Fluid mechanics question

[alphatarget1]

Okay resident CEs and MEs... Here is a question for y'all.

I have to find a pump that pumps water on a hill. 66' in height and the pipe is about 320' long. The calcs are shown below.

Hsys=Hpipe+Hheight+Hminor losses

We were looking at this pump with a flowrate of 120gpm. 2" diameter PVC pipe (assumed smooth) and avg. velocity is 12.25 ft/s

Reynolds # is 167416.4 and friction factor is .017 (sized up a little with the moody chart)

headloss from pipe will be (0.017)*(320')*((12.25ft/s)^2)/((2"/12")*2*32.174) =76.17 ft

headloss from tee is 3.5ft (assumed threaded tee with the K=2)

headloss from elbow is 4.7ft (assumed threaded with K = 1.5)

adding everything together the system head is 150'. That means a pump that provides 201' maximum head will work right? The shop people called the pump and told them what they needed to pump and the lady said we need a pump that has 350' and costs 2x as much. This is the first time I'm doing these types of calculations so any advice is appreciated!

 

[Tiamat]

Although that question is easy, I am ChemEng

My advice would be to ask them why they think you need the expensive one - make them do the calculation in front of you. I have a feeling they are trying over oversell ya.

 

[bonkers325]

is this a real project you're working on or a theoretical textbook example thing? it looks ok, but i didnt do so hot in fluid mechanics last year. but if you are confident in your calculations, then yes 201' of maximum head will suffice

 

[alphatarget1]

Originally posted by: Tiamat
Although that question is easy, I am ChemEng

My advice would be to ask them why they think you need the expensive one - make them do the calculation in front of you. I have a feeling they are trying over oversell ya.

I think it's an online place.

 

[alphatarget1]

Originally posted by: bonkers325
is this a real project you're working on or a theoretical textbook example thing? it looks ok, but i didnt do so hot in fluid mechanics last year. but if you are confident in your calculations, then yes 201' of maximum head will suffice

It's real!

I just want a little bit more assurance of a confirmation. I took fluids last semester also.

 

[silverpig]

<-- physicist. I can say what it'll theoretically take...

 

[iwantanewcomputer]

Originally posted by: Tiamat
Although that question is easy, I am ChemEng

My advice would be to ask them why they think you need the expensive one - make them do the calculation in front of you. I have a feeling they are trying over oversell ya.

me too, you should be ok as long as you are doing the calcuolation right. you might need to account for assumtions and estimates in those numbers

 

[habib89]

well all your equations and numbers look right.. i'm gonna assume you did the math right..

yah, i think they are trying to over sell you... have them redo the math.. do your math, and scan it, and fax it to them.. they can either say you're wrong and make them explain what is wrong, or they are gonna go by your numbers..

if you're the one making the decision, stick to it.. they might say you need the bigger one so that you can guarantee the proper flow rates.. if that's the case, look at the budget, and if you have the extra money, then get the bigger pump.. it's not always bad to spend more money, depending on where your funding comes from and how it is attained

 

[dullard]

<- PhD in Chemical Engineering. I haven't done any math like that in years. But what you did looks correct (I'm not going to go look up friction factors, so I will assume you did that properly). I would like to make a possible suggestion though.

Is it possible to use a larger diameter pipe? 12 ft/sec is a little on the high end. While it can be done, I like to keep liquids <= 5 ft/sec. That should drastically reduce your pipe loss and take a lot of strain off your pump. Of course you have to balance the cost of the pipe with that of the pump and and physical space constraints that you might have.

If you can't go to a bigger pipe, I might take the bigger pump. Those friction factors are quite approximate. As mineral deposits form in your pipe, friction will likely go up. At least the velocity will go up as it has a narrower cross-sectional area. If we assume a 33% margin of error, you are right at a minimum of 200'. A 201' pump will really strain if this happens (they may have built in a margin of error in that 201' number, but can they guarantee that?). Also consider your future needs. Will you ever need more than 120 gpm? Will you ever need to pump higher? Will you ever need to do repairs (each of which will add more minor losses)? Will the pump get less efficient as it ages?

 

[alphatarget1]

Originally posted by: dullard
<- PhD in Chemical Engineering. I haven't done any math like that in years. But what you did looks correct (I'm not going to go look up friction factors, so I will assume you did that properly). I would like to make a possible suggestion though.

Is it possible to use a larger diameter pipe? 12 ft/sec is a little on the high end. While it can be done, I like to keep liquids <= 5 ft/sec. That should drastically reduce your pipe loss and take a lot of strain off your pump. Of course you have to balance the cost of the pipe with that of the pump and and physical space constraints that you might have.

If you can't go to a bigger pipe, I might take the bigger pump. Those friction factors are quite approximate. As mineral deposits form in your pipe, friction will likely go up. At least the velocity will go up as it has a narrower cross-sectional area. If we assume a 33% margin of error, you are right at a minimum of 200'. A 201' pump will really strain if this happens.

Pipes are already installed, what happened was they're putting some landscaping at this valley to filter the sediment going into a lake and this is going to be temporary (meaning a year or two) and will be removed. I haven't taken the mineral deposits into account, though. Thanks for the suggestions!

 

[stephenw22]

where on the pump's curve will you be operating? what kind of pump are you using?

are the pump seals/components rated for the kind of water you'll be putting through it? (i.e. is there sediment or junk in the water that would wear out your pump).

is 120gpm the pump's flowrate with a 0' head, or at some other head? if i remember right, the flowrate drops off quickly as you increase head (for a centrifugal pump). you might only get a trickle out the other end if you're using 3/4 of the max head of the pump.

if you don't care about flowrate, a small positive-displacement pump might end up being cheaper. They're not as durable, but if you only need one for a couple of years, that shouldn't be a problem.