Design and Construction of CNG Containers (NFPA52 - 4.4)

4.4 Container Capacity. Containers are described by their liquid capacity and their design and allowable service pressures. The liquid capacity [ft³ (m³) of water] is the volume of liquid that is required to fill the container. The gas storage capacity can be calculated from the liquid capacity and allowable service pressure.

The amount of gas stored in a container can be estimated by using the information in Table A.4.4 referenced to 70ºF (21ºC). The gas quantity, in standard cubic feet (Scf), can be estimated by multiplying the container water capacity [standard ft³ (m³)] by the stored volume [Scf/ft³ (standard m³/m³)] factor at a given pressure. One Scf of natural gas weighs approximately 0.0456 lb (0.0207 kg) and has an energy content of about 930 Btu (2725 W) (on a lower heating value basis).


4.4.1 Containers shall be fabricated of steel, aluminum, or composite materials. The container shall be designed for CNG service and shall be permanently marked “CNG” by the manufacturer.


4.4.2 Containers manufactured prior to the effective date of this standard shall be permitted to be used in CNG service if recommended for CNG service by the container manufacturer or if approved by the authority having jurisdiction.

4.4.5 Pressure vessels shall be manufactured, inspected, marked, and tested in accordance with ASME Boiler and Pressure Vessel Code, Section VIII or Section X, and shall be suitable for CNG service.
Adherence to applicable ASME Boiler and Pressure Vessel Code, case interpretations and addenda shall be considered as compliance with the ASME Boiler and Pressure Vessel Code.

Best Industry Practice: In the past it was common to use a large number of small U.S. Department of Transportation (DOT) bottles manifolded together to provide the required storage volume. While this approach had a lower initial cost, it required a retest/recertification of the bottles at 5 year intervals. The large number of fittings also tended to cause an unacceptable number of leaks under routine operation. DOT bottles are currently used mostly for older small storage systems only.

In the last 10 years the use of DOT bottles has declined considerably and has been prohibited in some jurisdictions. ASME storage systems have very few potential leak points, can store large volumes of gas on a small foot print and can be outfitted with condensate drain valves. Most jurisdictions do not require any retest or recertification beyond that which is required by ASME or the owner’s insurance company.

4.4.6 The + (plus) and * (star) markings on DOT and TC cylinders shall not apply in accordance with DOT and TC regulations for cylinders for flammable compressed gases. The star marking shall be removed or obliterated. The removal of the marking shall be by peening and otherwise shall be in accordance with DOT or TC regulations. Grinding shall be prohibited.

4.4.7 Welding or brazing for the repair or alteration of an ASME pressure vessel shall comply with the documents under which the pressure vessel was fabricated. Other welding or brazing shall be permitted only on saddle plates, lugs, or brackets attached to the pressure vessel by the pressure vessel manufacturer.

4.4.7.1 The exchange or interchange of pressure vessel appurtenances intended for the same purpose shall not be considered a repair or alteration.

Cng Cylinder Container