Liquid Level Flow Control Loop

Figure 1 (ImageSource: ResearchGate)

The increase/decrease height of the liquid level in a process vessel / tank  depends upon the inlet and outlet flow rate of the processing fluid. If any imbalance in the inlet/outlet flow rates occurs,  results a corresponding change into the height of the liquid level (which falls under control objective). Here our purpose is to maintain a desired height of the process fluid. In respect of that a level transmitter is placed which measures the ΔH (Height maximum – Height minimum); respectively by comparative study between the measured output and fixed set point, and  the error (difference between set point and measured output) signal transferred to the controller, as a resultant as per the deviation from the set point, Action taken by the controller and regulates Final control element (Here by flow control valve’s in the output stream, %opening / %closing) to achieve a desired height.

Control Objective : 



In order to develop a complete control strategy for this control loop, it is very important to identify the process variables first or one have to identify first what is controlled variables, what is manipulated variables and what falls under disturbance variables list.

Here the height of process fluid (H) is considered as controlled variable, which is your desired output.

Manipulated Variable : Outlet flow rate -controlled by  control valve; as more fluid passes / flow rate increases from outlet valve  height of liquid level decreases in the process vessel / tank and vice-versa . Here, outlet fluid flow rate is considered as manipulated variable. The fluid flow rate depends upon pump speed and all.

Major disturbances : Those parameters which  affects  the process fluid height:

  • the flow rate of upstream/inlet  processing fluid.

Control Action : 

  • In this loop  reverse action controller is required, assuming a signal-to-regulate control valve:
  • Here feed forward scheme is required, whenever any change in the height of the liquid occurs. 
  • In case of feed forward, what ever the disturbances occurs that gets accounted ( Here by means of level transmitter  measures the height variation, including disturbance effects)  and a comparison done between set point and measured output, whatever the error is their—–on that controller takes necessary action, give message to the flow transmitter and accordingly flow rate control valve, % open / %close taking place. In this way inlet/outlet flow rate increases/decreases or, decrease/increase responsible for level control of the tank.

Note : 

  • Continuous liquid level sensors are mainly get used in process industries which continuously measures liquid level  of an entire system and keeps fluid level within a range.

ImageSource : Control Notes

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