Pressure Vessel Risk Based Inspection


A Risk Based Inspection is used to determine the interval of inspection, the decision of type of next inspection (whether it will be external, internal or on-stream) and the extent of next inspection. Normally a Risk Based Inspection Assessment based on the probability and the consequences of a pressurized equipment failure, is conducted to determine the risk. In this situation the pressurized equipment is kept under control or within the limits to reduce the severity of that risk. It shall include the evaluation of both the probability and the consequence of an equipment failure. 


1. Accessing the Probability of Failure
The assessment on the probability of failure shall consider the all types of mechanical damage; cracking, metal loss (either it is general or localized), creep, fatigue, embrittlement etc. The assessment shall include the determination and evaluation of equipment present condition and the effectiveness of corrosion monitoring program.

2. Accessing the consequence of failure
It includes the release of fluid which ensued from the equipment failure. The assessment shall consider all those consequences that may take place as a result of fluid release. So the consequences that could be expected are;
  • Fire                                                    
  • Explosion
  • Release of poisonous substances
  • Health impacts
  • Environmental impacts and pollution

After All the factors are determined in an RBI assessment, the upshot is used to define the interval of inspection and the following precautionary measures to minimize the probability and consequence of failure.
  • In-service Repair
  • Use of Inhibitors to prevent metal loss
  • Process Changes to reduce the severity of equipment failure

What can be defined using an Equipment Risk-Based Inspection?

Usefulness of RBI Inspection
The combination of both the assessments in an equipment risk-based inspection helps to define the following;
  1. The type of future inspection (external/internal/on-stream) to be included in the inspection plan.
  2. The appropriateness of non-destructive examination on the equipment.
  3. The parts of equipment that ought to be examined in the future inspection.
  4. The use of pressure testing if require
  5. The use of repair techniques (whether it will be permanent or temporary)

Role of a Certified Inspector
A certified inspector is accountable to the equipment user to ensure the maintenance and QA/QC inspection programs in accordance with appropriate design codes and standards. He is to perform the visual inspections in order to determine all the factors which are required in both probability and consequence assessments.

Comments
Once an RBI assessment is conducted on a vessel, it should be revised after every vessel inspection, process change and repair work and whenever an unpredictable breakdown takes place.

References
American Petroleum Institute API 510; 3.1.65, 5.2
American Petroleum Institute API 580 Section 1.1.1
American Petroleum Institute API 581

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