Corrosion damage: Repair versus fitness for service assessment
Inspection Engineer Ashfaq Anwer discusses corrosion damage on static equipment, and says some might be repairing equipment unnecessarily
All sorts of static equipment at petrochemical, oil & gas, refining or other energy operations require periodic inspection and, to some extent, at least one repair in their lifetime. Equipment installed for 30 years of design life could be threatened by at least one degradation mechanism, against which some sort of action is required. That action can be a repair or a fitness for service (FFS) study to keep the asset running with mitigation in the form of regular inspection monitoring. Less likely, but possible: A piece of equipment that requires no action to mitigate damage whatsoever during its life time.
For those pieces of equipment which get damaged due to a degradation mechanism, decisive action is required in order to mitigate the damaging effect and to bring the equipment back to its original specified form in order to function normally. The one major factor which controls the functionality of a piece of static equipment (including pressure vessels, heat exchangers and tanks) is the health of its pressure boundary.
Anything happening inside would also be important, possibly to a larger extent, but the one major factor to be taken care of is the pressure boundary envelope. This pressure boundary envelope can be attacked externally or internally, or could have an inherent defect which might open up over the course of its continuous operation, and yet remains masked in its operational history.
The pressure boundary of static equipment can be attacked externally by atmospheric corrosion mechanisms if uninsulated or by corrosion under insulation, if insulated. This corrosion under insulation (CUI) can act as external chloride stress corrosion cracking in cases where the pressure boundary is made of stainless steel.
Internally, there are many damage mechanisms which can attack the pressure boundary, depending upon factors like the operating fluid, its temperature, other chemical properties, pH, corrosivity and the material of construction.
Questions arise when there is an inspection finding on this pressure boundary from either the external or internal side. The main question: what caused the problem?
Important considerations are whether the pressure boundary was attacked by corrosion, which would eat at the metal and cause the issue, or if it might be stress corrosion cracking affecting the health of the pressure boundary. Finally, is it time to repair this fault, or perform an FFS study?
The latter cannot be answered until inspection determines whether the active damage mechanism is listed on Risk Based Inspection (RBI) or whether it is something unforeseen.
After this comes the significance of FFS evaluations. With the current circumstance and foresight of industrial operations in the oil and gas industry, the question of performing FFS has increased in value as it can lead to savings on inspection, maintenance and the overall operational cost of assets.
In cases where damage is well within acceptable limits, there is no need to perform repairs, but the definition of ‘acceptable limits’ has to be judged by qualified personnel.
Previously established practice used in many industries is still relevant today. In situations where repair would fix the issue and make the equipment as-new, repair would be the first choice for inspection personnel, and they would recommend the same without taking the earlier step of FFS assessment.
However, if the situation is not so clear, and FFS proves that the defect is within permissible size and dimensions, repair work should not be performed.
If the active degradation mechanism causing the defect is different than the one listed in RBI, then a quick Root Cause Analysis (RCA) session should be done, which should identify the root cause of failure. If not identified, then putting the equipment back in service would be acting ignorantly about its health.
Once identified, the root cause must be addressed and eliminated, followed by an FFS study.
However, if the active damage mechanism is listed in RBI data, then no action is required and operators can move to the next step, which is performing FFS to identify whether the defect is within the permissible limits or not. If the FFS study demonstrates that the defect is within acceptable limits, than a decision to repair the defect would not be cost effective, or necessary.
The cases outlined are true only in when a careful FFS assessment can be made in-house, or is not costly if done through outside resources. If that is not true, then operators might consider repairing the defect using the appropriate repair methods to bring the equipment back to its normal operation. It comes down to cost and expertise.
Operators need qualified, in-house resources to perform FFS and to decide what requires repair. Moreover, the in-house team should be able to perform assessments swiftly to avoid delay and potential downtime.
Facilities lacking FFS resources could be unnecessarily investing resources into repairing static equipment even when repair is not required.