Use Refill to Downsize a Water Storage Tank?

5/22/2023

This may sound like a strange question, but here it goes.

Is there a way to downsize a fire water storage tank based on the refill rate supplied by the city?

I know that in Houston there was a company that supplied calculations showing that the refill rate could supply the tank at a refill rate based on the flow test.

Could anyone substantiate this principle?

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15 Comments

Anthony

5/22/2023 07:41:23 am

This is called a break tank. Ch 14.5 NFPA 22-2018 ed.

james evans

5/22/2023 08:28:32 am

I have designed break tanks, but this is a scenario where the city does not have the required flow and the tank is a static tank. In my case it is about 300,000 gallons if i cannot downsize it due to the city supply flow. A break tank is used when the city has enough flow but they want an air gap to insure no back siphonage.

5/22/2023 06:05:00 pm

NFPA 22 standardizes the construction of fire protection water storage tanks for many purposes (including the tank water supply for a fire pump), defining a "break tank" as a tank providing suction to a fire pump whose capacity is less than the fire protection demand.

Here's the elephant in the room:
Can a break tank be sized less than the 15-minute duration of a fire pump operating at 150%of the rated capacity when calculating the tank Refill Mechanism (using a dependable water supply) as required by Chapter 14 (NFPA 22)?

Dan Wilder

5/22/2023 08:04:37 am

I've completed a couple over the years, not a break tank specifically, but an allowance to reduce the size of the main water tank based on available refill rates from the local water department.

We requested a letter from the water department (we did a flow test for the other) on available, worst case water refill to a tank (GPM IN), took the 150% demand from a fire pump (GPM OUT), and subtracted the two for actual usage. With AHJ approval, we had to prove at time of commissioning that everything worked as modeled...this was an exception due to space issues on the site more than a workable VE item.

ParkUSA is the company we last talked with about an actual Break Tank application (if that helps).

james evans

5/22/2023 08:30:53 am

Is there any place in code where this is allowed? I am not sure if it has been considered in water supplies for fire pumps. Yes the break tank concept is allowed but this is when the supply is sufficient.

Jesse

5/22/2023 08:07:40 am

I'm pretty familiar with the break tank requirement in Houston. And yes, you can consider the tank refill rate in your calcs. Not just for break tanks, but static supply tanks as well.

james evans

5/22/2023 08:32:24 am

Where in code is this allowed? I am more concerned with the static tanks, I have designed break tanks before.

sean

5/22/2023 11:14:26 am

The code does not restrict it. The code allows automatic water supplies and tanks. No where that i know of says you can't combine them.

Paul Pinigsi link

5/22/2023 08:08:54 am

Yes, we do then possible. It is essentially the same concept as a break tank. We have used this approach several times.

Glenn Berger

5/22/2023 08:12:27 am

I have done this in remote locations. Need to note, the reason that you have a water tank is that the city utility is unable to provide the required fire flow. Proceed with caution.

SCHULMAN

5/22/2023 08:34:32 am

Glenn's point is a good one ... Why do you have a tank if the city water is reliable enough to base calculations on? Yes, please proceed with caution especially if you are in an area where development is continuing as "city water refill rates" are subject to change / diminish over time. We don't count on "city power" when we are designing life-critical systems that require backup power as an example.

Casey Milhorn

5/22/2023 08:57:19 am

Yes, refill can be considered for storage tank size, as long as the source is deemed as a Reliable Automatic Refill. This is per NFPA 22.

Franck

5/22/2023 09:09:19 am

You're right saying that break tanks are normally used to create a "no connection zone" between city water supply and sprinkler system. The break tank is only used to be refilled by the city water at the rate the pump is taking water from it.

But as indicated above, your configuration is allowed as it is some kind of break tank anyway (but with a greater water capacity in your break tank).
When you are considering the refill rate to be sufficient to provide the duration (between the tank capacity + the refill capacity for the duration) with the pump operating at 150% as indicated by Dan, you have more than sufficient margin (no fire will require your fire pump to operate at 150% for the entire duration... otherwise you will be in an overtaxed system from the very first minute and this means that you won't be able to control the fire anyway).

But if you have no space limitation and if this is not an existing situation (i.e., existing tank but change of occupancy / hydr. calc. ends up with a higher demand), it would be better anyway to provide a dedicated tank of the dedicated size.
That way, if you have city water supply shortage (works on the city water mains, for example), you don't end up with an impairment on which you have no control.

Jay

5/22/2023 11:23:31 am

4.1.6 *
A tank shall be sized so that the net capacity plus automatic refill shall meet the system demand for the design duration, except as modified by Section 14.5 for break tanks

Alex

5/22/2023 04:18:38 pm

As others have mentioned, sizing is similar to a break tank.

The sizing of the tank is governed by the 2013 edition of NFPA 22.

Section 14.5.1(3) states that break tank shall be used to augment the city water supply when the volume of water from the city supply is inadequate for the fire protection demand.

Section 4.1.5 states that the size of the break tank shall be 150% of the rated capacity of the fire pump for a duration of 15-minutes.

When designing a break tank, also consider the required flow rate, the gallons per minute being supplied into the tank, and the tank's initial volume.

Thanks,
Alex

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Use Refill to Downsize a Water Storage Tank?

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