What is 150% Water Level for Vertical Turbine?

11/24/2021

We have a project where we're using a vertical turbine fire pump.

NFPA 20 makes reference to a 'water level' at 150% of pump rated capacity. What exactly does this 'water level' mean?

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5 Comments

Alex

11/24/2021 05:33:50 am

Water level refers to your usable water supply. For example, if you have a tank located 100 feet away from your building, you are going to battle friction loss through the pipe, meaning once you get down to the final few feet of water in your tank, there won’t be enough pressure to drive the water to the pump.

NFPA 20 4.16.3.2 states that you are allowed to drop down to -3 PSI at the suction flange when you’re at 150 percent of rated flow with the lowest water level (after system demand has been provided).

Franck Orset

11/24/2021 10:22:39 am

Alex, this is true for horizontal fire pumps taking suction from a tank, but the question is for a vertical turbine fire pump, where there is no pipe between the tank and the pump with friction loss.

Jesse

11/24/2021 09:54:55 am

I started to respond. Then had to refresh my screen and saw the Alex had already done so, and covered it all. Nothing to add to Alex's response. J

Jack G

11/24/2021 10:00:15 am

From memory, 20 states the required submerge certificate “ shall be obtained from the manufacturer”, somewhere, Chet 7. ( wet pits)
That being said, the concept is the same for pumping from underground tanks. A tank creates an “ imaginary low water level”, below which the water is unusable by the pump. The minimum submergence plus the 12 inches tge strainer sits above the floor easily translates to a few thousand gallons easily.
So the tank has to be larger ( usable plus unusable volume) below the submergence level, or the pump has to be installed below the tank in order to utilize all the water in it.
This is usually accomplished with the sump below the tank, or in a sump pit next to the tank connected by a larger pipe.
Now the question of “ designing the supply for system demand, plus hoses if the building has stand pipes, times duration”, or 150% of the pump times the duration. Code requires the first.
A vertical pump generally has a flatter curve and produces much more pressure at 150 % of flow. The tank and discharge pipe should be so sized ( even though this is common sense and not a requirement ) to hit the 150% as quickly as possible.
Now in the instance the tank is far from the building, and there is an underground supply to a wet pit, the underground should so be sized, so the 150% mark can be achieved, and the pressure gain is not lost in the underground. The higher pressure that you have to achieve with a vertical will make that supply “ larger then you need, with just calculating the system demand”.,
( envision sucking a milkshake with a thin straw).

Franck

11/24/2021 10:30:02 am

You may refer to pump submergence with well installations.
For a vertical fire pump taking suction from a lake, a pond, a river, or a wet pit, there is no particular issue as the water level will no go down immediately. It may never go down if you take suction from the Great lakes or from a large river, or slowly go down if you take suction from a limited size reservoir or pond.
But for a pump taking suction from a well, you are dependent of the water table, and the water table level.
And if you take water from a water table, the water table level may drop down immediately.
This is why NFPA 20 is taking some precautions which are the following:
The bottom of the fire pump (suction nozzle) should be covered /submerged by:
- at least 10 ft for minimum submergence
- plus an elevation corresponding to the draw down value when pumping water at 150% of the rated pump capacity.

This is to ensure that if your pump is operating at 150%, the water level is still at least 10 ft above your suction point.

There is a nice figure in the Annex section of NFPA 20 that shows the static water level before pumping, the draw down level at 150% pumping capacity and the minimum 10 ft additional submergence

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What is 150% Water Level for Vertical Turbine?

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