Valve control in action: Flow control

Using PID to control the flow within an HVAC system with a valve

Variable chilled or heated water systems will restrict the flow to loads – such as radiators – to control the temperature of the surrounding environment.

When doing so, the flow rate through the chiller or boiler will be affected. This means the supply of cold or hot water to the entire system will be much greater or less than what is demanded.

To prevent this scenario, a bypass path is built into the system and can then be used to restore the flow rate to the proper level.

How to regulate valve controls

A common solution to regulating the flow rate of a chiller or boiler system is to install a bypass branch that contains a control valve.

The control valve will open and close in order to keep up with the change in the flow rate.

As the flow rate in the system decreases, the valve will open further to restore the flow to the correct setpoint to maintain chiller or boiler flow.

Figure 1: Illustration of a basic chiller system

Best method to control valve position through flow rate feedback

Proportional, Integral, Derivative (PID) control is the best method to control the position of a valve based on flow rate feedback. The error between the setpoint and the feedback from an analog device can be fed into a PID function.

The output of the PID can then directly control an analog output to a control valve. As the flow rate fluctuates, the valve position will dynamically adjust to automatically regulate it.

Figure 2: Illustration of flow control by adjusting the valve position using PID

Achieving desired integral gain

The easyE4 nano programmable logic controller (PLC) allows a programmer to intuitively set up PID in its software. The programmer is able to select which gains they want to enable by setting EP, EI, or ED and simply has to set I1 to be the setpoint and I2 to be the feedback from the system.

Additionally, the proportional gain KP is configured directly on the faceplate. This value will have to be determined empirically based on the desired response.

Calculating the integral gain

The easyE4 nano PLC calculates the integral and derivative gain slightly differently. The integral gain, KI=KP*TC/TN. Where KP is the proportional gain, TC is the scan time of the function block, TN is the integration time. The integration time parameter (TC) is the value that the user adjusts to increase or decrease the integral gain.

Likewise, the derivative gain KD=KP*TV/TC, where KP and TC are the same parameters and TV is the differential time. The differential time (TV) is the value that should be adjusted to increase or decrease the differential gain.

Like the proportional gain, the integral and derivative gains need to be determined empirically. However, for valve position control, it is recommended that the derivative term should not be used.

The output of the PID, QV then directly sets the output to the system, which in this case is the valve position reference.

Figure 3: Illustration of a PID function block used to create an analog signal for valve position

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Valve control for HVAC control applications

Valve control can be used to address a variety of control applications. These include:

Flow control

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