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GATE 2022 CH – Question 21

Instrumentation and Process Control · Controller modes, control valves and tuning · 1 mark · Multiple choice

A control system on the jacket side of a reactor is shown in the figure. Pressurized water flows through the jacket to cool the reactor. The heated water flashes in the boiler. The exothermic reaction heat thus generates steam. Fresh boiler feed water (BFW) is added to make-up for the loss of water as steam. Assume that all control valves are air-to-open. The controller action, 'direct' or 'reverse', is defined with respect to the controller. Select the option that correctly specifies the action of the controllers.

a jacketed reactor whose jacket water is circulated through a boiler. The pressure controller PC on the boiler operates the steam valve and its setpoint comes from the reactor temperature controller TC. The level controller LC operates the boiler feed water (BFW) valve.
  1. PC: Reverse, LC: Direct, TC: Reverse
  2. PC: Direct, LC: Reverse, TC: Direct
  3. PC: Direct, LC: Reverse, TC: Reverse
  4. PC: Reverse, LC: Direct, TC: Direct

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Show answer and explanation

Correct answer: (C) PC: Direct, LC: Reverse, TC: Reverse

Explanation

PC: when the boiler pressure rises, the steam valve (air-to-open) should open more, so the output should rise with the measurement: direct. LC: when the level rises, the BFW valve should close, so the output should fall: reverse. TC: when the reactor temperature rises, more cooling is needed, which means a lower jacket pressure and a lower setpoint for the PC, so the output falls as the temperature rises: reverse.