Inspection, Rating, Repair, Alteration, and Rerating of In-Service Piping
Rating: the arithmetic inspectors live by
Rating starts with thickness. Readings at CMLs feed the applicable code formula (ASME B31.3 for process piping) to establish t-min, the retirement thickness at which the component can no longer satisfy design conditions. Compare measured t to t-min. Short-term and long-term corrosion rates are computed from at least two surveys; remaining life = (measured t minus t-min) divided by corrosion rate. The inspection interval is then set so it does not exceed one-half of remaining life and does not exceed the maximum interval for the piping class; Class 1 services, which include injection points and high-deterioration services, take the shortest intervals.
Worked example: measured 0.375 in, t-min 0.250 in, recent rate 0.010 in/yr gives a remaining life of 12.5 years. Half of that is 6.25 years; if the line is Class 1 with a 5-year maximum, the interval is 5 years. Using the long-term rate when an upset has accelerated loss is the classic trap.
Repair, alteration, rerating
Repairs restore design capability: weld repair of pitting, weld build-up, flush patch, section replacement, clamp or enclosure. Temporary repairs require engineering approval and a defined removal date. Welding demands a qualified WPS/PQR, qualified welders, code preheat and PWHT, and hardness control in wet H2S service. Pressure testing after repair follows the applicable code (hydrotest commonly 1.5 times design pressure) unless the owner/user authorizes an alternative such as NDE in lieu of testing. Rerating requires engineering evaluation of every component, including relief devices and supports, may trade pressure for temperature, and must be documented and reflected in the inspection plan.