We’ve all seen this situation: A quarter-turn valve is in the closed position. A pneumatic actuator applies an opening torque that exceeds what’s required to begin to open the valve. Suddenly, the valve “jumps” or “pops” open to as much as 45 degrees or more, resulting in a sudden surge of potentially disruptive flow. Why did this occur and how can we avoid this phenomenon?
Some time ago, the makers of a wave pool decided to use high-performance butterfly valves to control the flow of pressurized air to induce wave motion. The valves supplied initially, however, refused to open smoothly. Instead, they jumped from fully closed to half open before control could be established. Because of this, wave control was not possible using these valves.
Another time, a butterfly valve was under cycle testing as part of a development program. The high-seating torque exhibited by the valves caused them to stick in the closed position.
Once the valve disc moved out of the seat, the valve jumped so much that the actuator end caps failed from impact by the pistons.
In both instances, the valves exhibited high seating torque and minimal torque once they broke free of the seat. Blame for the opening jump can be placed on a valve’s torque characteristic; but in fact, the actuator is typically just as much to blame as the valve.
WHY THIS IS
For pneumatic actuators to operate, pressurized air or gas fills the entire void volume of the actuator. Essentially, this volume of compressed air can be thought of as stored energy contained by the piston’s resistance to movement (i.e., the valve torque). Once the valve moves away from the seat, resistance decreases, and the compressed air is free to expand until the pressure equals the valve’s resistance.
Figure 1 demonstrates how this works. It shows a valve torque characteristic where there is a high initial unseating torque followed by minimal torque requirement once the valve leaves the seat (represented in this graph by the solid line). Note that this graph shows the actuator pressure required to operate the valve at each travel position.
Resulting actuator travel and valve position are shown for various actuator void volumes. To establish a mathematical model, we’ve used a 6-inch piston with a 3-inch piston travel for full 90-degree travel in this example.
With a 40-cubic-inch void volume, the example actuator will cause an opening jump of nearly 80 degrees. Decreasing the void volume to 20 cubic inches reduces the jump to 40 degrees, and a 5-cubic-inch void volume results in a jump of only 10 degrees.
While the example cannot be used to predict the opening jump for specific applications, it clearly demonstrates that the void volume of an actuator significantly contributes to opening valve jump and that minimizing the void volume will result in less jump.
Actuator void volumes vary by type, design and direction of travel. Consider the example actuator shown in Figures 2 and 3. Void volume is considerably greater if pressure is applied between the pistons to cause valve opening than if that pressure is applied to the outer surfaces of the pistons. In this example, opening valve jump can be reduced by the choice of actuator orientation.
Clearly, to reduce the amount of opening valve jump, a pneumatic actuator with minimal void volume should be selected.
Alternative solutions for a jump situation are:
- Adding spring to the actuator (Figure 4) will help because the spring’s resistance to motion increases with travel, reducing the jump. Preferably, select a spring return actuator with a minimum of void volume.
- To a lesser extent, the actuator mechanism can lessen the effect. For instance, while a rack and pinion actuator produces a linear torque curve throughout travel, scotch yoke types decrease their torque output at mid-travel; therefore, they may cause less valve travel from the stored air pressure.
- Another option is using a hydraulic or electric actuator, neither of which will typically produce any opening jump.