Losses for standard bends < 45 deg (11-1/4, 22-1/2)

[zabrab]

I’m working on a force main that has a few of 22-1/2 degree and 11-1/4 degree bends. My references have friction losses for 90 degree and 45 degree bends, but nothing for these smaller bends.

In the past, I have prorated the losses for 45 degree bends or just used the 45 degree bend friction loss for these bends and, although not technically correct, one or the other approach has been accepted with no problems as it provides an increased friction loss over straight pipe for the bend, and, to be frank, because no one had a technical approach or reference that addressed these bends either.

Now I have a situation where a reviewer will not accept either of the above approaches without some type of source as back-up as there isn’t a technical basis for either and he will not provide friction losses for these bends that he would accept. Basically, he is taking the position of what I’ve proposed is unacceptable, that it is my problem to provide losses that are acceptable to him, and that, as the reviewer, it isn’t his problem to provide a what would be acceptable friction losses or even a source for these losses.

I guess what torques me about this situation is that the issue is one that doesn’t make any difference. Assuming all of these bends as 45 degree bends only adds 0.1 ft to my friction loss for the 3,000 ft force main at design flow.

Let me get off stage before I start whining and I say something I’ll regret. Let me just say that if this were a situation that would affect the design and/or a clear-cut technical issue addressed in various references, I would agree that it clearly is my “row to hoe.” However, I think we are more in a professional judgment/preferences situation and, as such, the reviewer should provide sources or methodology that are acceptable.

Back to my actual question:

Is there a source for friction losses in standard bends of less 45 degrees or an accepted mythology for approximating the friction losses?

 

[stanier]

Idelchik or Miller would have the data if you can get hold of a copy of their books.

You may also be better off going to the ductile iron supplier in your country.

http://www.acipco.com/adip/

http://www.tycoflowcontrol.com.au/tycowater/ductile_iron_pipelines

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[LHA]

If you can find Equivalent Length of the fittings:

k = Darcy's f for the material * Leq / D.

If not, the losses can't be 0.0, and have to be less than the amt. for 45d bends. Try identifying the operating point, using the system curves with both 0.0 and the 0.1, or whatever you got assuming all 45's. If both still produce points within acceptable operating range for the pumps, you've met due diligence and proven the design.

Engineering is the practice of the art of science - Steve

 

[bimr]

If you look in Cranes Technical Paper No. 410, there is documentation for the K Factors for mitre bends on page A-29. It will probably suffice.

 

[kchayfie]

We generally use a K-value of 0.2 for a 22.5 degree bend and 0.1 for a 11.25 degree bend (as opposed to 0.35 for 45 degree bends). I believe they come from Miller but its been so long since I looked it up...

But given that you are taking about losses of 0.1ft or less, I would argue that it is within the accuracy of the equations being used to estimate friction losses anyway.