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Closed loop compressor performance test comparisons
An ASME Power Test Code 10 (PTC-10) is a closed loop performance test for centrifugal compressors. It is often undertaken in the factory prior to shipping of a new compressor. It is a rather expensive test since it requires complete installation and commissioning of the compressor in the factory.
There are two alternative procedures to test the centrifugal compressor offered in PTC-10:
i. A test with a gas that is very close in composition and thermodynamic properties to the specified (design or guarantee point) gas at pressures and power demand close to the contractual values, called a type 1 test or
ii. A test using a substitute gas that is different from the contractual gas but allows to emulate the aerodynamic operating conditions, usually with an inert gas mixture like nitrogen or carbon dioxide, and at pressures lower than the specified gas, called a type 2 test.
Considering the distinction between the two tests, it appears obvious that by definition the type 1 test is the more desirable choice since it utilizes the actual gas composition rather than an emulated gas. But sometimes the obvious choice is not necessarily the best option or right answer.
Before we dig into the question, let’s provide more details about these tests. PTC-10 tests are tests to confirm aerodynamic compressor performance. Nothing else. There is nothing in PTC-10 about verifying mechanical limits or characteristics. The compressor has a contractually agreed guarantee performance point that is to be verified in a PTC 10 test. i.e., this is a test for compressor power, efficiency, and aerodynamic operating range only.
The operating point is defined by a gas flow, a suction and discharge pressure, a suction temperature, and a gas composition. While a Type 1 test requires the use of a gas composition very similar to the actual design gas composition, a type 2 test allows the use of a different gas, typically an inert gas like nitrogen or CO₂.
In a type 2 test, test conditions are selected so that aerodynamic similarity is reached. The volume reduction due to the compressibility of the gas is retained. Machine Mach numbers are the same as for the contractual operating conditions. This means that all velocity relationships within the compressor are the same as for the contractual conditions. It is not required to operate at the same pressure or flow of the specified conditions, nor will the compressor operate at the same speed or power consumption as required for the compressor at the specified condition. The ASME power test code provides a precise methodology to accomplish this, and the rules for reaching an aerodynamically similar point are well proven, allowing a precise representation of the operating conditions.
The key advantage of using a gas as allowed by a type 2 test is, that the gas, typically a pure or binary gas mixture, is available with a precisely known composition. The thermodynamic behavior, that is the relationships between pressure, temperature and volume, is well known. The use of similarity laws is well established, Mach numbers are retained, and this allows to maintain aerodynamic relationships. The gas mixture used in a type 1 test will generally not provide that advantage. Therefore, a type 2 test will generally provide more accurate performance test results.
Furthermore, the setup time, test duration, and costs for a type 1 test are significantly larger than for a type 2 test. Test stand capabilities, and the available power of the test stand drivers have to be considered when deciding since a type 1 test usually requires more power and thus larger drivers. For a type 1 test, the gas must be carefully mixed from several components, and the mixture concentrations have to be maintained throughout the test to avoid uncertainties.
On the positive side, the operating conditions required for a type 1 test can provide information about the rotordynamic and mechanical behavior of the compressor. However, the test code by itself does not provide any requirement to monitor or measure these characteristics. The ASME PTC10 code is strictly an aerodynamic test code. There are other tests, mentioned in the API 617 standard, such as a full load, or full speed and full pressure tests, that are designed to verify the rotordynamic performance or the mechanical behavior.
In other words, the extra effort to conduct a PTC 10 Type 1 test does not lead to more precision regarding the aerodynamic and performance results than a type 2 test. It may actually result in reduced accuracy since there are higher gas composition measurement uncertainties. And it certainly will significantly increase the cost and time consumption for a PTC-10 test.
Klaus Brun is the Director of R&D at Elliott Group. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.
Rainer Kurz is the Manage of Gas Compressor Engineering at Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and the past chair of the IGTI Oil and Gas Applications Committee.