We have an existing horizontal split case fire pump and ground level water
storage tank. The storage in the protected building is changing, and the
amount of water needed for fire protection is going up.
We are changing the tank to a “break tank”, but even then there is not much
water left in the tank after the required supply duration, only a few feet left
in the bottom of the tank.
The pump is right next to the tank, so not much loss due to friction in the
connecting pipe.
With the tank full, before the pump is turned on, the gauge on the suction side
reads about 15 psi, due to the height of water in the tank. When the pump is
running at 150%, this drops to about 7.5 psi.
At the end of the required duration I will only have about 1.5 psi of head from
the height of water in the tank. So if I lose another 7.5 with the pump on,
the gauge would read -6 psi.
Is this a problem for compliance with 2019 NFPA 20, or since the pump is
already running does this make the negative 6 psi gauge reading OK?
2019 NFPA 20 4.16.3.2
Thanks,
J
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