CNG Cylinder Inspection and Regulations 2 — Questions and Answers
Question 1: According to CNG cylinder inspection standards, which condition requires immediate removal from service?
- Surface rust covering less than 5% of the cylinder body
- A visible crack, burn mark, or bulge anywhere on the cylinder (Correct answer)
- Paint fading due to prolonged UV exposure
- A dent less than 1mm deep on the cylinder shoulder
Correct answer: A visible crack, burn mark, or bulge anywhere on the cylinder
Cracks, burn marks, or bulges indicate structural compromise and require immediate cylinder removal from service.
Question 2: Which standard specifically governs the design, testing, and labeling of compressed natural gas vehicle fuel containers in the US?
- DOT 49 CFR Part 178
- ANSI/NGV-2 (Correct answer)
- NFPA 52 Section 5
- ASME Boiler Code Section VIII
Correct answer: ANSI/NGV-2
ANSI/NGV-2 is the American National Standard specifically for CNG vehicle fuel containers, covering design, manufacture, and testing.
Question 3: A Type 4 CNG cylinder is best described as having which construction?
- Seamless steel construction with no liner
- Steel liner with partial carbon fiber wrap at the ends only
- Aluminum liner fully wrapped with fiberglass composite
- Plastic (non-metallic) liner fully wrapped with carbon fiber or fiberglass composite (Correct answer)
Correct answer: Plastic (non-metallic) liner fully wrapped with carbon fiber or fiberglass composite
Type 4 cylinders use a non-metallic plastic liner (typically HDPE) fully overwrapped with composite fiber material, making them the lightest cylinder type.
Question 4: What is the standard maximum service life of a DOT-specification CNG vehicle cylinder from its manufacture date?
- 10 years
- 15 years
- 20 years (Correct answer)
- Unlimited with passing annual hydrostatic tests
Correct answer: 20 years
Most DOT-approved CNG cylinders carry a 20-year maximum service life from the stamped manufacture date, after which they must be replaced.
Question 5: What is the primary purpose of hydrostatic pressure testing performed on CNG cylinders?
- To flush and clean residual moisture from the cylinder interior
- To verify the cylinder can withstand test pressures significantly above service pressure (Correct answer)
- To calibrate the accuracy of the attached pressure gauge
- To confirm the cylinder valve seals correctly at normal operating pressure
Correct answer: To verify the cylinder can withstand test pressures significantly above service pressure
Hydrostatic testing subjects the cylinder to pressures well above service pressure (typically 5/3 of service pressure) to verify structural integrity and detect permanent deformation.
Question 6: Which of the following is NOT required to be permanently marked on a DOT-specification CNG cylinder?
- The cylinder serial number
- The service pressure in psi
- The owner's contact information and vehicle identification number (Correct answer)
- The date of manufacture (month and year)
Correct answer: The owner's contact information and vehicle identification number
DOT required markings include serial number, service pressure, and manufacture date, but owner contact information is not mandated on the cylinder itself.
Question 7: What is the maximum permissible depth of an isolated gouge on a Type 1 (all-steel) CNG cylinder wall?
- 5% of measured wall thickness
- 10% of measured wall thickness (Correct answer)
- 20% of measured wall thickness
- No gouges of any measurable depth are permitted
Correct answer: 10% of measured wall thickness
Gouges exceeding 10% of the cylinder wall thickness on Type 1 steel cylinders require rejection, as deeper material loss compromises pressure-bearing capacity.
CNG Cylinder Inspection and Regulations 2
According to CNG cylinder inspection standards, which condition requires immediate removal from service?