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 there was a complete loss of pilot air being delivered to the valve actuator? Illustration GS-0051

Correct Answer: Option C — The valve would fail in the fully open position, most likely resulting in a high fuel oil temperature alarm condition.

This valve is designed as 'air-to-close' (fail-open). Upon loss of control air, the spring force dominates, driving the valve to its fully open position. In a fuel oil heater application, this allows maximum steam flow, causing the fuel oil outlet temperature to rise, triggering a high-temperature alarm condition.