Why Do Fire Storage Tanks Require 8-Hour Refill?

9/21/2020

NFPA 22, 2018 Edition regarding filling requirement and duration -

Section 4.2.1.4. states "The water supply shall be capable of filling the minimum required fire protection volume within the tank in a maximum of 8 hours" and at the same time Section 14.4.3 states "The tank shall be kept filled, and the water level shall never be more than 4 in. (102 mm) below the designated fire service level."

If I understand correctly, as soon as the water goes below 4-inches from the normal filled level in the tank (HHL), the makeup water supply should be starting to replenish and the pipe sizes should be provided in such a way that the tank can be filled within 8-hours.

In a fire, as the water is going down, the tank will always start full.

What is the case where the tank would be emptied out (except maintenance) and need to be refilled within 8-hours?

What if we store water in two tanks to allow maintenance - would we not need to meet this 8-hour requirement?

I'm unsure of how these two requirements work together. Appreciate any feedback.

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4 Comments

Peter

9/21/2020 10:11:34 am

The idea behind a water storage tank in most cases is that the water supply is not capable of meeting the system demand for the required duration. So as a default, over time, the tank will empty because the system demand will out last the tank supply. In some instances you can calculate a water supply to refill the tank to shrink the size of the tank so it meets system demand. However, sometimes that is not possible such as in a rural application where there is no water supply that is really capable. I have seen, in this instance, that the refill of the tank is nothing more than an irrigation pump pulling water some a small lake or a well. The 8 hour requirement is to make sure that that water supply (irrigation pump from a well, lake, or other) is capable of refilling the tank within 8 hours so that the fire protection system can be restored.

Franck

9/21/2020 11:41:16 am

And as always with requirements for duration, the limit is set arbitrarily.
It could have been 6 hours or 10 hours, but a consensus was probably found for 8 hours.
Maybe because it is the duration of one shift? One normal working day? So that if you start filling the tank, you can check during the day and son't have to stop and restart the next day, or to overfill the tank for hours because of a shift change.

If you have 2 tanks, this will give you 100% redundancy and you may argue with the AHJ if you can only fill it entirely in 10 or 12 hours, but the limit is normally also 8 hours.

You may have the same question for the fuel tank for diesel fire pumps. Tank supply is also 8 hours while requirements for duration is normally 1.5-2 hours...

Safety factor to avoid possible problems...

Bruce Verhei

9/21/2020 02:12:15 pm

The tank has infrequent requirement to be emptied for ITM.

When work is complete it is necessary to refill promptly.

Best.

9/23/2020 01:33:17 pm

If the water tank is not "full" then you have an impaired water supply. NFPA 25-15.5.2(4) will allow a preplanned impairment of up to 10 hours before the big-ticket mitigations like evacuations or fire watches come into play. An 8-hour fill time keeps it under the 10-hour impairment.

Why Do Fire Storage Tanks Require 8-Hour Refill?

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