NGVConnection Newsletter - October 2015

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CNG Fuel System Inspection Challenges for Heavy-Duty Fleets

By Annalloyd Thomason, Vice President/General Manager, NGVi 

Preparing for CNG cylinder inspectionRecently I was asked by the American Trucking Association’s Technical Advisory Group to meet with them to discuss the challenges they face with CNG fuel system inspections and to help them frame potential solutions. Perhaps the greatest challenge for many heavy-duty NGV users is that the fuel system inspection interval (3 years or 36,000 miles, whichever comes first) occurs every 11-12 weeks in high-mileage fleets. The frequent inspection interval, combined with the fact that a CNG fuel system inspection is a fairly detailed process, makes the labor burden and vehicle downtime for heavy-duty CNG fleet operators significant. This article discusses the CNG fuel system inspection background, the history of CNG fuel system inspection intervals, and possible solutions and the safety responsibilities of technicians who conduct the inspections.

The Confusion About CNG Fuel System Inspections
Responses to CNG fuel system inspection requirements are all over the map. Many NGV users aren’t even aware that fuel system inspections are necessary. Others argue that inspections just “aren’t that big a deal.” Most don’t understand the real reason why they are necessary beyond the fact that they are recommended/required by regulations or standards. And countless others have no idea the high degree of risk management their companies place in the hands of technicians who are responsible for performing the inspections.

The Importance of CNG Fuel System Inspections
CNG fuel system inspections are necessary, required and important because natural gas is stored onboard CNG vehicles at extremely high pressures—at least 3,600 psi. While CNG fuel systems are extremely safe due to their rigorous manufacturing, testing and installation standards, if a fuel system is fully pressurized and a component has been installed incorrectly, damaged or otherwise compromised, the result can be severe injury or loss of life--preventable injury or loss of life.

Codes, Requirements, Regulations or Law?
In 1992, the NGV industry first published NGV2, a voluntary industry manufacturing standard for cylinders (ultimately integrated through the American National Standards Institute (ANSI) standards process) which included an in-use inspection interval. NGV2 states that “Each container shall be visually inspected at least every 36 months, or at the time of any re-installation, for external damage and deterioration…”

Inspecting PRDs and connections on CNG cylindersSubsequently in 1995, the National Highway Transportation Safety Administration, within the U.S. Department of Transportation, published a similar manufacturing standard (FMVSS 304) that also included an end-user inspection interval that is slightly different. FMVSS 304 states that CNG containers “should be visually inspected after a motor vehicle accident or fire and at least every 36 months or 36,000 miles, whichever comes first, for damage and deterioration.”

One of the problems is that both of these standards are manufacturing standards, and the federal standard (FMVSS 304) that would seem most like “law” uses the phrase “should be visually inspected,” not the “shall” word that usually accompanies a regulation or law. Add to that the fact that both NGV2 and FMVSS 304 “require” inspection of the cylinders, and not the entire fuel system, while industry best practice (and the CSA Group CNG Fuel System Inspector Certification process) calls for inspection of the high-pressure portion of the CNG fuel system—not just the cylinders, and there’s room for a lot of confusion.

Enter the Heavy-Duty CNG Truck Challenge
Due to their high annual mileage, many heavy-duty CNG trucks require CNG fuel system inspections every 11-13 weeks. The affected heavy-duty fleet operators find this requirement to be inconvenient and expensive, and as a result, at least two fuel system manufacturers have offered documents allowing a one-year/100,000 mile inspection interval. However, because FMVSS 304 is treated as a regulation, many end-users are unwilling to adopt the revised standard due to the potential risk. The thought principle goes like this: If there is litigation following an incident involving a CNG fuel system where the inspection interval or process is in question, and the vehicle owner did not adhere to the 3 year/36,000 mile inspection interval found in FMVSS 304, what is the likelihood that the courts would give a pass to the vehicle owner because they did not follow the most commonly adhered to standard. It’s a hypothetical scenario, for sure, but one that crosses the minds of fleet managers every day.

