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Tech Series on CNG High-Pressure Systems: Part 2 – High-Pressure Fuel Lines and Fittings

By Kasia McBride, Marketing Manager, NGVi 


This article is the second in a series, providing technicians who will be diagnosing and repairing NGV fuel system components with useful information about characteristics, maintenance and requirements of different components included in the high-pressure side of the CNG fuel system. 

High-Pressure Fuel Lines and Fittings

The high-pressure tubing and fittings are critical components of NGVs because they carry fuel throughout the entire CNG high-pressure fuel system. CNG vehicles use reliable high-pressure stainless steel, seamless fuel tubes, as well as fittings which are typically double-ferrule compression or O-ring face seal type. Where  a bend in the tubing is complex or difficult, flexible  tubing  is used. The common brands of  tubing and fittings used in the high-pressure fuel system on CNG vehicles are Hoke, SSP, Parker and Swagelok. 

Because the U.S. standards for CNG fuel are 3,000 psi (200 bar) and 3,600 psi (250 bar), CNG systems require specific types of tubing and fittings designed to withstand the burst pressure of four times the nominal fuel pressure of 3,600 psi at 70 degrees F. Traditional tubing, and fittings used in gasoline and diesel vehicles, such as those used to carry fuel, on brake systems, A/C or power steering, are inappropriate for high-pressure CNG fuel systems, and should never be used in natural gas vehicles.

High-pressure stainless steel fuel lines and fittings are durable, and usually maintenance-free. Should external damage to CNG fuel lines occur, such as dents, kinks or severe scratches, they must be replaced. This also applies to flexible tubing. It is not possible to simply splice in a new section of fuel line. In other words, if a fuel line or fitting is damaged, the entire section must be fabricated and replaced.

Additionally, fuel system fittings should be checked for leaks. The more fittings installed in the fuel system, the greater the possibility of leaks. O-rings used in O-ring face seal fittings, typically made of Buna-N material, are replaceable, but the replacement O-ring must be compatible with natural gas. The replacement procedure used must follow the manufacturer’s specific recommendations, which will specify whether a lubricant is needed when replacing the O-ring face seal. 

Technicians often need to recreate the bends in a length of tubing that needs replacing. Bending stainless steel tubing requires specific skills including proper measurement, cutting, deburring, and proper use of bending tools designed for stainless steel tubing. Because stainless steel tubing changes in length when bent, it is important to follow proper layout measurements and use correct bending tools. When recreating a length of tubing that needs to be replaced, technicians can use the old section as a model, unless this section has been bent or kinked. 

Here are a few useful tips to use when working with tubing:

Routing of the tubing must be located in the most protected area to avoid being exposed to a road environment of debris, moisture, hot components, rotating parts, etc.

In the case of creating many lines that run parallel to each other, the fittings must be arranged so one fitting and line doesn’t have to be disconnected to get to another one next to it.

Because the size of tubing varies with temperature change, technicians working on the tubing should make sure to not make straight runs, and always incorporate relief bends into the layout.

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