Inspection and Testing

If the API standards are used, the inspection requirements are reasonably well outlined. At this point in the specification, the type of inspection to be used as well as any special inspection should be stated. The details can be resolved later, but the vendor should be given some idea of what inspection methods are used, such as visiting, resident, or no inspection. The tests that are to be witnessed should be covered. API defines two categories of test witness. One is the formal witness test, generally where the vendor runs a pre-test prior to running the customer witness test. Since this is double testing, it is more expensive to the vendor who may, if he feels the market will allow, pass the extra cost along in his bid price. The user is usually kept guessing at that point. The other test is the observed test. This works reasonably well with resident inspectors who are technically able to act as test witnesses. The problem is the schedule of the first test is not too predictable, as it depends on vendor shop loading, last minute problems, and dozens of little irritating items totally unpredictable yet ever present. The attraction to the observed test is that it is the only test and, therefore, should not carry an extra charge to the customer. With the witness test, there is some advance notice required. Because the test has been run, the formal test should be of a minimum duration. The evaluation for which type of test to use is strictly a user decision. If engineers must attend the witness test (where the engineer's time is critical), then the best choice is to take the witness test. While extra performance testing can be specified, outside the code performance test and the selection of the ASME testing, there is not too much incentive for the vendors to go further. The ASME type code tests are expensive, but when reliable operation and performance are important, they do help determine field performance and give a checkpoint from which to proceed if problems develop in the field. Also, they do catch problems while the compressor is still in the shop. There is a significant increase in cost to repairing problems in the field compared to doing it in the shop.

Full power, string tests and the other optional tests, as used in centrifugals, must be evaluated for each application. There is a strong feeling that string tests do find problems, particularly since contract couplings are used and criticáis or vibrations due to overhung weight can be uncovered.

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