ITMC Pneumatic Systems 5 — Questions and Answers
Question 1: During preventive maintenance on a pneumatic system, the technician notices the lubricator oil level is not decreasing. The MOST likely cause is:
- Excessive system pressure
- The flow rate is too low to create the pressure differential needed to draw oil (Correct answer)
- The oil viscosity is too thin
- The receiver tank is too large
Correct answer: The flow rate is too low to create the pressure differential needed to draw oil
Lubricators are venturi-operated and require a minimum flow rate to create sufficient pressure differential to draw oil into the air stream.
Question 2: What is 'water hammer' in a pneumatic distribution system and what causes it?
- Excessive compressor speed causing vibration in pipes
- Pressure shock waves caused by sudden valve closure trapping liquid slugs (Correct answer)
- Overheated air expanding rapidly in pipes
- Condensate striking the compressor intake
Correct answer: Pressure shock waves caused by sudden valve closure trapping liquid slugs
Water hammer occurs when accumulated liquid condensate is driven at high velocity into a closed valve, creating destructive pressure spikes.
Question 3: Which ISO cleanliness class applies to instrument and control air requiring the highest quality?
- ISO 8573-1 Class 5
- ISO 8573-1 Class 1 (Correct answer)
- ISO 8573-1 Class 3
- ISO 8573-1 Class 7
Correct answer: ISO 8573-1 Class 1
ISO 8573-1 Class 1 specifies the strictest limits for particulates, water content, and oil content for critical instrument and control applications.
Question 4: A regenerative desiccant dryer uses two towers alternating between drying and regeneration. Regeneration is accomplished by:
- Heating the desiccant with electric elements while bypassing compressed air
- Purging the offline tower with a portion of dried air to drive off absorbed moisture (Correct answer)
- Flooding the tower with nitrogen to displace water vapor
- Applying vacuum to pull moisture from the desiccant
Correct answer: Purging the offline tower with a portion of dried air to drive off absorbed moisture
Heatless desiccant dryers direct a small fraction of dried compressed air through the offline tower to strip moisture from the desiccant beads and exhaust it to atmosphere.
Question 5: A pneumatic system uses a pressure-compensated flow control valve. Compared to a standard flow control, what advantage does it provide?
- It automatically adjusts for changes in supply pressure to maintain constant actuator speed (Correct answer)
- It eliminates the need for a directional control valve
- It increases maximum system pressure
- It prevents pressure from dropping below the set point
Correct answer: It automatically adjusts for changes in supply pressure to maintain constant actuator speed
A pressure-compensated flow control maintains constant flow rate (and thus actuator speed) even when supply pressure or load pressure varies.
Question 6: When sizing a compressor for a pneumatic system, the designer must account for which additional load factor beyond the total calculated SCFM demand?
- Voltage drop in the motor wiring
- A leakage allowance (typically 10–30% of demand) for system inefficiencies (Correct answer)
- The weight of connected actuators
- The distance from the compressor to the furthest actuator
Correct answer: A leakage allowance (typically 10–30% of demand) for system inefficiencies
Industry practice adds a 10–30% leakage and future-growth factor to calculated demand when selecting compressor capacity.
Question 7: A technician is troubleshooting a pneumatic circuit where a cylinder extends but will not retract. The 4/2 solenoid valve shifts correctly on both signals. The MOST likely cause is:
- The pressure regulator is set too low
- A blocked or failed flow control on the retract exhaust path or a damaged rod-end port seal (Correct answer)
- The compressor cut-out pressure is too high
- The cylinder bore is oversized for the load
Correct answer: A blocked or failed flow control on the retract exhaust path or a damaged rod-end port seal
If the valve shifts but the cylinder won't retract, a blocked exhaust path (clogged muffler or flow control), crushed tubing, or a failed rod-end seal preventing pressure buildup are the prime suspects.
ITMC Pneumatic Systems 5
During preventive maintenance on a pneumatic system, the technician notices the lubricator oil level is not decreasing.
The MOST likely cause is: