Hydrostatic Testing of Heat Exchange
Hydrostatic testing has been effective in removing stress corrosion cracks greater than the critical size corresponding to the high level of pressure applied during hydrotesting. As the critical crack size for normal operating pressure is much greater than the failure flaw size at hydrotest pressure, a margin of safety is achieved following a hydrotest. In the United States, pipelines operating at a maximum allowable operating pressure (MAOP) of 72% of SMYS are required to be hydrotested at 90% of the SMYS or higher, and the minimum high-pressure hold time is 8 hours In Canada, pipelines known to be affected by SCC are required to be hydrotested at a pressure that is up to 110% of SMYS, and the hold time is at least several hours.
In a full-scale hydrotest study, cracks previously growing in a simulated soil environment were subject to a full hydrotest load spectrum, and then these cracks were tested again under lower pressure with some degree of pressure fluctuation. It was found that all cracks grew at much reduced rates after the hydrotest and this was attributed to the creation of compressive residual stress at and near the crack tip during the hydrotest over-loading.
Shell side Hydrostatic Testing
After manufacturing completion, the heat exchange shell side will be subjected to the hydrostatic testing. The test pressure amount shall be consistent with the value indicated in the approved drawing. The holding time shall be based on the supplier approved test procedure. The pressure gages calibration and range shall be controlled. The pressure gage range shall be within 1.5 thru 4 times of test pressure. The shell body and welding seams shall be inspected. The tubesheet also shall be checked. No leakage is allowed. The test pump hose shall be disconnected, and no pressure drop is allowed. The test result will be satisfactory if no leakage being observed and no pressure being dropped while the shell side was under pressure.
Tubeside Hydrostatic Testing
After successful completion of shell side hydrostatic testing, the bonnets are assembled, and tube side is subjected to hydrostatic testing. Similarly, the amount of test pressure shall be as the one indicated in the approved drawing. Other requirement is similar to the shell test. No leakage and pressure drop shall be observed while the tube side is under pressure.