The scope of a pressure relief system audit is to evaluate all equipment for the need for overpressure protection from potential sources of overpressure. API 521 addresses the potential sources of overpressure and is used as the guide to define the possible overpressure scenarios that need to be evaluated. Scenarios that are deemed 'not applicable' are described with supporting data as needed. All potential scenarios that can result in a relieving event are documented and calculated. The piping pressure drops for each scenario are evaluated at the maximum relief valve capacity and are used to determine the maximum capacity of non re-closing devices such as rupture disks.

Most of the required process safety documentation (OSHA 1910.119) is involved in or used as a basis for relief system design. As such the logical location for retention of these documents is with the relief system analysis and will be collected and assembled as a part of the relief system audit. The scope of work can be divided into three phases.

Phase 1:

1. Review P&IDs to establish general location of relief devices, preliminary identification of equipment data requirements
2. Develop pressure loop or protected system sketch showing protected equipment, associated equipment and instrumentation, main flows, and relieving devices
3. Assemble needed equipment information from plant records and files
4. Conduct field inspection of relief devices and associated equipment to obtain nameplate data. Field inspection information includes additional information such as equipment elevations, insulation type, and area drainage. Field inspection data is compared to file data and any conflicts are resolved
   • Develop piping pressure drop sketch (isometric) from the protected equipment to the flare header
   • Compare equipment nameplate data to records and identify any discrepancies
   • Load all required information into iPRSM® and run the calculations

Phase 2:

1. Review all possible causes of OVP and identify any applicable and non applicable scenarios
   • Review 18 standard causes of overpressure based on API 521 or a revised list based on specific plant operational concerns or procedures
   • Document for each non-applicable scenario the engineering logic that ensures overpressure cannot occur. Include any applicable calculations
   • Document basis for determining applicable required relieving rates in cases where overpressure is a possibility
2. Calculate required relief rates for each case thoroughly documenting any capacity credits or reductions taken
3. Calculate the required orifice areas, relieving capacity for pressure relieving devices and pressure drops for all relieving scenarios
   • Use API standard calculation methodologies and standard engineering practice as appropriate for the specified flow type
   • Two phase flow analysis using either Omega (API 520) or Client specific requirements in accordance with DIERs methodologies
   • Calculate actual relieving capacities based on actual orifice areas and manufacturer’s certified flow coefficients or for rupture disk type devices actual piping configuration and manufacturer’s certified flow resistance coefficient
4. Evaluate pressure-relieving system for inadequacies or deviations from code requirements or recommended practice
5. Document results of analysis

Phase 3:

1. Identify all global relieving scenarios for flare and header analysis. Normally include fire, instrument air failure, power failure and cooling failure
2. Review affect/cause relationships between failures to identify required relieving rates for affected equipment. (i.e. In the event of a power failure will cooing water be lost? What is the redundancy available and are periodic testing of spare emergency equipment adequate to reduce anticipated loads?
3. Prepare piping isometric or pressure drop sketch of header piping
4. Analyze potential pressure losses for defined global scenarios
5. Apply correct variable back pressure for each scenario to each affected relieving device
6. Recalculate corrected back pressures during relieving scenarios
7. Identify any relieving devices with excessive back pressure or those that capacity is affected to below the required capacity with the scenario specific back pressure from the header
8. 2-phase/flashing flow analysis where applicable
9. Define liquid loading possible on header or flare system
10. Evaluate liquid handling capability of flare knock out or vent disposal system
11. If requested, perform a flare stack/tip analysis, flare radiation study and vapor dispersion model