Planning for field performance tests

Field testing of gas turbine compressor packages requires the accurate determination of efficiency, flow, head, power, and fuel flow in sometimes less than ideal working environments. Field test results have significant implication for the compressor and gas turbine manufacturers and their customers.

The challenges of field tests arise not only in applying the laws of physics and engineering that govern the behavior of turbomachinery, but also in the depth of preparation and the organization of the necessary tools, conditions, and personnel required to conduct the tests and analyze the results.

This article contains excerpts from the paper, “Field performance testing of gas turbine driven compressor sets” by Rainer Kurz, Klaus Brun and Daryl D. Legrand at the 28th Turbomachinery Symposium.

Governing Codes

The American Society of Mechanical Engineers (ASME), the International Organization for Standardization (ISO), and the Verein Deutscher Ingenieure (VDI) have issued specifications covering thermodynamic calculation methods, instruments, site preparation, and the reporting of turbomachinery test results in various degrees of detail. Compliance with such specifications, as listed above, is a relatively easy matter in a factory environment where facilities are designed specifically for testing; qualified support personnel, instrumentation, and calibration laboratories are available; and real-time online computers routinely monitor the test progress. This is usually not the case at actual installation sites designed for commercial operation of turbomachinery.

Site performance tests generally require concerted planning and execution, including development of a unique test agenda prepared jointly by the manufacturer and the equipment end-user. Such an agenda should communicate the tleld conditions and equipment layout, list the instruments to be used and their location, describe the method of operation and the pressure and temperature limits of the facility, and specify any deviations from normal operation that may be necessary to conduct the test. It also should describe the methods of data reduction, of determining the test uncertainties, and the acceptance criteria. The items of such a test agenda can and should be discussed in a very early stage of the project. Preparations also include discussions on available operating conditions and operational limitations. In many cases, a specified operating point can only be maintained for a limited period of time (for example, because the pipeline operation depends on the tested package) or at fixed ambient conditions (if the necessary gas turbine power is only available on cold days). Because the installation of instrumentation is part of the overall station design, the requirements need to be communicated early.

Details, such as the necessary immersion depth of thermowells, as well as more onerous items, such as determining what flow measurement to use (taking into account the tradeoffs between pressure losses, efficiency, and cost), have to be agreed upon. The selection and calibration of the test instrumentation are extremely important. Generally, the instruments supplied for monitoring and protection of the packages are not accurate enough to achieve the small uncertainty margins necessary for a field test. This is mainly due to the necessarily more stringent calibration requirements for a field test. Whenever possible, laboratory quality instrumentation should be installed for the tests. The accuracy of the instruments and the calibration procedure should be such that the measurement uncertainties can be eliminated from future discussions regarding the pe1formance of the unit. The requirement for special instrumentation is especially important for tield tests of compressor sets with a low pressure ratio.

Test uncertainties and building tolerances

Test uncertainties need to be dearly distinguished from building tolerances. Building tolerances cover the inevitable manufacturing tolerances and the uncertainties of the performance predictions. The actual machine that is installed on the test stand will differ in its actual performance from the predicted performance by the building tolerances. Building tolerances are entirely the responsibility of the manufacturer. Test uncertainties, on the other hand, are an expression of the uncertainty of the measuring and testing process. For example, a machine tested with 84 percent efficiency may have an actual efficiency somewhere between 82 percent and 86 percent, assuming two percent test uncertainties. The test uncertainty is basically a measurement of the quality of the test. An increased test uncertainty increases the risk of failing the test if the turbomachine is actually performing better than theacceptance level, but it reduces the risk of failing if the machine performance is lower than the acceptance level. Because it is normal practice to use a lower performance than predicted as an acceptance criterion, it is in the interest of the manufacturer as well as the user to test as accurately as possible.

Agenda prior to the test

The customer and the manufacturer should agree upon and document the parameters of interest for the test, as well as the criteria (minimums and maximums) for acceptance. Gas turbine power and fuel flow, and gas compressor efficiency generally are the primary parameters, while compressor flow range and surge margins are examples of other common performance parameters. In a very early stage of the project, discussions should be started about necessary instrumentation and the site preparation to allow for the installation of the required test instrumentation, such as flow metering runs, thermowells, and pressure taps. Inevitable shutdowns and the effect of the test on production need to be addressed. In this phase, the tradeoff between various options of installing instrumentation and the effect on conducting the test can be evaluated.