January 2nd, 2018
Sometimes requirements change and it becomes necessary to replace an existing manual process valve with an automated on/off or control valve. But, before you pull the old valve out of service and replace it with a new valve, you may want to consider automating your existing valve. If your valve is in good condition and it has the means to mount an actuator, you can save time and money by automating it.
The preferred method is to remove the valve from the line and automate it in your maintenance shop but that is not always possible. If you chose to automate the existing valve in line, first and foremost, make sure the valve is not under pressure and you follow your plant’s lock-out tag-out procedures before you work on the valve.
Know What You’re Working With
To start, identify the size, make, and model of your valve. Consult your valve manufacturer’s literature or consult with a manufacturer’s representative for the torque (rotary valves) or thrust (if the valve is linear) requirement of the valve. Ask the manufacturer or their representative if it is recommended to automate their valve.
Make sure that your valve has an actuator mounting pad or body bolts that can be used to mount the actuator without compromising the integrity of the valve. Most modern ball valves have pre-drilled mounting holes for actuation. If using body bolts to mount the actuator, make sure your bolts are long enough to engage the bracket and valve body once you reinstall them. Ask your valve manufacturer for their recommendations for length and grade of bolts if you must replace existing bolts.
Remember, the valve is a pressure vessel and death or serious injury can occur if you use the incorrect bolts or if the existing bolts do not engage the body properly.
Selecting an Actuator
Determine the additional safety factor you want to add to the manufacturer’s recommended torque and add it to manufacturer’s torque. This safety factor will ensure that your valve will operate even if your air supply drops slightly or if the mounting kit binds slightly and increases the required torque. Also as an actuator wears and air bypasses the pistons, the torque output will start to decrease.
Determine the desired function of your actuator and make sure it is the proper size. Once you choose your actuator, consult with your valve manufacturer or rep and obtain the proper mounting kit to adapt the actuator to the valve. Please note, there are custom bracket manufacturers that will adapt just about any brand actuator to just about any brand valve, so don’t worry if the valve and your actuator are different name brands. The important thing to look at is to make sure you are using a rotary actuator on rotary valves and linear actuators on linear valves. Your actuator supplier can help you obtain the correct mounting kits in most instances.
Before you install the actuator, make sure the valve position matches the actuator. In other words, if the actuator is a fail closed actuator, make sure that the valve is in the closed position. When you mount an actuator on a rotary valve, leave the mounting bolts slightly loose and stroke your actuator a couple of times. This will ensure that the mounting bracket is aligned properly and is not binding.
Torque the bolts to the proper tightness. Once you tighten the bolts, stroke the actuator fully open and fully closed a few more times to make sure nothing is binding and the valve opens and closes smoothly. Most linear valves and actuators use a 2-piece stem coupling (clamshell coupling). The linear actuator will include a yoke that attaches to the valve. Some linear actuators require you to add low-pressure air to slightly open the actuator before attaching the stem clamshell. This will preload thrust to hold the valve tightly in the closed position. Consult the valve and actuator manufacturer.
Finally, add any controls you need such as solenoids, limit switches, or positioners to the assembly. Attach it to your plant air and wiring and test one more time before returning the valve to service.