Question 261General Subjects - Assistant Engineer

70% to pass

Suppose the illustrated pneumatically operated diaphragm actuated control valve is used to control the fuel oil outlet temperature of a steam-heated heavy fuel oil heater by controlling the steam flow. What would be the result if the stem packing was over-tightened and the stem was unable to vertically move in either direction? Illustration GS-0051

Diagram for USCG General Subjects Assistant: Suppose the illustrated pneumatically operated diaphragm actuated control valve...

AThe valve would fail in the fully closed position, most likely resulting in a low fuel oil temperature alarm condition.

BThe valve will lose control of the fuel oil temperature with the result that the fuel temperature will fluctuate with changes in fuel demand.

CThe valve would fail in the fully open position, most likely resulting in a high fuel oil temperature alarm condition.

DIt is not possible to predict how the valve would respond to over-tightened valve stem packing.

AI Explanation

The correct answer is B) The valve will lose control of the fuel oil temperature with the result that the fuel temperature will fluctuate with changes in fuel demand. Explanation: If the stem packing is over-tightened, the stem will be unable to move vertically in either direction. This means the valve will not be able to respond to changes in the fuel oil temperature and modulate the steam flow accordingly. As a result, the valve will lose control of the fuel oil temperature, causing it to fluctuate with changes in fuel demand, rather than maintaining a stable temperature. The other options are incorrect because: A) the valve would not fail in the fully closed position, C) the valve would not fail in the fully open position, and D) it is possible to predict the valve's response to over-tightened packing, as explained above.

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