We hear it all the time: Doctors recommend that everyone exercise on a regular basis to stay healthy. The same can be said for process valves found in numerous industrial applications.
Valves are utilized in systems because there is a need to control, allow or block flow. Like individuals, valves require regular movement to maintain peak performance. Valves do not like to remain in any one position for extended periods of time. When this occurs, the likelihood of the valve malfunctioning in a time of need increases dramatically.
Without proper periodic cycle testing, there is a probability that when the operator needs the valve to perform, it may fail. An inoperable valve can affect the performance of the entire system for which it is designed. When a situation arises that requires an operator to isolate a valve, users report that, on average, 20 percent of all valves are deemed inoperable, or reported as needing some form of maintenance.
Many factors can affect a valve’s safe operation, and one of the major causes is lack of proper maintenance. Chemical treatments, line sediment, particle shedding, corrosion, instrument air quality and unsatisfactory electrical connections are just a few issues that can affect a valve and, therefore, the system performance. Oftentimes, the plant has no intention of cycling a valve, or system restrictions prevent the valve from being operated for extended periods. As a result, many valves remain in either the open or closed position for prolonged periods. This time can extend from months to — in some cases — years.
The average process distribution system utilizes several types of valves found in various locations and installation positions. The typical system operates trouble-free with little or no preventive maintenance required until there is a system fault. Once a fault is detected, preventing downtime and servicing the equipment become priorities.
Many justifications are offered as to why manufacturing plants consider modern process systems as automatic. Most systems are designed for continual operation. Once the system is properly programmed and is operating at peak performance, the PLC monitors the system operation automatically. The pumps, temperature and pressure sensors, flow rate monitors, automatic on/off valves, modulating valves (in throttling service) and related subsystems work in harmony and are designed to deliver reliable trouble-free operation.
With the system in continual operation, it is understandable that operators and maintenance teams do not want to include cycling testing valves as part of periodic maintenance. When the system requires maintenance, or an emergency occurs that requires a change in valve position, however, it is often then that inoperable valves are discovered. This is not the time to discover you have a valve that will not operate. The system maintenance plan must be immediately upgraded to include a valve repair or replacement — and whatever prompted the valve operation must be dealt with as well.
Consider implementing a valve-exercise program into your facility. The benefits can increase productivity and reduce downtime
Putting Planned Valve Maintenance to Work
A common question asked by valve users is: How often should a valve be cycled? Let’s address this concern and offer an outline for a planned-maintenance program. When properly implemented, the following recommendations can improve system reliability and performance.
Start by evaluating the complete distribution system and identify each valve within the process. Review and understand the valve manufacturers’ operation and maintenance procedures for various valve types.
Regular and consistent execution of a preventive maintenance program should be ensured. Consider regular inspection and exercising all the valves in the system. Prioritize work by selecting the valves that are crucial to the system’s operation and work toward the lower impact packages. It is recommended that every valve be cycled fully open and closed on an annual basis — and even more frequently, if possible. If a reduction in time between cycle testing is allowable, then adjust the cycle-testing schedule accordingly. Simply exercising the valves on a regular basis will improve the system’s reliability, performance and life expectancy.
The following are suggestions to follow before exercising the valve:
- Safety first. Ensure all lock-out/tag-out procedures are followed.
- Be sure the valves casing is clean. External debris buildup can allow dirt or trash to enter through seals or cavities and cause complexities or even failure in proper operation.
- Gear operators, lever operators and various types of automation equipment should be cleaned and free of excess debris.
- Inspect all electrical connections. Water ingress is one of the leading causes of system failures.
In addition, have any questionable connections inspected and repaired by a qualified electrician. Ensure all local electrical codes are followed and that lock-out/tag-out procedures are implemented.
Once the visual inspection is complete, communicate with the control room, reliability engineers or maintenance managers to create a plan to safely operate the valves. In some cases, a valve cannot simply be opened or closed during normal operation. Other valves can be easily operated without any safety risk or system damage. Communicate as a team, formulate and work your plan.
This diaphragm valve is shown with an external electro-pneumatic positioner, which is used to control pneumatically operated process valves with single- or double-acting linear or quarter-turn actuators and to detect the position of the valve.
Begin the exercise program with any valve deemed suitable for safe operation. It is recommended to have open radio communications with an operator in the control room and a technician standing near the valve to observe the operation. Have the control room operate the valve through one complete open and closing cycle. Note any abnormalities during this exercise and keep accurate records as you move through the system. Manual valves also should be cycle tested during this time. A valve that is not properly sealing in the closed position will leave tell-tale signs. The control room can assist with providing valuable data by observing flowmeter readings or reporting upstream and downstream pressure indicators. Limit switches, positioners and visual-position indicators should all be observed for proper operation and readout.
Switching focus to any valve that cannot be fully cycled presents another challenge. It is recommended that these valves also be tested for proper operation, but how to safely address this concern?
Before beginning, have a complete emergency plan in place. If the valve is not responding to any input signal, immediately stop all testing. Then, determine and correct the cause of failure.
This butterfly valve has a digital electro-pneumatic positioner with an optional integrated process controller to control pneumatically operated process valves with single-acting or double-acting linear or quarter turn actuators.
The challenge becomes how to operate the valve safely without causing system damage. The suggestion is to slowly move the valve in the opposite direction from its current position. Specify open left (counter-clockwise) or open right (clockwise). A fully open valve normally can be moved 10 to 20 percent for a short period of time without any major system disruption. Have the control room begin the closure cycle with a plan to reverse the operation immediately if any alarms sound or signs of failure exist. Test each valve in the system until this exercise has been completed.
The same holds true for a closed valve, but additional considerations and precautions need to be addressed. You need to know if the system can accept any increase in flow or if the valve will open to drain. Confirm that there is a steady amount of pressure in the pipeline and slowly open the valve. Begin with five to ten hand-wheel rotations, and reverse to the closed direction two to three turns. Accurately count the number of turns. Listen closely to the sound: You should hear the flow increasing. This will help to determine if the valve is moving. Repeat this procedure a few times, then return the valve to the fully closed position. Have automated valves operated from the control room or select local operation to test the performance.
The condition and performance of each valve should be noted. Any valve deemed inoperable or showing signs of sealing issues, external leaking or damage should be noted and placed into the maintenance schedule. Consider all integral parts of the system. The operators, switches, solenoids, air systems and wiring all require attention because they are integral parts of the system’s proper operation. Consider implementing a valve-exercise program into your facility. The benefits can increase productivity, reduce downtime and result in an increase in revenue. PC