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I have in my house installed a water mist fire protection system. Currently it is connected directly to the main water system, which is of lower pressure than the nozzles are designed for.

Current solution

I would like to add a pump and an expansion/pressure tank to increase the pressure.

With pump and tank

My question is what kind of water pump can I use for this? It is an absolutely requirement that it will not significantly reduce the pressure or amount of water flowing through it in case of missing electricity. How well will centrifugal pumps let water flow through without power?

A battery backed solution that turns on/off a bypass path in case of power loss might be acceptable, but my initial thought is that this is added complexity and something that could fail so I think I would prefer not to have that.

Since the normal use case will be that the pump starts once to fill the tank and then never start again, power consumption and efficiency are of no relevance.

  • Where on this planet are you, and are you fed by a city supply or a well? – ThreePhaseEel Apr 20 '17 at 0:10
  • Norway, city supply. – hlovdal Apr 20 '17 at 0:18
  • Your question states that mains pressure is not high enough to meet minimum required by mist system, why then is it so important for lower pressure water to continue to (ineffectively) supply the system? – Jimmy Fix-it Apr 20 '17 at 2:45
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    Do you really have an actual mist system (commonly called a water fog system where I am from) or do you have a plain fire sprinkler system? The reason I ask is because true fog systems work differently and pressure is usually augmented a different way. – Jimmy Fix-it Apr 20 '17 at 2:47
  • Yes, it is a water mist system. The nozzles are of type Prev2exp. – hlovdal Apr 20 '17 at 7:04
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A simple solution would be to install a full sized bypass with a check valve, which ties in downstream of the pressure tank. This would allow full system flow in the case of pump failure. You could then use a pump of whatever type fits your needs most efficiently and effectively, and would need not be concerned about flow restriction through the pump.

You do not mention the nozzle activation mechanism but we must assume that the heads are closed unless opened due to fire melting a fusible/frangible element at the head(s) or other mechanical means. So... under static conditions the system would remain at the pressure held by the tank diaphragm, which would be above mains pressure (because it was pumped). Upon opening/activation of a spray nozzle then the tank would deliver until pressure drops enough for mains pressure to then lift the check valve and flow into the system, around the pump and tank assembly.

enter image description here

sorry for crude drawing of bypass with check valve

  • Perfect solution! – Mark Apr 20 '17 at 20:21
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So at mains pressure, will your nozzles work at all? Because if not, then there's no point in any system that can't keep the pump running.

Next, have you calculated how long the proposed ballast tank will keep the output going? If that's sufficient to evacuate the house, I'd stop right there. First, the chances of a fire and electrical blackout occurring simultaneously are slim to epsilon (barring an all-out regional disaster). Second, I doubt the system, running at full pressure, will douse a significant house fire, so it's really only there for small stuff or to give you time to evacuate.

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