State-of-the-art design of industrial processing plants involves the extensive use of preventive measures to avoid hazardous situations. Specific measures must be taken to prevent the pressures inside of processing equipment from exceeding safe, acceptable levels. The use of instrumentation to intervene where pressures may exceed the acceptable threshold is a common approach.
The objective of plant operators is to maintain maximum operating time without unplanned interruptions due to excessive pressure incidents. Despite the use of highly sophisticated risk assessment modeling, the risk remains that critical safety instrumentation may not be functional when needed. Where hazards such as overpressure or vacuum pressure cannot be avoided, pressure relief devices are commonly used to protect the pressurized systems from catastrophic failure.
Managing the Perils of Excessive Pressure
Pressure relief systems act as the last line of defense against overpressure events in process plants. Aging and modified infrastructure, combined with a reduced level of on-site expertise, has left many plants and refineries with relief systems that may place people and assets at risk. With tightened budgets, lower resources and a growing list of ‘must do’ issues in maintenance, relief systems are unfortunately often not given the required priority.
One of the most critical steps is assessing the risks in the process. All possible service conditions need to be considered and then the most appropriate safety system configuration can be selected. The identification of potential hazards must be approached from a wide-angled perspective, as dangerous situations can arise from many root cause situations. Based on the results of risk assessment, pressure equipment can be correctly designed and the most effective safety system components can be selected. The process equipment should be designed to:
- Eliminate or reduce the defined hazards
- Provide adequate protection against hazards that cannot be eliminated
- Inform the system user of any existing residual hazards
- Indicate the appropriate protection measures and
- Prevent misuse of applied safety systems
API 521 Update Influences System Design
In January 2014, The American Petroleum Institute (API) published significant changes to its 521 standard “Pressure relieving and depressuring systems”. The Occupational Safety & Health Administration (OSHA) considers API 521 to be the Recognized and Generally Accepted Good Engineering Practice (RAGAGEP) for pressure relief system design. In the past, industry practices too often involved relying on operator intervention or standard instrumentation to stop an overpressure event. One of the lessons from the reported incidents is that the industry should not rely solely on operators or potentially unreliable instrumentation to eliminate an overpressure scenario. If you would like to find out what the most important changes made to API 521 were in 2014, please read the whole white paper.
The determination of the best solution for protecting industrial processes is done by considering a number of technical and economic parameters. Some relief devices will offer the best performance only where specific operating conditions are present. Deviations from these conditions will potentially result in unexpected performance reductions.
Different types of pressure relief devices are used to ensure protection of installation subject to pressure:
- Reclosing devices
- Non-reclosing devices
- Combination of both
Reclosing devices (usually safety or pressure relief valves) allow for continued operation, even when occasional overpressures occur. The problem with these devices is that they may be inefficient in applications where leakage, fouling, plugging or icing are a concern.
Non-reclosing pressure relief devices (rupture discs) tend to be the more economical solution, though they require that the process be shut down or redirected while replacing a burst device. They are used as the primary relief solution, but also as a secondary or backup system.
A combination of pressure relief valves and rupture discs is becoming an increasingly popular option because it delivers the advantages of both solutions. The most commonly used combination is where the rupture disc is installed upstream of the pressure relief valve. The rupture disc provides a pressure and chemical seal between the process and the downstream valve. This reduces operational and maintenance costs due to leakage, repair, corrosion, etc., and it improves safety by avoiding the risk of polymerizing or plugging of the valve.
The use of rupture discs on the downstream side of pressure relief valves is considered in cases where corrosion or fouling of the valve trim is a risk or where backpressure could occur on the downstream side of the valve.
Safety is the Bottom Line
A pressure safety system is typically used as the ultimate measure – the last line of defense – to protect pressurized industrial equipment from exceeding allowable limits. It also provides a means to prevent a potentially hazardous situation from leading to injury or catastrophic equipment damage.
In order to ensure the proper and expected operation of safety devices as well as sustainable relieving capacity and to provide the required level of safety and minimize downtime when designing a pressure relief system, it is essential to consider not only the pressure relief devices, but the entire pressure relief system and all process risks.