Trash pump discharge sizes

jhark123 · Aug 1, 2022

Hi all,

I have a project coming up where I'd like to use a 6" trash pump with a 4" discharge pipe. Will this just reduce the discharge capacity or will it cause problems with the pump?

Thanks

redneckracin · Aug 1, 2022

why in the world would you want to put a 4" discharge pipe on a 6" pump for?? Depending on what the hose is rated for and how you are using it, the pressures may be to much for the 4" pipe to handle.

jhark123 · Aug 1, 2022

1. I already have a 6" Trash pump
2. I already have thousands of feet of 4" SDR 17 Fusable HDPE and a fusion machine
3. Discharge needs to be crossed by vehicle traffic for which I am to build ramps (tough to get clearance on a 6" pipe for low cars).
4. I already have ramps that I built for 4" pipe for a similar situation.

Another option would be to put in a Y and run two 4" pipes

redneckracin · Aug 1, 2022

Those are pretty good reasons.

2-4" lines would be approximately equivalent to the 6" line. 12.5 sq in for 4" vs 28.3 sq in for 6". you need to get 6" into the "Y" however or there will be a noticeable increase in back pressure the more flow you try and push down the pipe.

terex herder · Aug 1, 2022

Are you using the trash pump for the need to pump water with solids, or do you just need a water pump? Your trash pump is rated to pass 3" solids. How many gallon per minute do you expect to pump? Is this an engine drive pump (variable speed, so variable output), or electric (fixed speed, fixed output)? Do you have the pump curves for your pump?

If you are wanting to pump less than, say, 300 gpm, a single 4" will be fine. More than that and you will start paying more money to overcome friction head.

Delmer · Aug 1, 2022

It will increase the friction, so increase the output pressure, so decrease the volume. As long as the pressure volume is within the pump curve then the pump will be fine. Higher head will be LOWER volume and power on the pump.

Run the numbers as to the head and friction and compare that to the pump curve. Assuming this is an engine driven pump and your static head is reasonable, I'd bet you'll find that reducing the speed of the pump will make the pump operate quite well with the 4" discharge.

If you are pumping toward the higher end of the pressure, therefore need to keep speed at the max, and have sandy water, that will maximize the abrasive pump wear, but in the scheme of things that's not usually significant.

jhark123 · Aug 1, 2022

@terex herder Just clean water (should be lol) it's a stormwater bypass. It'll be engine drive.

Steve Frazier · Aug 1, 2022

If the 4" pipe will move the volume you desire, you should adjust engine rpm to find the sweet spot where you're running at the capacity of the discharge pipe. Exceeding that capacity will result in heat and cavitation, both of which are bad for the pump impeller.

Jonas302 · Aug 1, 2022

Done it many times

Welder Dave · Aug 2, 2022

Ran 2" hose a long distance of a 4" trash pump with no ill effects. You get more pressure and less flow but as long as the pump has water, should be OK. Less RPM would be good because higher would just waste fuel and not deliver any more water.

John C. · Aug 3, 2022

Centrifugal trash pumps don't care about output. Close down the output and the power drain on the motor or engine will decrease. The only issues of doing that is the amount of water being moved needs to be enough to keep the pump cool. Posts above are talking about moving the water in the most efficient volume for the amount of fuel used. Pressures will rise only to the capability of the pump. Trash pumps start out with close tolerances and wear sloppy as gallons are moved and sediment levels increase. The two killers of output are the sediment load and the vertical head the water has to move.

cfherrman · Aug 3, 2022

I was concerned more about 'thousands of feet' then the size being smaller.

I think you would just have to put it together and see, thousands of feet might be a lot head to overcome too.

John C. · Aug 3, 2022

Head refers to the vertical distance above the pump where the water has to discharge. A trash pump handling clean water and in good condition in my experience will only pump around 300 to 400 above the level of the pump. Load it with sediment like sand or gravel and it gets less real fast. You can have suction head as well. You might have problems if the pump is more than 30 feet above the water level.

cfherrman · Aug 3, 2022

Head is short for ft. Head and when your measuring length of pipe even going horizontal it adds to the ft head of the pump, even the suction side adds to the ft head.

terex herder · Aug 3, 2022

John C. said:
You might have problems if the pump is more than 5 feet above the water level.

There, fixed it for 'ya

Delmer · Aug 3, 2022

cfherrman said:
Head is short for ft. Head and when your measuring length of pipe even going horizontal it adds to the ft head of the pump, even the suction side adds to the ft head.

Yes, head is the vertical distance you're pumping (static head), or the equivalent pressure. And the friction of horizontal flow adds to the dynamic head or the actual pressure required to move the flow. But head is a measure of pressure, just another scale, like inches of mercury or any of the others.

Jhark didn't say he was going to use all of his thousands of feet of pipe. And it would be much easier to calculate the pressure drop than put it all together and find out it doesn't work. Even estimating and guessing at a pump curve will tell you if you're safely within the curve, or you need to measure closer, or it won't work at all.

FarmWrench · Aug 5, 2022

I didn't catch what you are pumping 1000s of feet across a road. If it's clean water and you don't have to do it for long or at high volume. One would be fine.

In fire hose if I remember we loose 100psi at 1000gpm in about 1000' of 4"? We run 5" and loose a lot less at that flow. Haven't pumped in years and getting rusty don't even carry my friction loss slide rule. The coupler has a lot of impact compared to welded HDPE.

If you got the pipe and it's all together then drag it into place and run two or three.

Trash pump discharge sizes

jhark123

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redneckracin

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jhark123

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redneckracin

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terex herder

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Delmer

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jhark123

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Steve Frazier

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Jonas302

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Welder Dave

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John C.

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cfherrman

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John C.

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cfherrman

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terex herder

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Delmer

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FarmWrench

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