The NGV Industry Is Trying to Address the Challenge
Over the past several years, representatives of NGVAmerica and the Clean Vehicle Education Foundation have met with NHTSA to begin a dialog about revising the FMVSS 304 standard. A few years back, the reported result of a NHTSA meeting was that NHTSA told the industry to “go back to the manufacturers” to get resolution. However, because FMVSS 304 is treated like a federal regulation, many cylinder manufacturers have indicated they will not revise their requirements until NHTSA/FMVSS 304 blesses the move. And there you have a circular problem.

Using depth gague on CNG cylinder to assess damageIn the past few weeks, a renewed effort has been made between NGV industry representatives and NHTSA, with a meeting to open another conversation regarding inspection intervals. It was reported that the meeting went well and at least put the discussion on the table (again), but government is not known for its speed and in my opinion, the process of changing the intervals through NHTSA will be a slow one.

Other Solutions on the Table

It has been suggested that to solve the problem expediently, heavy-duty trucking advocates should approach another U.S. Department of Transportation entity—the Federal Motor Vehicle Carrier Safety Administration (FMVCSA)—to seek their approval of the adoption of a one-year/100,000 mile inspection interval. The logic behind this suggestion is that FMVCSA already requires an annual inspection and that the CNG fuel system inspection could/should become another element of that inspection. Opposition to this suggestion comes in the form of concern that the required level of knowledge by FMVCSA regarding CNG and CNG fuel systems—including cylinders—would be high and that in general, federal agencies do not get that deep into what might seem like a “specialty” subject.

So in the Meantime…
In the meantime, heavy-duty fleet managers are developing solutions to the need for frequent CNG fuel system inspections based on their company’s risk tolerance. Most of NGVi’s clients have decided to adhere to the three-year/36,000 mile inspection interval, but have worked with us to help them streamline the CNG fuel system inspection process for their specific vehicles.

Using telescoping mirror to see other side of CNG cylinderFor example, because heavy-duty trucks use CNG cylinders covered by extremely sturdy shields, it can take a significant amount of a technician’s time to remove and then replace those shields every time a CNG fuel system inspection is due. There is no requirement to remove cylinder shields--as long as the technician conducting the inspection can get a full 360 degree view of the cylinders. The use of mirrors and GoPro-type cameras to help visualize all surfaces of CNG cylinders can help avoid the need to remove shields.

Providing effective training for technicians—and providing that training using the actual fleet’s vehicles—can go a long way towards reducing the inspection time. And speaking of training, the safety of any CNG fleet is only equal to the knowledge and skill of the technicians performing inspections. Companies cannot afford the risk of an untrained or poorly trained inspector—whether that inspector is an employee of the company or a third party.

Looking forward, it seems that CNG fuel system inspection intervals for heavy-duty trucks may be increased. Where the responsibility for that decision winds up is anybody’s guess.

Basic Safety Practices for CNG Technicians
By Kasia McBride, Marketing Manager, NGVi

Tech checking filters on CNG engineEvery technician who works on or around CNG vehicles should be familiar with the basic safety concerns for these vehicles. While CNG has three main hazards, including high pressure, fire and asphyxiation, the primary hazard is high pressure. This is because the nominal fill pressure of compressed natural gas (CNG) stored onboard natural gas vehicles is 3,600 psi, and that pressure can be deadly if it is not treated with respect.

To address high-pressure concerns, all CNG fuel systems are equipped with many distinctive safety features. One of them is that, by manufacturing code, the high-pressure CNG fuel system components other than CNG cylinders are designed to withstand four times their rated pressure, which allows them to withstand at least 14,400 psi without bursting.

CNG cylinders also are manufactured to rigorous federal standards, and are made from much sturdier materials than gasoline or diesel fuel storage tanks. According to the standards, all CNG fuel storage cylinders must be manufactured to withstand 2.25 times their fill pressure. This means that all CNG cylinders have a minimum burst pressure of 8,100 psi which is far above the fuel delivery pressures of CNG fueling stations.

