Objectives of preventive-maintenance and mechanical-integrity programs are to ensure that devices, systems, and equipment will perform satisfactorily during the interval between inspections or tests [I&T]. Thus, "leading indicators" (such as corrosion rates, and detection of incipient failures) are important in establishing appropriate I&T frequencies, rather than "present conditions" (such as hydrostatic tests, and detection of existing failures) or "lagging indicators" (such as historical failure rates). Of critical importance is the recording of "as-found" conditions, prior to repairs, calibrations, or replacements. Suggestions are provided for adjusting inspection-and-test frequencies and intervals, both upward and downward, as based on a "predictive-maintenance" program.
Of current interest is the testing of the shut-off devices on blowout preventers at wellheads beneath oil platforms. Typically, the gate valves are of a fail-safe design, where loss of air or gas pressure would cause the spring-loaded valves to close. Frequent testing is required to ensure that these emergency shutdown valves [ESD] will function properly when needed, to protect personnel and property. Although the ram preventer (pinch device) cannot be tested without destroying the piping, the actuating systems need similar frequent testing.
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