How to Calculate Refill from Existing Well?

6/5/2023

We have a Fire Marshal that is asking us to "show how the fill line can refill the [water supply] tank in a maximum of 8-hours, per NFPA 22 Section 14.4.2".

The tank we're proposing will be supplied by an existing well, but we do not have information on well capacity.

What parameters should we be looking for on well capacity? Do we have to test for both flow and time?

Is the computation on refill as simple as the flow rate (gpm) x time (8-hours) to get the contribution from the well?

Thanks in advance.

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6 Comments

Pete H

6/5/2023 06:52:26 am

Do you have the existing well's discharge and flow rate?

Cause yeah, it sounds like it's as easy as "Does the GPM from the well into our tank refill our full tank capacity in eight hours"

And while you may not know the full well capacity, being able to actually answer "And this will definitely not deplete the well in such a way to void other uses" because the well's capacity is that much larger than your tank's.

Take with a grain of salt, this is not something I normally ever deal with.

Dan Wilder

6/5/2023 08:02:26 am

Any historical data on the pump available (actual rated head/GPM, how long has it been there, any issues with low flow)?

Ya, as simple as flow x time....or the real world test when the tank is filled the first time. Maybe a flow test (providing it doesn't flood the site) over an hour or two, hopefully not the entire 8 hours.

Anthony

6/5/2023 08:08:21 am

A domestic well most of the time won't have the fill rate you'll need for your tanks. So that'll be a non-starter in most cases.

I suggest getting a contract with a local pool filler or water supplier.

Yes, you'll need you tank required volume/480 min. that'll get you your required GPM re fill rate.

Glenn Berger

6/5/2023 08:09:24 am

There are specialized civil engineers (maybe hydrologists) who can provide this information

Adam

6/5/2023 08:24:56 am

I've had the same question in the past (not on this forum), and never reached a solid conclusion.

My concern was that the flow from the well may deplete over time. You get a steady flow for the first couple hours say, and then it slows to a trickle before the 8 hours is up. Or over years, the water table may change.

How do you ensure that the well is up to the task?

In the case of a new system, perhaps perform a two-hour flow test to give you some confidence, and then monitor the initial fill to see the real-world performance. If the refill rate slows to an unacceptable pace, be prepared to contact a water truck company.

Water tables fluctuate with the seasons, so this adds some more uncertainty. What time of year would yield the most stringent results?

Pete D.

6/5/2023 01:46:48 pm

Tap into the fill line with a mechanical tee and flow it to obtain a pitot reading.

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How to Calculate Refill from Existing Well?

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