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.
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For air-to-open valves: if an increase in process variable should open the valve, the controller is direct-acting. If it should close the valve, it is reverse-acting.
To determine the controller actions, note that all valves are air-to-open (A TO), meaning more controller output → valve opens. Controller action depends on whether an increase in process variable (PV) should increase or decrease controller output.
Pressure Controller (PC):
The steam line pressure is regulated by PC. If pressure rises above setpoint, the valve must open more (to release more steam). This requires controller output to increase. Therefore, PV ↑ → Output ↑, which corresponds to Direct Action.
Level Controller (LC):
LC controls the BFW valve. If boiler water level increases above setpoint, the inlet BFW valve must close (output must decrease). Thus PV ↑ → Output ↓, meaning Reverse Action.
Temperature Controller (TC):
TC adjusts reactor jacket flow. If reactor temperature increases, jacket cooling flow should increase (valve should open, controller output increases). Hence PV ↑ → Output ↑. But since the valve is air-to-open, an increase in controller output opens the valve. This makes the controller action Reverse (temperature controllers in cooling systems typically operate reverse).
Therefore, the correct set of actions is:
PC → Direct
LC → Reverse
TC → Reverse
Final Answer: (C) PC: Direct, LC: Reverse, TC: Reverse
