Sprinklers Required Underneath Large Tanks?

8/5/2020

I have a water treatment plant with 3 tanks, 9 feet in diameter. The fire protection contractor feels we need to install a sprinkler under each tank, following the greater than 4 wide rule for obstructed construction. The tanks are located on a concrete floor. The tanks are supported on legs, and the clearance from the bottom of the tank to the floor is about 2 feet.

Remembering that code tells us where to install sprinklers; my argument is this is not an occupiable space and sprinklers are not needed. The tanks are part of a water filter system, and not a hazardous type chemical storage.

Are sprinklers required underneath these tanks?

Any thoughts or input is most graciously appreciated.

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8 Comments

FIREPE25

8/5/2020 10:10:34 am

Sprinklers are required. Any obstruction greater than 18" below the deflector that is greater than 4' wide needs sprinklers. I believe the 2016 edition of NFPA 13 added some context about not needing them if the obstruction was close enough to the ground, but I am not sure on the exact language.

James

8/5/2020 10:11:40 am

Additional protection under the tank should not be required provided the bottom of the tank is 24" or less from the floor.

8.5.5.3.1.4 Sprinklers shall not be required under noncombustible
obstructions over 4 ft (1.2 m) wide where the bottom of the obstruction is 24 in. (600 mm) or less above the floor or
deck.

Paul

8/5/2020 11:16:13 am

Keep in mind, the 24 inch clearance was only added in the 2016 Edition of NFPA 13, so may not be adopted everywhere. This has not stopped me from submitting a variance/equivalent design to the AHJ's for approval, which most circumstance a valid argument/discussion allowed it to be approved.

Also, what are the sprinklers protecting, contents of the tank? The standard upright/pendent may not be applicable, just make sure.

GLENN BERGER

8/5/2020 11:58:23 am

Sprinklers are required. It will be easy for combustibles or other materials to eventually find its way to this "obstructed" space.

Mark C.

8/5/2020 01:34:03 pm

Is there a possibility to enclose the area below the tanks? If a noncombustible or limited-combustible concealed space is created, Section 8.15.1 allows for certain concealed spaces to not require sprinkler protection. If access for any type of piping or maintenance is required below the tanks, 8.15.1.2.2 specifically may still apply:

8.15.1.2.2 Concealed spaces of noncombustible and limited-combustible construction with limited access and not permitting
occupancy or storage of combustibles shall not require sprinkler protection.

Jonathan Joseph

8/5/2020 01:49:43 pm

I would put some kind of wired mesh screen and wrap around the legs so debris will not collect under the tanks if that is an issue.

Paul and James have mention the codes that meets the requirements of not having sprinkler protection as long as it is not over 24 inches in height and the contents inside the tanks are not hazardous storage.

Franck

8/6/2020 10:44:55 am

I'm afraid that every body is right (or wrong) and all refernecs are correct (even though they might be contradictory).
This will all depend on your AHJ or Insurance company and their interpretation of the situation.

I would personally not require a sprinkler underneath the tanks (but again, personal opinion) as long as we ensure that:
- the tank is non combustible
- the content of the tank is non combustible/flammable
- there is no other tank with flammable/combustible liquid in the same area
- there will be no combustible storage underneath (with 2 feet, there is little chance anyway to have combustible storage).

This being said, if combustible/flammable liquids are present in the area, it is a good practice to provide a sprinkler protection underneath. The same with combustible storage of goods.
With fl. liquids, sprinklers are usually installed upside down (i.e. pendent sprinkler in the upright position) to cool down the underneath of the tank, as it was required by a standard named Discuss (used for the process of distilleries of alcohol) which is quite a good document if you can read it.
The purpose of that sprinkler is that there is no pool fire underneath the tank that could expose the flammable/combustible liquid inside.

Be careful with the 4 in. rule and sense of judgement is of high importance.
There is no requirement, for example, to provide a sprinkler protection underneath tables, even though they are more than 2 ft, because it is highly improbable that storage will be present underneath.
The 4 ft dimension is justified, according to NFPA handbook, through past practice and experience, not through fire testing.
But some AHJ apply the zero-tolerance rule... And no argument can convince them.

Dale Morris

8/10/2020 10:57:31 am

I want to thank you all for your input. At the time I posted the question, I was not informed of the actual space under the tank. The clearance is 9 inches from the bottom of the tank to the floor. I love the idea of placing some mesh around the bottom to keep trash from accumulating under the tank.

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