vertical turbine pumps and underground tanks 1

[6877263]

I have a question about Vertical turbnie pumps.

In chapter 7 of NFPA 20 there is a requirement of minimum submergence from the second bowl to the surface of the water.

There are only two options in the NFA 20:

1. A well-

In this case the subergance must be at least 10 ft below the pumping level at 150 percent of rated capacity.

2. A wet pit-

In this case the submergence must be below the lowest pumping level in the open body of water supplying the pit.

My question is what is the minimum submergence in the case of a vertical turbine pump that is installed in a underground storage tank?

Thanks

 

[skdesigner]

The pump manufacturer will state the minimum required submergence in the pump datasheet.

R M Arsenault Engineering Inc.

http://www.rmae.ca

 

[6877263]

Thank skdesigner for the answer!!

Where is that stated in the NFPA 20?

I searched in Chapter 7 and didn't find that the standard permits the submergence to be according to he pump data sheet.

 

[skdesigner]

NFPA 20-2013 7.2.2.2.3 states "The required submergence shall be obtained from the pump manufacturer."

Even though this comes from the section for wet pits, it is the same concept when pumping out of a tank with a vertical turbine. With a tank, the minimum submergence creates an imaginary low water level, below which the stored water can't be considered usable. Don't make the mistake that many do and size a tank based on the fire protection system demand, and drop a vertical turbine into it expecting it to suck up every last drop. The minimum submergence plus the 12" (or more) that the basket strainer has to sit above the bottom can easily translate to a few thousand gallons of water at the bottom of the tank that aren't useable for the pump. This means the tank either has to be larger, to accommodate both usable water and the unusable water below the minimum submergence level, or the pump has to be installed below the bottom of the tank in order to use all of the water stored in the tank. This is typically either done with a sump below the tank, or with the pump sitting in a wet pit adjacent to the tank with a pipe connecting the two.

Xerxes and Containment Solutions both have pretty good literature illustrating how they accomplish this with their FRP tanks.




R M Arsenault Engineering Inc.

http://www.rmae.ca

 

[6877263]

thanks again for your comment.

I'm a bit worried about the fact that paragraph 7.2.2.2.3 is in the section for wet pits only.

I spoke to a system designer about this issue. He claimed that the NFPA did not mention underground tanks at all in chapter 7, and that speaks for itself. Therefore any installation in a tank should have the same guidelines as a well. He claimed hat a wet pit does not need much submergence because it takes water from an open body of water, and therefore as long as you install the screen below the lowest pumping level, there will be no chance of surface vortex. In underground tanks, there is a limited amount of water, and since the refill mechanism is not calculated as part of the volume of the tank, we should use the guidelines of well construction.
Do you suppose there might be a reason that underground tanks are not discussed at all in chapter 7?

 

[stookeyfpe]

I'm assuming that your hydraulic demand is for an automatic sprinkler system including hose streams. If this is for a standpipe system or fire flow, you're client is about to purchase a lot of tanks if these are off-the-shelf.

http://www.zcl.com/assets/documents/library/water industry update may 2015.pdf