How Different is the 2016 NCEES Practice test from the 2011 one?

[amats42]

Excuse the crude adobe job, but this was my method. There is no way that I can see that the head won't increase.

View attachment tyKTLBp.pdf

 

[Nevill24]

Forgot to add.  So if you see that the operating point of 3 pumps in parallel is 500-520 units (lets gpm as you pointed out its unit less).  If you were to measure the flow at the discharge header on 3 pumps you would measure the 500-520 gpm.  What would you measure if you took a flow measurement directly at the discharge of each pump? 500 to 520 divided by 3 so ~170 GPM which is where I get the 167 GPM.  

 

[amats42]

Forgot to add.  So if you see that the operating point of 3 pumps in parallel is 500-520 units (lets gpm as you pointed out its unit less).  If you were to measure the flow at the discharge header on 3 pumps you would measure the 500-520 gpm.  What would you measure if you took a flow measurement directly at the discharge of each pump? 500 to 520 divided by 3 so ~170 GPM which is where I get the 167 GPM.  

From engineering toolbox:


Pumps in Parallel - Flow Rate Added


When two or more pumps are arranged in parallel their resulting performance curve is obtained by adding the pumps flowrates at the same head as indicated in the figure below.

Centrifugal pumps in parallel are used to overcome larger volume flows than one pump can handle alone.

• for two identical pumps in parallel and the head kept constant - the flowrate doubles compared to a single pump as indicated with point 2
  

Note that for two pumps with equal performance curves running in parallel

• the head for each pump equals the head at point 3
  
• the flow for each pump equals half the flow at point 3
  

 

[Nevill24]

From engineering toolbox:


Pumps in Parallel - Flow Rate Added


When two or more pumps are arranged in parallel their resulting performance curve is obtained by adding the pumps flowrates at the same head as indicated in the figure below.

Centrifugal pumps in parallel are used to overcome larger volume flows than one pump can handle alone.

• for two identical pumps in parallel and the head kept constant - the flowrate doubles compared to a single pump as indicated with point 2
  

Note that for two pumps with equal performance curves running in parallel

• the head for each pump equals the head at point 3
  
• the flow for each pump equals half the flow at point 3
  

That's exactly what I am doing!  The flow does not triple because it most follow the system curve!.  You tripled the flow and found where it intersected the y-axis for head.  This assumes there is absolutely no friction and the system curve is flat.  See below it's kind of sloppy, but hopefully it makes some sense.

 

[amats42]

I see what you're saying now. Hammering through questions I didn't question it and just took 3x the flow rate and moved on. I didn't look too closely at what a properly plotted 3xQ would look like.

I'm afraid I'm not any help, then. Perhaps you can report it to NCEES and hope to save future engineers the headache if they verify the graphs (unless I'm also now missing something!).

 

[Nevill24]

I see what you're saying now. Hammering through questions I didn't question it and just took 3x the flow rate and moved on. I didn't look too closely at what a properly plotted 3xQ would look like.

I'm afraid I'm not any help, then. Perhaps you can report it to NCEES and hope to save future engineers the headache if they verify the graphs (unless I'm also now missing something!).

Well it doesn't change the answer that would be selected as Audi points out using logic 65 ft is really the only thing that is event remotely feasible.   Personally I just thought it was a bit wacky of a best choice and while Audi may think I'm crazy at least I'm not the only one that is!

 

[amats42]

No, I definitely agree. I don't work with pump sizing so I just had it in my head that parallel means 3xQ and sped through it. Seeing the "correct" answer when going over the solutions cemented that false method in my head and I was happy (didn't think I needed to look back over curve material). Turns out I could use more review because I was making an easy mistake.

 

[Audi Driver P.E.]

Ok, at least I see where you're coming from now.  There is still some problem with your methodology, but there are two things going on here.  One,you're looking at the wrong operating point, from which to basis your calculation/plot.  You need to look at the intersection of the pump curve and the system curve.  That point is at roughly 38 ft and 230.  Your point on the green line is sort of pulled out of thin air... you did not move from point 1 to point 2 on the engineers edge chart, you did something else to arrive at your 55 ft /170 flow point. You have the final point on your pink line plotted correctly, but there should be a point on it that corresponds with 38 ft and 3 * 230= 690 (which is one of the only other points you can really plot with any accuracy).  That point is obviously NOT the capacity for the three pump system because you need to move back up your pink line to the system curve.  IF you've plotted your pink line correctly, at that intersection you should be approx 60 ft or so (by my reckoning).  It appears to me, you've drawn your pink line a bit too flat.  Even if your pink line is dead on the answer is still the same, yes?

The second thing that is compounding the confusion in this, is that they changed the provided pump/system curve from what was provided for the 2011 exam, but not the answers.  Rightfully so (for the plot anyway), because you had to do a lot more guessing out to the 690 point because the plot of the system curve is too short in that exam booklet.  When they made the new graph for this booklet, they altered the system curve so that it isn't as steep as what they provided before.  So, It's suggesting a bit lower value than what was more obvious in the other booklet.  Here is a photo of the original pump plot:

 

[Nevill24]

Ahhh ha...That's a completely different system curve than what is included on the 2016 practice exam.  Hopefully that is what you have been basing everything off of. Go back and look at the curves we have posted and you will see the difference.

 

[Nevill24]

OK I didn't notice in your last post that you acknowledged the difference...But taking that pump curve which is completely different than the 2016 practice exam using the method that I say is correct you get guess it...65ft.  My guess is NCEES expanded the plot, changed the system curve, but did forgot to change the options?  

Going back to the 2016 exam curve. I am looking exactly at the intersection pump of the parallel pump curve and the system curve.  That's how I am getting the ~170 gpm and ~55 ft of head.  The basis of my calculations/plot is using the single pump curve and creating a parallel pump curve by multiplying the flow at each point of head by 3.  Then plot the system curve and where the system curve intersects the parallel curve is where the system will operate.

The existing operating point of 38 ft and 230 IS taken into consideration in the combined parallel pump curve.  At 38 ft of head the parallel system creates 690 gpm (230+230+230).  At 0 feet of head 900 gpm (300+300+300).  At ~45 ft of head 600 gpm (200+200+200).  At ~65 ft of head  300 gpm (100+100+100). and so on. If you keep doing this you get at ~55 ft of head ~510 GPM (170+170+170).  

 

[Audi Driver P.E.]

Gotcha.  That approach is entirely correct.

 

[Nevill24]

Plotted everything in excel which hopefully will clarify anything for anyone that is interested down the road.  I just did smooth lines so it stands out better, but for more accuracy you can do a trend line....Thankfully we will not have to do this on the actual exam!  

View attachment Parallel Pump Problem.xlsx

 

[amats42]

Thanks y'all for the detailed responses. I definitely think it's worth bringing this up to NCEES. Like I said, if it wasn't for this thread I wouldn't have questioned my very wrong methodology. Well done!

 

[spacebanjo]

I had the same question on this problem. 



I believe it is an error and reported it to NCEES early this week. 

 

[Audi Driver P.E.]

To be clear: there is no error.  I don't believe there is anything they will or even should fix.  The question is a LOGIC question that is answered in a minute or less depending on how fast you can read.