Shop performance test of turbo-compressors

Turbo-compressor operators have realized the value of and payback from a proper shop performance test and shop mechanical run test. Careful analysis of the shop test results of the turbo-compressor can help to detect malfunctions and problems, before delivery. Any correction or modification at site would be expensive and requires special considerations.

An ASME-PTC-10 type-1 performance test should be done wherever possible. This is a shop performance test with the gas at conditions very close to site operating conditions. When a type-1 test is not possible due to any reason, the ASME-PTC-10 type-2 shop performance test can be conducted. For instance, it is possible that flammable process gas may not be available or used at the site due to safety concerns or cost. A type-2 performance test procedure permits the use of a substitute test gas but it should not lead to excessive power consumption. Also, it should not lead to excessive discharge temperature. This is nearly always an inert gas or mixture although mixtures are not desired and not usually used. The alternative gas should be readily and cheaply available. Substitute gases such as Helium (He), Nitrogen (N₂), air and CO₂ are usually employed.

A type-2 test accepts extensive differences between shop test conditions and specified site operating conditions. There are only a few limits on some essential fluid-dynamic parameters of the test conditions compared to the specified site operating conditions. The specific volume ratio and flow coefficient should be within 5% deviation.

The Mach number should be kept within 0.1 deviation and the turbo-compressor’s Reynolds number should be within 0.1 to 100 times. Beside these limits, a typical type-2 test allows considerable deviations in the test speed, turbo-compressor’s capacity, mass flow, pressures, temperatures, and power. For example, a natural gas (MW=16) turbo-compressor can be type-2 performance tested using CO₂ (MW=44) with around half the inlet flow, approximately 20% of the mass flow, around 50% of the speed, approximately 6% of the consumed power and the discharge pressure less than 10% of specified site operating discharge pressure. The concept behind a type-2 performance test is performing a test with a different gas, following which the formulations of fluid mechanics are used to estimate the turbo-compressor performance in the specified operating conditions using the recorded fluid-dynamic details. Major fluid-dynamic characteristics are within certain limits.

Type-1 vs type-2

There has always been a question regarding accuracy and usefulness of a type-2 performance test. Some engineers believe that an ASME-PTC-10 type-2 test is not an actual performance test.

Arrangements and details of such a type-2 test should be fixed in the bidding stage of a turbo-compressor and before the order placement. Turbo-compressor vendors usually prefer the simplest arrangement for a type-2 test. Unfortunately, ASME-PTC-10 type-2 code allows vendors to use the simplest and cheapest test arrangements. Usually type-2 test gas selection and arrangement are discussed after the turbo-compressor order or even near the test time; it has often resulted in considerable changes in costs or a poor test arrangement.

The molecular weight is a major factor for the selection of type-2 test gas as any significant difference between molecular weights of the test gas and final site gas can result in a different test speed and consequently completely different conditions. This is a challenging decision as often a suitable gas mixture should be used to achieve close resemblance with the job gas. Therefore, great care is needed for the selection of an alternative gas.

The test speed should preferably be as close as possible to the site rated speed and it is strongly preferred that the test speed be between 80% and 100% of the site rated speed. In other words, the speed difference below 20% is desired. A properly arranged type-2 test can give a useful prediction on the turbo-compressor behavior and indicate any problem or short-fall early in the shop, before delivery.

Shop mechanical test run tests

A pressurized shop run test is always preferred compared to an under-vacuum run test. This is usually done with nitrogen. To start the shop run test, the turbo-compressor speed increases to the maximum continuous speed; the operation continues until bearing metal temperatures, lubrication oil temperatures and shaft vibrations are stabilized. The turbo-compressor should be operated for 15 minutes at the trip speed. After that, continuous 4-hours operation at the maximum operating speed is performed. The main focus of a run test is on the vibration, bearing performance and lubrication oil system. During the shop run test, the lubrication oil temperature rise through bearings should not exceed certain limits, say a maximum of 30°C temperature difference. Operation at extreme conditions should be tested such as maximum and minimum lubrication oil pressure and temperature although limits should not be exceeded.

After the run tests, all bearings should be removed, inspected, and reassembled. Subsequent minor cosmetic repairs of bearings do not justify repetition of the shop run test. If melting, smearing, overheating or distinct wear occurs in bearing components (such as shoes), these parts should be replaced, the root cause of defect(s) should be investigated and eliminated; and then the mechanical run test should be repeated. After the run test, the compressor casing is leak-tested to evaluate joints and seals. The assembled compressor is pressure tested to the maximum operating pressure for 30 minutes or 1 hour. Removal and inspection of dry gas seals is not recommended. Post-test inspection of casing internal is also not recommended due to the advantage of delivery of the proven and tested turbo-compressor to the site.