TDH - Complex Rising Main 2

[SMIAH]

Profile attached.

I want to check the pump total head.

Should I average the
minimum head = 143.4-140.5 + total pipe flow friction losses
and
maximum head = ((154.4-144.4)+(149.0-147.0)+(149.0-148.5) + total pipe flow friction losses)?

Or simply TDH = 154.4 - 143.4 + pipe flow friction losses from this line.

Thanks!

 

[cvg]

I would not be surprised to see a comb air/vac valve at the high point. this would maintain atmospheric pressure on the downhill leg(s), allow air into the line and promote gravity flow. In fact, there may be one at each high point. regardless of how this system operates, you will need a pump that can fill the first leg of the pipeline all the way to the top. You might find quite a few relevant discussions of sewer force mains hydraulic performance in the Waste disposal & treatment Forum

http://www.eng-tips.com/search.cfm?...+main&t=&h=&searchSite=1&searchArea=1&d=1&s=1

 

[LittleInch]

Once you allow air into it then all bets are of though I do understand why water / sewage systems work this way, I still think it's better to keep it all at positive pressure.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

There is an air valve on top on the 1st leg but it's closed/stuck and not operating anymore.
I wanted to change that design to allow the pressure to be above atmospheric all the way. To do so, I have to increase the TDH (pump) though.
I'm gonna do a drop test next week to see what this pump can discharge (with the air valve closed/on).

 

[SMIAH]

Back on Geometric head: If I want P > atm all the way : is it (154.4 - 143.4) + (149 - 147) + (149 - 148.5) = 13,5 m or...

 

[LittleInch]

I thought we'd sorted that out earlier. Your calculation method is not correct, see my post earlier with the graph attached. For a full pipe design you need to concentrate on friction losses plus any end point head losses/ gains and forget about the intermediate hills. You need 23m at the start to keep the pipe full up to the last hill when it starts to run away from you. Unless you put on some sort of restrictor at the end point or greatly increase flow rate, this is just physics.

LI

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

LittleInc, comments on Page 50-51 (esp. point 3.7.2) :

http://www.grundfos.com/content/dam...olutions/waterutility/pdf/sewage-handbook.pdf

 

[LittleInch]

Yes, that's a very interesting book, I'll keep that for another day, but those pages only apply when you have air or gas in it. You're trying to operate this like a "proper" pipeline with pressure greater than atmospheric at all points. When you do that, that calculation isn't valid and the book says that.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[cvg]

this is effluent, it naturally contains large quantities of air and in addition, the BOD and COD will also create other gases. So it is critical that you maintain sufficient pressure and velocity throughout the pipeline in order to avoid pockets of gas filling the high points. That will create a "vapor lock". Also, be aware that if you increase the pressure and eliminate additional oxygen intake into the pipeline, it will almost certainly maintain anerobic conditions all the way. Since most lift stations do not pump either constant flow rates or constant flow, this can be challenging. And if your pumps cycle on and off, then you will definitely need to maintain back pressure or you will allow your pipeline to fill with air.

 

[SMIAH]

cvg : I'll go read the Waste disposal & treatment Forum.
See that there's alot of posts about similar issue.

 

[SMIAH]

Did the drop test.

15.1 L/s.

Putting it in my calculation sheet : TDH = 15 m.
The pump was supposed to be 36 L/s with TDH of 17 m.

 

[LittleInch]

The pump might be abe to pump that, but if the pipe resistance is greater than it can pump then the flow reduces. There maybe some reduction in diameter / silting up going on here but also that the system curve is not accurate. I don't thin we've ever seen a system curve or diam / type of pipe to be able to judge your calcualtion sheet.

I'm a bit confused about this now...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SMIAH]

The discharge line might be clogged (silted) if air vent is closed and vacuum effect is not there to help it flow by gravity after the 1st leg.
I'm not really surprised as I need at least a 24 m pump to pump 34 L/s.
Simulating it to keep the discharge line above atm. pressure, I need more than 28 m at the pump to pump 34 L/s.

What I don't know is what will happen if I put a 34 L/s - TDH 28 m in that wet well.