PRD on a CNG cylinderFurthermore, to protect CNG cylinders from rupture in the event of fire or other over pressurization, pressure relief devices (PRDs) are installed which “activate” (open) at a specified temperature or pressure, and thereby relieve the CNG cylinder of its pressurized contents if the pressure becomes excessive. Each cylinder is equipped with at least one of these mandatory safety devices—and many cylinders have two or more PRDs.

While these features help assure safety, it is important to remember that any high-pressure fuel system or component can be dangerous if maintained using unsafe procedures. It is therefore crucial that technicians who have daily contact with the high-pressure systems are aware of the appropriate maintenance and repair practices and safety procedures to assure their safety, and to avoid possible hazards of pressure.

Below are some general rules each NGV technician must follow while servicing, maintaining and repairing NGVs.

  1. Since manufacturer requirements cover specific types and brands of fuel system components, technicians should always refer to them as the primary source of information.

  2. It is essential that before they start working on NGVs, technicians must be familiar with each component within the CNG fuel system. They should also be able to identify whether it is a high- or low-pressure component.

  3. Technicians should always remember to turn off the gas supply to the specific section of the fuel system prior to working on it.

  4. They should depressurize the area of the CNG fuel system where repairs are necessary, and if needed, they should defuel all or a portion of the fuel system using the correct defueling procedure.

  5. Finally, as an important safety precaution, technicians should treat every fitting and component on a fuel system as if it contains high pressure. Unless they are certain the pressure has been relieved, they should never attempt to tighten, loosen or service any high-pressure fitting or component.
Special procedures must also be followed while performing service or repair of CNG cylinders, valves or PRDs. Before servicing these components, technicians should always remember to close the quarter-turn valve to shut off the flow of gas to the remainder of the high-pressure fuel system. When necessary, they should also use the defueling procedure specified by the manufacturer to completely defuel the affected cylinder(s).

Defueling panelDepending on whether the high-pressure CNG fuel system does or does not have a defueling port or bleed valve, there are two different types of system depressurization procedures. In the case when a defueling port or bleed valve is present, technicians should first close the cylinder valves or disconnect the cylinder electric solenoid valves prior to depressurizing, thus preventing the flow of gas from the cylinders to the remainder of the CNG fuel system. Then, they should use the defueling port or bleed valve to depressurize the remainder of the high-pressure fuel system.

For vehicles that do not have a defueling port or bleed valve, the typical depressurization procedure starts with closing the cylinder valves or disconnecting the cylinder electric solenoid valves, and then performing the rundown procedure (outlined below), which is commonly used to depressurize portions of the CNG fuel system (not including the cylinders):
    1. Shut off all of the individual cylinder valves or disconnect cylinder electric solenoid valves.

    2. Start the engine and let it run until it stalls or shuts off.

    3. After a short while (about two minutes), try to restart the vehicle--which should not start. To verify that the cylinder valves have sealed in the closed position and have shut off the gas flow from the cylinders, repeat the attempt of restarting the vehicle until you are sure the engine will not start.

    4. If the engine will not run, the high-pressure fuel system should have very low pressure—less than 150 psi.

    5. To confirm this, check the high-pressure fuel gauge. It is essential to verify that the system has very low pressure before you proceed. Also, remember that only the section of the fuel system downstream of the closed cylinder valves are depressurized and can be safely disassembled.

    6. Use two wrenches to open a fitting just to the point where gas begins to flow.

    The following is a generic description of one procedure for depressurizing the low-pressure CNG fuel system components.

    • Whenever possible, follow manufacturer’s procedures.
    • Close all cylinder valves or disconnect the cylinder electric solenoid valve.
    • Using the vehicle’s defueling port or bleed valve, depressurize the high-pressure fuel system.
    • Next, use a bleed valve (if present) to complete the depressurization process. If no bleed valve is present, technicians may need to use two wrenches to disconnect the fitting of the component to be serviced. Use one wrench to hold the body of the fitting and the other wrench to turn the fitting nut. This will prevent damage to the fitting.

