Parallel Pump Operation - friction loss 6

[STrunk]

For pumps in parallel operation, do I calculate the combined flow at the point of force main connection?

For example: Assume Pump "A" has 75 gpm, Pump "B" has 50 gpm. Based on the attachment, does Pump "B" see the friction head loss of 50 gpm @ 100 ft. of fm length + 125 gpm (50+75) @ 500 ft. of fm length? Or does Pump "B" only see 50 gpm @ 600 ft. of fm length?

 

[RWF7437]

Looks like homework.

You need more information, including:
Pump Curves for each pump ( Head vs Flow )
Pipe sizes for all pipes
Pipe roughness for all pipes
Then plot system head curves for each pump running separately through only those pipes which carry flow.

Then add flows of Pump curves together.
Then plot system head curve for all pipes in the system.

Then intersect the combined pump curve with the total system head curve. The point of intersection will be where the pumps will operate.

good luck

 

[STrunk]

I agree, it does look like homework. I made up the problem b/c I'm just trying to understand the basics.

In order to "plot the system head curves for each pump running separately through only those pipes which carry flow", I assume that this is single/simplex operation of each pump. How do I then add the flows of pump curves together (for combined pump curve)? What if the pumps are subtantially different from each other....can they still be added?

 

[semo]

I'll say your first thought is correct. With your scenario, Pump "B" will see the friction head loss of 50 gpm @ 100 ft. of fm length + 125 gpm (50+75) @ 500 ft. of fm length?

Now to see how they operate together with separate pump curves will be an iterative computation. You can estimate the flows for the two pumps. When you do the HL calcs based on these flows, go back to the curves with that HL and see what flow you get from the pump. It will be different so take the revised flows and repeat the complete process. As you repeat the process, You will narrow down towards the correct flows.

If the pumps are substantially different, one pump could overpower the other and cause it to be at shutoff head.

Once you get the curves(pumps) you want, you also need to look at the system with each pump running separately to make sure you don't run off the right side of your curve as well.

 

[tkall]

I know this post is going to cause some problems but I have been thinking about this for quite some time. It is probably better suited for the ethics forum but the questions Strunk is asking makes this a reasonable place to open the can of worms.

First I must say that there are some brilliant people contributing to these forums. BigInch immediately comes to mind as does binmr (did I get that right) and I am not arbitrarily complaining. The problem I see is answering questions for engineers/designers who represent that they are "designing" some thing for some one and, based on the nature of the question, proclaim that they do not understand the most basic, fundamental concepts necessary to complete a successful engineering design. When a question(s) is asked that is so elementary and the fact that it is asked online rather than looking it up in at least one of hundreds of texts available on the subject I have great concern for the client that is paying for the project and the people that will use/work with/build the design. Clearly, when considering the types of works being discussed here, there is far more to the job than the simple calculation of pipe friction. And, if the designer does not know how to do that, should that designer really be working on the project at all? Furthermore, if there is a failure, can the designer implicate the PE from this forum who provided the advice?

Strunk, I do not mean to be picking on you but, in my opinion, your questions clearly demonstrate that you are engaged in work that is far beyond your ability to competently perform the duties you have described.

I heard of a case where a blog entry was used against someone in court. I am interested to hear what others think.
Tom

 

[PEorl]

tkall,

I disagree completely. Maybe STrunk did look up the concepts/basics in a book and is trying to ensure that he understands all the concepts. This person is less likely to make a mistake then the person that goes at it alone. You are not being asked to QA/QC this work, and sign & seal it. I don't see how you can be sued for giving your opinion/advice on how you would do something when you haven't been paid to do it. This forum is engineering tips and it is a great forum to share ideas and learn.

 

[PEorl]

STrunk - To add flow rates for two pumps in parallel, see example at:

http://www.engineeringtoolbox.com/pumps-parallel-serial-d_636.html

 

[tkall]

PEorl:
For instance, if you as a PE happen to be on a construction site where someone is engaging in dangerous behavior, say installing pipeline in a deep trench with inadequate shoring. You see them and don't say or do anything because you were just over there chasing you dog that was loose (not being paid) and the trench collapses and they die, you better believe you would be dragged into court by a slick lawyer. You might be found to have no liability (I doubt it) but that would be for a jury to decide.

I was just wondering if a disservice is being done by answering such sophmoric questions. And I would imagine that if you gave bad advice here and it was followed and someone got hurt, that during a deposition the designer would say a guy on the forum told him to do it that way. If you have anything to take, they will try to get it.

But don't take this so personally...it's just a guestion and I think it is a reasonable one.
Tom

 

[cvg]

interesting rhetoric, but hardly bears on the questions or answers in this thread. I hardly think any of us can assume what strunk is doing unless we ask. And he already said he is just trying to learn. hopefully he will also read books, search this forum and ask a mentor many questions, perhaps the same ones asked on this forum. It also seems unlikely that he will be stamping and signing a pump station design, but wait - I am making an assumption...

Tkall - by the way, next time please do not hijack threads, that does not conform to site policies. please start another one and refrain from personal attacks.

see the following posting policies:

http://www.eng-tips.com/market.cfm

 

[tkall]

cvg:
To what personal attack do you refer?

 

[bimr]

The question that was posed is actually more complicated that it appears at first reading.

In addition, the civil field is quite broad. Sometimes you end up working on a civil engineering project that is out of your area of expertise. So that a simple problem for a structural engineer is complex for a water guy and vice versa

So I would tend to give someone the benefit of the doubt.

In regards to the problem, you have to figure the system head curve as three separate segments. Each of the segments will have a different curve, but the curves should join together at the point that represents where the pipes meet since each pump would have to have the same pressure at that point.

You will also have to talk to the pump supplier. The pump supplier will have to select each pump such that the pumps are capable of operating at the combined pressure head.

If the pumps are not designed for the combined head (with bigger motors), then the operating point of each pump will slip leftward on the pump curve and pump a lower volume.

It would be a simpler problem with equal sized pumps. With two different size pumps curves, it complicates the issue.

After you have obtained the curve of the combined output of the pumps, you can overlay the curve onto the system head curve.

And thanks for the complement, tkall.

 

[STrunk]

Thank you for the responses.

And to the person on this thread that is complaining about elementary questions:
I take my responsibilities as an engineer very seriously. I know and understand my obligations both ethically and to my clients. I do not deny that the problem that I have presented is out of my expertise. However, this does not mean that I cannot pursue information to help gain knowledge about this topic for myself as part of my professional development. And yes, I have researched different text on the matter (with very limited information on the subject); and yes, I have contacted others with experience in this area who do NOT deny this is a complicated problem; and yes, I am formally trained in the field of civil engineering.

Furthermore.......did you not pursue a formal education in engineering? If so, why? You could have "read the text" and self-educated yourself on theory. For most, including myself, this is not practical. The thought that it is more responsible for an engineer to research different engineering situations on their own in a vacuum without input from fellow colleagues is ridiculous. I must wonder why someone like yourself is even a member of this site if you don't believe it is proper to share information among people in the same field. I have drawn on the knowledge and experience of others. To do otherwise shows a lack willingness to use all available resources which can very much compromise professional development.

Thanks to those on this website that enjoy learning from each other. I have found it helpful and hope that I will be able return the favor someday.