PRV monitoring is necessary for environmental protection compliance and can avoid expensive fines, and possible process unit or plant shutdowns. Monitoring also prevents waste of costly material and energy, and avoids bad publicity. Historically, Pressure Relief Valves (PRVs) have been difficult to monitor because they are simple mechanical devices by design. Monitoring methods typically have included manual inspections of tell-tale signs. Plants can monitor PRVs by observing process pressure, but when the system pressure is close to the operating limit, the peaks and valleys makes it difficult to determine when the PRV actually opened or closed. For these reasons, pressure measurements are susceptible to false positives and inaccuracies and provide no real-time insight into the health and status of the individual PRVs. Monitoring how many times PRVs activate and how long each was releasing product helps plant personnel understand processes better, but does not give visibility on leakages caused by PRV malfunction.
When the process pressure fluctuates around the PSV setpoint value, the blocking disc will lift to allow the chamber to fill and lift the stem.
The process fluid vents to the discharge pipe, reducing the pressure, but not opening the valve completely. This occurrence is called simmering. Simmering can also cause material buildup on the disc seating and create stem misalignment, which prevents the valve from closing completely. The discharge caused by simmering and it’s side effects are not usually detectable by conventional monitoring methods and account for considerable emission volume and consequent economic losses, aside from fines and eventual plant shutdowns.
In addition to the challenges of compliance monitoring there are other issues to contend with, such as PRVs that leak, don’t close and reseat after an event, or have their bypass valves in the wrong position due to human error. A significant part of the difficulty when designing and installing a comprehensive PRV monitoring system is that a typical plant will have several different PRV makes, models, sizes and operating pressures from various vendors.
