(84c) Good Engineering Practices - Inspection and Test Frequencies

The "Process Safety Management" standard of the Occupational Safety and Health Act [OSHA] requires that "The frequency of inspections and tests of process equipment shall be consistent with applicable manufacturers' recommendations and good engineering practices, and more frequently if determined to be necessary by prior operating experience." Guidance concerning inspection and test frequencies is provided in publications of the Center for Chemical Process Safety, and an objective of this presentation is to compile sources for the available data.

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.