Difficulty: Medium
Correct Answer: Air-to-close valve with the controller indirect (reverse) acting
Explanation:
Introduction / Context:For an exothermic continuous stirred-tank reactor (CSTR), cooling reliability is critical to prevent thermal runaway. The control loop usually manipulates cooling water flow via a control valve on the jacket or coil. Choosing the correct valve fail action and controller action ensures both safety and proper loop direction.
Given Data / Assumptions:
Concept / Approach:An air-to-close valve (fail-open) will open upon loss of air, increasing cooling water flow and enhancing safety. Loop direction: when temperature rises above set point, the controller must open the cooling valve. For an air-to-close valve, increasing controller output pressure would close the valve; therefore the controller must be indirect (reverse) acting so that a rise in measured temperature decreases controller output, opening the valve.
Step-by-Step Solution:
Select fail-open hardware: air-to-close valve on cooling line.Match loop direction: rising temperature → more cooling → valve should open.Choose reverse (indirect) acting controller so higher temperature yields lower output pressure and thus opens an air-to-close valve.Verification / Alternative check:Typical P&IDs for exothermic services show fail-open cooling valves with reverse acting temperature controllers to achieve the correct sign.
Why Other Options Are Wrong:
Air-to-open with direct acting: air failure would shut cooling (unsafe).Air-to-open with reverse acting: wrong fail-safe and may complicate loop direction.Air-to-close with direct acting: sign mismatch—hotter reactor would drive valve to close.Common Pitfalls:Overlooking fail-safe hardware intent; always design for safe state on instrument failure.
Final Answer:Air-to-close valve with the controller indirect (reverse) acting
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