How do Scotch Yoke Valve Actuators work?
Scotch Yoke Valve Actuator is a 90-degree driving mechanism. Which is driven by compressed air or hydraulic oil from an external source.
What are the advantages of Scotch yoke valve actuators?
It is important to consider several factors. So When choosing an actuator for a particular application. Such as the environment, the accuracy required. Also, the speed at which the actuator must close or open. The Scotch-Yoke Valve Actuator automates a wide variety of quarter-turn valves. Like butterfly valve, ball valve, plug valve, and damper valve. With various torque profiles to provide a “best fit” for every application. The main difference between a Rack & Pinion and a Scotch Yoke actuator is the torque they can generate.
What are their functions?
Scotch yoke actuators typically consist of a crank and piston system inside a hollow cylinder. Pressure utilizes a piston side inside the cylinder. As a result, the piston moves along the axis of the cylinder. Which rotates the actuator stem 90° with pilot air application. A spring returns the piston to its original position after the pressure is relieved.
A scotch yoke actuator works with what types of valves?
Scotch-yoke actuators are only suitable for 90-degree valves. Like butterfly valves and ball valves and are ideal for variable torque valves.
The torque required to operate a valve
A valve’s operating torque varies along its stroke. At the beginning and end of a valve’s stroke, it usually requires more torque.
In order for a butterfly valve to start opening from a beginning position. Higher torque is a must to overcome the friction between the seat and valve body. To keep the valve moving in the middle section of the valve stroke. The lower torques apply.
A scotch-yoke actuator’s torque output changes with the position of the stroke. Resulting in higher torque at the beginning and end of the stroke.
What volume of air needs to stroke an actuator?
We need to determine the amount of air to drive the valve for one full stroke after we have sized the actuator.
We need to determine the amount of air to drive the valve for one full stroke after we have sized the actuator.
From the selected model number, we know that we have a “CSY” with a “25” module size and a “350” cylinder size. We can calculate the volume of air displaced by the actuator. We have to use the following information:
- air volume closed = 1,159 in3
- inlet size= ½””
- Therefore, the max volume of air is needed. To move the actuator a full stroke = air volume closed = 1,159 in3
We can calculate the required flow coefficient for the accessories and air consumption based on the volume of air required to stroke the valve and the time to stroke the valve.
An industry application for scotch-yoke valve actuators
- Oil & Gas: Actuate valves to isolate pipeline flow
- Pneumatic valves for isolating nozzles in rock washing lines
- Actuate valves to isolate feed lines, filters, and tanks for water and wastewater
The internal parts and common malfunctions
Two major problems usually lead to malfunctions in scotch-yoke actuators:
01. pressure loss and power loss
02. Because of a lack of lubrication
03. in the air supply
