During hands-on practical skill building, technicians use a depth gauge to assess damage on a compressed natural gas (CNG) cylinder.

During hands-on practical skill building, technicians use a depth gauge to assess damage on a compressed natural gas (CNG) cylinder.

At A Glance

Three key points public agencies need to remember about compressed natural gas (CNG) fuel system inspections are:

  • CNG fuel system inspections are required by federal regulation
  • They require a high level of accuracy and precision 
  • Proper technician training is critical

Over the last decade, fleet operators have asked, “Should we transition to natural gas, and if so when?” With the favorable economics that have evolved, shifting away from diesel has made so much more sense that many fleets are already deep into the conversion. Now with more than 40% of new refuse trucks and more than 25% of new transit buses being built to run on natural gas, the question has become: “How do we train our technicians, drivers, and other fleet support personnel to handle natural gas vehicle systems?” Technicians well versed on working with diesel or gasoline-­powered vehicles usually have no knowledge of high-pressure gaseous fuel systems.

Compressed natural gas (CNG) fuel systems have been proven to be safe due to stringent federal and manufacturer requirements, as well as due to the unique characteristics of natural gas. But they require very specific and very different safety procedures than traditional liquid fuels, including routine CNG fuel system inspections.

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 natural gas vehicles (NGVs).

Start with a General Visual Inspection

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.

Prepare for a Detailed Inspection

Before the detailed visual inspection can begin, technicians must gather information such as vehicle history, previous inspection records, any vehicle records of collision damage, or any accidents, vehicle identification number, 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 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.

Technicians need the equipment pictured for the detailed visual inspection.

Technicians need the equipment pictured for the detailed visual inspection.

The equipment required to conduct a detailed visual inspection include:

  • 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 location 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.

Determine the Damage

While conducting this inspection, NGV technicians will examine all components of the high-pressure system including cylinders and shields, brackets, pressure relief devices (PRDs), valves, fittings, lines, hoses, regulator, coalescing filter, and fuel receptacle. These components should be inspected for damage and 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 0.01 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 damage, so technicians should always consult the manufacturer of the cylinder if damage exceeds 0.01 inches for their specific requirements.

Technicians must verify that pressure relief devices (PRDs) are properly attached to the cylinder and are not deformed, bent, or corroded.

Technicians must verify that pressure relief devices (PRDs) are properly attached to the cylinder and are not deformed, bent, or corroded.

  • Level 2 damage includes any scratch, gouge, or abrasion with a damage depth of 0.011 to 0.05 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 0.05 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.
  • 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 his or her findings to management or the vehicle owner.

Technician Training is Essential

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.

Once the inspection of the CNG cylinder is complete, technicians should ensure the mounting brackets are properly secured and in good condition.

Once the inspection of the CNG cylinder is complete, technicians should ensure the mounting brackets are properly secured and in good condition.

CNG fuel system inspections allow technicians to identify unsafe conditions that 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 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. 

About the Author:

Kasia McBride, director of marketing for the ­Natural Gas Vehicle Institute (NGVi), has nearly a decade of experience in the natural gas vehicle industry, authoring multiple articles on the subjects of NGV fleet safety and CNG/LNG technology. She can be reached at [email protected].