Single Size Manifolds friction loss calculator or equation 2

[bluepearl]

Hi everyone, I have a question regarding my project. I need to calculate the net friction loss (psi) based on the following criteria:

a.) flow rate per outlet (gpm)
b.) outlet spacing (ft.)
c.) manifold lenght (ft.)
d.) Slope (%)
e.) pipe type (pvc, aluminum)
f.) nominal pipe size (in.)
g.) Pipe class (100, 125)

This is the equation I'm using but I'm not sure whether I missed out something

F = Q^1.852 x D^-4.8655 x L x 0.01 x pipe class

where:

F = Friction loss in psi
Q = Flow rate in gpm
D = pipe size in inches
L = Lenght of pipe in feet
pipe class = 100 or 125


Can someone teach or show me how to compute for the net loss in psi and the flow in gpm. or how can I solve this in excel. Do you have any excel formulas to solve this problem? Your help will be greatly appreciated. Thanks.

 

[KernOily]

If I understand your question correctly, there is no cut-and-dried way to do this. Usually what we do is to design the system so that the dP across the header is << than the dP across one of the outlets. That will ensure uniform flow through the header and more importantly uniform/equivalent flow out of all the outlets. Is that what you are trying to do?

 

[RWF7437]

You did NOT say what fluid is flowing. What is it ?
It appears you are using the Hazen-Williams Equation which can only be used for water at normal temperatures and pressures.

If your fluid is not water or you are not using the H-W Eq. then you must use some other equation. The Darcy Weisbach Eq. is often used but there are others.

I don't think you will find any "built in" formulae in Excel but once you define what you want to do and how you want to do it, as suggested by KernOily, you should find Excel can do the grunt work for you.

Finally, be very careful of any formula that does not clearly state the units of measurement being used. Hydraulics is full of mixed units, empirical formulae to lead you to very wrong answers.

good luck

 

[bluepearl]

The liquid inside the pipe is water at normal temperature. Is there any modified formula to calculate the friction loss on the manifold with multiple outlets?

 

[RWF7437]

Use the Bernoulli Eq. It is a special case of Newton's Law of Conservation of Energy. Sketch the Hydraulic Grade Line (HGL) backward from the known flows at the outlets.

You can do this in a spreadsheet or you can model it in the free program EPANet or you can spend a lot of money on fancy software to do it. Your choice.

There is no "modified formula". You'll have to modify it yourself by understanding the problem and applying the basic physical principles.

good luck

 

[KernOily]

Model the header and the outlets using the Darcy equation. Treat the outlets as pipe exits or orifices. That will give you a good enough approximation for what you are doing.

This is an iterative problem. If you have a lot of outlets, a software package helps but otherwise just do it by hand. Assume the flow is equally split across all the nozzles and claculate the pressure drop. Iterate the nozzle dia until you get what you want.

Your answer will not be exact - but nothing ever is in this business. Close enough for horse shoes and hand grenades.

 

[BigInch]

If you like, you can download my Churchill equation based flow loss spreadsheet. Once you use Churchill you'll never use Darcy, Colebrook-White, Hazen-Williams again. Churchill is a NO iteration solution.

http://rapidshare.com/files/40413717/pipe_churchill.xls.html

It will do the flow losses, but you must add the static head differentials yourself.

http://virtualpipeline.spaces.msn.com

 

[KernOily]

BigInch,

Great post as usual. Correct me if I'm wrong here, as I usually am. The Churchill eqn is a friction factor eqn, yes? There are other explicit eqns one can use for f as well. The Darcy eqn is a friction loss eqn, not a friction factor eqn.

The fundamental pipe fluid flow problem is still an iterative one, especially if a piping network is involved. The problem here at hand in truth is a piping network. I have designed header and manifold/spray nozzle systems exactly like this using the Pipephase software and it is an iterative procvedure. The software determines f, but I still have to make initial guesses at flowrate or source pressure or sink pressure and then run the model.

Regardless of which equation one uses, and they are all just forms or adaptations of the energy equation, there is always one too many variables to get a closed form solution in a piping network. One must always make an assumption of rate or diameter, calculate the answer, and see if that answer meets your criteria. Thanks!

 

[bluepearl]

Thanks a lot.

 

[BigInch]

Thanks for the compliment KernOily.

6 of one, half-dozen of another. IMO, they're all friction factor equations that when all the multiplication is over yield what you're really looking for, the head loss, in one way or another.

The difference between Churchill and the Colebrook-White equation for example, is in C-W the friction factor f is a function of 1/f, so an iterative procedure was required. In the Churchill equation friction factor is independent of f so no iteration is required.

Well you are correct in saying that hydraulic design usually involves an iterative solution in some manner or another, but that's really because we usually do not have our problem well defined to begin with, or its because we are really not too confident that some of the variables we know arn't at their optimized values, or because our bosses always think they have a better solution to one or the other of them.

If you take a flowing pipe, there's a number of things that we are usually faced with determining before we think the pipe is hydraulically solved. They usually amount to 5 primary variables, assuming the other extraneous variables such as viscosity, specific gravity, temperature, compressibility, etc. are all known.

So anyway they are,

1.) inlet head,
2.) outlet head,
3.) flowrate,
4.) length and
5.) diameter.

As all are dependent on one another, 4 of those 5 variables would have to be absolutely defined in order to arrive at a non-iterative solution for the last one, so it is possible to have a non-iterative solution for one pipe, given 4 of the 5 variables above.

As long as we can chain together one pipe after another, we can make a large determinate system, if for any segment we know 4 of the 5, even if they are a different known set for each segment. Theoretically we can write the one unknown variable per segment in terms of the known variables and solve directly for the system.

The chaining can be extended to include various simple "networks", if the network does not involve a loop and we are willing to define a boundary condition or two. A single pipe splitting into two branches can be broken into two chains and solved without iteration, if you can hold the a certain backpressure at one of the outlets. Often that's not too difficult to do, since the downstream pipeline or tank often defines some boundary criteria that we should meet anyway. Sometimes Mother Nature helps, but telling us that the outlet pressure can't be less than 0 psia, etc.

Comples networks (with loops) however can be another story. There an iterative proceedure is necessary, because in the simplest terms of flow determinations, we would not know what net flow could be circulating in any given loop, so we have to eliminate that one unknown variable per loop, or solve for it by iteration.

Most of the time we know or can guess at some of the variables, such as a good pipe diameter to allow us to arrive at a "direct solution", but it really depends on how well some of the 5 primary variables are defined to begin with, or how good your experience factor is at making great guesses that you don't have to iterate later. At other times, we need to eliminate some variables by shortening the time period of interest, to eliminate line packing effects from our solution, or shorten the length of the segments of pipeline to eliminate compresibility effects, temperature changes, or define some convenient boundary conditions.

But all good guesses aside, the only real common denominator for all of the variables is end cost of the various alternatives, so to do a "proper job" without relying on guesses or predetermined diameters, wall thicknesses, pressure drops, etc. amounts to a cost optimization and sensitivity analysis. Still you're not sure you've picked the right one until you get a hard bid on the construction cost, so it can be really difficult to do a good job at times even with very good guesses on your side.

http://virtualpipeline.spaces.msn.com