How to Work Around PSI Gaps When Sizing Relief Valves

A pressure relief device is the last layer of defense in chemical plants, refineries, and upstream facilities. Documentation of relief system design is required by OSHA to ensure safe plant operation. Before you begin the Pressure Relief Analysis (PRA) design or revalidation, gather all relevant process and equipment information. This Process Safety Information (PSI) may include P&IDs, PFDs, equipment data sheets, equipment drawings, U1 forms, H&MBs, and one-line diagrams. Data collection becomes an important role to perform PRA. Good documentation will help engineers to eliminate unnecessary risk during relief valve sizing; however, many facilities may not have complete and up-to-date PSI. Incomplete and out-of-date information can be confusing, put a facility at risk of citation, and create nuisance rework.

What do you do if there is a lack of information? Not every relief system can be done with complete and updated information. If you get into a situation where some of the information is not known, you may consider:

• Talking to the process team or operators at the facility to come up with reasonable assumptions;
• Making assumptions based on similar system/equipment and historical data;
• Becoming familiar with the standards, practices, or procedures from the facility;
• Becoming familiar with the codes and standards, such as API 520 Part 1 & 2, API 521, API 2000;
• Performing the analysis based on worst case.

The following are some examples of assumptions that may be made for pressure relief systems when the PSI is not complete:

• MAWP or design pressure of the system not known: MAWP or design pressure conservatively assumed to be same as set pressure of the relief device;
• Operating pressure of equipment not known: Operating pressure can be conservatively assumed to 90% of the set pressure (operating pressure is not recommended to be higher than 90% of the set pressure of relief device for stability reason – to minimize the risk of fatigue due to chatter or flow induced vibration);
• Hydrotest pressure not known: Hydrotest pressure can be assumed to 1.5 times MAWP (for equipment built before 2003) or 1.3 times MAWP (for equipment built after 2003);
• Fail open or close position of control valve not known: Fail position assumed to be the position that will provide worst case.

Upon completion of a pressure relief analysis, a list of assumptions and concerns should be included in the PRA deliverable for follow up. Each assumption should have an accompanying recommendation to verify the accuracy of the assumption with a follow-up action to either update the PSI if the assumption was correct or to update the PRA deliverable if the assumption was wrong. In conclusion, sound engineering judgement and reasonable assumptions are the keys to performing a quality pressure relief analysis (PRA).