Basics of industrial final control elements 8

The ACTUATORS

The topic of CONTROL VALVE is quite a long and exhaustive one. So far we have seen the different parts of the VALVE side and their importance and functions. Though still when we talk about control valves, we come across the terms flashing and cavitation as well. Since these terms are not included here as these are rather process conditions, these will be discussed in a separate article, when we delve deeper into the subject. For now lets expand this topic of the CONTROL VALVE as we travel a little up to see what is the other part, the ACTUATOR is doing?

ACTUATOR is the power which in turn is controlling and throttling of fluids through the valves. One thing to note here is though the control valve actuators are powered pneumatically, hydraulically or electrically, but first we will take up the most commonly used type which is air powered or as we technically mention them as pneumatic actuators.

But....but when we talk of pneumatic actuator, here as well we have a wide variety. There are...

Single acting linear, spring return type diaphragm operated actuators        

Double acting, linear actuators without spring action        

Single acting, rotary, spring return actuators        

Double acting, rotary, piston type actuators....just to name a few.        

Let's have a quick look. Here we mentioned two things as we start our explanation about actuators. Firstly the force driving the actuator is air. Secondly it is moving with a linear motion (which accounts for the total valve travel). Thirdly it is a direct acting actuator. This is our first target .

Have you struggled or seen people struggling to haul furniture while shifting homes or just rearranging in a house? For small items one or two people are enough...isn't it? But for heavier ones need more manpower. Same is true for the actuators. For smaller valves with less pressure we need less force, hence we see actuators with smaller air chamber (smaller diaphragm). But for bigger valves larger ones are required. In a small plant also many sizes of actuators are observed. Basically it is the main muscle of the control valve driving (throttling) the flow as required. When we discussed different instrumentation signals in our articles, we have seen that pneumatic signal had been gradually replaced with analog (4-20 mA), then HART and Fieldbus signals. But apart from pneumatic signals other signals don't have the force to directly operate control valves specifically for the pneumatic actuators which is the topic of our discussion.

Here the pneumatic signal prevails which most of the actuators use to open and close the control valves. For this, different converters like current to pneumatic or fieldbus to pneumatic converters condition the signal transducing the received signal into a pneumatic signal which drives the control valve. (converter portion will be taken up separately). Now let us see what actuators actually are! As the name suggests an actuator actuates the valve plugs up and down (since we are discussing here linear actuators).

Stop.....

Have we missed something here. Yes, it is the middle part which JOINS the CONTROL VALVE'S stem with the ACTUATOR stem.

Hence, let us have a quick look on how the linear actuators are coupled with the control valve itself and what are the major parts which form this section before entering the main topic of different actuators actions and working principles. Reminder: here the actuator being considered for coupling with the control valve is the first type mentioned above where we have seen some classifications of the actuators. The other type of actuators have different coupling arrangements.

The actuator's diaphragm chamber and the valve body has in between what is known as a YOKE. Though the two parts i.e. diaphragm chamber and yoke combined is known as an actuator, but to clear the things we are sub dividing it for better understanding. Diaphragm chamber is bolted on the top, while on valve side it is connected through yoke claiming nut shown in the picture above. The valve stem and the one from actuator side is coupled in the middle of the yoke using a valve coupling. This is normally a two piece set of machined metal block with TWO sets of threads. One is for the valve stem, while on the other side the actuator shafts threaded end is coupled, in most cases, using coupling screws or bolts/nuts. However some manufacturers have actuators shaft with female threads inside, which accommodates the threaded valve stem and a lock nut locks the threads. In between a thin disc is inserted which acts as the travel indicator. Whatever the arrangement, the two parts actuator and valve meet in the yoke portion.

To see how much is the valve opening or closing a travel indicating plate (travel scale) or valve scale is installed in the inside wall of a yoke which is positioned properly when a valve is calibrated and have marks engraved which show the valve opening or closing percentage from zero (0%) to maximum (100%). The Yoke is normally the portion of the valve which accommodates various accessories for the valve like a valve positioner, position transducer, air pressure regulator, etc. Also in some cases a HAND JACK or HANDWHEEL is also there in the yoke portion to operate the valve manually if there is failure in driving the control valve from the control signal.

Since most of these items will be discussed in another article down the line, for the moment we will skip the details here, though each of these items play vital role in the performance of a control valve. The details of the ACTUATOR or rather pneumatic actuators with linear movement will be dealt in our next article so sit tight and wait as our subject, the basics of our industrial final control elements unfolds more information in the next issue.

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