Basics of industrial final control elements 1

When we talk about controlling a parameter in industry, many different variables need to be controlled and for each type a different final control element may be required. In normal processes, we have seen that what is being measured, weather it is flow, pressure, temperature or level, after being measured it is either indicated ,recorded or controlled or can be a mix of these actions.

If it is a process variable which needs to be controlled, another element or rather the FINAL CONTROL ELEMENT /CONTROL VALVE is used. So in this article and in some of the articles coming up in the near future, we will be talking about this part of a control strategy. But what is a valve? In our daily life we use them several times in a day. When we are washing, bathing or using water from a tap, we open the VALVE to start the water flow. When we are filling the fuel for our cars, the fuel will start to flow only when pressing the lever on the fuel dispensing nozzle opens the flow of fuel. That is also a valve.

Most of the times four types of valves are in use in general. To understand what a valve does let's have a look on how they start, stop or control fluid flow. So in below figure there are some common valves use in the industry.

1- Gate valve: Most common is a gate valve used in the lines to allow or stop passing of fluids. There is a gate connected to a threaded spindle which has a wheel on the outside. Turning this wheel will slide the gate inside or outside, to close or open it allowing any fluid to pass downstream. (bottom left)

2- Globe valve: A globe valve (which has similar construction as a control valve in most cases) has a seat and a plug. The globe valve is operated by an external wheel as well which move the valve plug. In closed condition plug will sit on the seat and will not allow flow to pass. When it is moved up it starts passing flow and full flow will be passing once it is fully open (bottom right)

3- Butterfly valves also are used in some applications, where the valve is basically a disc, which rotates inside the valve body to open and close the line by an external handle and has a maximum movement of ninety degrees. (top right)

4- Similar to a butterfly valve but using a metallic ball with bored hole inside, is the ball valve. This also operates up to 90 degree angle from full open to full close position when ball is rotated. (top left)

There are many other types but since our main aim is to target the control valves so Saunder's valves, three way valves, four way diverter valves, needle valves, check/non - return valves, solenoid valves etc. will be dealt separately.

Now let's take a simple example of a vessel having some liquid. We have measured the level, using one of the numerous methods mentioned in our articles. Now if it is a part of a continuous process where the incoming and outgoing fluids need some control to maintain certain level in the vessel then either need to control input into the vessel, i.e.. when the desired level is reached we should close or throttle the inlet in such a way that the particular set value is maintained, if there is no control on the outlet flow. In other scenario, if the in-flow is uncontrolled then outlet should be throttled in such a way that the level should be maintained at desired height in the vessel by throttling the outlet flow. Or a combination of both, which means that inlet and outlet are both throttled to keep certain level in the vessel.

One way is to install some manual valves discussed above at inlet or outlet or at both locations and while a continuous watch is kept, these valves are throttled from time to time by a process operator to maintain the level as it was done in Industry 1. Sounds exhaustive and also if the measurement of the vessel level is critical, human errors may complicate things. Here the automation kicks in which uses a LOOP to maintain the level automatically without human intervention using three important pieces of instrumentation, namely LEVEL TRANSMITTER (working as eyes of the operator), which is continuously monitoring the level and sending a signal to a CONTROLLING UNIT (the brain of the operator which is controlling his actions). Here this controller will compare this input signal with a set point or desired level value, set by an operator to generate an output that will drive a FINAL CONTROL ELEMENT (hands of the operator) to throttle it for maintaining the level. Since this throttling action will cause the measured level to change a bit (or a lot) the measuring device, monitoring this change, will again send a signal to the controller to reposition the final control element (our main topic) to compensate for the level changes and this is continuously repeated over and over to keep a balance and maintain the desired level inside the vessel. Since the TRANSMITTER, CONTROLLER & CONTROL VALVE all three are working in harmony and are looped together, hence we call it a CLOSED LOOP CONTROL.

The CONTROL VALVE or the final control element which is our present topic of discussion, has two main parts. The actuating mechanism or actuator, which positions the VALVE to throttle the influx or out flux of the liquid. The other part is the valve itself through which the liquid's flow is maintained. Both parts of the final control element are important. The actuator which is the muscle of the control system is powered either electrically (motorized valve) or hydraulically using liquid power or using air (pneumatically).

The last option is the most common one used in almost all process industry. Even if the controller output is a current signal of 4-20 mA or a fieldbus digital signal, it is converted into pneumatic signal using a current to pneumatic or I/P convertor or a fieldbus convertor/positioner for actuating this final control element.. Here we have used a word positioner (valve positioner) as is known technically. This device is used as part of our control circuit to properly position the valve for precise throttling, to change the action (will discuss separately) or to increase the power (signal strength) as well. (This part will be taken up when we discuss valve positioners).

Control valves come in all shapes and sizes and colors (in some process organizations) and have different controlling properties like an on-off, linear, equal percentage or quick opening type. Action wise it is categorized as fail open or fail close. Direct or reverse acting...etc. Again the valve may be classified in accordance with its leakage class, which means how much quantity of fluid is permissible in closed condition through a control valve for the specific application. All these factors affect the way a valve is used in a process and what criteria must be taken into account when selecting a control valve for a particular application.

Control valve basically is a mechanical device, but since it has very close relation to how it performs its operation in controlling a process, hence it becomes a part of instrumentation and control system. In certain industry its maintenance is a part of mechanical discipline, but understanding its functionality is still very very important for a professionals of instrumentation and control field. There are certain other instruments which are closely related to the operation, performance and feedback of the control valves. These are the control valve positioners, limit switches, solenoid valves, position feedback transmitters, current to pneumatic converters (fieldbus to pneumatic converters), lock up relays, booster relays etc. which can be part of this final control element. These are all known as ancillary devices for a control valve and rarely we find any control valve without using any one a combination of some of these devices in industrial applications. Here mentioning self regulating valves which directly control mostly pressure or back pressure of a line without using a normal instrument closed loop is also necessary which are also part of the process industry.

We will first take the details of how the control valve itself is performing the throttling operation by way it is constructed for a particular use. What are the different types and characteristics associated with these valves and how it can effect the integrity of the overall process in general in our next article. So sit tight and wait as we unveil the details of all the things which we have mentioned in the article above ONE by ONE...

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(Please note that though the accuracy of the information provided here is based not only on reliable sources, but also based on personal experiences, but it is for providing a general picture and the author is not liable for any loss or damages arising from the use of this information in any instance)