By adding a spring to an actuator to provide a fail safe action the torque output is reduced by about half. This is because some of the air pressure is used to compress the spring and only the balance is available to produce output torque. Where full torque is required in the non-fail position the actuator torque is reduced to one third of the double acting rating. These facts result in larger actuators having to be selected for the application. Large springs are expensive and very heavy. On large actuators sizes the cost implication of providing a fail action is very significant.
An alternative option that is more cost effective than a mechanical spring is to use a double acting actuator in conjunction with an air tank that stores air at the highest supply pressure - this can then be released by means of switching valves in the event of air failure to drive the valve to the required fail position.
This reduces the available torque produced by the actuator in the event of air failure but the advantage is that in normal operation the full torque is available. The larger the volume of the air tank relative to the volume of the actuator cylinder the smaller is the down rating. The air tank can be free standing or can be incorporated into the actuator design.
The air tank can be free standing or can be incorporated into the actuator design.
The advantage of a double crank mechanism is that the pivot points are bushed and so backlash is negligible and friction is minimized. The torque characteristic of the actuator can be changed to suit the application by manipulation of the arm lengths and starting angles.
|Open position - low torque||Closed position - high torque|
The ideal situation is to match the output torque characteristic of the actuator to the valve required torque characteristic - but this needs to be done in all positions in both directions. Many valves require much higher torque in the closed position. For example, a butterfly valve a lot of force is required to get the disc into and out of the seat but not much is required to move the disc throughout its stroke. If the fluid is liquid, however, the hydrodynamic forces on the disc are high in the mid position and the actuator must be capable of overcoming these. A metal seated ball valve needs high torque throughout the whole stroke.
|CFD showing Hydrodynamic Torques occur in butterfly fly due to the pressure differences on either side of the disc.|
Modulating valves operate most of the time in the mid position and so the actuator needs to produce its most torque in that position. Higher torque output will result in more accurate positioning and stiffer action. Both of these will result in improved and more precise control. The double crank mechanism designed to give this high mid position torque with its low backlash is the best way of achieving the desired result. Double acting cylinders (with air tanks if a fail action is required) are strongly recommended for control applications. For on-off applications, the valve will either be open or closed. These valves will typically require more torque in the closed position than in the open position. For an example of the difference between the torque developed by the Double Acting On-off and the Double Acting Modulating EDA400-350 Rotary Actuator, click here.
Adjustable end stops on the actuator are essential on most valve types. Butterfly valve life can be extended and torque requirement reduced by closing the disc only as far as necessary to create tight shut off. Ball valves should be accurately positioned in the open position to ensure the flow pattern of the medium is undisturbed by protruding lips of the ball.
|Example of Adjustable End Stops.|
The bespoke nature of this actuator range - where each one needs to be individually designed and manufactured to suit the valve and the application - means that the methods used for design and manufacture need to be simple and automatic. This ensure the costs are kept low and delivery periods short. One of the cost reduction concepts is the direct mounting of the actuator to the valve obviating the need for brackets and couplings. There is no actuator shaft as the rocker plates are mounted directly to the valve shaft. The actuators have one (or two) pair of cylinders mounted in such a way that the side forces on the valve shaft are balanced, as shown by the two red arrows in Figure 8 below. This eliminates the need for outboard bushes on the valve shaft. Stainless steel is used for the internals and to reduce the cost of painting and manufacturing time.
|Free Body Diagram showing that because there are two actuators there are no side loads.|