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RCramer
November 23, 2016, 12:51 PM
I had accidentally left a manhole on a forcemain at a manifold location. Set up like this, the pumps matched drawdowns. When I changed it to a break node, it pumped significantly less then expected based on the drawdown.
I can't think of a reason why the manhole would drop the headloss on the forcemain unless somehow it was allowing a siphon?
Any thoughts?

Andy Bolden
November 24, 2016, 11:12 AM
I had accidentally left a manhole on a forcemain at a manifold location. Set up like this, the pumps matched drawdowns. When I changed it to a break node, it pumped significantly less then expected based on the drawdown.
I can't think of a reason why the manhole would drop the headloss on the forcemain unless somehow it was allowing a siphon?
Any thoughts?

Assuming you are talking about ICM here.

Usually when resolving issue with a Forcemain it is something annoying simple which needs corercting!

If you haven't already done so I would suggest looking at the rising main long sections and replaying the simulations for both. Does the hydraulic profile make sense? Perhaps your manhole floodtype was set to lost (so it flooded, head didn't build up and gave the pump an easy life)?. The manhole's plan area might also be sufficient that the head doesn't build up as quickly on each pump cycle as would occur in a closed system so the pumps don't get exposed to the full downstream head which would occur in practice. You have not said, but presumably you have a rotodynamic pump. Is the duty point where you expect it to be? (Look at the Pump Operation graphic). Do you have the correct pump curve in your model? Is the real pump operating at a higher speed than the pump curve you have used? Have you put your pump curve in the right way round - ICM defies normal graphing logic and expects 'y' values (head) in the first column and 'x' values (flows) in the second! Are the internal diameter and roughness of the rising main correct?

By drawdowns do you mean drawdown tests on the pumps or the rate at which the sump draws down?. Is the sump area correct in your model? - perhaps you are getting a false match.

RCramer
November 28, 2016, 07:13 AM
The duty point is only where it is supposed to be when the node is a sealed manhole. Interesting thought about the volume of the manhole making a difference. The pump is rotodynamic and was set based on pump curve minus age based on field drawdowns. I have had trouble with this forcemain when I've modeled it in excel, it doesn't behave the way you would expect it to mathematically. In fact, the prediction for behavior in excel more closely matches what the pump should do if it is set as a break node in the model (about half of what it actually pumps based on drawdown).