best sump pump inverter

Trash pump discharge sizes

Question: 6" Trash Pump with 4" Discharge Pipe

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?

Reasons for using 4" pipe

I already have a 6" Trash pump. I already have thousands of feet of 4" SDR 17 Fusable HDPE and a fusion machine. Discharge needs to be crossed by vehicle traffic for which I am to build ramps. 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.

Considerations for 4" pipe

Depending on what the hose is rated for and how you are using it, the pressures may be too much for the 4" pipe to handle. Two 4" lines would be approximately equivalent to the 6" line. 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.

Pump Usage and Flow Rate

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.

Impact on Pump Performance

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.

Clean Water and Engine Speed

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

Practical Experience

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.

Centrifugal Pump Behavior

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. 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.

Distance Considerations

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.

Understanding Head

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 5 feet above the water level. 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. 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.

Practical Advice

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.