Hydrostatic Test

It is conventional to perform a hydrotest on all pressure-containing casings for all compressors. For the reciprocating compressor, where there is no casing, it is the cylinders that are tested individually, based on VA times the maximum working gas pressure, but not less than 20 psig. The 20 psig minimum is generally recommended for the rotary, positive-displacement casing as well as dynamic compressors.

Reciprocating cylinder water jackets are tested at VA times specified working pressure for the cooling fluid. API 618 recommends a pressure of no less than 115 psig.

For compressor casings designed for different pressure levels, the hydrotest is complicated by the requirement for split-level pressure testing. The casing has to be divided with special closures to permit the isolation of the pressures. It is not unusual for the closures to leak, complicating the test interpretation, but this is not a cause for rejection. Generally, these types of casing designs are discouraged, but nonetheless are furnished on occasion.

The duration of the test pressure in most cases is 30 minutes minimum; however, the test must be held for long enough beyond that time to permit a thorough examination for porosity and casing wall leakage.

Interpretation of joint leakage is somewhat more complicated because the pressure profile causing the leakage may not always be a true representation of the ultimate service conditions.

Casings are designed for controlled deflection, a more stringent criteri -on than stress. To avoid a misunderstanding, stress is not ignored, but because the deflection requirements inherently keep the stress relatively low, it is not the primary concern. Obviously, in those areas where deflection is not the limiting factor, the stress is then used for the design basis. This makes the testing of casing integrity somewhat more difficult. Hydrotesting is used as in pressure vessels, and the casing is inspected for leaks. Since the hydrotest is based on a value of 1.5 times the maximum allowable working pressure, the test checks the basic safety of the casing to ensure that the stress levels were not exceeded in any area. Only when design modifications are made is the casing instrumented to directly monitor deflections. The tendency is to use joint leaks as a measure of casing suitability. Of course, if the casing doesn't leak, everything is acceptable; however if the joint leaks, the problems begin. The joint may well leak from the distortion due to the hydro pressure; however, it can be argued that this is not a proper test since the joint should never experience this level of pressure during operation. If the casing is given a gas test at the maximum allowable pressure at the time of final assembly when the joint is bolted and made ready, as it is intended to operate in the field, a better judgment may be made. This little dissertation will not settle the arguments, but, if the objectives and methods are considered as the designer intended, perhaps the discussions will not be as heated.

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