Figure 1. Comparison of probability of failure on demand (PFD): Valves with and without partial stroke testing (PST).


Gain Insight into Valve Performance

Wouldn’t it be great if our personal automobiles could run hundreds of thousands of miles without the unexpected surprise of a sudden mechanical failure? Fortunately, modern autos do monitor numerous critical systems and give forewarnings such as “check engine” or “service engine soon” warning lights, notification of high coolant temperature, low oil pressure, tire pressure problems, and much more. These warnings allow the motorist to be warned ahead of time about potentially damaging conditions, providing an opportunity to take action before a total breakdown occurs.

For boiler operators, a similar desire exists for longer periods of safe operation. But that goal is usually limited due to safety concerns, as the risk of failure on demand increases over time.

Unlike today’s modern autos with self-monitoring features, boiler operators sometimes find themselves lacking information about some of the critical combustion system components and how well they are able to protect the operation, plant assets, and personnel, should an unforeseen emergency condition arise.

Safety instrumented functions found on a boiler system normally consist of a sensor, a logic-solver, and final elements. The final elements are fuel safety shut-off valves in most applications, and they are documented to be the weakest link. Hence, the probability of failure on demand for a safety instrumented function is heavily influenced by the valve’s safety integrity level (SIL) performance.

HOW DO YOU KNOW IF YOU HAVE AN UNSAFE VALVE?

A safety shut off valve will typically prove itself unsafe only when it is required to function, i.e., in an emergency shut-down situation. At this point, prevention is no longer a choice, and the result can be catastrophic. As a partial solution to this problem and to ensure a basic level of safety, for example, NFPA 85 calls for redundancy - the use of two agency-listed safety shut-off valves for burner applications.

Agency approvals provide a reasonable assurance of valve performance, but cannot account for stressors present in a fuel delivery system, such as corrosive fuels or dirty instrument air, which degrades valve performance. These stressors increase the probability of failure on demand.

Figure 2. An example of Maxon’s patent-pending PSTrend functional logic.

IMPROVING VALVE PERFORMANCE

IEC 61508 and 61511 standards mandate a more quantitative approach for safety systems by determining failure probability through statistics. The adherence to a SIL for safety instrumented functions is not only required at initial installation but must be maintained over time.

As shown in the accompanying - Figure 1, which compares an untested valve with a valve having scheduled partial stroke testing (PST), one can see the increased potential for maximized boiler uptime while maintaining, for example, SIL 2 performance. This graph suggests that maintenance would be required within two years for an untested valve vs. six years for one with monthly PST testing1. In short, partial stroke testing allows an operation to run safer for longer periods of time.

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 which have remained in an open position for an extended period of time.

There are currently three known methods of partial stroke testing. One of these is the Maxon-developed PSTrend, a PLC-based system which pulses the power (on-off) to the valve solenoid, resulting in partial movement of the valve disk without interrupting burner management system functions. That is, this method of PST does not require shutting down boiler operation while testing the valves.

Maxon’s patent-pending PSTrend functional logic is depicted in Figure 2. The diagnostic test for a normally-functioning valve usually lasts less than 5 seconds, and that time depends on fuel pressure, the actuator spring force, instrument air pressure, and friction characteristics of the valve.

PSTrend tracks 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 easily retrofitted to an existing system. 

Figure 3. Cycling and leakage requirements for a typical 3-in. gas valve from various agencies.

PST CAN BENEFIT VALVE PERFORMANCE

Linear-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 and should be considered 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.  TB

REFERENCES

  •  ANSI / ISA 84.00.01-2004 standard
  • NFPA 85 Boiler and Combustion Systems Hazards Code, 2007 Edition
  • Paul Gruhn, P.E., Harry L. Cheddie, P.E. Safety Instrumented Systems: Design, Analysis and Justification, 2nd Edition. ISA.


Sidebar 1

IEC 14.11.3 & 4 mandates 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.

Sidebar 2 - What is an Agency Listing?

Valve manufacturers often seek agency (third-party) approval of their valves to meet local laws, codes, or insurance regulations.

Manufacturers submit their products for evaluation for form, fit, and function. This includes rigorous tests addressing construction, mechanical and electrical performance. Figure 3 shows cycling and leakage requirements for a typical 3-in. gas valve from various agencies.