Design Considerations
for Pressure Relief Systems

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 pressurized systems from catastrophic failures. 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 affects the operational characteristics and flow capacity of safety relief systems.

A safety system typically is used as the ultimate limitation to protect pressurized equipment from exceeding its allowable limits or a means to prevent a potentially hazardous situation leading to injury. Overpressure protection is applied in industrial installations to ensure adequate safety levels are met, and investments are secured against the effects of unacceptable pressure levels.
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. Indeed, it has been known that operating problems can often occur in pressure safety systems because of incorrect selection of the appropriate device or because a correctly selected device was adversely affected by improper handling, incorrect installation, lack of maintenance or incorrect/incomplete design considerations.

Use of Regulating, Monitoring and Safety Systems in the Process Industry
The safe operation of pressure systems in the industry is controlled through the use of a coordinated range of design measures:
a) Regulating Systems: this is typically a system designed to adjust or control a particular parameter (pressure, level, temperature, etc) to remain within predetermined limits.
b) Monitoring Systems: these systems will periodically check a defined number of given operation parameters and highlights immediate attention to those which are outside of “normal” operating limits. Normal operating conditions of the process can be restored either directly through the actions of the monitoring system, or through the intervention of an operator.
c) Safety Systems: a system which prevents 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.

Role of Process Risk Assessment and Risk 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. This requires a realistic assessment of the risks by means of risk analysis and risk evaluation. A graphical presentation of the essential steps is presented in Figure 2:
Identifying the potential hazards during operation must be done from a wide-angle approach; dangerous situation can occur due to for example
• operational conditions
• human errors
• unreliable functions
• unsafe process loading
• maintenance
• physical characteristics of the process media (adhesion effects, exothermic reaction, etc)
• chemical characteristics of the process media (corrosion, toxicity, stability, etc)
• specific site conditions (vibration, wear, external fire, etc.)
Each of these factors can result in the pressure exceeding the defined pressure limits of the equipment.
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. As a general rule it will be inevitable to include safety systems such as pressure relief devices in the design of the process equipment. Pressure Equipment Legislation will generally not allow otherwise. Safety systems shall be designed to operate independent of any other functions, and shall operate reliable under all conditions determined by the risk analysis (including start up, shut down and maintenance and repair situations).

Pressure limitation measures:
Industry uses following types of pressure relief devices to ensure protection of installations subject to pressure:
a) Reclosing Devices,
b) Non-reclosing devices, and
c) Combinations of reclosing and non-reclosing devices.
The choice of best choice is driven by a number of individual parameters, both technical and economical. Most applications can be either categorized as primary or secondary relief application; in the case of a primary relief function the pressure relief device will be used as the sole means of over-pressure protection (figure 3). In cases where reasonable doubt may exist that the primary relief device may not function, or may not cope with the amount of pressure generated under special conditions, an additional pressure relief device may be installed (figure 4). The set-to-open pressure of the secondary device will be higher then the set pressure of the primary device, but under no circumstances the set pressure can exceed the design pressure of the equipment to be protected.
The use of non-reclosing pressure relief devices will offer in most cases lower cost solution, but requires upon activation 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 devices is installed upstream of the safety or relief valve (figure 5). 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 polymerizing 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. The use of simple breather devices (see Figure 6) offers an acceptable solution.
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 " xxxx " and ANSI/ISA S84.01 "yyyyy".
Table A provides an overview of the general advantages and disadvantages of the individual pressure-limiting device.
If none of the pressure safety options can be selected for the application then the equipment shall be designed to withstand the maximum possible pressures.

Specific design considerations:
The safety system design and selection of most suitable pressure relief device must be done taking into consideration all operating conditions likely to exist during the lifetime of the pressure equipment.
One of the often-underestimated issues that may lead to unexpected lack of performance of safety systems is the effect of superimposed backpressure on safety relief devices.
Laboratory tests on an extensive range of commercial safety valves, ordered specifically for the purpose of operating under backpressure conditions, show a difference between the performance guaranteed by the manufacturer and the actual valve performance. This difference may be so great that the protected equipment might operate at pressures higher than the maximum allowable pressure. The application of specially developed balancing bellows can assist but does not solve the problem. Inlet piping to pressure relief devices shall be as short and straight as practical, and in the case of using safety or relief valves the pressure drop to the valve inlet shall not exceed 3% of the set pressure of the valve. The total inlet pressure drop is calculated using the actual flowing capacity of the selected valve, and shall include for any effects of a combination with bursting disc devices or other components. A basic requirement of an inlet pipe design for pressure safety systems is that the equipment must be protected against excessive pressures at all times during operation; the use of isolation valves in the inlet or outlet pipe configuration is, with few specific exemptions, strictly forbidden.
An important area of attention is the design and configuration of outlet systems. The discharge of pressure and media must be done with appropriate level of safety, suitable preventive measured must be made to avoid unintentional flow into other connected equipment (such as installation under maintenance or out of service) and means for access, inspection and drainage must be made. Discharge pipes must be as straight and short as possible and have a diameter adequate to evacuate the (combined) capacity of the connected relief devices. Finally the design of the discharge pipe shall be such that the discharge velocity shall be subsonic. <<