    Fleet safety should be routine, and not reserved for special occasions. With sufficient training, including all appropriate procedures and appreciation for system pressure, NGV technicians can perform CNG fuel system maintenance safely and routinely as well. It is only with proper training, however, that this level of competency can be achieved.

Upgrading Your Vehicle Maintenance and Repair Facility for NGVs Series
(Part 5 of 5): Welding and Hot Work

By Annalloyd Thomason, Vice President/General Manager, NGVi 

weldingNone of the codes governing NGV maintenance facilities require building modifications to perform welding or hot work, but industry best practices describe precautions that should be taken to minimize risk. These precautions should be taken not only when welding or hot work is performed directly on a natural gas vehicle, but also when it is being performed on other vehicles parked inside the maintenance facility at the same time NGVs are present.

The first and most basic safety procedure to implement for welding on or around a CNG or LNG vehicle is to protect all the fuel system components with a welding blanket or other fire retardant device. This includes not only CNG cylinders or LNG tanks, but also any exposed components including stainless steel tubing and fittings, regulators, etc. This is necessary because if welding slag comes into contact with NGV fuel system components, it can cause damage that may compromise the safety of the fuel system.

Caution - Hot Work Permit Required in This AreaA second basic practice is to either (1) close the quarter-turn manual valves on each CNG cylinder and depressurize the remainder of the CNG fuel system using the run-down procedure; or (2) completely defuel the fuel system, including the cylinders, to minimize the amount of fuel onboard the vehicle.

In addition to the basic precautions described above, other safety measures may be advisable, depending on the circumstances and the level of risk protection your company chooses. These may include:

  1. The use of a Hot Work Permit process in advance of the welding procedure that is monitored and signed off on by a management representative.

  2. Ensuring that available sprinklers, hose streams, and fire extinguishers are in service and operable.

  3. Ensuring that the hot work equipment is in good repair.

    Welder using fire-resistant tarpaulins
  4. Within a safe distance (35 feet) of the hot work area:
    1. Removing any flammable liquids, dust, lint or oil deposits from the welding area.
    2. Ensuring floors are swept and clean.
    3. Ensuring that any combustible floors are wet down, covered with damp sand or fire-resistant sheets.
    4. Remove other combustibles where possible. Otherwise protect with fire-resistant tarpaulins or metal shields.
    5. The use of fire-resistant tarpaulins suspended beneath the hot work area.

  5. Institute a fire watch/hot work monitoring system where
    1. Fire watch is provided during and for 30 minutes after work.
    2. Monitor hot work area for 30 minutes after job is completed.

  6. Other precautions that may be taken could include
    1. Protect the welding area with separate smoke, heat, or flame detection.
    2. Use appropriate lockout/tagout procedure.

Finally, when the welding or hot work is complete, the CNG/LNG fuel system should be inspected for leaks before returning the vehicle to service.

This is the fifth and final article in the series “Upgrading Your Vehicle Maintenance and Repair Facility for NGVs”. If you have questions, or would like to speak with one of our experts about Vehicle Maintenance Facilities, call us at 800-510-6484 or email our Customer Solutions Manager Sabrina Dodd at

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CNG Fuel Price Report
From Clean Cities Alternative Fuel Price Report published by National Renewable Energy Laboratory (NREL) for DOE's Clean Cities Program

Overall Average Fuel Prices (as of July 2015)


Nationwide Average Price for Fuel This Report

Nationwide Average Price for Fuel Last Report

Change in Price This Report vs. Last Report

Units of Measurement

Gasoline (Regular)




per gallon





per gallon





per GGE

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Bus Fleet Replaced With Models Running on Compressed Natural Gas

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Natural Gas Vehicle Institute is North America’s leading provider of training and consulting on natural gas as a transportation fuel.

Our services address the full range of natural gas vehicle and fueling issues, including:

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