Chapter 18    Level and Inventory Control

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Check Your Reading             Study Questions             Thought Questions

 The questions will be categorized according to the key concepts in the chapter.

Level Process Control Objectives

Feedback Control











Check Your Reading

Solidify the Concepts

The liquid level affects the pressure at the bottom of the tank, and thus, the driving force for the flow.  Is the level below
self-regulating or non-self regulating?

Level Process

Non-self-regulatory variables are unstable and must be controlled.


It is self-regulating because the level affects

  dL/dt = (Fin - Fout)/A

It is non-self-regulating because the level does not affect  

dL/dt = (Fin - Fout)/A


It is approximately non-self-regulatory because the pressure effects due to level changes are small compared with the pump head.














What is the guideline for the liquid volume in the liquid level in this flash drum?

Level Process

We want the least volume required for smooth operation.


V = 300 m3

V/Fliq = 1.0 min


V/Fliq = 7.0 min

V/Fliq = 100 min










Tight level control is useful when

Control Objectives

Do we apply feedback or feedforward?


the uncontrolled flow has high frequency variation

the uncontrolled flow changes infrequently


the level should be maintained close to it's set point

your coffee cup is filled to the brim












Averaging (loose) level control is useful when

Control Objectives

Which control algorithm do we use?


The range of the manipulated flow is limited,
 |(Maximum Flow)-(Minimum Flow)|< maximum

The volume of the tank is large


The rate of change of the manipulated flow is limited,
             | dF/dt| < maximum.

A drain is near the tank to dispose of overflow.













The method for tuning level controllers is easy and gives good performance. Why didn't we use this method in Chapter 9 for all control loops?

Feedback Control

The Ciancone correlations are valid for what processes?


The professor wanted to confuse the students.



We could not obtain analytical solutions for the general feedback loop with dead time



We considered several control objectives simultaneously.



The level approach could not prevent instability.











When we want to implement
averaging level control, what factors contribute to good performance?

Control Objectives

Good performance comes from more than tuning.


A large value of V/F.

Small variability of the uncontrolled flow


Proper controller tuning