SOLUTIONS MANUAL - Control, 3 rd Edition By Romagnoli & Palazoglu ...

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SOLUTIONS MANUAL

For Introduction t o Process Control, 3 rd Edition By Romagnoli & Palazoglu 1 / 4

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SECTION I

(Introduction)

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I.1. What is the significance of James Watt’s flyball governor from the viewpoint of automatic control?

Solution:

James Watt’s flyball governor was developed to control the speed of the steam engine. This is achieved by adjusting the supply of steam. The governor consists of a vertical spindle (A) driven by the engine. Two heavy brass or iron balls (C) are pinned to the head of the spindle. These balls or their arms (B) are connected to a sliding piece (E) on the spindle by rods (D). This sliding piece engages a lever (F) as it moves up and down the spindle, depending on the speed of the engine. The lever closes or opens the throttle valve to vary the supply of steam. At a steady speed of the engine, the balls maintain a certain fixed distance from the spindle and thus keep the supply of the steam constant. If the speed of engine increases, the balls fly away from the spindle and move the lever to close the throttle-valve, reduces the supply of steam and slows down the engine. If the speed goes below the steady level, the balls come closed to the spindle and open up the valve to increase the supply of steam thus increasing the speed of engine. This is an inherently stable closed loop control system. Speed of the engine (output variable) is measured by the distance of the balls from the spindle and control action is taken by adjusting the steam supply (manipulated variable). See Figure I.S1.

FIGURE I.S1 James Watt’s flyball governor.

I.2. Two liquids are mixed together in a tank and the product is removed through an exit stream at the bottom of the tank. We know that changes in the feed stream flow rates can cause the tank to overflow or completely drain, and this is undesirable. Is it possible to modify the design of the tank to avoid instability? How?

Solution:

A B D

C

E F 3 / 4

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Yes, it is possible. One possibility is shown in Fig. I.S2.

FIGURE I.S2 Level control using an overflow weir.If the flow rates of A or B or both increase, the level increases and the mixture C exits from the top. If the flow rate decreases, the liquid level increases until it flows out through the exit (does not drain the tank). The outflow design is referred to as an “overflow weir.”

I.3. Neutralization of the pH of acidic effluents is a very important problem in wastewater

treatment processes. Figure I.1 depicts such a process:

FIGURE I.1 A pH neutralization process.The incoming stream contains mixed acids with varying buffering characteristics and a standard base stream is available for neutralization. Define all the relevant variables for the process as required.

• State the control objective(s) for this process.
• What are the possible controlled variables, manipulated variables, and

disturbances?

• Suggest a feedback and a feedforward control strategy for this process and show

A B C A B C

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