【2025 Safety Systems】SIS (Safety Instrumented System) and Safety Valves (Pressure Relief Devices)
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This post was last edited by The one on 2025-12-10 07:10. In process safety design, overpressure is a common risk scenario that can lead to severe property damage or injuries; it is therefore a top priority in preventing safety accidents in chemical enterprises. SIS (Safety Instrumented Systems) and safety valves (pressure relief devices), as typical protective mechanisms, play a key role in preventing and controlling the risk of overpressure in processes. As a preventive protection layer, SIS prevents dangerous consequences through interlocked actions. A safety valve relieves the effects of overpressure by releasing pressure, and serves as a protective mechanism for mitigation. Accidents must be prevented before they occur; they need to be stopped in their infancy! It seems that the functions of SIS are more in line with the concept of preventing problems before they occur, making it less costly and more advantageous to control accident risks. However, almost everywhere in the world, pressure vessels exposed to overpressure risks are required to be equipped with independent mechanical overpressure protection devices (such as safety valves), while there are few such requirements for SIS; companies can decide whether to install them based on their specific needs. The reliability of safety valves is much higher than that of SIS.
Factors affecting the reliability of safety valves and rupture disks as independent protection layers for IPL: As the final line of defense in a pressure-bearing system, pressure relief devices activate automatically when earlier protection measures fail, thereby releasing pressure and preventing explosions caused by continuous increases in pressure within equipment or pipelines. They play a crucial role in ensuring the safety of manufacturing processes. Commonly used pressure relief devices in industry include safety valves, vacuum break valves, rupture discs, and emergency pressure relief manholes.
In the protective layer model, the pressure relief device acts as an independent protective layer, preventing explosions by releasing pressure. However, as a relief IPL, although the pressure relief device can prevent overpressure, it cannot completely eliminate risks and may introduce new risk scenarios. For example, releasing flammable gases into the atmosphere may cause fires or explosions, requiring separate analysis and the implementation of protective measures.