CNG Compression Technology and Equipment 1 — Questions and Answers
Question 1: What type of compressor is most commonly used in CNG fueling stations for achieving the high pressures required for vehicle fueling?
- Centrifugal compressor
- Axial flow compressor
- Reciprocating piston compressor (Correct answer)
- Rotary screw compressor
Correct answer: Reciprocating piston compressor
Reciprocating piston compressors are the industry standard for CNG stations because they can achieve the high pressures (3,600 psi) needed for vehicle fueling.
Question 2: What is the standard service pressure for CNG vehicle storage cylinders in the United States?
- 1,800 psi (124 bar)
- 2,400 psi (165 bar)
- 3,000 psi (207 bar)
- 3,600 psi (248 bar) (Correct answer)
Correct answer: 3,600 psi (248 bar)
3,600 psi (248 bar) is the standard service pressure for CNG vehicle cylinders in the US, as established by NFPA 52 and industry standards.
Question 3: What is the primary purpose of an intercooler between compression stages in a multi-stage CNG compressor?
- Lubricate piston rings between stages
- Remove oil contamination from the gas stream
- Cool the compressed gas to improve efficiency and protect downstream components (Correct answer)
- Regulate final discharge pressure to the storage bank
Correct answer: Cool the compressed gas to improve efficiency and protect downstream components
Intercoolers cool the compressed gas between stages, reducing the work required for subsequent compression stages and protecting valves and seals from heat damage.
Question 4: In a multi-stage CNG reciprocating compressor, what happens to the gas at each successive compression stage?
- Pressure remains constant while only volume decreases
- Pressure progressively increases as volume decreases at each stage (Correct answer)
- Pressure alternately increases and decreases between stages
- Pressure only increases significantly in the final stage
Correct answer: Pressure progressively increases as volume decreases at each stage
Each compression stage progressively increases gas pressure while reducing volume, with the cumulative effect delivering gas at full CNG service pressure at the final stage.
Question 5: What is the function of a moisture separator or coalescing filter in a CNG compression system?
- Separate nitrogen from methane in the gas stream
- Remove particulate matter from inlet gas before compression only
- Remove water vapor and entrained liquids from compressed natural gas (Correct answer)
- Filter compressor lubricating oil from the discharge gas stream
Correct answer: Remove water vapor and entrained liquids from compressed natural gas
Moisture separators remove water vapor and liquid water from compressed gas to prevent corrosion, hydrate formation, and contamination of vehicle fuel systems.
Question 6: What material are piston rings and seals in oil-free CNG compressors commonly made from?
- Natural rubber compounds
- Lead alloy ring inserts
- PTFE (polytetrafluoroethylene) and carbon composites (Correct answer)
- Neoprene rubber
Correct answer: PTFE (polytetrafluoroethylene) and carbon composites
PTFE and carbon-filled composite seals are used in oil-free CNG compressors because they are chemically resistant, maintain integrity at high pressures, and require no lubrication.
Question 7: What is 'cascade storage' in a CNG fast-fill fueling station?
- A system where multiple compressors run in a pre-set timed sequence
- Multiple storage vessel banks maintained at different pressure levels to optimize vehicle fill speed and efficiency (Correct answer)
- An emergency overflow storage system that activates during peak demand periods
- A backup storage system that activates only during compressor failure
Correct answer: Multiple storage vessel banks maintained at different pressure levels to optimize vehicle fill speed and efficiency
Cascade storage uses banks of cylinders maintained at low, medium, and high pressure levels, drawing from lower-pressure banks first and topping off from high-pressure banks to maximize fill efficiency.
CNG Compression Technology and Equipment 1
What type of compressor is most commonly used in CNG fueling stations for achieving the high pressures required for vehicle fueling?