Uprating an Ageing Air Receiver: Code Thickness Versus Local Pitting
A plant owner wants to raise the compressed-air header from 8 to 10 barg and has asked you to endorse the existing vertical receiver for the higher pressure. The records show: shell to SA-516 Gr 70, outside diameter 1219 mm, nominal thickness 9.5 mm, longitudinal seam spot-radiographed (joint efficiency 0.85), allowable stress 138 MPa, built in 2004 with a 1.5 mm corrosion allowance. Two ultrasonic surveys of the same band 200 mm above the bottom tangent line give minimum wall thicknesses of 7.1 mm (2012) and 6.2 mm (2016). Within that band there are isolated pits measured at 2.5 mm deep. The 2016 survey did not cover the heads or the nozzle reinforcement. There is no record of a pressure test after 2004, the safety valve is tagged 10 barg, and the owner wants the rerate plus a 10-year inspection interval.
1. Using the survey data, determine whether the shell band is acceptable at 10 barg, and state whether a 10-year interval is supportable.
Read the worked answer
Required thickness (ASME VIII Div 1, UG-27(c)(2), longitudinal stress governs because the longitudinal seam is the radiographed one):
t = PR / (SE − 0.6P) + CA
- R = (1219 − 2 × 6.2)/2 = 603.3 mm
- P = 1.013 MPa (10 barg)
- SE = 138 × 0.85 = 117.3 MPa
- t = (1.013 × 603.3)/(117.3 − 0.608) = 611.1/116.69 = 5.24 mm
The corrosion allowance has already been consumed, so 5.24 mm is the absolute floor for the corroded condition. The band average of 6.2 mm passes.
But the pits do not pass. Local remaining wall = 6.2 − 2.5 = 3.7 mm < 5.24 mm. The average-thickness check is not a valid assessment of local metal loss; a fitness-for-service assessment (API 579 Part 5 for local loss, Part 6 for pitting) or a repair is required before the vessel can be kept at 10 barg.
Interval: corrosion rate 2012–2016 = (7.1 − 6.2)/4 = 0.225 mm/yr. Remaining life at the band = (6.2 − 5.24)/0.225 = 4.3 years; half-life = 2 years. A 10-year interval is not defensible; the maximum interval is about 2 years, and the pits are on a shorter clock than that.
2. The owner says the shell 'averages 6.2 mm, so it is fine'. Set out the options you would put to the owner and what pressure you would actually certify.
Read the worked answer
The average thickness is not the governing number. The options, in order of preference:
- Fitness-for-service assessment. Run API 579 Part 5/6 on the pit field using the actual pit density and depth. If the remaining strength factor and the local thickness criterion are met, the vessel may continue at 10 barg with a documented, shortened interval.
- Derate to match the pits. Back-calculate the pressure that 3.7 mm of sound metal can carry:
P = SEt/(R + 0.6t) = (117.3 × 3.7)/(603.3 + 2.22) = 0.717 MPa ≈ 7.1 barg. Certify at 7.0 barg with margin. This is often the cheapest immediate answer and keeps the plant running while a repair is planned. - Repair. Weld build-up over the pit field or a insert patch, with a qualified WPS, PWHT if the original required it, and full NDT afterwards. This preserves the 10 barg rating.
- Replace. For a 2004 vessel with an active 0.225 mm/yr corrosion mechanism, replacement is usually the lifecycle answer - the pitting will recur unless the water carry-over from the dryer is fixed.
I would not sign a certificate at 10 barg on the 6.2 mm average. I would issue a certificate at 7.0 barg (or the FFS-qualified pressure) with an explicit 2-year interval, and require the cause of the internal corrosion (condensate, dryer carry-over) to be corrected.
3. Apart from the shell, what must be verified before any uprating certificate is issued?
Read the worked answer
An uprating is a change of the pressure envelope, not just a wall-thickness sum:
- Safety valve: capacity and set pressure re-established for 10 barg (plus the allowable accumulation), not merely retagged. The relieving rate must be re-based on the compressor/dryer discharge and any fire exposure.
- Downstream equipment rating: dryer, filters, headers, hoses, tools and the pressure-reducing stations must all be rated at the new pressure. The header is the weak point more often than the vessel.
- Nozzle reinforcement and heads: the 2016 survey did not cover them. UT thickness and reinforcement checks (UG-37/UG-32) are required before the whole vessel, not just a shell band, can be certified.
- Pressure test: a hydrostatic test at 1.3 × MAWP adjusted by the stress ratio (UG-99) - the 2004 test record is missing, which alone blocks certification.
- Statutory route: the uprating is a modification, so the written scheme of examination must be revised and the vessel re-examined by the Authorised Examiner before return to service.
- Cyclic life: if the header pressure now swings over a wider range, check the fatigue screening in UG-22/Div 2 Part 5.
Any one of these gaps makes the certificate unsupportable regardless of the calculation.