chp14SQ

14.10
How many process reaction curve experiments are required to tune the controllers in a two-level cascade?

IMPLEMENTATION

One step change to the valve to determine the dynamics of both controlled variables.

Two steps, one to the valve and the other to the secondary controlled variable.

Two steps, one to the valve and the other to the set point of the secondary variable.

14.11
Cascade control was designed for the fired heater in Example 14.2 which is repeated below. The cascade is effective for which of the following disturbances?

DEGREES OF FREEDOM

A sticky control valve

A disturbance in fuel temperature

A disturbance in fuel heating value (composition)

A disturbance in fuel source pressure

14.12
The non-isothermal stirred tank chemical reactor in Example 13.12 is shown in the figure below. We want to control AC-1. What is the best cascade design for the disturbances shown in the figure?

DESIGN CRITERIA

14.13
The cascade control system for the packed bed reactor in Example 14.1 is oscillating as shown in the figure. What is the likely cause and what initial step would you take to correct the situation?

If you want to see a figure of the process, click on the hint.

The secondary controller is nearly unstable; place the secondary controller in manual.

The primary controller is nearly unstable; place the primary controller in manual.

We are really not sure. Place both controllers in manual and implement the cascade until the problem occurs.

14.14
When tuning the PRIMARY controller in a cascade system, we expect excellent results from the Ciancone tuning correlations.

IMPLEMENTATION

14.15
Cascade control is very effective for what secondary disturbance frequencies?

DESIGN CRITERIA

Lower than the critical frequency of the primary loop.

Higher than the critical frequency of the primary loop.

Lower than the critical frequency of the secondary loop.

14.16
The packed bed reactor from Example 14.1 is considered here. A significant pressure disturbance occurs in the feed pressure, which upsets the flow rate and product quality. How would you change the control design?

DESIGN CRITERIA

Control the reactor outlet concentration by adjusting the feed valve, v2.

Control the reactor outlet concentration by adjusting the set point to a feed flow controller via cascade (A1 -> F1->v2) .

Control the reactor outlet concentration as shown and control the feed flow rate by adjusting the feed valve.

14.17a
The following design is proposed for a cascade control system. Decide whether the cascade control design is acceptable or not, based on gains and dynamics responses.

DESIGN CRITERIA

K_p = gain, t = time constant, q = dead time

First system Kp in engineering units, t and q in minutes
Primary process		Secondary process
Kp1	2.3	Kp2	0.15
t1	53	t2	10
q1	12	q2	3

14.17b
The following design is proposed for a cascade control system. Decide whether the cascade control design is acceptable or not, based on gains and dynamic responses.

DESIGN CRITERIA

K_p = gain, t = time constant, q = dead time

Second system Kp in engineering units, t and q in minutes
Primary Process		Secondary Process
Kp1	0.02	Kp2	0.15
t1	53	t2	10
q1	12	q2	3

14.17c
The following design is proposed for a cascade control system. Decide whether the cascade control design is acceptable or not, based on gains and dynamic responses.

DESIGN CRITERIA

K_p = gain, t = time constant, q = dead time

Third system Kp in engineering units, t and q in minutes
Primary Process		Secondary Process
Kp1	2.3	Kp2	0.15
t 1	53	t 2	30
q1	12	q2	25

	AC-1 -> TC-1 -> vc
	AC-1 -> AC-2 -> vc
	AC-1 -> AC-2 -> va