When recalculating MAWP for continued use, what thickness is used?

When recalculating MAWP for continued use, what thickness is used?

• A. The nominal thickness
• B. The actual thicknesses determined by inspection minus twice corrosion loss before the date of the next inspection
• C. The original thickness
• D. The thickness after corrosion allowance

Answer: (B)

When continuing service, the MAWP calculation must reflect the pipe wall that is actually left, not what it was designed to be or what it was originally. You start with the actual wall thickness measured at the most recent inspection, and you apply a conservative adjustment for future thinning. Specifically, you subtract twice the corrosion loss that is expected to occur before the next inspection from that measured thickness. This gives the thickness to use in the MAWP calculation, ensuring the pressure rating remains safe throughout the interval. For example, if the measured thickness is 0.50 in and you expect 0.05 in of corrosion before the next inspection, you would use 0.50 − 2×0.05 = 0.40 in in the MAWP calculation. Using nominal or original thickness would ignore actual thinning, and using just the thickness after corrosion allowance would not accurately reflect the current, in-service condition.

When continuing service, the MAWP calculation must reflect the pipe wall that is actually left, not what it was designed to be or what it was originally. You start with the actual wall thickness measured at the most recent inspection, and you apply a conservative adjustment for future thinning. Specifically, you subtract twice the corrosion loss that is expected to occur before the next inspection from that measured thickness. This gives the thickness to use in the MAWP calculation, ensuring the pressure rating remains safe throughout the interval.

For example, if the measured thickness is 0.50 in and you expect 0.05 in of corrosion before the next inspection, you would use 0.50 − 2×0.05 = 0.40 in in the MAWP calculation. Using nominal or original thickness would ignore actual thinning, and using just the thickness after corrosion allowance would not accurately reflect the current, in-service condition.