Basics of industrial final control elements 9

Actuator types

Here in this part, we will have a look on how an actuator looks like in real life and how they are working. Have you been to a construction site ever, or have seen cranes, dumpers, dozers etc. When these machines lift or move things, they are actually using actuators, or hydraulic actuators to be more precise to perform their heavy work. In industry these actuators are also found, but in instrumentation and control, there are many other different types of actuators also inclusive of those mentioned in the example.

Actuators come in all sizes and shapes and types. Direct and reverse acting (hinted in earlier article) single and double acting, failure to open or failure to close are some of the aspects which categorizes how different actuators work. Then there are rotary actuators in addition to linear control valve actuators. All of these are now the topic of our discussion here onwards one-by-one. The figure below shows parts of a single acting actuator. The actuator itself under discussion can be divided into two segments, the YOKE (middle part) and the diaphragm chamber (actuating mechanism). Yoke we have discussed in our previous article. Here again we are mentioning a spring loaded actuator below which, in case of loss of air, will return to their original position.

Actuator itself can be categorized as either direct acting or reverse acting. Here the important point is that we are ONLY considering it from actuator's action point of view.

Why we have mentioned it as an important point?         

The details are coming in below as we discuss the overall action of a CONTROL VALVE.

Lets compare the direct verses reverse acting actuators first.

From actuator point of view, reversing the air signal input results in an actuator changing its action from direct to reverse. However to look at the whole picture, the plug and seat arrangement consideration decides if the CONTROL VALVE is a direct acting or reverse acting. This becomes more clear if we compare it with the figure above specifically mentioned in this article to see the main differences. It can be observed that the left side actuator is labelled as a direct acting actuator in the bottom figure. Nonetheless, the top figure which is giving the whole picture claims the control valve is a reverse acting one, though the similar direct acting actuator is used there (1st Figure). This is because though the ACTUATOR is a direct action one, but it is CLOSING the valve reducing the fluid flow.

Therefore to perceive in a process or system only actuators action can not decide if the control valve is a direct acting one or a reverse acting one. It is always better to see the whole action plan, rather than the position of the air signal. That has many times confused the professionals, and that's why it is clarified here in details.

Then comes the next part which is the air to open and air to close control valves and their fail safe condition which is air failure to open or air failure to close. As a thumb rule whichever control valve is air to open (while we discuss spring loaded actuators as per our example above) will be an air failure close (fail to close) control valve, and vice versa. This is because when air failure happens, the spring action will bring the valve back to its original position. Unless in special cases where either a minimum mechanical stopper, or lock up relays are used.

In the control valves, we have discussed single seated and double seated valves. In actuators we have single acting and double acting ones. The examples above are for the single acting actuators. Double acting actuators are somewhat having a different shape, as it uses a piston and cylinder as an actuating mechanism. Lets have a detailed look on how a double acting actuator works.

Signal from a controlling device weather digital, analog or pneumatic comes in as single value for each position of the valve. For example if the analog signal output is 8 mA, it will generate an equivalent signal to open/close valve by 25%. In a single acting actuator if it is fed directly also, it will move the valve to the desired position. But in a double acting actuator it is not the case, since it requires air on both sides of the piston. Though more commonly it is used for rotary type valves, but in some cases it is also used for linear control valves. Here valve positioner plays an important part. The incoming signal to the valve positioner is used by this device to increase or decrease the pressure in the top and bottom chambers in such a way that the desired valve position is obtained. While a single acting actuator can work without a valve positioner, it is required (specifically) for a double acting actuator to function properly.

Since we have mentioned rotary valves, let's have a look at some other actuators which perform this function. Most air dampers, turbine inlet guided vanes, gas turbine's nozzle control and similar devices though do not have a linear motion, but the actuators used for these devices are single or double acting pistons with linear movement. Lever system converts the linear movement to rotary movement and it opens and closes the final control device accordingly.

The other type creating a rotary motion itself is shown in the below figure. As the name states, a rotary actuator creates a rotary movement and is mostly used for butterfly or ball valves (V notch ball) or a full bore ball valve as well. These actuators are coming in all shapes and size. ESD valves also employ these actuators mentioned below. These are used in industry commonly for emergency shut off rather than for control. Mentioning these actuators here is only for information as we are already discussing the topic of actuators, and because it is also found in abundance in industry.

The rotary action has in some cases a gear mechanism while in other actuators use scotch yoke arrangement. The below figure shows the two arrangements.

As we have seen so far, the actuator can be a linear one, or a rotary one. Though a linear actuator can sometimes be used for rotary motion of the valve, a rotary actuator is specifically for a rotary valve only. Unless someone converts this rotary motion into linear movement through mechanical means, but in industry it seldom happens. One thing worth mentioning before we close our article for the day is that, though the rotary actuators are used mostly for 90 degree direction change, used with ball or butterfly valves, its movement in some cases in not limited to quarter turn, as it is technically defined.

This topic will continue in our newsletter as still there is much to be explained in this series of basics of industrial final control elements. So hold your horses, for more information is in the line in our upcoming articles. By the way, a horse can be a good actuator for the carriage or cart...wouldn't he???

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