Pressure relief valves are everywhere in process plants. Anywhere a system is closed, there has to be a way for pressure to escape in an emergency. Following piping around a process unit will invariably yield multiple PRVs of various sizes and configurations. The problem is, you can’t tell if they’re working properly just by looking at them.
For someone trying to improve the performance of a plant or process unit, this is a serious problem, and Marcio Donnangelo examines the situation in his article in the November 2018 issue of Chemical Engineering, Wireless Monitoring for Pressure Relief Systems. So why is PRV performance a problem?
Pressure relief systems might release the contents to atmosphere if the products involved are environmentally benign, but more often, individual relief points feed into a system to collect liquids, while sending gases to a scrubber, recovery system or flare. Any product released will typically be unrecoverable or recovered for a less noble application, such as feeding energy to boilers and furnaces. This loss adds to the cost of an incident, along with possible environmental consequences and fines. Since the header connection covers the outlet of either type of device, it may not be clear to operators trying to troubleshoot the situation which valve or disk has opened if the answer is not identifiable from normal process operating information.
So, a valve that is operating within its rated pressure but not shut positively is wasting product at least, and contributing to a safety or environmental incident at worst. Some maintenance people might be able to tell if it is closed or not, provided they can be right next to the valve, assuming such is practical and safe. But how can you tell otherwise?
Acoustic monitoring devices are now available that are designed for mounting directly to pipes adjacent to valves, PRVs, steam traps and other common fluid handling equipment. They capture sounds transmitted directly through the metal at high frequencies, above human hearing limits, so ambient noise is not a concern. Fluids flowing through the devices mentioned make various sounds, particularly when some part of the mechanism is affected.
So, the sound gives away a malfunctioning PRV, just as it does for a steam trap. (Read a different article on how acoustic transmitters can be added to steam traps.) The acoustic transmitter, a Rosemount 708, sends its information via WirelessHART to the control room, maintenance office or other location, so technicians can respond appropriately. A Rosemount Wireless Pressure Gauge can also be used to check the condition of a rupture disk used ahead of a PRV to make sure no pinhole leaks have developed. Marcio goes into more detail, so give the article a full read. As he sums up:
With prompt action, the overall effectiveness of the unit’s safety system can be assured while avoiding product loss and potential environmental consequences. Many equipment condition monitoring devices are now available that can help determine how well many types of assets are functioning. This information can help optimize maintenance efforts and avoid costly unplanned shutdowns.
You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the WirelessHART and Device Diagnostic Groups and other specialty areas for suggestions and answers.
Posted by Deanna Johnson, Rosemount Measurement & Analytical Global Marcom Manager, Emerson Automation Solutions
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