SlayPE Pump Problem Question

[Vel2018]

Hello guys,

Just needed some advice if my approach and solution are correct? 

I solved this problem but did a different solution path, my solution was quicker than whats presented in the solution page, I did get the correct answer before looking at the solution. So I was thinking maybe I just got lucky and do I really have to do the solution path presented to answer this problem correctly? Those are some questions in my mind.

What I did was to calculate velocity for each pump on a 12in ID pipe based on 1000gpmx.002228ft^3/sperGPM/0.7854=2.84fps

Now this pump can put out approximately 76ft of head at that flow. But I don't know yet whats the TDH.

Now I assume that 2 pumps in parallel add up the flow rate capacity, so since area is constant I assume the velocity inside the pipe is the only variable that will make the Q double from the 2 pumps so, 2 times 2.84fps=5.7fps

I input this into the friction head loss and minor loss equation. I got TDH of 54.73. I used this TDH to get the Q in the graph and got it approximately 4500GPM. 

 

[MikeGlass1969]

What I did was develope a TDH equation as a function of pipe velocity and plotted it on the pump curve. This is a system curve.   I also guesstimated what a second pump would add flow wise.  Where the two curves crossed was my answer. 

I hope the answer is (D)....

 

[Vel2018]

What I did was develope a TDH equation as a function of pipe velocity and plotted it on the pump curve. This is a system curve.   I also guesstimated what a second pump would add flow wise.  Where the two curves crossed was my answer. 

I hope the answer is (D)....

Thanks for sharing Mike!

I did a check on my work.

For example, if I follow my approach and started at say getting velocity based on 3000GPM the available head of both pumps should be around 60.8ft

Then I calculated velocity based on 3000gpm 1ft diameter and multiply the value by 2 for two pumps running. Then input that velocity value to the equation of friction and minor loss. I get the required head greater than 60.8ft, it tells me that both pumps can't be on 3000gpm each and also I can't use the required head calculated to plot in the graph. Then I tried 2000gpm, in this flow the available head is greater than the required head so I know this can work. 

So then at 2200GPM for each pump, this is where the Available Pump head based on the curve should equal the required head from calculation. 

Please let me know if it makes sense? Any thoughts? 

Yes the answer is D.

So its also plotting the system curve based on one pump. 

 

[MikeGlass1969]

In hind sight,  I should have delveloped the equation as a function of gpm instead of velocity.