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Tech Series on CNG High-Pressure Systems: Part 1 – Fill Receptacles

By Kasia McBride, Marketing Manager, NGVi 


Since one of the main characteristics of CNG vehicles is their high-pressure fuel system, NGV technicians are required to fully understand specific safety standards and procedures and know how each component within the fuel system should operate. Specifically, the high-pressure side of the fuel system includes the fueling receptacle(s), defueling receptacle, check valves, quarter-turn valves, mechanical gauge(s), cylinder valves, storage cylinders, PRDs, solenoids, coalescing filter, lines, fittings and high-pressure regulators.

This article is the first in a series which provides technicians with the information they need to better understand the components included in the high-pressure side of the CNG fuel system. It also provides details on safety procedures–as well as some useful tips for maintaining NGVs.

The Fill Receptacle

Each CNG-powered vehicle is equipped with one or more fuel receptacles designed to meet the NGV-1 standard. This standard, established in 1993, defines the external size, location and design of the fastening groove, and the type of valve. It ensures a mechanical fit between fill nozzles and receptacles used for CNG vehicles.

These receptacles are a quick-disconnect style, made of stainless steel, and are designed for either 3,000 or 3,600 psi operation. They have a one-way check valve built in which prevents the backflow of gas (which would be at high-pressure). For faster refueling, heavy-duty vehicles may be equipped with multiple fill receptacles with a larger diameter option.

The service pressure of a vehicle’s CNG fuel system is recorded on a durable, readily visible label at the fueling connection receptacle on the vehicle. Based on the service pressure for which they are designed, CNG dispenser nozzles are sized and color-coded: 3,000 psi nozzles are blue and 3,600 psi nozzles are yellow. 

This color coding helps to prevent overpressurization of a 3,000 psi cylinder. A vehicle with a 3,600 psi system can be filled with a 3,000 psi nozzle (blue), but a 3,000 psi vehicle cannot be filled with a 3,600 psi (yellow) nozzle. There is a one millimeter difference in diameter between a 3,600 psi receptacle and a 3,000 psi receptacle, which prevents the use of a 3,600 psi nozzle from filling a 3,000 psi vehicle.

One of the main safety procedures for CNG fill receptacles is regular leak checking. An electronic leak check must be conducted during the federally required CNG fuel system inspections after every three years or 36,000 miles, or after any fire or accident. Some manufacturers and fleet operators recommend leak checking more frequently, such as annually or semi-annually. Additionally, each time a vehicle is brought into the maintenance facility, a receptacle leak check must be performed prior.

In order to ensure a gas-tight seal between the receptacle and fueling nozzle, an O-ring is fitted in the receptacle. Drivers and technicians must remember to visually check O-rings before each fueling. If the O-ring is missing or damaged, it must be replaced prior to fueling, otherwise leakage may occur. Also, in cold temperatures, O-rings can lose their elasticity and become dislocated when the nozzle is removed after fueling. In many cases, ice can form in receptacles and cause the check valve to hang open when the fueling nozzle is removed. Sometimes the ice can be cleared by pushing the nozzle back and removing it again, but paying attention to the O-ring’s condition on the receptacle is extremely important in order to avoid any leakage issues.

To improve safety in the event of a drive-away (where the vehicle driver forgets to disconnect the fueling nozzle from the receptacle before leaving), the receptacle must be mounted securely enough to withstand more than 150 pounds of pull force from the fueling nozzle. To prevent this from happening, CNG fueling hoses are designed to release at 110 pounds.

Finally, natural gas used for vehicular applications, and delivered from a CNG fueling station, must be free of any solid material, water, or oil. The easiest place to check for this is at the receptacle, which should be visually inspected for oil routinely. The heart of the natural gas fueling station is the compressor and most compressors require lubrication (oil). Over time, it is likely that some amount of the lubricant could leak into the fuel stream (unless the station is extremely well-maintained). Any accumulation of oil in or around the receptacle is a sign of oil carryover from the CNG compressor, and the oil may be transferred to the vehicle during the routine fueling process. Oil in the CNG fuel stream onboard vehicles can cause serious operational problems and damage—therefore, monitoring for oil at the receptacle is essential.

More information about receptacles and other high-pressure components of the CNG fuel system is covered in-depth in NGVi’s Heavy-Duty and Light Duty NGV Maintenance and Diagnostics Classes. For more information about NGVi’s training programs, click here.