NFPA 52 is displayed as the shadow of NGV 6.1

Are You Confused by the Requirements of NFPA 52 and NGV 6.1?

Until recently, the primary code governing CNG fuel system installations has been NFPA 52. However, the 2023 version of NFPA 52 has introduced ANSI/CSA NGV 6.1 as a standard that also governs these installations, and, by default, inspection requirements.

The team at NGVi has performed a thorough analysis and comparison of both documents, and has found a few inconsistencies between them. These will likely be addressed in future versions, but should be brought to your attention in the meantime.

Before we discuss those inconsistencies in more detail, here’s a quick refresher on why NFPA 52 and ANSI/CSA NGV 6.1 are both important to the natural gas vehicle industry.

What is NFPA 52?

NFPA 52 is the fire code that governs natural gas-powered vehicles and fueling stations to mitigate hazards associated with compressed natural gas (CNG) and liquified natural gas (LNG). It was first published in 1984 and is updated every three years (except for this most recent edition which was delayed due to COVID). It should be noted that NFPA 52 is an NGV fuel systems code rather than a standard, and focuses on safety rather than performance. Each edition gives fuel system installers updated requirements and details for major components, such as where they’re permitted to be located on natural gas vehicles.

What Is ANSI/CSA NGV 6.1?

The 2019 edition of NFPA 52 introduced the performance-based standard, ANSI/CSA NGV 6.1, which started as a list of recommendations for fuel storage and delivery systems for on-road vehicles prior to becoming mandatory. NFPA 52-2019 explained that installers could abide by its requirements or by those found in NGV 6.1. Now, NFPA 52-2023 mandates that NGV 6.1 is the required installation standard to be used in addition to NFPA 52

Initially, this sounded like a smooth handoff from code to standard, until we reviewed both of their latest editions. The differences that we found mean that: 1) CNG vehicle fuel system installers and inspectors will now be required to be familiar with and apply both documents; and 2) CNG fuel system inspectors must be aware of these differences for performing accurate pre-service, general visual, and detailed visual inspections.

Changes to NGVi’s Training

Because the required location of some CNG components have changed, our curriculum design and course content are both being updated to reflect them. We want you to know that any technicians who take our classes will continue to receive the most up-to-date training that accurately explains these documents and how to apply them.

Important Differences Between NFPA 52-2023 and NGV 6.1-2021

Until we see any update from NFPA or ANSI/CSA, here are nine (9) differences that CNG fuel system installers and fuel system inspectors should know about between both documents.

1. Location of the fuel system automatic isolation valve.

NFPA 52 states that an automatic shutoff valve should be installed between fuel storage and the first-stage pressure regulator.

NGV 6.1 requires any automatic fuel system isolation device be located either at the outlet for fuel storage, or as close as possible to it.

The takeaway: While the location of this valve is not consistent, NGV 6.1 most closely describes where to place the automatic isolation valve relative to its intended purpose – to protect the high-pressure portion of the vehicle fuel system.

2. Messaging for pressure relief device (PRD) vent locations.

NFPA 5.2 states that PRD labels must read in all caps: “ATTENTION: CNG VENT LOCATION.”

NGV 6.1 states that the letters “P-R-D” should be centered on the labels for these vents.

The takeaway: While the messaging is not consistent, NFPA 52 most closely describes the best wording to label PRD vent locations.

3. The number and type of PRDs per cylinder.

NFPA 52 states that one or more thermally activated PRDs be used to protect the container/cylinder, and that the container manufacturer must specify the number, part number, and location.

NGV 6.1 states that each fuel container/cylinder must be protected by a PRD that adheres to both ANSI/CSA PRD 1 and the container manufacturer’s specifications.

The takeaway: While the number and type of PRDs are not consistent, NFPA 52 most closely describes the PRD requirements to adequately protect a cylinder, as determined by the cylinder manufacturer. 

4. Labels in the driver compartment.

NFPA 52 states that a permanent label should be present and visible in the driver’s compartment if the vehicle uses roof-mounted cylinders. This label should list the maximum total height of the vehicle in its unloaded configuration.

NGV 6.1 does not mention any requirement for a label in the driver’s compartment of a vehicle with roof-mounted CNG fuel storage cylinders.

The takeaway: While driver compartment label requirements are not consistent for vehicles with roof-mounted CNG fuel storage cylinders, NFPA 52 more clearly states the safety related provision.

5. Locations for a manual shutoff valve or manual fuel system isolation valve.

NFPA 52 states that the manual fuel system isolation valve should be located between the fuel system isolation solenoid valve and first-stage pressure regulator.

NGV 6.1 does not specify the location of a manual shutoff valve, but does indicate that it should be easily accessible and identifiable by a label (using appropriate colors and letters), and rotate no more than 90 degrees when opening/closing.

The takeaway: The nomenclature for these valves and their locations are not consistent. Neither NFPA 52 nor NGV 6.1 clearly states where a manual shutoff or manual fuel system isolation valve should be located. Knowledge and experience dictates that this valve should be located downstream of the fuel system automatic isolation valve. 

6. Identifying PRD fuel lines.

NFPA 52 has no label requirement for PRDs that use pressurized fuel lines to connect with container valves or end plugs.

NGV 6.1 states that any PRDs that connect to container valves or end plugs must use pressurized fuel lines that are labeled, “Pressurized at all times.” These labels may also be color coded.

The takeaway: While labels and color codes for PRD fuel lines are not consistent, NGV 6.1 more clearly states the need for them. 

7. Using thermally activated PRDs.

NFPA 52 states that each cylinder should have one or more thermally activated PRDs attached to them.

NGV 6.1 states that temperature sensors connected with temperature-activated PRDs should occupy the same space as the cylinders they monitor.

The takeaway: Technically speaking, NGV 6.1 does not require the use of thermally activated PRDs, while NFPA 52 historically has (and will likely continue to do so in future code revisions).

8. Manually activated PRDs.

NFPA 52 does not mention these PRDs at all.

NGV 6.1 states that manually activated PRDs should adhere to ANSI/CSA PRD 1.

The takeaway: While manually activated PRD use and requirements are inconsistent, NGV 6.1 does allow them to be used if the customer requests them.

9. Defueling system labels.

NFPA 52 makes no mention of having a label near the defueling system that contains specific information about the fuel system.

NGV 6.1 states that a label must be present near the defueling system that contains specific information about the fuel system.

The takeaway: While label requirements for defueling are inconsistent, NGV 6.1 provides an increased level of safety for technicians using the defueling system.

Why Is NGVi Offering This Information?

Our goal is to ensure that CNG fuel system installers and inspectors have the most up-to-date, accurate information about the components and systems they work with every day. We’re providing this comparison to clarify the intent and application of each document, and to aid in CNG fuel system inspections.