Lecture 7 - Level Control

Question 1

Why Level Control Differs from PID

Answer 1

• Not self regulating - if the flow is changed, the level continues changing instead of settling
• Tuning is different than PD - more often important to keep steady flowrate not constant level
• Cascade rules don’t always apply

Question 2

Variables to be Tuned for Level Control

Answer 2

V -Volume between 0 and 100% indication on level gauge
d - maximum tolerated deviation from set point
f - normally expected flow disturbances
F - range of flow controller
ts - controller scan interval

Question 3

Tight Level Control

Answer 3

Used when:
- level control must stay close to setpoint
- tank is small
- safety risks asociated with too high level control
- downstream process is sensitive to level change
– Flow may fluctuate when level is controlled tightly as flow varies to hold level at setpoint

Question 4

Average Level Control

Answer 4

Used when
- tank can act as a buffer (large surge volumes)
- main goal is to keep flow steady
- temporaru level changes have minimal impact on process
– Flow is kept stable and the level is allowed to flow within safe limits

Question 5

Cascade for Level Control

Answer 5

Cascade is used in level control to improve response time and stability by managing disturbances before they effect the level too much
- Level controller is the master controller, flow controller is the slave controller and controls the flow into or out of the tank
Cascade control is level control is used as the flow controller acts to handle fast changes, while the level controller handles the slower changes
Used when
- want to reduce the load on the level controller
- using averaging level control

Question 6

Level Control in Design

Answer 6

• make sure the level sensors cover the entire vessel height (very important in averaging level control)
• avoid noise in the measurements

Question 7

Non-Linear Level Measurement

Answer 7

Where some vessels have a nonlinear relationship between level height and volume (horizontal or spherical tanks) - instead of controlling based on level %, control based on the maximum measureable volume as better reflects the actual amount of liquid in the tank