Two or three vents required for diesel fuel tank?

1/24/2025

We have a 119-gallon double-wall fuel tank for our diesel fire pump.

Our tank rep believes we only need (2) vents total: (1) an emergency vent for the interstitial space and (1) a combined normal/emergency vent for the primary tank.

We have received pushback that we need (3) vents: (1) normal for the primary tank, (1) emergency for the primary tank, and (1) emergency for the interstitial space.

We are having difficulties finding any requirements either allowing or not the combination of the emergency and normal vent lines of the primary tank (NFPA, building code, or UL 142).

Do any of you know if the combination of these vents is typically allowed?

What do you typically see for the venting of a diesel fire pump fuel tank?

Note, this job does fall under 2022 California Building Code, but I am still interested in what is common even outside of California.

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6 Comments

Dan Wilder

1/24/2025 08:35:45 am

Are you speaking to the actual tank openings or how to pipe those tank openings out?

Looking back at the last diesel fire pumps, our tank had 4" vent and a 4" dual wall vent with the fuel return line connected to the main vent but another project had 3 vents (additional one was dedicated on the fuel return line so it was a smaller diameter, there were still 2x - 4" vents for the main tank and the containment area)...both indicate compliance with UL142 and only about a year apart.

Older version of UL142 referenced below but it is allowed to combine vents.

Section 8.1 Each primary containment tank and each compartment of a compartment tank shall have provision for both normal and emergency venting. The openings for these vents shall be located at the top of the tank. The interstitial (annular) space of a secondary containment tank shall have provision for emergency venting. The opening for this emergency vent shall be located at the top of the secondary containment and shall terminate vertically above the top of the primary tank. These vent openings shall be in addition
to the fill, withdrawal, and liquid level gauge openings.

Section 8.4 A vent opening that provides for emergency venting shall have a capacity not less than that specified in Table 8.1. A vent opening that provides for both emergency and normal vents shall also have a capacity not less than specified in Table 8.1. Emergency vents are not prohibited from use for normal venting of the primary tanks if the tanks are marked as specified in 48.1.1(e).

Section 48.1.1 Each tank shall be marked with:
e) Identification of the emergency vent openings

Below are the two configurations we have seen

https://blog.anvil-fire.com/2021/01/new-ac-fire-fuel-tank-sizesdesign-2021.html

https://blog.anvil-fire.com/2012/04/diesel-fuel-tank-accessories-for-fire.html

Ken Thackery

1/24/2025 10:15:35 am

If you have a double wall tank you need 2 vents, the tank will come with 2 threaded openings. Tank size dictates vent size. We did 3 750gpm diesel pumps this year and vent size was 2", we're doing a 2500 gpm now and they're 4", due to larger tank size. The pump submittals will tell you vent size and the fuel tank will be ordered accordingly through your pump rep. Also if you go to Clark fire.com there is tool to size pump exhaust and louvers for combustion/exhaust air. note: the inner/outer wall space is very very small so it can look like there is no gap but there is.

https://www.clarkefire.com/design-tools/pump-room-calculators

https://www.peerlesspump.com/products/fire/pes/

Jack G

1/24/2025 10:38:16 am

NFPA-20, 2019 11.4.1.6 indicates the vents for the diesel tank.
Aurora diesel fuel tank sketches for diesel tanks, single and double wall, Aurora sketches 916, page 257. And single wall, 915- 256.
Basically the main vent for both are the same ( a 2 inch screened vent that tees off just below the emergency vent which is the same size of the tank outlet. Standard or 4 inch vent sizes. Both have a connection to the fuel return line just above the tank ) .5 inch.
Double wall adds a vent for the interstice space size as determined
By the tank sketch/ Aurora sheet. Both vents are tee d off to side with a 2 inch screened outlet looking up. E- vent straight thru. ( for both interstitial and emergency)
Pipe and fittings to be black and mal- iron. Sizes of vents and fittings may change for 4 inch vents.
NFPA 20, 2019 indicates in ll.4.1.6.4 indicates the vents must be separate and not combined.
Fittings to be black and mal- iron. ( diesel heat melts the gala- turns it to sludge) Aurora adds NFPA 30 and UL-142 compliance.
I add a valve to the bottom drain plugs, to facilitate draining the tanks ( rated for fuel ) I use the blue pipe dope— rated for diesel fuel.,( not pipe dope rated for water )
I pipe it just like the Aurora sketches so as not to void the warranties.
So NFPA 20, 39- ul 142 indicate 1 vent single for single wall tanks and 2 separate for double wall tanks. Both show a vent for the fuel line piped into the main vent just above the tank.. was the third separate vent for the fuel return line- I m guessing?!
Your pump cuts from the supplier should include tank, tank piping and sizing sketches to follow.

COLIN LUSHER

1/24/2025 11:31:01 am

I'm interested to see the responses here too. Most are saying only 2 vents required for a double wall tank. But I work with Patterson tanks mostly, and each tank has four (4) vent outlets; 1 main vent for inner and outer tank, and one emergency vent for inner and outer tanks. I've always piped them separately. Would love to know if any of these can be manifolded. NFPA 20 is clear that inner tank and outer tank vents cannot be manifolded, but can the main vent and emergency vents for each tank be manifolded? Four vents out the wall seems excessive.

Todd E Wyatt

1/24/2025 02:51:16 pm

Per the "2022 California Building Code, Title 24, Part 2 (Volumes 1 & 2) with July 2024 Supplement updated", the applicable Referenced Standard is NFPA 20 - 2019 which is referenced in 412.2.4.1, 913.1, 913.2, 913.2.1, and 913.5.

The (4) 913 sections deal with Fire Pumps which would be applicable to the diesel fuel tank serving it.

NFPA 20 - 2019 references "diesel fuel supply tanks" in Chapter 11 Diesel Engine Drive :

11.4 Fuel Supply and Arrangement.
11.4.1 General.
11.4.1.1 Plan Review.
Before any fuel system is installed, plans shall be prepared and submitted to the authority having jurisdiction for agreement on suitability of the system for prevailing conditions.
11.4.3 Fuel Supply Tank Location.
11.4.3.1
Diesel fuel supply tanks shall be located above ground in accordance with municipal or other ordinances and in accordance with requirements of the authority having jurisdiction and shall not be buried.
A.‍11.4.3
Diesel fuel storage tanks preferably should be located inside the pump room or pump house, if permitted by local regulations. Fill and vent lines in such case should be extended to outdoors. The fill pipe can be used for a gauging well where practical.

This section refers to the AHJ and municipal ordinances so you should both of those sources.

Vents are discussed per the following :

11.4 Fuel Supply and Arrangement.
11.4.1 General.
11.4.1.6 Vent Piping.
11.4.1.6.1
Vent piping shall be arranged so that the vapors are discharged upward or horizontally away from adjacent walls and will not be trapped by eaves or other obstructions.
11.4.1.6.2
Outlets shall terminate at least 5 ft (1.5 m) from building openings.
11.4.1.6.3
Outlets shall terminate at least 12 ft (3.7 m) above the finished ground level.
11.4.1.6.4
Vents from interstitial spaces of DOUBLE-WALL TANKS shall meet the requirements of 11.4.1.6.2 and 11.4.1.6.3 and shall not be manifolded together with a vent from the primary compartment of the tank.

Jassica @ Diesel Storage Tank link

7/26/2025 12:37:23 am

Helpful post! It’s good to know that most diesel tanks need two vents—one for the main tank and one for the space in between. Clear and easy to understand.

Two or three vents required for diesel fuel tank?

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