Modern design of process plants will involve the use of overpressure relief devices (mainly safety relief valves and/or bursting disc devices) where considerations are taken to ensure proper and predictable functioning under all standard and upset conditions. It is most important to remember that a pressure relief device is a safety device employed to protect pressurised systems from catastrophic failures. Roger Bours reports.

One of the design criteria considered by the design engineer is the existence of 'special' operating conditions, such as superimposed or built-up backpressure, which could strongly affect the operational characteristics and flow capacity of safety relief systems.
A safety system typically is used as the ultimate limitation to protect pressurised equipment from exceeding its allowable limits or a means to prevent a potentially hazardous situation leading to injury.
It is essential to consider not only the pressure relieving devices or safety-related systems, but also the complete pressure relief system so as not to reduce the relieving capacity or adversely effect the proper and expected operation of the safety devices.
The safe operation of pressure systems in the industry is controlled through the use of a coordinated range of design measures:
* Regulating systems: typically a system designed to adjust or control a particular parameter to remain within predetermined limits.
* Monitoring systems: will periodically check a defined number of given operation parameters and highlights immediate attention to those which are outside of 'normal' operating limits.
* Safety systems: prevent the process parameters from exceeding the ultimate limiting values and ensures that risks for personnel injury and damage to the environment or property are eliminated or controlled.

Pressure limitation measures

Industry uses following types of pressure relief devices to ensure protection of installations subject to pressure:
* Reclosing Devices.
* Non-reclosing devices.
* Combinations of reclosing and non-reclosing devices.
The choice of best solution is driven by a number of individual parameters, both technical and economical.
The use of non-reclosing pressure relief devices will offer in most cases lower cost solution, but requires that the process is shutdown or redirected through alternative safety systems to allow for replacement of the burst device. Subsequently non-reclosing pressure relief devices will only be selected as primary relief solutions in cases where loss of process media, or shutdown for repair is tolerated. The selection of non-reclosing devices as secondary or backup systems is however a widely accepted solution.
Reclosing devices 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 can not be tolerated. The potential for leakage, fouling, plugging or icing can however render these critical devices inefficient.
Reclosing relief devices are mainly safety valves or relief valves (either direct loaded or pilot operated), whereas the non-reclosing relief devices are bursting disc devices. Combinations of safety valves and bursting discs are becoming most popular as they offer best of both individual solutions.
The most commonly used combination will be a design where the bursting disc evice is installed upstream of the safety or relief valve. In such a configuration the bursting disc device will provide 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 bursting disc devices on the downstream side of safety or relief valves may be considered in cases where:
* Corrosion or fouling of the valve trim may be a concern (a common problem in systems using common headers to evacuate process media) or
* Backpressure could occur on the downstream side of the safety or relief valve, changing the set pressure of the safety system.
In all cases where combinations of bursting disc devices with safety or relief valves are used, measures must be made to avoid that the space between the valve seat and the bursting disc are kept at atmospheric pressure.
Any increase of pressure in this cavity, due to for example temperature changes, minute pressure leaks, etc, will result in a dramatic and uncontrolled change in opening pressure of the safety system.

Alternatively in cases where pressure relief can not be applied due to environmental or safety issues, the use of controlled safety pressure relief systems (CSPRS) or safety related measurement, control and regulating devices (SRMCR) may be evaluated. Such systems will generally be developed to interact with the process to avoid the occurrence of situations possibly leading to unsafe conditions. Such systems will need to be carefully selected, taking into account guidance regarding safety redundancy specified in design documents such as IEC 61508 'Functional safety of electrical/electronic/ programmable electronic safety-related systems', IEC 61511 and ANSI/ISA S84.01.

Risk assessment and reduction

One of the most critical steps in establishing the appropriate role and settings of the individual systems will be the risk assessment for the process. Engineers will need to consider all possible service conditions to select the most appropriate safety concept to ensure safe operation under all conditions.
Identifying the potential hazards during operation must be done from a wide-angle approach; dangerous situations can occur due to many root cause situations.
Based on the results of the risk assessment, the pressure equipment can be correctly designed and the most effective safety system can be selected. Basically the process equipment shall be designed to:
* Eliminate or reduce the hazards as defined.
* Provide adequate protection measures if the hazards can not be eliminated.
* Inform the system user of the existence of residual hazards.
* Indicate the appropriate protection measures used, and
* Prevent misuse of safety systems as applied.
Under all circumstances preference will be given to inherently safe design solutions. Safety systems shall be designed to operate independent of any other functions, and shall operate reliably under all conditions determined by the risk analysis (including start up, shut down and maintenance and repair situations). l

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Roger Bours is Manager, Pressure Relief Products, Fike Europe Bvba, Herentals, Belgium. www.fike.com