Centrifugal compressor tests

API 617 lays down several tests for centrifugal compressors. Full-pressure, full-load, full-speed tests and stability tests are major ones. These tests can be performed in combination too.

 This article contains excerpts from a paper, “Review of centrifugal compressors high-pressure testing for offshore applications” by Leonardo Ishimoto of Petrobras, Fabio de Norman et d’Audenhove of Royal Dutch Shell, Gary M Colby of Dresser-Rand, Marcelo Accorsi Miranda of Petrobras, Raphael Timbó Silva of Petrobras, Edmund A Memmott of Dresser-Rand.

Siemens single shaft centrifugal compressors with horizontally split casings in the STC-SH series[/caption]

 ASME PTC 10 Type 1

These tests are performed with the specified gas at, or very near, the field operating conditions. The combined effect of inlet pressure, temperature and molecular weight shall not exceed 8% of deviation in the inlet gas density. Limits for specific volume ratio, flow coefficient, machine Mach number and machine Reynolds number are specified. These parameters guarantee that the test is in similitude conditions with site and the performance can be confidently evaluated. The test evaluates the compressor performance from overflow (or overload) to close to the surge condition. The mechanical integrity of components is also evaluated, since the compressor is operating at full pressure and full speed. The rotordynamic behavior can also be verified, although not the stability margin.

 Full-pressure / Full-load / Full-speed test

API 617 states that the conditions for this test should be discussed and developed jointly by the purchaser and the vendor. The discussion below addresses each one of these conditions, some additional tests and how the conditions can be combined to develop a test that meets the purchaser objectives while taking into account the OEM’s test facilities limitations, as well as limiting cost. Since the combination of these requirements can result in a very different type of test, in this work we are going to refer to these tests in a generic way as “Full Load test”

Full Load

The term full load refers to an absorbed power equal to or higher than specified. This requirement alone is very vague as it does not give an indication on which parameters will be evaluated by the test. A vibration and mechanical assessment might be possible but, as mentioned, the term alone is not precise in defining the test procedure and what to expect from it. Therefore, it should be combined with other requirements to fulfill the purchaser objectives.

Full Speed

This requirement is especially important regarding the vibration evaluation. Although most machines that undergo a FPFLFS test would already have passed a mechanical running test at maximum continuous speed, such tests could replace the mechanical running test without compromises. In some cases, it might be necessary to reduce the speed during certain steps of the test. As an example, if the test procedure requires an investigation on the low flow portion of the operating map, the speed might have to be reduced to avoid over-pressurizing the test loop. In most cases, the test is conducted on an inert gas medium having a higher k value than the specified gas resulting in higher discharge temperature. Discharge temperature might also limit the test speed.

Full Pressure

The full pressure condition permits the evaluation of the compressor for the mechanical integrity of its components, such as pressure containing components (not only the casing), dry gas seals, as well as bearings mechanical behavior and temperature rise. This is an important verification, especially for the axial bearing, which is not subjected to significant load during the mechanical running test or the performance test. A full pressure test would also improve the evaluation of the rotordynamic behavior, since substantial aerodynamic cross-coupling forces, typically absent in a MRT, are introduced to the system.

Full Density

Although the API 617 8th Edition (2014) does not specify the term “full density”, it does use the position of the compressor on a plot of flexibility ratio (the ratio of the maximum continuous speed divided by the first critical speed on rigid supports) vs. average gas density in the Level I screening criteria. API 617 8th Edition says to use the machine condition at the normal operating point unless the supplier and purchaser agree upon another operating point. The position of the compressor on this plot does not determine if the compressor will be stable or not. The position on the plot and the results of an API 617 Level I log dec analysis with the journal bearings, squeeze-film dampers (if used) and oil-film seals (if used) and an anticipated cross-coupling as in Memmott (2000), API 617 7th Edition (2002), and API 617 8th Edition (2014) are used to determine if a more detailed API 617 Level II log dec analysis with inclusion of the gas annular seals is needed.

Stability Test

A stability test consists of the measurement of the logarithmic decrement (log dec) by using nonsynchronous forced excitation applied during the test. Stability tests can be done in conjunction with the traditional full load tests to measure the log dec, while the answer of a full load test when the stability test is not done is either stable or not stable (at the one specific condition). A main advantage of the stability test is to foresee the risk of instability in operational conditions other than the rated (7th Edition term) or normal operating point (8th Edition term). If stability testing was done only during a mechanical run test it would help to validate the combined rotor-bearing-pedestal system model, but it would not validate the dynamic modeling of the annular gas seals and that of the load related anticipated cross-couplings. The effect of the annular gas seals is critical for high-pressure high-density compressors.