Pipe network pressure drop analysis
Pipe network pressure drop analysis
(OP)
Hello everyone.
I have a question for pipe network analysis. This can be done by computer programs but I would like to know if there is a hand calculation (spreadsheet) method available.
Even if someone can point me to a reference book, I would be greatful.
I tried searching for this on internet but was unsuccessful.
My question is.
1. If we have a pipe network, how do we find the total head developed so that the pump can be selected.
2. If we have heat exchangers in parallel, how do we find out the pressure drop in each line (is it similar to electrical resistance in parallel) and afterwards the total head developed for the pump calculation.
3. If a pump is pumping to a single pipe which then splits in to two lines going to two tanks (open to atmosphere) how do we know what will be the flow rate in each tank if these lines dont have a flow control valve. How can we find out the total head developed by the pump. To make this question interesting what if the pipe size going to these tanks are different and that the fittings are also different for both the pipes and that the tanks are at different elevations.
4. For a cooling tower there are several discharge points for a single line, how do we find out the pressure drop and the flow rate in each outlet if these outlets dont have a flow control valve or even a butterfly valve.
Till date I was using Crane technical paper 410 to do the calculations for pressure drop, but that was for a single input and single output pipe with just one pump connected. there were cases of 2 x 100% pumps, but I simplified the calculation by neglecting one pump. I took the additional tee in the calculation for pressure drop.
The reason for asking all these questions is that they were in my head for a long time and now that client has asked us to perform similar calculation, I thought it is the best time to ask other questions too.
Thanks
Dharmit
I have a question for pipe network analysis. This can be done by computer programs but I would like to know if there is a hand calculation (spreadsheet) method available.
Even if someone can point me to a reference book, I would be greatful.
I tried searching for this on internet but was unsuccessful.
My question is.
1. If we have a pipe network, how do we find the total head developed so that the pump can be selected.
2. If we have heat exchangers in parallel, how do we find out the pressure drop in each line (is it similar to electrical resistance in parallel) and afterwards the total head developed for the pump calculation.
3. If a pump is pumping to a single pipe which then splits in to two lines going to two tanks (open to atmosphere) how do we know what will be the flow rate in each tank if these lines dont have a flow control valve. How can we find out the total head developed by the pump. To make this question interesting what if the pipe size going to these tanks are different and that the fittings are also different for both the pipes and that the tanks are at different elevations.
4. For a cooling tower there are several discharge points for a single line, how do we find out the pressure drop and the flow rate in each outlet if these outlets dont have a flow control valve or even a butterfly valve.
Till date I was using Crane technical paper 410 to do the calculations for pressure drop, but that was for a single input and single output pipe with just one pump connected. there were cases of 2 x 100% pumps, but I simplified the calculation by neglecting one pump. I took the additional tee in the calculation for pressure drop.
The reason for asking all these questions is that they were in my head for a long time and now that client has asked us to perform similar calculation, I thought it is the best time to ask other questions too.
Thanks
Dharmit





RE: Pipe network pressure drop analysis
JMW
www.ViscoAnalyser.com
RE: Pipe network pressure drop analysis
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Pipe network pressure drop analysis
So when you finally believe your manual calculations are right you can check them.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
RE: Pipe network pressure drop analysis
Take your example number 3. You know the flow rate into the tee. The head loss from tank 1 to the tee must equal the head loss from tank 2 to the tee. (Don’t forget to include the surface elevations in the tanks.) So you have to change the flow in each line until two conditions are met: 1) the flow of the two lines out of the tee equals the flow into the tee, and 2) the head loss across both pipes are equal. Finally add this head loss to the head loss from the pump to the tee and you have the total discharge head loss. This example is relatively easy to do with excel. But only slightly more complicated networks get very messy to solve this way.
If the client wants you to perform these calculations then I suggest you get software to do it. I use AFT’s Fahom software which costs around 1,500. Still, even with the software you can get into a lot of trouble if you don’t have a good understanding of what’s going on “behind the scenes.” So go to the library and study any book on fluid dynamics. Learn how to do simple models by hand before using software to solve more complicated ones.
RE: Pipe network pressure drop analysis
"Simulation and Analysis of Gas Networks", A.J. Osiadacz, Gulf Publishing Co.
In the meantime you might occupy yourself with this page. I think it has the answers to most of what you're interested in.
http://www.isesco.org.ma/pub/Eng/water/Chap6.htm
http://virtualpipeline.spaces.msn.com
RE: Pipe network pressure drop analysis
As for starters, for small projects, I can take consideration of advise from khardy and do the calculation. Though it will be a bit tedious doing a manual calculation, but it will clear my doubts and I will gain confidence in the analysis.
I downloaded the Epanet software, thanks to Stanier, and am trying to work with it. It is a good utility for starters and will help me to check my answers from manual calculation.
I went on the webpage mentioned by BigInch and that will help me too.
Thanks for the post.
If you have any other comments please let me know. It will help me to improve the knowledge.
Dharmit
RE: Pipe network pressure drop analysis
http://virtualpipeline.spaces.msn.com
RE: Pipe network pressure drop analysis
The system is really two systems with about half the gas on the top of the mesa and half in the wash with a 14-inch line between them. It only takes 1/2 psi change in suction pressure at either of the sites to change direction of flow in the 14-inch. Less than 1/4 of the inch-miles of pipe are piggable and all the others have standing water in them (they are fiberglass so corrosion is not an issue).
