Calculating Friction
Calculating Friction
(OP)
I am a recent graduate who recently got hired by a commercial irrigation company. Got hired, did a bunch of manual labor etc.. to learn the industry and now my boss has me learning how to do the calcs etc... for how to design these systems.
I suppose I should qualify this really quickly, neither my boss nor his boss (the owner) are engineers. I mean, we are in kansas, so anybody could potentially be an "engineer", but the owner is an accountant and my boss is just a very sharp guy who never went to college and got promoted to his position by his abilities. They both realized, however, that having somebody with a solid understanding in math would be beneficial as their company expanded and hired me at a bargain basement price (haha, which I am a little bitter about but I had to take an offer asap to pay back my loans).
Anyway, going through my boss's spread sheets I realized he was using the Hazen-Williams Equation to calculate friction loss. I know that the Darcy-Weisbach equation is the more "correct" equation but that it might be more difficult to impliment into a spreadsheet due trying to pull values off of the Moody Diagram. However, all the irrigiation water around here is well water and the pipes are all buried so the temperature of the water would remain constant as it goes through the pipe correct?
My boss has been away for about a week and he basically told me to go through and learn how to use his spread sheets etc.. and come up with a list of questions for when he gets back.
SOP according to the irrigation books I have read is to use the Hazen-Williams equations, but what is SOP in other industries? Does it matter? I am probably going to try to make a spread sheet for comparison but I was hoping some people with much more experience and knowledge in this area could help alleviate my ignorance.
I suppose I should qualify this really quickly, neither my boss nor his boss (the owner) are engineers. I mean, we are in kansas, so anybody could potentially be an "engineer", but the owner is an accountant and my boss is just a very sharp guy who never went to college and got promoted to his position by his abilities. They both realized, however, that having somebody with a solid understanding in math would be beneficial as their company expanded and hired me at a bargain basement price (haha, which I am a little bitter about but I had to take an offer asap to pay back my loans).
Anyway, going through my boss's spread sheets I realized he was using the Hazen-Williams Equation to calculate friction loss. I know that the Darcy-Weisbach equation is the more "correct" equation but that it might be more difficult to impliment into a spreadsheet due trying to pull values off of the Moody Diagram. However, all the irrigiation water around here is well water and the pipes are all buried so the temperature of the water would remain constant as it goes through the pipe correct?
My boss has been away for about a week and he basically told me to go through and learn how to use his spread sheets etc.. and come up with a list of questions for when he gets back.
SOP according to the irrigation books I have read is to use the Hazen-Williams equations, but what is SOP in other industries? Does it matter? I am probably going to try to make a spread sheet for comparison but I was hoping some people with much more experience and knowledge in this area could help alleviate my ignorance.





RE: Calculating Friction
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Calculating Friction
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Calculating Friction
I am going to recommend you stick with H-W. You are on a process industry forum, and these guys live for Darcy Weisbach, probably even use the two-K loss coefficient method..
You don't need any of that. If your boss already has the C factors for the PVC fittings and pipe, and equivalent lengths, then stick with that because the difference in the formulas won't affect the results.
Learn to link the spreadsheets together for different diameter runs, maybe spiff up the spreadsheet with drop down selection boxes (for example drop down box to pick diameter, or types of fittings). I would work on your people and business skills while in this job, then take the first opportunity that comes along.. It is tough out there right now, good luck
There are probably a blue million darcy weisbach spreadsheets out there available for downloading. I recommended buying a copy of Crane 410 manual if you want to create your own.
RE: Calculating Friction
You need some tough love! You should be happy you have a job, and a good job too! So get over this crap about your bosses qualifications and get on with the task you were given
Learn to use the spreadsheet! When he gets back and gives you an assignment that requires the use of the spreadsheet, what will be your excuse for not knowing how to use it?
Do not become a know-it-all all of a sudden, else you'd be on the dole in a New York minute!
So just pass the first test before you; learn to use the spreadsheet!
RE: Calculating Friction
RE: Calculating Friction
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"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Calculating Friction
How accurate do you truly need to be? I have found that I can get quite accurate results by setting up a Newton-Rhapson iteration routine for the Colebrook equation to determine the friction factor. You may have to return absolute values of the difference between Fn and F(n+1) and pick a low positive value for your initial seed number; I discovered years ago that the shape of the curve is such that you should always be able to converge to the correct value if you do this.
Failing that, I have found that for Reynolds numbers less than 100,000 (depending on how much less), you can "eyeball in" a f=0.025 (or some such number) pretty easily, and for higher numbers, f=0.015 (or some such number). You could set up a look-up table following this approach.
I have never used Hazen-Williams myself, but I have never heard of anything completely going haywire if you do use it.
Regards,
SNORGY.
