Water Pumps and pressure

[ken pino]

Hi, I am new to this forum and hope I am in the right place. We have a pond that supplies water for our hydro-electric plant. It has become plugged up with silt and debris. I have a 4" trash pump. Trying to pump the silt out was not working very well. The silt is not fluid enough to flow down to the pump inlet hose. I currently have things set up so the output pipeline from the pond bypasses the hydro plant so it can be used to flush the silt and debris out of the pond and back to the river. The velocity of the water coming out of the pump is not that great but the volume is good. I can get a 4" to 2" reducer for the pump output to increase the velocity at the discharge so I can use a 2" hose as a "jet" to break up and wash away the silt and debris. What I want to do is also get 2" to 1" reducer that I can put at the end of the 2" hose to act as a nozzle to increase the velocity even more. This way I can choose between a 2" or 1" output with only one 2" hose. The hose manufacturer says the maximum pressure the hose can handle is 150PSI. The pump manufacturer said that the pump will work fine with the reducer. The maximum discharge head on the pump is 98ft which is about 42 PSI which the hose can handle.
My question (which the pump manufacturer could not answer) is: when the 2" to 1" reducer is put on the end of the hose, will the pressure in the hose increase? The pump is not positive displacement, so it seems to me that 98 feet of discharge head is the maximum pressure the pump could ever generate and that the reducer would only increase the discharge velocity. Am I correct? If not, how much pressure increase should I expect?
Bottom line is, do I risk bursting the 150 PSI hose by putting a 2" to 1" reducer at the end of it?
Hopefully this is clear. Thank you.

[BWG]

It's possible for this type of pump the mfg rated a little conservative to always be sure pump delivers stated rating. Doubtful it would be much over. You are correct in thinking this is the pumps max pressure.

The real issue is throttling a large 4 inch trash type pump down to 1 inch. That's a huge restriction for a pump this large not designed to operate like this. Cavitation or excessive friction might occur or maybe nothing. Bottom line you're still only at 42 psi.
Always listen for smooth and consistent pump and engine sounds. Knocking, clicks or anything irregular adjust engine speed or shut down if engine speed doesn't clear up.

Better to rent a higher head 2 inch pump if you want velocity using a nozzle to blast away and loosen material. Kind of like a high pressure washer that only does 4 gpm but at 4,000 psi vs a 15 gpm 30 psi garden hose.

Had two friends who once wanted to remove a sand bar from their favorite private ski area. After 10 days with 2 trash pumps, one loosening and a big one moving material they gave up. This was many years ago and I think tried both a high head 2 inch pump and a high pressure washer with 16 HP engine to put the material in suspension.

My opinion for this case and bottom sediment using as large as you find gas engine air compressor and a 3/4 to 1 inch line with pipe attached and injection in the muck will stirrup things faster. Air bubbles are very powerful lifting and breaking apart material.

[ken pino]

Thanks for the info. It is very helpful. I had thought of the compressed air a few years ago when the buildup first started getting bad. I abandoned it because I thought it might stir up so much debris that it would clog the inlet filter screens to hydro plant pipeline (Penstock). At that time I had no way to clear those screens other than to drain the pond (thru the penstock/hydro plant) and manually remove the debris. Once the screens are plugged, draining the pond is a long term project as the penstock/hydro plant is the only outlet from the pond other than a high water overflow. Trying to drain the pond through debris build up on the screens can take a really long time. But now I have made some modifications where I can bypass the hydro plant and then remove the filter screens and have the penstock flush directly back to the river. Now the compressed air is looking good again.

[danzcool]

The maximum head height is equated with the maximum pressure the pump can handle. By introducing a reduction in size you are adding dynamic head. Assuming your 42psi number is correct for 98' head, the pump will never exceed that 42psi, so the hose will be fine even if you put a cap on the end. I did a quick cal on the 98' and it is within tolerance of your 42psi, I got 1 or 2 higher, but still the 150PSI rating of the hose would require 328' of head pressure, so 98' is eazy peazy.

[Grumpy]

I don't know how large your pond is, but when the slurry/muck is difficult to "pump", a positive displacement diaphragm pump can work well. It will move the heavy slurry, sand, small rocks better than a trash pump and requires much less water. We used air powered positive displacement diaphragm pumps and they could excavate using very little water, it is just slow.

[BWG]

They are using the hydro pipe as a drain and using the pump to spray and put the muck in suspension and able to flow. Still think air injected into muck will do a better job.

Trying to move material any distance out of a pond with any type of pump will require an expensive and higher HP unit and pipe that is capable to transport a distance away. Size of the pond hasn't been mentioned. Mechanical removal with an extended boom digger might be an option if reachable from sides.