CNG Compression Technology and Equipment 2 — Questions and Answers
Question 1: What should an operator check FIRST if a CNG compressor fails to start?
- Rebuild the compressor's piston rings immediately
- Replace the inlet gas filter cartridge
- Check electrical power supply, control circuit status, and any fault indicators on the control panel (Correct answer)
- Manually pressurize the inlet line to force the startup sequence
Correct answer: Check electrical power supply, control circuit status, and any fault indicators on the control panel
Electrical and control system faults are the most common and easily corrected causes of compressor startup failures, making power supply and control panel diagnostics the logical first step.
Question 2: What condition is most likely indicated by an abnormally high final-stage discharge temperature on a CNG compressor?
- Excessive lubricating oil accumulation in the crankcase
- Failed or worn compressor valves, or inadequate intercooling between stages (Correct answer)
- Excessively low inlet gas supply pressure
- Moisture contamination in the incoming gas stream
Correct answer: Failed or worn compressor valves, or inadequate intercooling between stages
Faulty or worn compressor valves that allow gas blow-by, or inadequate intercooling between stages, are the primary causes of excessive final discharge temperatures.
Question 3: What is the purpose of an unloader valve on a CNG reciprocating compressor?
- Prevent backflow from storage banks into the compressor
- Release excess gas automatically if the system exceeds maximum allowable working pressure
- Allow the compressor motor to start without compression load before gradually applying load (Correct answer)
- Control gas distribution between individual cascade storage banks
Correct answer: Allow the compressor motor to start without compression load before gradually applying load
Unloader valves allow the compressor motor to start without compression resistance, reducing starting current and mechanical stress, with compression load applied gradually as the motor reaches operating speed.
Question 4: Which parameter is most critical to monitor continuously during CNG compressor operation to detect developing problems early?
- Ambient humidity level outside the compressor building
- Crankcase oil level in the compressor only
- Stage discharge pressures and temperatures at each compression stage (Correct answer)
- Inlet gas odorant (mercaptan) concentration levels
Correct answer: Stage discharge pressures and temperatures at each compression stage
Monitoring stage discharge pressures and temperatures reveals compressor valve condition, intercooler effectiveness, and compression efficiency, providing early warning of developing mechanical failures.
Question 5: What action should an operator take IMMEDIATELY if a CNG compressor's lubricating oil pressure drops below the minimum safe operating level?
- Add oil and continue operation at reduced output capacity
- Immediately shut down the compressor to prevent bearing and mechanical damage (Correct answer)
- Switch the unit to manual bypass control mode
- Increase compressor speed to restore oil pressure through pump centrifugal action
Correct answer: Immediately shut down the compressor to prevent bearing and mechanical damage
Low oil pressure causes rapid bearing and crankshaft wear; immediate shutdown is mandatory to prevent catastrophic mechanical damage and potential gas release from seal failure.
Question 6: What does an increasing pressure differential across a CNG compressor inlet filter indicate?
- Normal wear that requires no operator action until the next scheduled service
- A change in the Btu content or composition of the supply gas
- The filter is becoming clogged with contaminants and requires cleaning or replacement (Correct answer)
- Compressor valve clearance needs adjustment
Correct answer: The filter is becoming clogged with contaminants and requires cleaning or replacement
A rising pressure drop across an inlet filter indicates debris accumulation that restricts gas flow, reduces compressor efficiency, and requires filter service to restore proper operation.
Question 7: How frequently should compressor valve inspections typically be performed in a CNG fueling station?
- Only when a valve failure is suspected based on performance degradation
- Once every five years as part of a major equipment overhaul schedule
- Per manufacturer specifications, typically every 1,000 to 4,000 operating hours (Correct answer)
- Daily external visual checks are sufficient for valve maintenance requirements
Correct answer: Per manufacturer specifications, typically every 1,000 to 4,000 operating hours
CNG compressor valves are high-cycle wear components prone to fatigue failure; manufacturer-specified inspection intervals ensure problems are identified before unplanned shutdowns occur.
CNG Compression Technology and Equipment 2
What should an operator check FIRST if a CNG compressor fails to start?