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

Correct Answer: Option C — The valve will lose control of the fuel oil temperature with the result that the fuel temperature will fluctuate with changes in fuel demand.

Over-tightened stem packing introduces excessive friction, causing the valve to stick. This mechanical binding prevents the actuator from modulating the valve position in response to temperature changes. Consequently, the valve loses its ability to control steam flow, leading to uncontrolled fluctuations in the fuel oil outlet temperature.