Pressure-relieving devices (PRD) are vital components designed to prevent overpressurisation in pressure vessels and other related equipment by releasing steam, gas, liquids, or vapours. Out of the many types of PRD devices in use today, pressure safety valves and pressure relief valves are two of the most well-known and commonly used across various industries.
Both are designed to let pressurised media escape a system through a secondary passageway, effectively preventing pressure from building up beyond the application’s safe operating limits. However, while they virtually have the same purpose, there are some critical differences between the two and knowing which valve to use is essential to avoid dangerous consequences. Below, we delve into what sets them apart and how they each ensure proper pressure system operation.
Definition
A pressure relief valve limits pressure in a system to a certain level by releasing stored gas in proportion to excess pressure build-up. As such, these valves are typically installed in systems where pressure can rapidly increase and cause an explosion if left unchecked.
On the other hand, a pressure safety valve works to release gases instantly from a system that is already overpressurised to prevent potentially catastrophic incidents. These devices automatically activate before the pressure level exceeds an established threshold limit to ensure the equipment goes back to safe operating levels without delay.
Function
Pressure relief valves are a must in any system where preventing excessive pressure is vital to maintaining optimal functionality. In doing so, pressure relief valves also play a pivotal role in protecting equipment and preventing system failures. They are generally set to open gradually when the pressure level reaches a specific threshold level and only close once the excess pressure has been released.
Meanwhile, pressure safety valves are made for systems where overpressurisation could be catastrophic to the people, property, and processes in a facility. Therefore, these valves are likened to failsafes or last-resort options that release pressure to avoid severe damage to employees and systems should all other relief valves fail to effectively control the pressure in a system. Unlike their counterpart, however, safety valves are typically designed to stay open, and they must be manually closed after activation.
Setpoint
As mentioned, the two valves open when the pressure reaches a setpoint or a specific pressure level, but it should not be interpreted as the pressure set. This setpoint is measured in pounds per square inch and is based on a system’s maximum allowed operating pressure (MAWP) limit. Pressure relief valves generally cannot be used at maximal overpressure values equal to 3% of an application’s MAWP, which is in contrast to pressure safety valves that are made to operate beyond it.
Can the two terms be used interchangeably?
Having established the definition and differences between the two valves, one might wonder if it is still acceptable to use the terms interchangeably. Thankfully, industry professionals will know what you are referring to in most cases, regardless of which term you use. However, when working with both types of valves, it is important to use the correct term for each as they serve very different purposes. Doing so ensures everyone in the facility is on the same page, uses the right valve for its intended purpose, and follows the proper testing schedule.
Additionally, understanding the factors to consider when selecting a pressure relief valve or pressure safety valve is crucial to maintaining safety and efficiency in your operations.
Conclusion
Understanding the differences between pressure safety valves and relief valves is crucial for maintaining the safety and efficiency of any pressure system. While both devices are designed to release excess pressure, a pressure safety valve acts quickly to prevent catastrophic failure by opening fully in emergency conditions, whereas a relief valve gradually releases pressure to maintain safe levels under more controlled conditions. Knowing when to use each type of valve helps ensure the correct response to overpressure situations, protecting equipment, personnel, and the environment. Making the right choice between these valves is vital for achieving optimal safety and performance in your operations.
