There are many ways to manage pressure in production processes. A common way for handling over-pressure conditions is through the use of relief valves. Wikipedia defines a relief valve as:
They are designed:
Emerson’s Jonas Berge authored an article for Control Engineering Asia, Cut Your Losses with Relief Valve Monitoring.
…to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. When the set pressure is exceeded, the relief valve becomes the “path of least resistance” as the valve is forced open and a portion of the fluid is diverted through the auxiliary route. The diverted fluid (liquid, gas or liquid–gas mixture) is usually routed through a piping system known as a flare header or relief header to a central, elevated gas flare where it is usually burned and the resulting combustion gases are released to the atmosphere.
While these devices perform the function of relieving overpressure conditions, they can sometimes not completely seal close properly. Jonas notes:
After a release, it is not uncommon for the relief valve to not seat back properly. This causes the valve to pass internally, allowing valuable product, feedstock, or other material to escape unnecessarily during normal operation.
Given the number of relief valves that may be present in a process manufacturing or production facility these leaks can be difficult to find, especially when they are piped into a common flare system.
Jonas describes typical maintenance practices:
…to routinely pull relief valves into the workshop for inspection and overhaul, without much information about the expected condition of the relief valve.
Not only can this be labor intensive, but the relief valves may be located in difficult to access locations. One improvement to this preventive maintenance practice is to conduct:
With the introduction of wireless instrumentation over the past many years, continuous relief valve monitoring is now possible and feasible with wireless acoustic transmitters. Installing these devices in close proximity to relief valves allows them to:
…manual leak testing with a portable acoustic tester to survey the relief valves in-situ…
…detect and report both relieving events and leaking relief valves typically every minute or as quickly as once per second if needed.
Since these transmitters provide continuous monitoring, there:
…is no need to send personnel into the field to perform these diagnostics and there is no need to remove the relief valve or take it out of service. The relief valve remains in-situ, no shutdown or bypass is required, and it has no impact on the process.
From an installation standpoint:
…acoustic transmitters are non-intrusive, they are very easy to install and a low risk to deploy. The sensors clamps on to the outside of the valve using metal clamping bands or a bracket. There is no cutting, drilling, or welding required. No process penetrations are created.
And, the complexities associated with wiring are avoided, especially for installations in hazardous areas.
Jonas highlights the benefits of continuous monitoring:
…ability to detect leaking relief valves immediately and service them early to stop losses much sooner than was possible in the past.
…maintenance personnel spend less time pulling relief valves or manually testing for leaks, and can instead focus on repairs of those relief valves that are found passing.
…allows maintenance personnel to schedule overhaul of relief valves when they need it, no sooner, no later.
He highlights other advantages including improved turnaround planning, data-driven condition-based maintenance, fewer emissions and more.
Read the article for more on integration of these transmitters into plant automation and asset management systems, shared infrastructure with other wireless measurements, and the advantages of the IEC 62591 WirelessHART communications standard.