RE: Calculating Friction
Sizes of the pipes vary. Volumetric flow rates depend on the the well/size of the field/desire for future expansion of the system. Mainline is ~8" to 12", sub mains are ~4" to 8", the "drip tape" that runs under the field varies greatly depending on soil content/length of field etc.. and we use manufacturer's programs to calculate pressure/flowrates.
Basically here is how a system works: water comes out of well gets filtered etc... then gets piped underground through various fittings to tape inlets where it flows through the tape to the flush mains. Once the system pressurizes water starts to flow out of the tape onto the field where it waters crops from the roots up. So we know the pressure at submain manifold and at the flushmain at the other side of the field but since everything else is underground you can't measure the pressure anywhere else.
However, how accurate the readings need to be depends greatly on our customer's needs and some farmers are fine with just getting water somewhat evenly put on their entire field whereas GE seed companies want to know exactly how much is getting put where (with some % of error). The system is kind of "reversed" engineered since we want our inlet pressure at the drip tape to average some X psi and a variation in this pressure is going to effect exactly how much water is being put on some portion of the field that the tape waters.
RE: Calculating Friction
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Calculating Friction
Good luck,
Latexman
RE: Calculating Friction
Hazen-Williams is definitely the way to go with water. Hazen Williams is very accurate all the way up to 25 ft/sec velocity in the piping, and you are definitely going to be below that for sprinkler piping.
Of course if you want it to be real accurate your spreadsheet needs to account for when the sprinkler is at the 'dead end' of a flowing pipe vs just tee'd off a main.. The velocity adds to the pressure available to the sprinkler that is at the dead-end.. Your bosses spreadsheet may or may not have accounted for this.. I bet not one person here every adjusts their calcs for it, even if they are obsessed with calculating friction factor out to several decimal places. The reason being is that it makes the calculations cumbersome.. It is just something small to keep in mind if you are getting high line velocities and need very accurate calcs.
You might consider an option on the spreadsheet that asks "sprinkler on end of line?" and if it is, the spreadsheet accounts for the additional velocity pressure available for pushing water out of the sprinkler (Pv = 0.001123 * Q^2 / D^4, Pv=velocity pressure psi, Q=flow gpm, D=inner diameter inches)
This is very similiar to hydraulic calcs for a fire sprinkler system, and you might start this discussion over in the NFPA forum.. BTW you aren't calculating it "backwards". The calcs should always start at the known pressure.. So in your case its 0 psig (ambient air) + X psig required across the nozzle + line losses + elevation difference.
As I said probably the most important thing is that your boss has already gathered the pipe roughness (in HW term that is called C-factor), and the losses for the fittings (in HW its called 'equivalent length', in D-W it is either 'equivalent length' or 'K-factor').
RE: Calculating Friction
@Eleventy - You have stated that you are a young engineer starting out and that you are keen to learn. I admire you for that. Take my and Latexman's advice and compare H-W against D-W. This is the way you learn and develop your experience and judgement. Add in the Manning formula as well while you are at it. There is no doubt that D-W is the most accurate and flexible of the methods, but H-W is still mandated by some authorities for historical reasons, so learning it is not a wasted effort.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Calculating Friction
to everybody else: thank you, I mentioned this all to my boss and he encouraged me (when I have downtime) to create a spreadsheet comparing the different calculated pressure losses for different set of equations and then I am planning on creating a database so we can compare what the calculated pressures would be for separate projects with the as-built pressure losses with some % error for construction differences.
RE: Calculating Friction
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Calculating Friction
However, I would be a little hesitant about this, as some people may take offense to you showing them that what they've been doing for years is "wrong".
Maybe the first thing I would do would be to try to clean up the spreadsheet and maybe do something in VB with check boxes, etc.
RE: Calculating Friction
While that is not to mean that there wouldn't be any significant variations with different conditions, it does perhaps tend to tone down some partisan rhetoric (see also past discussion at http://eng-tips.com/viewthread.cfm?qid=165987 etc).
[And then again, there might also be value to going to the field occasionally and with instruments measuring/approximating however possible what is actually being achieved – this could also vary with different conditions. Let us know what you find.]
RE: Calculating Friction
RE: Calculating Friction
Other variations are also encoutered, presumedly because of roughness values changing with age of pipe. Roughness doesn't always increase with age of pipe and some pipe actally becomes smoother with age in some circumstances. The viscosity of water at 32F is 7 times its value at 212F. Why program an equation that immediately puts you at a disadvantage by not having the capacity to handle commonly variable temperature conditions, and why even write a program for an equation if it only works in rel few specific cases, or is more complex to program, yet gives no increase in expected accuracy, or even gives actual errors in others.
You don't have to irrigate in winter would seem like a lame excuse.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv