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CNG Fuel System Inspections: Technician Training is Key

By Kasia McBride, Marketing Manager, NGVi

CNG fuel system inspections are complex processes, during which technicians must be able to properly identify and determine the extent or significance of any damage discovered to system components, including cylinders, and be clear on what specific actions must be taken. Whether any damage or other concerns about fuel system integrity are due to accidents, misuse of system hardware, or simply through long-term wear and tear, the damage is not necessarily easy to identify for those unfamiliar with the components involved.

To conduct CNG fuel system inspections correctly, safely, and accurately, with the obvious goal of averting any unsafe driving or maintenance situations, technicians must be trained on the entire process of fuel system inspection, all related components, and safety practices specific to NGVs.

There are two different kinds of CNG fuel system inspections: the general visual inspection and the detailed visual inspection. General visual inspections are the industry recommended best practice. These are basic inspections of the system’s visible elements that should be conducted regularly as part of the vehicle’s routine maintenance schedule. During these inspections, technicians must look for any signs of obvious or gross external damage to the cylinder shields, loose or damaged mounting brackets, and any damage to the high-pressure components of the fuel system. If evidence of damage is found during the general visual inspection, then a detailed visual inspection becomes necessary.

In addition to conducting the detailed visual inspection when damage is found, this higher level inspection is required every three years or 36,000 miles (whichever comes first) of vehicle use, and after any fire or accident, according to standards ANSI/NGV-2 and FMVSS 304.

The detailed visual inspection covers the entire high-pressure fuel system, including cylinders, and must be conducted by sufficiently trained technicians. It should adhere to the cylinder manufacturer’s maintenance and inspection requirements, or secondarily to CGA C-6.4. – (Methods for External Visual Inspection of Natural Gas Vehicle and Hydrogen Vehicle Fuel Containers and Their Installations.) Because the manufacturer’s requirements cover information and guidance for specific types and brands of the cylinders installed on the vehicle, they are more stringent, always take precedence, and should be the primary source for information.

Before the detailed visual inspection can begin, technicians must gather information such as the vehicle history, previous inspection records, any vehicle records of collision damage, or any accidents, VIN, license plate number, and other potential identifiers. This information becomes the foundation of the inspection and the documentation.

The next step is to prepare the vehicle for inspection. This includes gaining access to the cylinder area by removing shields that cover the cylinders (if at all possible) as well as cleaning all of the cylinders. It is very important that the surface of each cylinder be free of dirt and other debris prior to conducting inspection so that the inspector can visualize any damage.

The equipment required to conduct a detailed visual inspection includes:

  • Telescoping inspection mirror or go-pro camera (or similar device) and flashlight which are helpful when inspecting cramped, awkward, or hard-to-reach areas.
  • Measuring tape or ruler to measure the length and width of any cylinder damage.
  • Leak-check solution to check for leaks in fittings, valve packing and seals, pressure chambers, and other leak points. A portable methane detector may also be used to detect the general locatio n of gas leaks.
  • Depth gauge, a precision instrument used to measure the depth of any damage to the cylinder
  • A camera to document the inspection, including any discovered areas of damage
  • “Failed Cylinder Inspection” sticker
  • “Passed” inspection sticker
  • Inspection form and pen

After all necessary information and equipment are gathered and the vehicle has been prepared, the technician can proceed with the detailed visual inspection of the CNG fuel system.

While conducting this inspection, NGV technicians will examine all components of the high-pressure system including cylinders and shields, brackets, PRDs, valves, fittings, lines, hoses, regulator, coalescing filter and fuel receptacle. These components should be inspected for damage, obstructions and that they are properly seated. Technicians should also leak check all fuel system connections and fittings including all solenoid valves, high-pressure coalescing filter, quarter-turn valves, high-pressure regulators and PRDs. They should make sure that the O-ring is present on the receptacle, is free of debris, and is not torn, punctured, or otherwise damaged.

PRDs must be checked for any visual signs of damage, including gouges, scratches, corrosion, rust, bulging, or plugged channels. Technicians must verify that the PRDs are properly attached to the cylinder and are not deformed, bent, or corroded. Also during the inspection process, they should inspect the interface between the PRD and cylinder valve and make sure it is tightly seated with no gaps or evidence of being loose.

During the cylinder inspection, technicians must be able to verify any damage to the cylinder and assess its type and level. The potential damage sources can include heat, impact, corrosion, abrasion and chemical attack (for example, battery acid or solvents). Correct assessment of the damage source is extremely important because, for example, in the case of heat damage, it is impossible to determine the amount of heat to which the cylinder was exposed, nor the extent of potential internal damage within the cylinder’s protective layers. In a case such as this, the cylinder would have to be condemned.

Next, the technician must measure the depth and length of any damage found, including cuts, scratches, indentations, punctures, or abrasions observed on each cylinder. The damage must be photographed and recorded on the inspection form, which will later help technicians assess the level of the damage, and to consequently determine the final disposition.

CNG cylinder damage is formally classified into three levels:

Level 1 damage is defined as no damage or damage depth of less than or equal to .010 inches. In this case the cylinder does not require any repair and can be returned to service. Note that some manufacturers have different limits for cylinder damages, therefore technicians should always consult the manufacturer of the cylinder if damage exceeds .010 inches for their specific requirements.

Level 2 damage includes any scratch, gouge, or abrasion with a damage depth of .011 to .050 inches. This damage requires repair, a more thorough evaluation, testing or destruction. The manufacturer’s guidelines will determine the final outcome.

Level 3 damage is defined as damage of a depth greater than .050 inches or any damage sufficiently severe that the cylinder shall not be repaired and must be condemned.
Once the inspection is complete, the inspector must determine the outcome of the inspection.

If all cylinders exhibit Level 1 or no damage and all high-pressure fuel system components are undamaged, do not leak, and show any signs of needing repair or replacement, the vehicle can return to full service immediately.

If any cylinders exhibit Level 2 damage or one or more components of the high-pressure fuel system need repair or replacement, or any cylinders exhibit Level 3 damage or any components of the high-pressure fuel system need repair or replacement, the vehicle has to be removed from service and the technician must report their findings to management or the vehicle owner

CNG fuel system inspections require a high level of accuracy and precision, especially since measurable differences between the three levels of damage assessment are very small. Failure to properly assess and diagnose damage can jeopardize the safe operation of the vehicle, and increase the risk of accident both in the shop and on the road. With proper training, including not only theory, but applicable hands-on practice, technicians can obtain actual experience before they begin inspecting CNG fuel systems on the job.

Technicians who perform CNG fuel system inspections must have sufficient knowledge of all of the codes and standards governing CNG fuel system installation and CNG fuel system components, be able to interpret them, and apply that knowledge routinely in practice. They also must understand the mandated inspection requirements, and know how to properly apply the cylinder manufacturer’s maintenance and repair guidance and recommendations during their inspections.

CNG fuel system inspections allow technicians to identify unsafe conditions which could result in life-threatening situations. To do their jobs adequately, technicians need not only classroom training (the theory), but also the hands-on practical skill-building essential to measure, record, document, and to accurately assess any damage found within the context of the manufacturer’s written specifications. As a result, technician training is the single most important key to reducing risk and maximizing operational safety.