Wet Pit Sizing

[Mikes54]

I have a customer with a pond fed fire pump with a wet pit. There is a 24" pipe coming from the pond, into the wet pit. Approximately 75' of pipe. The existing pump was designed and installed 35 years ago. It is a 1000 gpm pump. New addition on site will now require a 2000 gpm pump. I need to verify that the wet pit under the pump is large enough to feed the new pump. Can anyone tell me the proper way to size the wet pit so I can verify this? Thanks.

 

[Sdpaddler50]

I only work in CA, and we don't have these animals so I will let others chime in. One thing you should check is adequate screening /filtering of the pump inlet for this raw water source. If you can not find adequate direction from nfpa 20, you can also check out fm 3-7n.

 

[cdafd]

Not and engineer or FPE, but

Dtermine demand of system and duration of water supply needed, and will the refill of the pit meet all that


24.1.2 Capacity. Water supplies shall be capable of providing the required flow and pressure for the remote design area determined using the requirements and procedures as specified in Chapters 11 through 22 including hose stream allowance where applicable for the required duration.


24.2.6 Penstocks, Flumes, Rivers, or Lakes. Water supply connections from penstocks, flumes, rivers, lakes, or reservoirs shall be arranged to avoid mud and sediment and shall be provided with approved double removable screens or approved strainers installed in an approved manner.

 

[PEDARRIN2]

It is my understanding a wet pit under the pump serves to keep the impeller primed. As long as the pond level is kept a few feet above the impeller, the impeller would always be submerged. Increasing the flow from 1000 gpm to 2000 gpm through a 24" pipe only minimally increases the pressure drop.

You should be ok.

 

[LCREP]

My concern would be on the 24" pipe is it clear the entire diameter. Where are the screens at the pipe opening or at the sump? I agree a 24" pipe should be ok from a flow point of view. If the pipe is not clear all bets are off. If it has never been cleaned it may be time to check it out via camera or a very skinny diver. Ponds have a lot stuff that can grow In the pipe.

Why not do a flow test on the existing pump and see what kind of flow u are getting into the pit. You will at least get an idea of what you have.

 

[cdafd]

This might help you read the whole thing

Check page 15

http://georgiafiresprinkler.org/wp-...ationary-Pumps-for-Fire-Protection.pdf#page15

 

[cdafd]

This might help you read the whole thing

Check page 15

http://georgiafiresprinkler.org/wp-...ationary-Pumps-for-Fire-Protection.pdf#page15

 

[SprinklerDesigner2]

Being in Georgia I do a lot of these animals especially in south Georgia where we dig a hole and we get water.

It is important to remember size of screen and pump intake must be sized for 150% of rated pump capacity.

From NFPA #20 2013 Edition:

A.7.2.2.2 The velocities in the approach channel or intake pipe should not exceed approximately 2 ft/sec (0.7 m/sec), and the velocity in the wet pit should not exceed approximately 1 ft/sec (0.3 m/sec). (See Figure A.7.2.2.2.) The ideal approach is a straight channel coming directly to the pump. Turns and obstructions are detrimental because they can cause eddy currents and tend to initiate deep-cored vortices. The amount of submergence for successful operation will depend greatly on the approaches of the intake and the size of the pump.

A 24" pipe contains 23.49 gallons per linear foot which translates to 46.98 gallons per second @ 2 ft/second or 2,818.8 gpm.

The problem you will have is with the screen.

4.14.8.2 Double intake screens shall be provided at the suction intake.
4. 14.8.3 Screens shall be re movable, or an in situ cleaning shall be provided.
4.14.8.4 Below minimum water level, these screens shall have an effective net area of opening of 1 in.2 for each 1 gpm (170 mm2 for each 1 L/min) at 150 percent of rated pump capacity.

The following example illustrates how to correctly size a suction screen for a fire pump.

A 750 gpm (2839 L/min) vertical shaft turbine pump requires a screen to protect the wet pit
that takes water from an open lake or river. What size is needed for the screen?

Solution: Since the pump is rated for 750 gpm (2839 L/min), the pump delivers 1125 gpm (4258 L/min) at overload (150 percent of 750 and 2839). Therefore, the screen must have an area of 1125 in. (0.72m 2 2). Since 4.14.8.6 requires a 0.50 in. (12.7mm)mesh and a No. 10 B&S gauge wire, the total screen area must be 1800 in.2 (1.1 m2) (1.6  1125 in.2). This area is equivalent to a screen area of 12.5 ft2 (1.16 m2).

That's a screen measuring a minimum of 4'-0" wide at least by 3'-2" high. For a 1,000 gpm pump you would need this screen to be 4'-0" wide by 4'-2" high at the low water mark.

In my opinion pipes aren't the way to go... maybe for some it would be but I've never encountered a situation where it would be better.