Referring to the illustrated pneumatically...

🔍 Key Concepts

• Normally open vs normally closed: Think about what position the valve plug is in when there is no air pressure on the diaphragm – look at how the spring is arranged. • Direct-acting vs indirect-acting (reverse-acting): Consider whether an increase in air pressure on the diaphragm will move the plug toward the seat (closing flow) or away from the seat (opening flow). • Fail-safe position: On pneumatically actuated control valves, the word "normal" usually means the position the valve goes to on loss of air supply, when only the spring is acting.

💭 Think About

• In the illustration as drawn, imagine there is no instrument air connected. With just the spring acting on the stem and plug, is the flow path between INLET and OUTLET open or blocked? • Trace what happens when air pressure is applied at the diaphragm air connection: does the diaphragm force the stem downward or upward, and does that motion tend to open or close the valve seat? • Match your observations to the definitions: direct-acting means that an increase in air signal drives the plug in the same direction as the actuator’s motion; indirect (reverse) means the increased air moves the plug opposite to what the spring alone is trying to do.

✅ Before You Answer

• Identify clearly whether, with spring force only, the plug is on the seat (no flow) or lifted off the seat (flow path open). That tells you NC vs NO. • Verify the direction of air pressure force on the diaphragm (which side of the diaphragm the air enters) and how that force is transmitted to the stem and plug. • Before picking an option, double-check that your choice’s combination of direct/indirect and NC/NO matches both: (1) the spring-action/air-action directions, and (2) the fail (no air) position shown in the figure.

Referring to the illustrated pneumatically operated diaphragm actuated control valve, what statement is true? Illustration GS-0051

The control valve is direct-acting and normally closed (NC).

The control valve is indirect-acting and normally open (NO).

The control valve is indirect-acting and normally closed (NC).

The control valve is direct-acting and normally open (NO).

🔍 Key Concepts

💭 Think About

✅ Before You Answer