Reservoir Size for Sprinkler System

[Norm01]

I have large building with a number of sprinkler headers located at various locations. The building fire protection system is to be supplied with a reservoir and a vertical turbine pump. Based on hydraulic calculations of the sprinkler systems, the system futhest from the water supply has a system demand of 520 gpm @ 125 psi. A system closer to the water supply has a system demand of 688 gpm @ 105 psi. Both systems are ordinary hazard occupancy with a water supply duration of 60 minutes. In order to size the reservoir and pump system is it a matter of sizing the reservoir to meet the demand of the 688 gpm system (i.e. 41,280 gal.) and then size the pump to meet the pressure requirements of the 520 gpm system ?

 

[stookeyfpe]

I think you would be ok sizing for the 520 GPM @ 105 PSI demand. If a 500 GPM @ 100 PSIG pump is specified, the pump curve should easily be within 140% discharge pressure at 65% of the flow demand. I would size the reservoir based on the most flow demanding (e.g., 688 GPM) requirement.

I offer one caveat and that is it has been a long time since I looked at vertical shaft pump supplied from a reservoir so you will want to confirm my statement regarding the pump capacity at 140% of its discharge pressure.

 

[FFP1]

The fire pump should be sized to meet the highest system demand plus the hose streams for fire fighting efforts. The curve should meet both of the above system demands. Simply plot the theoretical curve and plot both demdns including hose streams.

The reservoir should be large enough to meet the highest flow rate PLUS the hose stream (probably 500 gpm in your example) for 60 minutes. Looks like a 75,000 gallon reservoir is in order, but remember 75,000 gallons must be available to the fire pump at all times. You might need to take into account the volume of water which is too low for the pump to access and you may need to consider evaporation or arrange for an automatic fill to ensure the reservoir level is maintained above the calculated minimum.

 

[TravisMack]

As FFP1 said, the pump is there to supply pressure. Size the pump to get the proper pressure for the system while working within the listed capacity. A 500 gpm pump can supply the 688 gpm demand, but, you need to check the pump curve to make sure you get a pressure that meets the 125 psi at 520 as well as 688 @ 105.

The reservoir is sized to supply the greatest demand of flow. This would be the 688 gpm area. As long as the reservoir does not supply hydrants or hose valves, you should not need to add the hose allowance. (11.1.5.3 Where pumps taking suction from a private fire service main supply sprinklers only, the pump need not be sized to accommodate inside and outside hose. Such hose allowance shall be considered in evaluating the available water supplies. ) Therefore, you are probably looking at +/- 45k gallon tank.

Good luck with the project.

Travis Mack
MFP Design, LLC