By David Douglas & Herb Wise
A valve and actuator wellness program can help users avoid downtime, lower cost, streamline repairs, and improve the overall health of a production facility. However, to fully appreciate the need for a wellness program, it is important to understand the causes of valve and actuator failure.
Each actuator is custom made for each particular valve. An actuator is sized by changing the gear sets, motor output torque, and speed to develop enough torque and withstand enough thrust to operate the valve with maximum differential pressure across the valve.
Motor actuators are mechanical gearboxes driven by an electric motor. The motor either turns (quarter-turn valves) or raises and lowers (gate and globe valves) a valve stem to operate the valve. With quarter-turn valves thrust is not developed in the actuator. The actuator must develop enough torque to make the stem turn against both the packing and system forces, but the valve contains all the necessary thrust.
Since quarter-turn valves don’t handle large amounts of thrust, they can be small and light. Actuators must have slow output speeds to allow enough time to respond with operating directions before the cycle is completed. Sometimes this requires the use of an external gearbox attached between the actuator and the valve.
Gate and globe valves generate thrust by overcoming packing and system forces as they raise and lower the stem into the valve seat. Therefore, the actuator must be able to withstand the thrust and the torque generated. Actuators for gate and globe valves can produce faster output speeds than quarter-turn systems, but they are also larger and heavier.
In all motor actuators, a worm gear is used to drive the output. The worm gear is the least efficient gear in the actuator and is highly affected by lubrication. Loss of lubrication in the gear is a frequent cause of bearing and gear problems within the actuator. The oil or grease level in the main gear case must be high enough to keep the gear fully immersed in lubricant.
Another problem occurs when water is allowed into the lubricated area of the actuator and/or the electrical area. When water is found in the actuator, it usually gets in through electrical conduits or is blown in by a packing leak on the valve. This can cause corrosion and failure of the actuator.
In some actuators, grease is used as the lubricant and gaskets are used as the seals. When idle for long periods of time, the grease will separate, and oil from the grease will leak at the gaskets. This not only lowers the level of lubricant, but also lowers the quality of the lubricant remaining.
As a result of the above issues, actuators are often oversized for normal valve operation (low differential pressure and normal voltages), and the torque switch is typically used to keep the output within safe limits.
The handwheel on an actuator allows manual operation when needed. Due to the gearing, handwheel operation must be used very carefully. The handwheel, when operated as one would operate a manual valve, can shut a wedge gate valve so tight that the motor cannot pull it out. This can cause direct damage to the actuator and/or valve.
Proper operation of valves and actuators improves reliability. While implementing improvements may seem demanding, a few key practices can ensure the health and wellness of valves and actuators, lower costs, and make repairs easier. Best practices include:
• Ensure the actuator is the correct one for the valve attached. Remember the actuator is customized for the valve. Stem diameter, stem pitch, stem lead, valve size, system operating, and maximum pressure will all have an effect on actuator operation. If any of these have changed, the actuator should be resized with the new parameters.
• Inspect all areas available on the actuator to identify such issues as: moisture, corrosion, tightness of electrical connections, and oil leaks from the internals. Using grease plugs, check the level of the lubricant and its quality and consistency.
• Run the actuator, listen for unusual noise, watch the overall operation for any unusual items, and watch the torque and limit switches for proper operation
• Develop a customized preventive maintenance program for the actuator that evaluates each actuator based on its unique needs
• Train all operators on the correct operation of the actuator.
About the Authors
David W. Douglas is the president of Paradigm Services LP and CPL Control Products of Louisiana, with over 25 years experience in global manufacturing and services for all phases of the oil and gas industry. His experience includes general management, acquisitions and divestitures, workouts and turnarounds, international and domestic alliance partnering, distribution channel management, and technical sales management. Paradigm Services is engaged in aftermarket service, repair, and remanufacturing of valves and actuators installed in refineries, petrochemical plants, power generation facilities, and pipelines and oil and gas production facilities. Prior to Paradigm Services, Mr. Douglas was President and CEO of ENERPRO International, Inc., He held various management positions within Baker Oil Tools, Baker Hughes Tubular Services, and Reed Tubular. Mr. Douglas can be reached at 281-478-5200 or firstname.lastname@example.org.
Herb Wise is the manager of Actuation Technology at Paradigm Services LP. He has over 30 years experience in all aspects of the motor-operated valve (MOV) industry, including sizing, modification, repair, root cause analysis, maintenance training, safety training. preventive maintenance program development, MOV mounting design, and diagnostic testing. His experience includes startup of five MOV repair companies and positions including: training manager, division manager, general manager, and production manager. Herb was a member of the Technical Advisory Group responsible for development of Technical Repair Guidelines for Limitorque Valve Operators published by the Nuclear Maintenance Applications Center (NMAC) unit of ERPI (Electric Research Power Institute, www.epri.com). He also served on the IEEE advisory committee for motor-operated valves. He is a member of IEEE Working Group 3.3. This group was responsible for writing Maintenance Good Practices for Motor Operated Valves. Mr. Wise can be reached at 281 478-5200 or email@example.com.
For More Information: www.paradigmserviceslp.com
Ensuring the Wellness of Valves & Actuators
Avoiding Downtime, Limiting Maintenance Costs
By David Douglas & Herb Wise