Pump Selection????

[SCPE]

When selecting a pump do you have to look at high points in the force main?

Ex. Wetwell elevation is 10 but the forcemain has to get over a highpoint of elevation 100 midway thru the pipe run and the discharge is elevation 10.

Your TDH would be headloss + 0 since the suction and discharge elevations are the same but dont you have to "get over the hump" in the middle of the run?

 

[RIP - VTEnviro]

Yeah, if you want to make shit flow uphill, you need to pump it.

Meaning that you need to figure out where your highest point is, and make sure your pump supplies enough head to get it over the hump.

You also need to consider minor losses throughout - friction, valves, etc. - to make sure your wastewater doesn't get over the hump and then die somewhere down the line and clog up the pipe.

 

[JasonT33]

Yes you are pumping to the highest point and then after that hump, it will be gravity flow. Kind of a strange design..

 

[RIP - VTEnviro]

Unless you have another local high point somewhere. Rolling terrain or something like that.

That's what I was getting at as far as making sure that not only will you pump over the highest point, but also get it through the rest of your system.

 

[DVINNY]

Yes you are pumping to the highest point and then after that hump, it will be gravity flow. Kind of a strange design..

In real world application, you are usually pumping UNTIL you can get it to a gravity flow system. I suppose if it were an outlying area and had to go pretty far, that it might go up and down hills as part of the force main.

In that case, you're friction factors would still be figured using the full footage of line, and you're elevations will use highest point throughout. But in reality, the velocity gained during the 'downhill' part would supply some head at the bottom of the next uphill run.

that would be tricky.

I doubt you'd see something like that on the exam.

 

[RIP - VTEnviro]

Something like given blah blah blah, what horsepower pump is necessary to satisfy the given conditions seems more likely, or pump curve stuff.

 

[SCPE]

Thanks guys, this is an actual real world application, about 20,000 feet of force main. I figured it had to get over the hump. Most pump selection books state never cover the high point issue in selection. What makes it tricky are the air release valves where the force main switches back and forth between pressurized and gravity flow.

 

[DVINNY]

How can it switch back and forth? without another pump?

You should have air release valves at the highest point of each run, but the line should really be force main even though it's running downhill. The contents should still be pressured going down hill, correct?

 

[DVINNY]

If it truely switched back to a gravity, you would be dumping into a manhole at the peak, and then the gravity line would be straight runs no more than 400 ft between manholes and the gravity line footage won't be figured into your friction losses for the pump.

then at the bottom, you'd have another pump and force mains back up hill.

If it's pressured line all 20,000 ft. then you don't have the manholes but will have the air release valves, and will consider all the pipe footage for friction loss.

 

[SCPE]

I am not really describing it well here. There are also manifolding pump stations tieing into the force main at various points along the way.

 

[3gorgesdam]

I hope this is for the afternoon section.