Yep it takes between 125,000 and 200,000 iterations to solve the model.
David
RE: Pipe network pressure drop analysis
http://virtualpipeline.spaces.msn.com
RE: Pipe network pressure drop analysis
My model had 0.05 psi tollerance built in. Any coarser and the reasults got progressively less meaningful. I was using the model to evaluate the pigging schedule, so fairly subtle changes were really important.
David
RE: Pipe network pressure drop analysis
The gathering field I worked in had the ultimate optimist geologists in a very tight lower Wilcox zone. They would be thinging 50 MM for 5 years, get 20MM for 30 days, then only 50M for the next 3 years. By the time they figured out it was only getting the small 50, they'd already dug 2 more and I'd already put in 10 miles of 12". But they could drill fast enough to keep up 800 MMSCFD from the system ... back then. Wonder what its doin' now.
http://virtualpipeline.spaces.msn.com
RE: Pipe network pressure drop analysis
Item 2 is a special case of item 1, and can be quite sophisticated if you follow it thru to the bitter end. For the case of a HX, you assume several different flowrates thru each HX, and for each flowrate, calculate the heat transfer and pressure drop. This calculation must include gravity head changes, which become very important for phase change cases ( ie boiling or condensing water). Each HX with its piping will then have a curve of DP vs W (flow) plotted. Since each set of the HX's with their piping are operating in parallel with the other HX's, they must have the same total DP, and the sum of all the flows must equal the total known W. Now plot a curve of DP vs (sum of all W's) for all HX's in parallel. Since you know the total W, read the DP associated with that W and then work back to the W for each HX at the known common DP. This works easily if the flow is stable and montonic in all HX's.
The problem becomes "sopisticated" if there is flow instability- ie, if the DP vs W curve for each HX is not a monotonically changing DP with W. Key phrase- Ledineg static flow instability, or parallel channel instability. This is a problem if it occurs at the range of W's that the process is expected to operate at.
For the case of a simpler piping system without HX's, a simailar process is used. All pipes working in series will have a common W and the DP's are additive, whiel all pipes in parallel have a common DP and the W's are additive.
RE: Pipe network pressure drop analysis
There is no way you can accurately predict the DP across a large series of randomly oriented bends in a piping system- the actual DP across each bend is really a function of what the relative configuration of each bend is with its next upstream bend, due to the effects of pre-swirl. So you have to add flexibilitly to the system by adding throttling butterfly valves and 10% mor pump flow than theoretically required.
RE: Pipe network pressure drop analysis
http://en.wikipedia.org/wiki/Hardy_Cross
RE: Pipe network pressure drop analysis
http://www.lmnoeng.com/Pipes/PipeNetwork.htm
RE: Pipe network pressure drop analysis
Thats good history lesson along with the online calculator to support it.
Is there a book that deals with specifically flow analysis, network analysis only and the finite element analysis.
Thanks
Dharmit
RE: Pipe network pressure drop analysis
http://virtualpipeline.spaces.msn.com
RE: Pipe network pressure drop analysis
Osiadacz’s book is good but you could try the reports below as a starter.
The Simulation of Gas Flow Dynamics in Pipeline Networks (free download)
http://www.prci.com/publications/L20005e.cfm
Using an Excel-Based Model for Steady-State and Transient Simulation
http://www.psig.org/papers/1990/9904.pdf
By the way, there are a number of free technical reports available on http://www.prci.com/publications/index_ebook.cfm
RE: Pipe network pressure drop analysis
http://virtualpipeline.spaces.msn.com
RE: Pipe network pressure drop analysis
You can hire the publisher of PipePhase to do this for you, or hire an engineering contractor who has the software to do it.
The original poster's problem is a liquid problem so it should be fairly straightforward to solve using one of the copmuter methods. No density change and no heat transfer simplifies things a whole lot.
zdas - 0.05 psi is a preeety tough convergence criterion, especially in a compressible flow problem. One good thing about Pipephase is it lets you adjust all the convergence criteria/parameters of the solver engine to help stabilize the solution. A lot of times a model will start to converge but then it suddenly runs off to bo-bo land, so you can adjust the various criteria to get the thing to solve. A lot of this is a function of your initial guesses and the network granularity. Pipephase has a lot of tricks to help you get a solution. Even with that I have built quite a few models that just would not solve, usually due to looping. Owners tend to build systems that defy analysis... it's "Ehhh, what's one more jumper gonna hurt?"
RE: Pipe network pressure drop analysis
Actually (I was holding my tounge there zdas) and think 0.05 is expecting a little too much from hydraulic analysis. The typical deviations of the actual values of other variables in the field vs their typ. input values in an analysis would not allow confidence in an answer depending on convergence at 0.05 psi. A liter of water could get picked up somewhere, a valve at 99.5% open, or a temperature change of 2ºF and you'd be flowing from the other direction again.
http://virtualpipeline.spaces.msn.com
RE: Pipe network pressure drop analysis
These books are for gases but will be of great help.
They make a good start
Thanks
Dharmit
RE: Pipe network pressure drop analysis
RE: Pipe network pressure drop analysis
RE: Pipe network pressure drop analysis
http://virtualpipeline.spaces.msn.com