Manipulated Variable in Level Control System

admin
January 11, 2026
No Comments

Q.23 The process and instrumentation diagram for a feedback control strategy to maintain the level
(h) of a liquid by regulating a valve (V) in a tank is shown below.
F₁ is inlet liquid flow rate, F₂ is outlet liquid flow rate,
LT is the liquid level transmitter, LC is the liquid level controller,
h_sp is the setpoint value of the liquid level,
h_m is the measured value of the liquid level and
P_v is the valve pressure.

The manipulating variable(s) is/are

(A) F₁ only

(B) F₂ only

(D) h_sp and P_v only

Manipulated Variable in Level Control – Detailed MCQ Solution

Control systems aim to maintain a process variable, such as liquid level, temperature, or pressure, at a desired setpoint by adjusting a manipulating variable.
In this question, we focus on a tank where the liquid level h must be controlled by regulating the inlet valve.

Given System Components

F₁ → Inlet liquid flow rate (controlled by valve V)
F₂ → Outlet flow rate
LT → Level transmitter measures $h_m$
LC → Level controller compares $h_m$ with setpoint $h_{sp}$
Pᵥ → Controller output signal to valve V

The target of the control loop is to maintain the tank level h.

What is the Manipulated Variable?

In a feedback control system, the manipulated variable (MV) is the variable directly adjusted by the controller to control the process.

Here, the controller output Pᵥ adjusts the valve opening, which changes F₁, the inlet flow rate.

Therefore:
➡️ Valve position (via Pᵥ) → adjusts F₁
➡️ F₁ controls tank level h

So, the manipulated variable = F₁.

Correct Answer

✅ (A) F₁ only

Option-Wise Explanation

(A) F₁ only — Correct

The controller sends a signal to the valve.
The valve adjusts inlet flow rate F₁.
F₁ changes the liquid level, making it the only true manipulated variable.

(B) F₂ only — Incorrect

F₂ is the outlet flow.
It is not controlled by the loop and remains uncontrolled/disturbance.
Hence, not a manipulated variable.

(C) $h_m$ and $P_v$ only — Incorrect

$h_m$ is the measured variable, not manipulated.
$P_v$ is the controller output signal, NOT the process variable changed; it only commands the valve.
Neither directly qualifies as the MV.

(D) $h_{sp}$ and $P_v$ only — Incorrect

$h_{sp}$ is the setpoint, chosen by operator.
$P_v$ is the controller output, not the final manipulated quantity.
These are critical parts of the loop but not manipulated variables.

Conclusion

The only variable actively changed by the controller to maintain level is the inlet flow rate $F_{1}$ .
Thus, Option (A) is correct.