A control valve is a type of valve that we use to control the flow, pressure, level or even the direction of the fluid according to the need of the process. Control valves have lots of different types such as glob valves, butterfly valves, diaphragm valves, and so on.
Control Valve Applications
We would almost always like to have control over different parameters of the process like temperature, pressure, level, and so on. To achieve this, obviously, we need a controller like Programmable Logic Controller or PLC for short.
We also need some sensors and transmitters to send data over to PLC or DCS. Finally, we need a piece of equipment to carry out the PLC’s commands which usually called as “Final Control Element.”
A Final Control element can be a pump, a heater, a control valve, and so on.
But very often by “Control Valves”, we mean the type of valves that are used not only to fully start and fully stop the flow, but also to control or adjust or in other words throttle the flow of the liquid (or fluid).
By throttling the flow, we achieve our final purpose that is, for example, controlling the temperature of a furnace or the level of a liquid inside a tank.
Different Parts of Control Valves (Globe Valves)
Well, this is a “Globe valve”; One of the most common types of control valves. Let’s check out its different parts;
- Body of the valve that the fluid will pass through it.
- Bonnet is connected to the body and covers its inner parts.
- Plug that controls, Stop or Start the flow by exposing the liquid flowing inside the pipe.
- The Actuator that transfers the mechanical power to the plug using the “Stem”.
- The Positioner is another part of the control valve that commands the Actuator.
The main role of the “Positioner” is to be an interface between the PLC and the Actuator to precisely adjust the plug for being open or closed. There are a variety of Actuator and Positioner types that we have checked them all at the end of this article.
How Control Valve Positioner Works
Let’s start with an example to understand the operation of the control valve’s Positioner and its relation to the PLC and Actuator.
In this example, we aim to control the temperature of a liquid within a tank by regulating the flow of an additive. This additive will generate heat by having a chemical reaction with the liquid already inside the tank.
Say, the PLC decides to send a 50% open command to the control valve. This command is based on two items;
– First, the PLC logic
– Second, the feedback it has received by the temperature sensor installed on the tank
This command signal often is in the form of a 4-20 mA DC electrical current and is sent from the PLC. The device that receives this signal on the field is the “Positioner” of the control valve.
As we are using a specific type of pneumatic actuator in this example which needs compressed air to become activated, therefore we chose an electro-pneumatic positioner to command it.
Don’t worry about the names! You’ll understand all of them.
In this case, the positioner plays the role of a “translator” and converts the PLC language to the actuator language! It means that the Positioner will convert the 4-20 mA signal to an air pressure signal.
You may ask how does this conversion happens?
Well, inside of an Electro-Pneumatic Positioner we have a converter unit which is called “I to P Transducer”. In fact, this transducer converts the 4 mA DC current to a 3 PSI air pressure and the 20mA DC current to a 15 PSI air pressure and of course, they are proportional in the middle range.
Let’s name the 3-15 PSI air pressure as the “Pneumatic Signal”.
With a simple calculation, we understand that in order for the PLC to open the valve for 50% of its full range, it should send a 12mA signal to the Positioner.
Then the Positioner will convert it to a 9PSI signal accordingly and will send it directly to the actuator.
But sometimes this amount of air pressure is not enough to move the actuator.
Therefore we should increase the air pressure so that it will overcome the spring force of the actuator and moves that.
The Air supply input will provide us with a clean, filtered, regulated air with sufficient pressure, thanks to a filter/regulator device.
After all, using the Air supply input and a built-in pressure amplifier, the Positioner will be able to convert the 9PSI air pressure signal to sufficient air pressure for moving the actuator to the right amount.
Same as the PLC that requires feedback by the sensor to decide about the command it is going to send, the Positioner also needs to receive feedback to precisely position the valve stem and open the valve to 50%.
This feedback is sent by the “control valve” to the Positioner using a mechanical mechanism.
In this way, the Positioner will decide how much pressure the actuator needs to move the valve stem.
Control Valve Positioner Types
In general, Positioners come into three different categories;
1 – The Electro-Pneumatic Positioners or I/P Positioners that we already discussed in the example.
2 – The Pneumatic Positioners, in which their control signal is a pneumatic signal and they do not need any I/P Transducer modules integrated.
3 – The Digital Positioner or Digital Valve Controller. In addition to an “I to P Transducer”, these positioners take advantage of a “Microprocessor” to fill in the place of the mechanical position feedback.
The input signal or setpoint from the PLC will directly send to the Microprocessor.
The valve position feedback which is measured electronically will also enter the Microprocessor.
Comparing these two electronic signals, the microprocessor is able to adjust the valve position quite accurately in comparison with the other types of positioners.
Using Digital Positioners, we are able to communicate with the valve by different types of protocols such as HART or Fieldbus protocols like Profibus.
By such digital communications, we can calibrate control valves way more easily using Hand-held communicators without tackling time-consuming and sometimes difficult mechanical adjustments.
Besides, we can send some feedback from the control valve to the PLC or DCS via these communication protocols.
Standalone I to P Transducer Instead of Positioner
The last point is that the standalone I/P transducers can control the valve independently in case that the accuracy is of less importance.
Meaning that they are not integrated into any kind of positioners and therefore there is no feedback in this case.
Now that we’ve got introduced to the positioners, let’s get into the Actuators and their different types.
Control Valve Actuator Types
Generally, we can classify the Actuators into 4 different categories;
The Pneumatic actuators are the most used kinds of actuators due to their:
– Simple design
– Fairly low price
– Intrinsically safe characteristic
Electric Actuators have an electric motor inside. They had been designed for:
– On/Off control
– Continuous control
– Where there is no access to compressed air (As their primary application)
In this article, we learned how a Spring-and-Diaphragm actuator along with an Electro-Pneumatic Positioner adjusts the valve in order for the valve to control the flow of the fluid according to the PLC commands.
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