CNG Facility and Ventilation Requirements 2 — Questions and Answers
Question 1: What is the minimum mechanical ventilation rate required for an enclosed CNG vehicle storage or repair facility?
- 0.75 cfm per square foot of floor area
- 1.0 cfm per square foot of floor area
- 1.5 cfm per square foot of floor area (Correct answer)
- 2.0 cfm per square foot of floor area
Correct answer: 1.5 cfm per square foot of floor area
NFPA 52 requires a minimum of 1.5 cfm per square foot of floor area for mechanical ventilation in enclosed CNG facilities.
Question 2: In a CNG facility, ventilation air inlets should be located at what height relative to the floor?
- At ceiling level to capture rising gas
- At floor level or as low as possible (Correct answer)
- At mid-wall height, approximately 4 feet
- Height is not regulated, only total airflow matters
Correct answer: At floor level or as low as possible
Ventilation inlets should be at floor level because natural gas, though lighter than air, can accumulate in low spots when equipment is cold-starting or leaking.
Question 3: Which code primarily governs CNG vehicle fueling facility design and ventilation in the United States?
- OSHA 29 CFR 1910.110
- NFPA 52 (Correct answer)
- NFPA 30
- API RP 500
Correct answer: NFPA 52
NFPA 52, Vehicular Gaseous Fuel Systems Code, is the primary standard governing CNG fueling and storage facilities including ventilation requirements.
Question 4: A CNG compressor room classified as a hazardous location must have electrical equipment rated for which hazardous location class?
- Class I, Division 2 (Correct answer)
- Class II, Division 1
- Class III, Division 1
- Class I, Division 1
Correct answer: Class I, Division 2
CNG compressor rooms are typically classified as Class I, Division 2 locations where flammable gases may be present under abnormal conditions.
Question 5: What is the purpose of explosion-relief panels or vents in a CNG compressor building?
- To allow fresh air into the facility during normal operation
- To reduce overpressure in the event of an internal explosion (Correct answer)
- To exhaust vehicle emissions from the building
- To maintain required humidity levels inside the facility
Correct answer: To reduce overpressure in the event of an internal explosion
Explosion-relief panels are designed to fail at low pressure differentials, venting blast energy outward and protecting the building structure if a gas explosion occurs.
Question 6: How should exhaust ventilation outlets be positioned in a CNG vehicle repair facility?
- At floor level on the same wall as inlets
- Near the ceiling to exhaust lighter-than-air gas that has risen (Correct answer)
- At mid-wall height opposite the air inlets
- Outlets are not required if inlets are properly sized
Correct answer: Near the ceiling to exhaust lighter-than-air gas that has risen
Exhaust outlets near the ceiling effectively remove natural gas that has risen due to its lower density than air, preventing dangerous accumulation.
Question 7: Under NFPA 52, when is continuous gas detection monitoring required in an enclosed CNG fueling facility?
- Only during overnight unattended operation
- Whenever the facility operates with the doors closed
- When the facility cannot provide the required ventilation rate (Correct answer)
- Only when the facility stores more than 1,000 scf of CNG
Correct answer: When the facility cannot provide the required ventilation rate
Continuous gas detection is required as an alternative safeguard when the required mechanical ventilation rate cannot be maintained, triggering alarms or automatic shutoffs.
CNG Facility and Ventilation Requirements 2
What is the minimum mechanical ventilation rate required for an enclosed CNG vehicle storage or repair facility?