In processes it is not enough to know the magnitude of a variable, it is necessary to be able to control it to obtain the desired values. The devices that intervene in the variable to modify its magnitude are known as final control devices or actuators.
Just as the variable needs a specific sensor element to be monitored, it needs a specific actuator to be controlled. For the purposes of this article, I would like to present some of the valves I have had to install and configure in some of the plants I have worked on and describe how they work. I have chosen valves because they are elements with which the 4 fundamental variables mentioned above, which are pressure, level, temperature and flow, can be manipulated.
The previous one is a photo of a control valve. The operating principle is that it receives a signal from 4 to 20 mA that represents a command for a percentage of valve opening according to the value of the current sent. The device that converts this electronic signal into a pneumatic signal is called I/P and can convert a current into a pressure. Valves can be of the air to open or air to close type depending on where you have the air intake, the chamber that fills with air has a pressure range in PSI that determines the opening percentages for certain pressures generally from 3 to 15 PSI. This type of valves are used to control pressure because if the opening percentage is reduced the pressure at the outlet decreases, the same happens with the flow and if it is placed to fill and or empty a tank, the tank level can be controlled. By controlling an airflow you can have control over a certain temperature, so you can control many variables using this type of valves.
In addition, the valves must be equipped with limit switches that are switches that close when the valve has traveled full, so that the operator can monitor the status of the valve whether it is fully open, fully closed or in transition. If a position transmitter is installed, the position of the valve for intermediate values can be determined. Due to its mode of operation, this valve is pneumatically operated with electronic control.
This is an on/off control valve. Unlike the first one, it can only be open or closed, with no intermediate values. It has a higher response speed than the previous one so it is used in critical blocking points in the case of emergency stop and venting. It also has limit switches and is activated by an electronic command through a solenoid valve that blocks or allows air to pass through the valve chamber for actuation.
This valve is of the same nature as the above in terms of control type and speed of response. However, it is equipped with a hydraulic system for manual opening and closing and a pneumatic system for remote opening and closing, in which case there is a solenoid valve to activate the opening and one to activate the closing, which can remain in either of the two states even if the control solenoid is deactivated.
In addition to these there are many other types of valves including motorized valves that depend on an electric motor for their operation. I hope this article has been to your liking and remember to leave your comments.
All images are my own. I allow their use as long as I am mentioned in the credits and the source is cited.
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