Qualitative RBI of a Commercial Refrigeration System

In light of Mike Hurley's recent post on Process Safety Management (PSM) and Commercial Refrigeration, I wanted to share the scope and results of a Qualitative RBI study employed to prioritize inspection and repairs for a commercial refrigeration unit ahead of a refurbishment project.

The client used a redundant refrigeration system to maintain product storage temperatures within a relatively tight tolerance that was required by the FDA. The system was fourteen (14) years old and had been maintained per the manufacturer's recommendations. Manufacturer's recommended maintenance did not account for the two (2) biggest damage mechanisms affecting the refrigeration unit: the water quality of the system and corrosion under insulation (CUI). Failures were already occurring in the process, and the client needed the highest level of reliability in this system to avoid product spoilage.

A qualitative RBI Assessment was recommended to address the needs of the client. The equipment was divided into basic components for a total of ninety-three (93) components. A non-standard risk matrix with three (3) criticality levels was developed based on the needs of the client.

Probability of Failure (POF) was determined by using the following:

• A simple hoop stress equation for each basic component.
  • Component specific structural tmin assumptions were assigned
  • Conservative assumptions were needed as the manufacturer's data was lacking in some areas.
• Internal thinning rates were calculated based on known values where available. Estimated rates from similar service and/or a Corrosion Specialist were used where no historical data was available.
• A standard External CUI rate was used for all components susceptible to CUI with no credit given for paint.
• The Remaining Life (RL) was then calculated and a factor assigned based on its value.
• Unique damage mechanisms (UDM) were assigned a qualitative POF factor based on a methodology developed with the client.
• Probability was finally assigned by using the more conservative factor (RL vs. UDM).

Consequence of Failure (COF) was determined with the client for each component. A methodology was drafted to capture different impacts (Safety, Environmental, System Operability and Storage Room Operability). Each component was then ranked from High to Low based on five (5) different criteria for each impact.

Risk was then defined for each asset by merging POF and COF, the results of which had shown:

• 65% of the equipment reviewed was ranked as low probability and 45% was ranked low criticality.
• Six (6) fixed equipment assets and seven (7) piping assets required immediate insulation removal for CUI inspection.
• Five (5) exchangers required immediate replacement due to water side corrosion. The small size of the exchangers meant inspection and repair were not cost effective.
• Internal inspections were required on water side components to validate estimated rates and install sacrificial anodes.
• Equipment replacement was required due to unique damage mechanisms.
• High Consequence piping was recommended to have more rigorous inspections and examinations to prove the findings of this assessment.

The application of qualitative RBI helped this client prioritize equipment damage mitigation in order to effectively scope a refurbishment project and achieve their system reliability goals. This scenario highlights a significant problem for some refrigeration system owner/users who may be maintaining their assets as prescribed but are unaware of the damage mechanisms that could affect the reliability of their assets and safety of their personnel.

We have found that refrigeration systems are often overlooked as their integrity is typically managed by personnel outside of the compliance-related, asset integrity industry.

• Depending on the refrigerant being used, integrity management is required by compliance agencies.
• Depending on the nature of the owner/user's industry, they may not be aware that certain assets require compliance management.

Qualitative RBI is just one of the tools we can use to help owner/users better understand and effectively manage their asset integrity and reliability.

Are the damage mechanisms properly identified and mitigated for your refrigeration assets?

If your answer is not a confident "Yes", read more about how our Risk Based Inspection Services can help develop, implement or improve your Asset Integrity program.

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