute to errors, how to evaluate raw data, calculation of performance parameters, correlation the test results and evaluation of the final report. Utilization of the engineering principles contained in the article will be helpful to the reader in performance testing of compressors in both settings.

 

 

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