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How To Know The Philosophies Of Control Of A Control Valve

During the design phase of an industrial process like that of the Blackhawk supply, the following question should be asked: What do you want to do with the valve?

The answer will help to select the type of valve and the optimal control philosophy for the process.

The most common control philosophies will be discussed below.

Discreet Control (All-Nothing)

A discreet control is usually carried out employing an on-off valve. This is typical where excellent control of the process variable is not required, such as controlling a tank's level.

In a tank, usually, the level doesn't need to be always constant. There is a certain margin so as not to have to pour water continually.

For example, 2 level switches can be used. A buoy indicates that the water level is below 3 meters, and another indicates that the water has reached 5 meters.

Tank With Level Switches

The difference between the 5-meter level and the 3-meter level will be called hysteresis. (5-3 = 2 meters hysteresis)

When the level drops below 3 meters, the tank will be filled by opening the on-off valve until it reaches 5 meters.

Discreet Control Of A Tank

Once the water has reached a height of 5 meters again, the valve will remain closed and will not open again until the level drops below 3 meters.

Typically, on/off control valves are used for this type of discrete control. However, although it is impractical, a regulating control valve could be used and an on-off control philosophy.

Modulating Control (Regulation)

When a finer control of the system is required, and the measured variable is required to remain at a certain point, hysteresis can no longer be used, and in this case, a modulating control will be applied. A specific set point will be established, which should be tried to maintain at all times.

For example, a set point of 5 meters.

Checking The Level Of A Tank

According to the drawing of Blackhawk supply, a transmitter has been installed in the tank to indicate the valve controller's level. Based on this information, the valve will try to maintain the setpoint of 5 meters.

To keep this level stable, the valve will have to open and close, pouring more or less water into the system.

Programming the valve plus the transmitter under this philosophy is called a control loop.

To use this philosophy, the most common is to use a regulating control valve. However, there are applications where an on-off valve associated with a PID could be used.

Split Range

Within modulating control, there is a particular application called split range control.

An example of this type of control is when the setpoint (the required value) has an extensive range.

Suppose a valve with the capacity to regulate well at high flows has been selected. In that case, it will not regulate correctly at shallow flows, so if in the design phase it is not possible to regulate adequately, the entire required range is widespread. Install two valves in parallel, in which one will be smaller than the other.

The smaller valve will be designed to regulate better at low flow rates, and the larger valve will regulate at higher flow rates. And when the process flow exceeds a specific value, the most oversized valve will begin to open, and the most miniature valve will begin to close.

Control With Intermediate Positions

For this type of control, the most appropriate is to use a specified valve to work in intermediate positions.

As indicated in this article, this type of valve will not work by modulating, and it will remain fixed at a particular opening percentage as long as the process requires it.

For example, a valve with intermediate positions has been installed to regulate the passage of hot water through the radiators and control a house's temperature.

Control Intermediate Positions

In this example, three intermediate positions have been established. In the morning, the valve will be set to 50%, at noon to 10%, and at night to 80%. This would allow that it warms more during the night and the morning and that at noon the temperature is maintained a little. This control example is straightforward and does not require modulated control.

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