🔍 Key Concepts

• Operation of a pneumatically operated diaphragm actuator and how air pressure and spring force move the stem • Difference between an air failure (fail-safe position) and a mechanical jammed stem • How a fixed steam valve opening affects fuel oil outlet temperature as fuel demand changes

💭 Think About

• If the stem packing is so tight that the stem cannot move up or down, what happens when the temperature controller tries to increase or decrease the air signal to the actuator? • With the steam valve stuck at one fixed opening, what will fuel oil outlet temperature do when fuel flow to the heater increases or decreases? • Does a mechanically stuck stem automatically drive the valve to fully open or fully closed, or does it simply prevent it from changing position?

✅ Before You Answer

• Be clear on the valve fail-safe action with air loss versus what happens when the stem is physically locked • Decide whether a stuck stem means the valve will be forced to an end position or held at whatever position it was in when it jammed • Relate a fixed steam flow through the heater to the likelihood of high temp alarm, low temp alarm, or fluctuating temperature with load changes