Pump/piping question 1

[drewnje]

I have two pumps operating in separate sumps and joining at tee and entering a tank through a single line. All lines are 2" O.D. HDPE SDR 11 pipe, shortly after the connection, the line enters a manhole and exits as a 4" SDR 11 HDPE Pipe. I have very little info on the pumps other than one was spec'ed at 55 GPM at 96' TDH and the other at 53 GPM at 85' TDH. I need to ensure this configuration is not a problem, especially in the 2" line before it expands to 4". Any insight on how to best check this? Thanks.

 

[77JQX]

What are the relative elevations of the sumps and manhole and eventual discharge from the 4" line, and the lengths of pipes connecting them? What is the rate of flow into the sumps? And how do you define "not a problem"?

 

[rconner]

Are you sure pipe is actually/exactly " 2" OD " (I only ask as this sounds a little strange to me, and actual OD and DR are important to determining ID, that is most important in flow calculations)?

 

[drewnje]

It is a 2" diameter HDPE pipe, the exact O.D. is not 2 inches, but 2.375". We are not worried about the flow into the sump, the pumps apparently were spec'ed to meed the inflow requirements. Ultimately, the water will need to be pushed up hill to the tank 70' higher in elevation approximately 1,110 feet away from the sumps. The 2 inch line accounts for approximately 60 feet of this distance before expanding to 4 inches. Not a problem being, if the two pumps are kicked on simultaneously, will they be working against each other as they join into one pipe at a single node? What will the resulting pressure and flow be at the junction and will the water make it to the take.

 

[drewnje]

The pumps were not sized taking into account another pump, they would only operate individually. Now the potentially exists that they will kick on at the same time.

 

[77JQX]

You seem to be an earnest young engineer. Rather than give you the answer, I'd prefer to give you guidance about how to solve it yourself. First, download EPANet 2.0 - it's free and easy to use. Set up your two sumps as reservoirs at elevation zero, and input your two sump pumps also at elevation zero, with "design point" curves at the given flow and head values. Connect the sump reservoirs to the sump pumps with short (1 foot) large diameter (99 inches) pipes to make the suction losses negligible. Establish the pipe junction node at the appropriate elevation, and connect it to the two sump pumps with the correct length pipes of the correct ID and C value. These can all be input manually, so you don't have to worry about making a scale drawing. Finally, set up the discharge as a reservoir at elevation 70, and connect it to the junction of the 2-inch pipes with another pipe of the appropriate length, dia., etc. Now you should be able to run the model with either, or both pumps simulataneously, and get the answers you want. Let us know if you have questions about this.

 

[cvg]

calc the system head curve and plot it
plot both pump curves on the system head curve
where they intersect will give you the approximate flow from each pump
now add the two pump curves together to get combined flow and plot that on the system curve.
intersection point will be the combined flow from both pumps.

this only requires graph paper and a calculator or better yet, Excel
you can find the procedure in any basic engineering text book for applied hydraulics

 

[drewnje]

77JQX,

Thank you, I was had downloaded EPANet, but was not sure how to model it. I still have a few questions, where can I input the design point of the curves? I think I am overlooking something. When I right click and select the properties of my pumps, that is not an input.

 

[77JQX]

drewnje - I have to apologise, I was doing EPANet from memory, and it truns out I remembered wrong (in my defense, I use another modeling program). You need to define a curve in EPANet for each pump, which can be as simple as a single design point - the program then makes assumptions about the shape of the curve. Without knowing more than you do about the pumps, that's about as good as you're going to get. You input the pump curve data through the little window called the "Browser," using the drop down menu to "curves" and clicking the "add" button. On the pump data input, you enter the name of the curve you defined.

 

[drewnje]

77JQX,

Thanks for all the help! May I ask what modeling program you use? One last question thought, the model ran and it shows my combined flow entering the tank; however, it is showing a negative flow downgradient of one of my pumps. It doesn't make sense because I said it is going into the tank. Any thoughts?

 

[77JQX]

Is the pump itself showing a positive flow? Pipe flows depend on the direction the pipe was draw in. Could you have draw the pipe in reverse? Look at the start and end node IDs for the pipe to check this.

We use Infowater modeling software.

 

[drewnje]

Thanks, that was it, just had to redraw the pipe in the direction of flow, thanks again for all the help!