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The role of risk assessment in the design of pressure relief systems

4th November 2016

Posted By Paul Boughton


Pressure relief systems are the last line of defence against overpressure events in process plants. Roger Bours reports

Designing industrial processing plants involves the extended use of preventive measures to avoid hazardous situations.

Measures must be taken to prevent the pressures inside of processing equipment to exceed safe, acceptable levels. Where hazards such as excessive (over- or vacuum-) pressure cannot be avoided the use of pressure relief devices is common.

Pressure relief systems are the last line of defence against overpressure events in process plants. Ageing and modified infrastructure combined with a lack of on-site expertise has left many plants and refineries with relief systems that may place people and assets at risk.

Engineers are required to consider all possible service conditions (including start up and shutdown conditions) and select the most appropriate safety concept for safe operation under various conditions. It is known that most incidents tend to occur when the installation is not in its ‘normal’ running condition.

Based on the results of the applied risk assessment, the pressure equipment can be correctly designed and the most effective safety system components can be selected.

Basically the process equipment should be designed to:

* Eliminate or reduce the hazards as defined

* Provide adequate protection measures if the hazards cannot be eliminated

* Inform the system user of the existence of residual hazards

* Indicate the appropriate protection measures used

* Prevent misuse of safety systems as applied.

Once the best suited pressure relief system design has been finalized the most appropriate system components need to be determined. Industry uses the following types of pressure relief devices to ensure protection of installations subject to pressure:

a) Reclosing Devices

b) Non-reclosing devices

c) Combination of reclosing and non-reclosing devices.

Reclosing devices (usually safety valves or pressure relief valves) allow for continued operation, even when spurious overpressures occur.

Consequently, reclosing devices will be preferred for primary relief applications where long-term opening of the process equipment cannot be tolerated. However, the potential for leakage, fouling, plugging or icing can render these critical devices ineffective.

Non-reclosing pressure relief devices (rupture or bursting discs) will usually offer a more economical solution, but require the process be shut down or redirected allowing replacing a burst device. In many cases this is not an issue, and rupture discs are used as the primary relief solution.

In addition, the use of rupture discs as secondary or backup systems is a widely accepted solution.

Combinations of pressure relief valves and rupture discs are becoming increasingly popular, as they offer the best of both solutions.

The most commonly used combination is where the rupture disc is installed upstream of the safety or pressure relief valve. In such a configuration the rupture disc provides a pressure and chemical seal between the process and the downstream valve, resulting in reduced operational and maintenance cost (leakage, repair, corrosion, etc) and improved safety (no risk for polymerising or plugging of the valve).

The use of rupture discs on the downstream side of safety relief valves are considered to reduce corrosion or fouling of the valve trim (a common problem in systems using common headers to evacuate process media) or where backpressure could occur on the downstream side of the safety relief valve, changing the set pressure of the safety system.

In all cases where rupture discs are used in combination with safety relief valves, provisions must be made to prevent pressure build-up in the space between the valve seat and the bursting disc.

Conclusion

A pressure safety system is typically used as the ultimate measure (‘last line of defence’) to protect pressurised industrial equipment from exceeding allowable limits or as a means to prevent a potentially hazardous situation from leading to injury or catastrophic equipment damage.

Under all circumstances preference should be given to inherently safe design solutions. Safety systems should be designed to operate independent of any other functions, and they should operate reliably under all conditions determined by the risk analysis, including start up, shut down, maintenance and repair situations.

The choice of selected pressure relief system components has to be done taking into consideration all pressure relief system parameters (both geometry and operating conditions) present prior and during the relief event.

Roger Bours is with Fike









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