Suppose the illustrated self-contained, internal-pilot, piston-operated temperature control valve is part of the temperature control system for a steam-heated heavy fuel oil service heater for a steam boiler. If there was a decrease in demand for fuel by the boiler, what statement correctly represents how the valve would initially respond? Illustration GS-0045

Correct Answer: Option D — The fuel oil heater fuel outlet temperature would increase, causing the remote bulb pressure to increase and the control diaphragm to flex downward and through lever action, further close the pilot valve.

A decrease in fuel demand causes the heater outlet temperature to rise. The thermal bulb expands, increasing pressure on the control diaphragm. This flexes the diaphragm downward, which, through the mechanical linkage, closes the pilot valve, reducing steam flow to the heater to maintain the desired fuel temperature.