How to Calculate Diesel Fuel Tank Vent Capacity?

4/7/2026

I have a plan reviewer requiring calculations (CFH) for a diesel fuel tank because the vent extends further than the UL label is calculated for.

What are the guidelines and how is this calculation performed?

The comment: "Provide the required emergency vent capacity, in Cubic Feet Per Hour (CFH), for the primary and secondary fuel tank emergency vents. The venting capacity should be indicated on the UL label on fuel tank. The shop drawings don't list the emergency vent capacity. Note: The capacity listed on the UL label is calculated with the vent within 12 inches of the tank body. If the vent is extended above the tank more than 12-inches, additional calculations are required to account for the additional piping and height above the fuel tank. These calculations are required to be computed by a registered engineer."

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5 Comments

Dan Wilder

4/7/2026 08:05:23 am

Looking at UL142, that scope is the tank itself and openings (for items like venting and fill), accessories are specifically excluded on the website. I would reach out to the MFG to discuss.

https://www.ul.com/thecodeauthority/knowledge/ul142-aboveground-flammable-liquid-tanks

Features not covered by the UL 142 Listing

UL 142 tank Listings covers the features and accessories described above, which are described in the individual Listings. Any other accessories or components that are shipped with the tanks, attached to the tanks or added to the tanks are not included in the scope of the tank Listing. It is anticipated that the code authority will approve the use and/or installation of any such accessories independent of the tank Listing.

Glenn Berger

4/7/2026 08:16:38 am

NFPA 30 (2021) - Section A.27.8.1.6 "Vent sizing formulas and prescriptive vent sizes, such as those established by UL 142, Steel Aboveground Tanks for Flammable and Combustible Liquids, are typically based on the direct installation of a venting device onto a tank. When the outlet of a vent must be extended to a remote location, such as for tanks located in buildings, which require vent discharges, to be located outside, a significant reduction in vent flow can occur unless the size of the vent and connecting piping is increased. In such cases, the size of vents and vent pipe extensions should be calculated to ensure that a tank will not be overpressurized during a fire exposure."

Dan mentioned going to the tank mfg for additional information can be a good starting point.

Joe Mc

4/7/2026 08:52:46 am

If the emergency vent is piped above the tank more than 12 inches:

You must recalculate effective venting capacity due to:

Pipe friction losses
Added vertical head
Fittings (elbows, screens, weather caps)
Engineering approach:
Use vent manufacturer flow curves
Apply corrections for:
Equivalent pipe length
Pressure drop

Ensure the adjusted capacity ≥ required UL venting rate.

In many cases, extended piping significantly reduces capacity, requiring:

Larger vent size, or
Multiple vents

Ricardo

4/7/2026 11:51:17 am

If the calculations are required, where is the information found to perform such calculations? This appears to border on aerodynamics of the tank, pipe and the end nozzle.

There is a section in the SFPE Handbook of Fire Protection Engineering, Smoke and Heat Venting that does discuss venting. Follow the calculations and understand with references. The calculations are all done in metrics and results answer flows to less than 1% change over a 100m elevation change. It's worth reading and understanding the minimal difference. These are the same calculations I recall in aerodynamics class.

My book is the 3rd edition but still relevant.

Ray

4/8/2026 11:07:35 am

According to the 2024 Edition of NFPA 30, the Boiler and Pressure Vessel Code dictate the sizing requirements for the emergency vents. See the code section below:

22.7.4* Extension of Emergency Vent Piping.

Piping to or from approved emergency vent devices for atmospheric and low-pressure tanks shall be sized to provide emergency vent flows that limit the back pressure to less than the maximum pressure permitted by the design of the tank. Piping to or from approved emergency vent devices for pressure vessels shall be sized in accordance with the ASME Boiler and Pressure Vessel Code.

How to Calculate Diesel Fuel Tank Vent Capacity?

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