Turnaround elbow: remaining life against an inspector's hold
At a crude unit turnaround with nine days left, a 10-inch long-radius elbow on the column overhead line was profiled by phased array. Nominal wall is 12.7 mm; the ASME B31.3 minimum required thickness is 7.1 mm; the lowest reading is 9.4 mm. The same CML read 9.9 mm eighteen months earlier. The unit inspector wants the elbow replaced before start-up and has written a hold; a replacement elbow is on six weeks' lead time. The RBI assessment places the circuit at medium risk with a four-year inspection interval, and the CML has no commissioning baseline. Operations says the flare system must be back in service on schedule.
1. Calculate the corrosion rate and remaining life, state the inspection-interval consequence, and give your conditional decision on continued service.
Read the worked answer
Rate. (9.9 - 9.4) mm over 1.5 years = 0.33 mm/y.
Remaining life. (9.4 - 7.1) / 0.33 = about 6.9 years to the minimum required thickness.
Half-life rule. Half of 6.9 is about 3.5 years, which is shorter than the four-year interval in the RBI plan. The interval as written is not valid and must be reduced - two years is the defensible number for this CML.
Add the measurement uncertainty. At ±0.1–0.2 mm on each reading, the true rate could be anywhere from near zero to about 0.5 mm/y. The pessimistic end still leaves roughly 4.5–5 years, so the elbow is not at risk of failing before the next planned opportunity.
Conditional decision: the elbow may remain in service to the next opportunity, provided that
- the full elbow is profiled on a proper grid, not just at the one CML, before the unit restarts;
- the interval is reset to two years and the RBI record updated;
- the finding is documented with the actual data, signed by the responsible engineer, and the inspector's hold is formally dispositioned rather than left open.
A six-week lead item on a 6.9-year remaining life is not the right use of the shutdown.
2. Give at least three reasons why a single 9.4 mm reading at one CML may understate the risk on this elbow, and what you would do about each.
Read the worked answer
The CML may not sit on the thinnest point. Overhead-line elbows usually thin at the extrados from droplet impingement and at the intrados from the liquid pool; a spot reading on either flank misses both. Profile a full grid around the elbow, especially the extrados and the 6 o'clock of the downstream straight run.
The two readings may not be the same point. Without a permanent marking and a recorded scan index, the 'same CML' can drift a few centimetres and manufacture an apparent rate - in either direction. Verify the physical stamp and re-establish the datum before trusting the trend.
There is no commissioning baseline. The 9.9 mm itself may already reflect 25 years of loss, and the service condition may have changed - higher throughput, a wash-water change, or the ammonium bisulfide dew point moving upstream. Check the operating history, not just the metal.
Localised mechanisms hide in averages. Phased array on a coarse grid can miss a narrow, deep pit. Complement with radiography or a denser scan where the profile shows irregularity.
What I would do: re-profile the whole elbow, compare the pattern with flow and dew-point data, and treat any isolated deep indication as the governing feature in the fitness-for-service assessment rather than the average wall.
3. How do you resolve the disagreement between the inspector and engineering, and what do you tell operations? Write the decision path and the record you would leave.
Read the worked answer
Separate the roles. The inspector's duty is to raise the condition and put the hold in writing; that is correct and should never be discouraged. The decision, and the accountability for it, belongs to the responsible engineer with the data. The disagreement is not personal and should not be settled by seniority in the room.
Decision path
- Table the rate, the uncertainty, the FFS basis and the four-year interval problem together, in one meeting with the inspector present.
- Get the additional inspection done inside the remaining nine days - it is cheap and makes the argument evidence-based rather than positional.
- Record the outcome as a formal disposition with conditions: re-inspection interval, monitoring owner, next review date.
- If the additional inspection cannot be completed in the window, take the conservative path - hold the line back or insert a temporary engineered repair - and replace at the next opportunity.
What operations needs to hear: the remaining life, the reduced interval, what will be monitored and by whom, and the residual risk in plain terms. Not a lecture on procedure.
Leave in the record: the raw readings, the calculation, the inspector's dissent, the additional inspection results, the sign-off and the revised RBI entry. If it is worth deciding, it is worth being auditable.