Important Aspects of Centrifugal Compressor Testing-Part 1

Every centrifugal compressor, whether it is new or has been in service for many years will most likely be tested to verify its thermodynamic performance.  For a new machine the testing may be conducted in the manufacturer’s facility under strict controlled conditions or in the field at actual operating conditions.  Older compressors that have been placed in service after maintenance or have been operating for an extended period of time may require testing to verify the efficiency and normal operation.  This TOTM will review ASME PTC-10 (also referred to as the Code) testing procedure and other topics that contribute to an accurate centrifugal compressor test results.

This two-part series will review the salient aspects of a performance test.  Part 1 will review the thermodynamic performance test objectives established in the Code as well as other factors to consider in a testing procedure.  While this code is primarily applicable to shop testing it can also apply to field testing.  Part 2 will review the Code assumptions and basic performance relationships.  It will also examine the three important principles that influence the operating conditions and ultimately influence the accuracy of the performance test.  They are volume ratio, Machine Mach Number and Machine Reynolds Number.

Introduction

The purpose of a performance test is to verify that a centrifugal compressor will perform in accordance with the manufacturer’s design at the operating conditions given in the specifications.  It also provides a method of confirming the shape of the compressor head-flow curve, efficiency, and the maximum and minimum flow limits at various speeds.  Frequently a performance test is conducted under field conditions with the specified gas and operating conditions.  However, if the performance test is conducted in the shop it may not be possible to test the compressor with the specified gas because of safety concerns or testing facility limitations.  Whether the test is conducted in the field or in the shop, proof of the compressor design is recommended and often necessary to demonstrate contractual obligations and mechanical integrity.

Frequently the gas composition used to confirm a compressor performance differs from the specified gas.  This is often the case regardless if the test is conducted in the field or in the shop.  For field tests, where the gas composition and operating conditions are set by the process, adjustments must be made in the calculations to confirm the compressor design specifications.  Typically, a shop test is conducted with a carefully selected mixture of gases blended together to form a gas that has physical properties that closely resemble the specified gas.  Even with a substitute gas, differences remain which influence the test results.

The original compressor design places limits on the thermodynamic performance.  The most important of these limits include flow rate, power, temperature, pressure and speed.  There are other design restraints which are not as commonly known but will also influence the compressor performance.  Such factors are volume ratio, Mach number and Reynolds number.  These limits were incorporated in the compressor design and are influenced by gas properties, operating conditions and the mechanical design.  To verify the design and operating limits for a compressor, it is necessary to test the machine.  For new machines, these tests are commonly performed in the manufacturer’s facility; however, the testing is sometimes performed in the field.  It may also be helpful to periodically test a compressor to trend the machine performance.  Testing conducted during commissioning will establish a baseline of performance.  Periodic field tests are often conducted to verify the overall performance and signal changes that may predict mechanical damage, internal fouling, or other deteriorating conditions.

Summary of ASME PTC-10 – Performance Test Code
The procedure presented in the Code provides a method of verifying the thermodynamic performance of centrifugal and axial compressors.  This code offers two types of tests which are based on the deviation between test and specified conditions.  A detail procedure is given for calculating and correcting results for differences in gas properties and test conditions.  The following briefly describes the guiding principles of the Code.

• Type 1 test is conducted with the specified gas at or very near to the specified operating conditions.  While the actual and test operating conditions may differ, the permissible deviations are limited.  See Table 1, 2 and 3 for deviation limits of testing variables of a Type 1 test.
• Type 2 test is conducted with either the specified gas or a substitute gas.  The test operating conditions will often differ significantly from the specified conditions.  The operating conditions are subject to limitations based on the compressor aerodynamic design.  See Table 2 and 3 for permissible deviations of operating conditions and test gas properties.
• The calculation method of a Type 1 and Type 2 test may conform to either Ideal or Real Gas laws.  Physical property limitations are given in Table 3 if Ideal Gas Law methodology is used.

The Code also gives procedures for calculating and correcting test results for difference between the test conditions and specified conditions.  It also gives recommendations for accurate testing including compressor testing schemes, instrumentation, piping configuration and test value uncertainties.  The following summarizes each topic.

• Thermodynamic calculations may utilize either enthalpy, isentropic or polytropic methods.  The Code provides equations and examples for determining compressor work (also referred to as head), gas and overall efficiencies, gas and shaft power, and parasitic losses.
• The Code gives a correction procedure for test gases and test operating conditions that deviated from the specified operating conditions.
• Compressor testing may be open-loop or closed-loop; however, the test results are subject to limits that may give preference to the test arrangement.
• Instrumentation methods and measurement uncertainties (refer to PTC-19 series of standards) used to test compressors are given.
• Recommendations for piping layout are also included.

