PRESSURE RELIEF MONITORING USING WIRELESS INSTRUMENTS

Challenge

Pressure relief valves are critical safety devices that act to protect equipment and personnel in the event of dangerous pressure build up in process piping and vessels. Normally closed, these devices are typically controlled by a spring force and have no auxiliary source of power. As the operating pressure approaches the opening point of the valve, the valve will simmer or weep much like a tea kettle before the whistle blows.

When the sealing surfaces are in good condition, emission and product loss is limited to the release of excess pressure, however as time goes on, a percentage of these valves will leak into the atmosphere or waste collection systems that normally go to flare.

These valves are usually mounted on the top of vessels and are often at inaccessible locations that require special equipment and safety procedures for insite inspections. The typical inspection approach is to schedule the removal and shop inspection of pressure relief valves on a time interval that is established based on historical or design criteria. It is common to find a large percentage of the valves fail seat leak testing at their scheduled inspection. 30% to 50% of valves tested do not pass pre-installation criteria during these tests. 5% to 10% of them are leaking so severely that they have become major sources of lost product and possible emissions.

Solution

Wireless sensor networks enable new best practices of pressure relief valve monitoring. In particular, the application of wireless acoustic monitors is very effective for a large component of the installed pressure relief valve population. The same sensors can also detect leakage through isolation and by-pass valves for many service conditions. While ultrasonic sensors provide a versatile, low cost method for detecting leakage, flow and release events, they are unable to distinguish the ultrasonic activity generated from leakage or flow from spurious noise that is generated by mechanical equipment in close proximity to the valve being monitored. The most effective combination of sensors for valve monitoring includes system gauge pressure at the valve inlet plus an acoustic sensor on thevalve body.

This combination provides additional data that confirms spurious mechanical noise as non-release events. If a short duration acoustic event occurs with no corresponding drop in pressure, then it is verified that pressure release has not taken place. Furthermore, with continuous pressure monitoring, it is possible to accurately calculate the mass discharged during a release event.

If acoustic activity above the normal background level is sustained without a corresponding drop in pressure, then it is an indication of valve leakage. The severity of leakage can be determined by evaluating the magnitude of the ultrasound level in conjunction with the valve inlet pressure. At higher pressures, the flow and the ultrasound level will be correspondingly higher. Low pressures, below 30 PSI, do not generate significant ultrasound for leak detection.

Source: Hydrocarbon Online Newsletter, February 18, 2011 - For more information see the following link: Pressure Relief - Wireless Instruments