# Converting a Pump from gpm to MPH



## Snakebitx1 (Jul 11, 2012)

I'm looking at a few pumps, but some have an output of MPH and some gpm. I know that gpm is a volume flow and MPH is a speed, but I need to compare several pumps with each other. For example, I'm looking at a pump that says "Max water flow (1150 gpm) @ 3,000 PSI." I was hoping somebody could help me figure out how to compare that with a pump that is given in MPH.


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## jase (Jul 11, 2012)

divide it by the outlet section area to convert the volume flow to a speed.


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## jase (Jul 11, 2012)

first you'll need to convert GPM to cuft/hour


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## blybrook PE (Jul 11, 2012)

Jase has nailed it. If you have mathcad, it'll take care of the other conversions for you, but divide the gpm by the outlet area and you'll get the speed.

IE: 2" outlet (Sch40), area is 3.356in2, 1gpm on the pump is equal to 0.065mph.


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## MA_PE (Jul 11, 2012)

MPH = Megagallons per Hour?

1150 gpm = 0.069 MPH


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## Snakebitx1 (Jul 12, 2012)

jase said:


> divide it by the outlet section area to convert the volume flow to a speed.


Thanks man, I was having a bit of a brain fart!


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## ptatohed (Jul 12, 2012)

Snakebitx1 said:


> jase said:
> 
> 
> > divide it by the outlet section area to convert the volume flow to a speed.
> ...


Are you sure MPH stands for Miles Per Hour? It seems that MA_PE was on to something. It's hard to imagine a manufacturer providing only pump discharge speed and not volume.


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## MA_PE (Jul 12, 2012)

That's where I was going. Pumps move fluid. You're typically concerned with volume/flow rate not outlet speed. Outlet/nozzle pressure maybe.

Do manufacturer's actually spec pumps by velocity?


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## MGX (Jul 13, 2012)

It also sounds strange to have the units in MPH. From what I've seen it is usually expressed in FPS.


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## Snakebitx1 (Jul 13, 2012)

ok, i've got another idea and question. I was thinking that instead of using a pump to just use a motor connected to a prop. Although, say that instead of using one prop with one motor I decided to use two props being run off the one motor.

How would the flow of water be effected? Would it double?

Would I need twice the HP in order to keep the props turning at the same speed as the individual prop was spinning?

Could I use the same HP and cut the RPMs in half?


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## MA_PE (Jul 13, 2012)

the prop blades would cut the treadmill runner and then the boat wouldn't fly.


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## Snakebitx1 (Jul 13, 2012)

I hope that makes sense. If not let me know and I'll try to explain it better.


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## Snakebitx1 (Jul 13, 2012)

MA_PE said:


> the prop blades would cut the treadmill runner and then the boat wouldn't fly.


Good thing it's not on a boat.


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## 2bsss (Jul 22, 2012)

Are you sure the MPH isn't cubic meters per hour (m3 /hr)? I have never heard of a pump being specified or described by discharge speed (specific spped yes, but that is a different thing).


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## Jonhnny123 (Jul 25, 2012)

Agreed, m^3/hr sounds way more reasonable than miles per hour.


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