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RSSGuidelines For Use Of Wireless Instruments For Valve Monitoring
September 29, 2009
•Application Note: Guidelines For Use Of Wireless Instruments For Valve Monitoring
By Control Microsystems
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. Therefore, these valves require a margin between the Maximum Allowable Working Pressure (MAWP) of the vessel or piping and the system operating pressure. Relief valve manufacturer's normal recommendation is that system pressure should not exceed 90% of valve set pressure (opening point), which is usually equal to MAWP. As the operating pressure approached 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 release of excess pressure, however as time goes on a percentage of these valves will leak to 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 in-situ 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. With the cost of energy and petroleum products reaching new high levels and air quality becoming an increasing area of concern, continuous relief valve monitoring can be a source of improving productivity and achieving compliance with environmental standards. Wireless sensor networks enable new best practices of continuous 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.
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