Centrifugal Compressor Class III Tests Performance Test

This test is the same as Class II except that compressibility factors are applied along with changes in “k” value from suction to discharge.

In Class II and III tests, the test speed and test pressures and temperatures are often greatly different than the specified values. The ASME code includes tables showing the allowable deviations for volume reduction, Q/N, machine Mach number, and machine Reynolds number for Class II and III tests. Similarly, permissible departure from specified conditions for Class I tests are listed including pressure, temperatures, specific gravity of the gas, speed, and capacity. See Figure 600-1.

Fig. 600-1 Typical Test Gases and Equivalence Values

Class II tests are seldom used for compressors in the petroleum industry. Class III tests are the most common. Test gases for Class III tests include carbon dioxide, nitrogen, Refrigerant 12 or 22, and mixtures of helium and nitrogen. Generally, it is preferred to run the test with a pure unmixed gas. With a mixture of helium and nitrogen, it is sometimes difficult to maintain a constant gas composition for the duration of the test. If makeup is required in the loop during the test, it is not easy to add the correct proportions of the two gases. In such cases, it may be advisable to require the compressor vendor buy an adequate quantity of certified pre-mixed gas from a specialty gas manufacturer.

Some Class III tests are run with a sub-atmospheric suction to reduce power requirements during the test. This procedure invites air leakage into the loop which will upset the gas composition. Therefore, flange tightness should be carefully checked prior to test startup.

Figure 600-1 shows some typical test gases used for various specified gases in Class III closed-loop tests. In general, the heavier test gases are used for heavy specified gases. Helium/nitrogen mixtures are used for hydrogen-rich gases such as ammonia synthesis and refinery recycle gases.

Note that the equivalent speed, capacity-speed ratio, and volume ratio, at which a Class III test is run, are generally compromises between the various departures allowed by the PTC-10 code (see Figure 600-2). CMP-MS-1876 requires that the test speed has a safe margin from the rotor’s critical speed.

The subject of Reynold’s Number corrections of the results usually comes up when the performance test agenda is being developed by the vendor and purchaser. The corrections suggested by ASME PTC-10 have been proven to be very misleading, and are inclined to favor the vendor. Depending on conditions, the ASME corrections could allow a specious improvement in efficiency of 6% or more. Most purchasers have disallowed any correction, and some vendors voluntarily decline to make corrections. In some special cases, 50% of the corrections would be allowed.

The effects of flow in different regimes of Reynolds Number is well known, and some correction should logically be applied. The problem is in developing suitable correlations of the complex flow path in the compressor.

In the early 1980’s, a group of eight major compressor manufacturers in the United States and Europe got together under the auspices of the International Compressor and Allied Machinery Committee (ICAAMC) to develop a new correction method. Test data were pooled, and good correlations between measurements and predictions were established. The ICAAMC method includes the friction factor concept that is used in the analysis of flow in piping. The method has been proposed to ASME for possible adoption in the next revision of the PTC-10 Code. It has already been used successfully on several compressors. The ICAAMC method should be considered for cases where the ratio of Reynolds Numbers for test and specified conditions are in the range of 0.01 to 100.

Modified closed-loop Class I tests have been run on high-pressure machines with discharge pressures ranging from about 3000 to over 9000 psi. Such machines are used for injection of natural gas into an oil field formation. For these tests, the test gas is formulated by blending several hydrocarbon gases and other gases to closely approximate the composition of the actual gas. The test is run at full pressure and full load. Sometimes the main objective of such a test is to determine mechanical behavior at high-pressure levels, and aerodynamic performance may be of secondary importance. A Class III test is usually run in addition to the modified Class I test. The full pressure test along with the Class III test will provide good data for predicting gas properties.

Performance tests are ordinarily specified for all machines in critical service where the process flow and pressure is crucial, or where the service is troublesome and unpredictable such as gas injection. If some components of the machine are unavoidably unproven, the machine should have a performance test. The performance test will often shake out mechanical problems.

The performance test is generally good insurance for machines in tough duty, since it is far less expensive and time consuming to modify a machine at the factory than at the jobsite.

When it is decided that a performance test is required, merely specifying an ASME test is seldom sufficient to obtain the desired results. The objective of the tests should be stated in order that the vendor and purchaser can work out an appropriate test procedure. In this regard, consultation with a mechanical specialist is strongly
recommended.

Allowable Departures From Specified Design Parameters ASME PTC-10

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