There are several controls used in a facility in various ways that directly affect the relief system design or design basis, and are critical to the design of a pressure relief system.  While there are a host of inputs that affect the relief system design basis, the particular risk-based focus here is on controls that are used to justify non-credibility of an overpressure scenario, or significant reduction of a required relief rate, for which the existing relief system would have been otherwise significantly undersized.  The decision to permit the use of pressure relief critical controls in the relief system design should consider:

• The need to clearly and adequately define and characterize requirements related to the use of the controls within corporate/site practices
• The need to have measures in place to ensure correct implementation of such requirements, such as additional focused process hazard analysis
• The need to have a more stringent inspection / maintenance program, proper management of change, and systems / practices to maintain the effectiveness of the controls

Intermittently used pressure relief critical controls having precedence in RAGAGEP include the following:

• Specifically planned and trained operator emergency intervention
• Operating procedures and administrative controls
• Mechanical integrity and inspection processes
• Pressure relief critical components

“Pressure relief critical components” as used here is intended to include physical elements in the field, such as piping fittings, instrumentation, or equipment, and warrant further discussion. These pressure relief critical components may include the following:

• Control valve mechanical stops
• Check valves
• Bypasses around control valves
• Restriction orifices or nozzles
• Mechanical locking elements
• Valve failure controls
• Heat tracing or chemical injection for plugging prevention
• Safety instrumented systems (SIS)

We believe there are five elements that represent a best practice for dealing with these pressure relief critical components:

• The definition and use of these components, including what credits are allowed under what conditions as well as minimum expectations, are outlined in corporate or site practices
• The components are identified (specially tagged) and tracked, capable of being pulled into a specific list for maintenance and inspection activities
• The components are specifically (and easily) identified in the relief system design basis
• A P&ID note exists that indicates changes to the components require relief system consultation
• The MOC process has a specific line item dealing with the components

The foremost disadvantage of this best practice would be the apparent documentation requirements; however, this is often considered minimal as these components are not in common use (and thus there should not be many of them).

The attention paid to risk-assessment principles in the application and maintenance of critical controls used in the pressure relief design basis will reap dividends in the future with resilient integrity of the last line of defense against overpressure.