Suction pipe for Fire Pump

[Kashawy]

Hello,
In our project we have a concrete fire tank which is lower in level than the fire pump room by 590 mm, taking in consideration the level at which the suction pipe will be fixed in the tank that means it will exceed 1000 mm from suction to bottom of tank.
Is this acceptable or it will affect the pump readings during the handing over to the local authority? (in addition to cavitation)

The second part of my question is regarding the drain of fire tank, in our case if we provide drain for tank inside pump room that means a part of tank volume which is below the pump room will not be drained, so what is your advice please?

*pump used is horizontal split case*

 

[SprinklerDesigner2]

It's Monday morning, and I haven't had my coffee yet, but I am pretty sure you can not have a negative suction pressure on the pump inlet.

I've done a number of projects where we used a "cistern" and I would use a vertical turbine pump. Solves a lot of problems.

 

[LCREP]

Vertical turbine pump is the only pump permitted by NFPA 20 with a negative head. A horizontal pump is not permitted with a negative head pressure on new installations.

From NFPA 20,2013ED

7.1.1* Suitability. Where the water supply is located below the discharge flange centerline and the water supply pressure is insufficient for getting the water to the fire pump, a vertical shaft turbine–type pump shall be used.

 

[MGXFP]

You still have the atmospheric pressure to work with regarding the suction piping. I would do some calculations to ensure the suction pressure does not fall below this level. You won't be able to use the pump to lift water so that 590 mm water level will be off limits.

 

[UFT12]

Technically speaking, it doesn't really matter where is the bottom of the tank, provided that the low water level (pump off) is at such elevation above the pump axis that creates a head to account for the pressure loss from vortex plate to the suction gauge for a 150% of the rated pump flow. The low water level must be reached for your design flow and at the end of the system supply duration. I don't know what is your design standard but see NFPA 20, par. 4.15, ed. 2016 (4.16, edition 2019) for reference. Once you have selected the pump, it s easy to do a quick calculation and determine this elevation.

Now, if you don't have this elevation, you will have to either do what the other posters wrote or use this tank as break tank. But this can be tricky. There are more requirements. At the end you will have to check with your AHJ.

 

[sv879]

Agree with UFT12.

As per NFPA20, ed 2019, 4.16.3.2, gauge pressure at the pump suction flange shall be permitted to drop to –3 psi (−0.2 bar) at
150 percent of rated flow with the lowest water level after the maximum system demand and duration have been supplied. Which means if you are keeping 2m distance from water tank bottom to the center line of suction, that will comply with NFPA 20 requirement, as long as NPSHa is above the NPSHr defined by fire pump manufacture & water inside the tank is good enough to cater the demand requirement.