Pressure relief valves are mechanical devices that are critical in safe plant operation. They are designed to open when the pressure in a process exceeds the design pressure of the system. When a pressure relief valve lifts, the process fluid is sent to a predetermined location. In certain process it is important to know when a pressure relief valve is lifting. Traditionally pressure relief valves are not automated and have no position monitoring, however there are new applications where position monitoring is proving useful. Traditional Pressure Relief Valves In critical process applications, it is important for operations to be made aware that a pressure relief valve is lifting. This is due to a number of concerns such as process upsets, potential environmental releases, flare knock out drum levels, etc. Finding a reliable and cost effective solution for this operation’s need is not always an easy task. Several industry standards include adding a flow switch or a temperature transmitter downstream of the pressure relief valve.
When the pressure relief valve lifts and the discharge piping from the PSV changes in temperature, the temperature transmitter allows the control system to alarm and informs operations of an issue. This application does work fine, however, temperature control loops are notoriously slow. It will take time for the process temperature to change the thermowell and RTD assembly within the discharge piping. In this time, it is possible that large volumes of process can be released. Position transmitters are typically not installed on non-critical pressure relief valves due to the amount of wiring that would be required. Pressure relief valves are generally mounted quite high, which would lead to expensive wiring costs. However, in critical applications, wiring is already nearby if a temperature transmitter, flow switch or other instrumentation is being utilized to detect a pressure relief valve lift. Smart Pressure Relief Valves There are new technologies that can be used to aid pressure relief valve monitoring. One method is to add a wireless position monitor, such as the Fisher 4320, to the pressure relief valve.
When using a position monitor, operations will know the moment the valve has lifted. If the facility has a wireless control network available, wireless position monitoring can be used. Wireless position monitors are designed to be easy to use and provide position monitoring in real time. Wireless position monitors are cost effective because there is no cost of wiring and they allow critical valves to be connected to the control system. Maintenance of valves will likely be minimized due to more accurate monitoring of the valve.
Installing position monitoring on valves will also aid in identifying chattering valves. Chattering is defined as the rapid opening and closing of the valve, which can result in misalignment, and valve seat damage. If this continues, mechanical failure of the valve internals and associated piping can occur. Pressure relief valve chattering can be caused by many issues, such as an excessive inlet pressure drop to the valve, excessive backpressure in discharge piping, and an oversized relief valve. Chattering may not lead to enough process being released to trip a differential temperature alarm or flow switch while still damaging the valve. Once valves are identified appropriate measures can be taken to replace damaged valves and make changes to prevent further problems.