Test Gas Selection
There are many gases commonly used to test compressors.  They are selected based on physical properties, toxicity, flammability and environmental concerns.  See Table 4 for a list of the most frequently used gases.  The manufacturers will sometimes blend the various gases to match the equivalency criteria and the test facilities limitations.  Following are recommendations to consider when selecting a test gas.

• The compressor mechanical design may impose constraints on the test.  Consider the machine rotor dynamics, overspeed, maximum temperature and power limitations when selecting a test gas.
• Avoid flow rate mismatch of impellers.  The volume ratio equivalency is the most important parameter in selecting a test gas.  This may also place limitations on the operating conditions.  More on this subject in Part 2. of this series.
• The test gas molecular weight should closely match the molecular weight of the specified gas.
• The test gas k-value should closely match the specified gas to duplicate the Machine Mach Number.  If this is not practical then the test k-value should be slightly greater to avoid possible stonewall limitations.
• Select a test gas with minimum Reynolds Number deviation from the specified gas.  This will minimize the efficiency and head correction factors.  This is especially important for machines with a low Machine Reynolds Number.

Table 4
Typical Test Gas Mediums (1)

Note:

1. From “Compressors 201” course at Turbomachinery Conference, 2009
2. Values from National Institute of Standards and Technology and Gas Processors Suppliers Association
3. Values at 60 0F (15.6 C) and 14.696 psia (101.3 kPa)
4. Gas composition and physical properties varies with local utility

Test Objectives
The following are some factors to consider as part of the performance test procedure.

• API 617 requires a minimum of five test points to be taken at the operating speed to demonstrate the surge point, stonewall, required operating point and two alternate points.  The user may optionally request additional test points to verify compressor performance at alternate speeds.  For example, extra data points may be needed to verify the surge line or critical process operating conditions for variable speed machines.
• The test may be performed as a Type 1 or Type 2 test.  Type 1 is normally more accurate and is typically reserved when test conditions can be made to closely match the specified operating conditions.  A Type 2 test is typically a shop test utilizing a substitute gas.
• If a Type 2 test is recommended, the test gas may be a pure gas such as those listed in Table 4, or a mixture of gases.  The composition of the test gas should be agreed upon before testing.  In addition, the composition of the test gas should be sampled before, during and after the test.  Some gas mixtures tend to stratify and give erroneous results.
• The physical properties of the test gas are critical to the outcome especially if it is a mixture of selected gases.  An agreement on the physical properties is recommended.
• Normally an agreement is made as to the “equation of state” used to calculate the results of the test.  Not all EOS programs give the same results, nor is there industry agreement as to which method is best.
• Discuss the specific driver used in the test.  Will a shop driver or the specified driver be used?  Will the driver be fixed or variable speed?  If it is variable speed, will it be motor, gas turbine or steam turbine?
• If a gear is part of the test, will it be manufacturer or user supplied?  Is the efficiency of the gear known?  Tests can be performed to verify gear efficiency.
• Will the gas be cooled with a water-cooled or air-cooled exchanger?  Is there temperature limitations on the coolant used in the test?
• Is the allowable working pressure of equipment and piping systems adequate for the test?  Will a pressure safety valve be needed to protect the system and is it properly sized?
• An agreement on how the input power will be measured is important.  Options include, heat balance, calibrated driver, dynamometer, and torque meter.  Review the specific method of measuring input power with the manufacturer.
• A piping and instrument schematic is recommended.  The drawing should show details of the test loop including the placement of major equipment, number and location of instruments, and piping size.  This is especially important for compressors with multiple sections, inlet sidestream, or back-to-back configuration.
• Before proceeding with a performance test a written procedure is recommended that outlines how the test will be conducted.  The procedure should clearly convey the scope of the test, the responsibilities of each party, test piping and instrument arrangement, measurement methods, uncertainty limits, calibration, taking of test data and how to interpret results, and acceptance criteria.

By Joe Honeywell

References

1. ASME PTC-10, “Performance test Code on Compressors and Exhausters”, 1997.
2. API Standard 617, “Centrifugal compressors for Petroleum, Chemical, and Gas Services Industries”, 1995.
3. Kurz, R., Brun, K, & Legrand, D.D., “Field Performance Testing of Gas Turbine Driven Compressor Sets”, Proceeding of the 28th Turbomachinery Symposium, 1999.
4. Short Course “Centrifugal Compressors 201”, Colby, G.M., et al. 38th Turbomachinery Symposium, 2009.
5. National Institute of Standards and Technology, Web Site for Properties of Fluids.