How Do You Know If You Have an Unsafe Safety Shutoff Valve?
May 1, 2012
Typically, a safety shutoff valve will prove itself unsafe only when it is required to function - as in during an emergency shutdown situation. At this point, prevention is no longer a choice, and a failure can be catastrophic.
As a partial solution to this problem and to ensure a basic level of safety, standards such as NFPA 85, the Boiler and Combustion Systems Hazards Code, calls for redundancy: the use of two agency-listed safety shutoff valves for burner applications.
The International Electrotechnical Commission (IEC) mandates functional safety devices as a part of overall safety equipment and systems. Safety Integrity Level (SIL) defines the level of risk reduction, on a scale of SIL 1 to SIL 4 (from least to most dependable) for safety instruments.
With regard to shutoff valves, IEC mandates that in order to maintain SIL 2 performance, the minimum hardware fault tolerance must be 1, supporting NFPA’s redundancy requirements (listed double-block valves) in combustion system fuel trains.
Agency approvals provide a reasonable assurance of valve performance, but they cannot account for stressors present in a fuel delivery system such as corrosive fuels or dirty instrument air, which can degrade valve performance. These stressors increase the probability of failure on demand.
What Is Partial-Stroke Testing (PST)?PST is a diagnostic technique used to test valves for dangerous, undetected failures that may be inherent in valves that have remained in an open position for an extended period of time.
Among the three common methods of partial-stroke testing is a PLC-based system that pulses the power (on and off) to the valve solenoid, resulting in partial movement of the valve disk without interrupting burner management system functions. One main advantage of this method of PST is that it does not require shutting down boiler operation while testing the valves.
The diagnostic test for a normally functioning valve usually lasts less than five seconds. (The actual amount of time depends on fuel pressure, the actuator spring force, the instrument air pressure and friction characteristics of the valve.) The PLC-based system tracks the results from periodic testing and looks for increasing test duration times that are indicative of degrading valve performance, possibly due to corrosion or other stressors.
This trending data can be made available through Ethernet for alarm and annunciation purposes, allowing boiler personnel to plan ahead for service or replacement of faulty valves.
Partial-stroke testing systems can be designed as an integral part of a combustion system or retrofitted to an existing system.
PST Can Benefit Valve PerformanceLinear-acting valves that utilize metal-to-metal seats - that wear in, not out, with use - can actually benefit from frequent testing. This self-cleaning feature makes corrective action possible for a degrading valve by simply increasing the test frequency, fully leveraging the valve’s “wear in, not out” characteristics.
Partial stroke testing is a proven, simple method for enhancing safe operation of a boiler system. It should be considered a part of every stringent safety program along with proper training, regular maintenance, and other recognized good safety practices. All of these inputs combine to reduce accidents, increase boiler uptime, and contribute to an increase in your operation’s bottom line.