Butterfly Valve Torque Characteristics: Scotch Yoke Actuators Vs Rack & Pinion
A BFV produces a high torque at the opening and closing phases, for about 15 degrees, as the disc comes into, and is leaving, the liner. During the remainder of the time of travel, to and back from the full open position, it requires a much reduced torque. This gives rise to a U-shaped torque characteristic.
The scotch yoke actuator pistons are connected to the shaft by a yoke and pin. The yoke has a slot in which the piston pin can move, to and away from, the centre line of the output shaft. The distance between the centre of the shaft and the connecting pin on the piston varies, depending on the position of the piston. When the piston is either full in or out, the variable distance is greatest. As the connecting length between shaft and piston is in effect a lever, it follows that the maximum torque output of the actuator is at the piston end travel, and the minimum output is at the middle position of the pistons, where the length is minimum. This produces a U-shaped output torque characteristic.
Matching the valve and scotch yoke actuator characteristics together makes an excellent pair. The benefit for the customer is smooth opening and closing of the total assembly. This produces a much longer actuator and valve liner life. The bonus is a much reduced possibility of water hammer, lower maintenance costs and better process consistency.
Conversely, rack and piston actuators only produce a straight line torque output. That is acceptable for a ball valve or low torque-demand quarter turn device. But BFV’s require different consideration. Use of a rack and pinion actuator on a BFV valve can frequently lead to structural stresses as the disc literally jumps out of the liner. This is because sizing is usually done to the average, rather than the peak, torque of the BFV. Accelerated wear and unsatisfactory performance are commonly experienced, plus water hammer. Customers rarely demand the correct actuator selection because they have not been educated by BFV sales people, who themselves are usually unaware of the benefits of scotch yoke. As a rack and pinion actuator is about 10% lower cost to manufacture, that design has become common. But the lower initial cost can quickly become a burden to the operator when the plant is in use.
In conclusion, our recommendation for long and satisfactory plant life is to use a properly matched scotch yoke actuator on a properly built BFV. These should be made by the same manufacturer.