NCEES Practice Exam TFS Prob 124

Hello,

I do not understand the solution to this problem.  Problem and solution picture.

My only explanation is that it is an error because the pump curve the show in the solution is different from the pump curve in the question.

Thanks.

I posted about this in another thread, but didn't get a response.  It appears to be an error to me as well. I get an answer closer to maybe 55ft, but out of the choices available would choose 65ft.

B is close enough.  A is out in left field and C and D are there to fool you.  Those are nearly a single pump or three in series.  This should be a one minute problem.   Good luck!

Agreed P-E and on my timed practices I selected this option for those reasons. However, looking at the pump curve in the question and selecting 65ft it is still very far off from the system curve (by a factor of 2). On the test I would probably burn a ton of time wondering what I am missing because all the answers suck. 

For reference, this is TFS problem 528 in the 2011 booklet.  The solution process is found in MERM 13 page 18-18.  Look at figure 18.15.  It shows how to add capacity for pumps in parallel.  In this case, you have three identical pumps so, the operating point moves up the system curve in additive fashion.  This is basically a logic question.  One pump delivers a FLOW of 210 (whatever units that is) at 38 ft (so you can rule those two answers out) and three pumps in parallel will produce flow that is roughly 3x that.   You also know that the flow and head is going to be MORE than the head that is produced by one pump at the intersection of the system curve (38), so you can also rule out 30ft as well.  So you know you need to move UP the system curve to some point.  The only reasonable answer is the HEAD that corresponds to what you think the flow might be (which is roughly 3x 210 = 630).  That puts you at an operating head that is not only closer to 65 than other option in the list, but 65 is the only remaining LOGICAL number given, because the head is not additive like the flow is.  A head of 210 is not achievable with these pumps in parallel.  If the pumps were in series, then the 210 answer comes more in play. So, be aware of how easy it would be for them to change this problem to the other answer, just by changing the question to series instead of parallel.