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-reclosing 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. iConduct 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
   v: Develop piping pressure drop sketch (isometric) from the protected
   equipment to the flare header
   vi: Compare equipment nameplate data to records and identify any
   discrepancies
   vii: Load all required information into iPRSM® and run the calculations
   

1. Review all possible causes of OVP and identify any applicable and nonapplicable
   scenarios
   1. Review 18 standard causes of overpressure based on API 521
      or a revised list based on specific plant operational concerns or
      procedures
   2. Document for each non-applicable scenario the engineering logic
      that ensures overpressure cannot occur. Include any applicable
      calculations
   3. 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
   1. Use API standard calculation methodologies and standard
      engineering practice as appropriate for the specified flow type
   2. Two phase flow analysis using either Omega (API 520) or Client
      specific requirements in accordance with DIERs methodologies
   3. 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
   

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 backpressure for each scenario to each affected
   relieving device
6. Recalculate corrected backpressures during relieving scenarios
7. Identify any relieving devices with excessive backpressure 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