NFPA 22, 2018 Edition regarding filling requirement and duration -
Section 22.214.171.124. 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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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.
9/21/2020 11:41:16 am
And as always with requirements for duration, the limit is set arbitrarily.
9/21/2020 02:12:15 pm
The tank has infrequent requirement to be emptied for ITM.
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.
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