Friction loss 3

[friend81]

Dear Friends,

I would like to get a clarification in calculating friction loss in a PVC pipe of 100m length.Using the formula

hfs = 4f L/D x V*2/2 X g,

Q = 30cub.m/h
L = 100m
D = 51.4mm (ID of UPVC pipe)
Kin.Viscosity of Water @ 30deg.C = 0.801 cSt

Results i got are as follows

V = 4.018119 m/sec
Re = 258164.17
f = 0.003509

And finally the head loss per 100m pipe is 22.50m.

But friction loss chart provided by the pipe manufacturer shows 19.88m for same diameter & flow.

So i applied Hazen Williams equ. but the result was 28m.

I would like to know was there anything wrong in my calculation ?

Please throw me some light.

 

[BigInch]

As a general rule, it looks like it is reasonable, however I'd hate to try to make such a general correlation. In pipelines, I've seen it vary between systems. Gathering systems are rougher than mid sections of major transmission lines.

If you double a roughness, that's probably a very good number to use for design. I typically use 0.0018 for steel, which seems to be just short of 2 times what comes from the mill and gives reasonable values for pipelines, since we don't count up all the angles and get the equivalent pressure drops for a million tiny little pipe bends anyway and, given the rest of the uncertainties, average temperatures and viscosities, densities over 1000 miles or so, somehow it gets pretty close no matter how old or new the pipeline is. For specific segment flows, I usually find roughness is always smaller than that value, unless there is some other problem with wax, hydrates, nothing to do with the actual roughness value of the pipe material that was choosen at design time.

All things considered, for pipelines the actual roughness value used gets pretty diluted in the end.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[friend81]

Thanks for all your experised comments finally i got the value closer to the manufacturer declared, by using the Blasius formula for Re~10^7,

f= 0.0008+{0.05525/(Re)^0.257}.

here i have taken the exact pipe diameter as 51.4mm and
Kinm.Viscosity of water as 0.000001 at 20deg.Celsius,

finally the frictionloss i got is 20.359m

 

[MortenA]

I have seen operational data for a 24" gas pipeline in the north sea that pshows the opposite og 23362's postulate: That the relative roughness decreases over time. Iniitally the dP was faily well predicted but over time the dP has dropped when comparing to expected so that the roughness have been adjusted from the traditional 0.05 mm (1/500 of an inch i belive?) to about half 0.025. For new pipelines installed by modern technology i have come to use this value as the default - because the original data for the 0.05 mm is quite old and relates more to "plant piping" than long pipelines with few fitting and very precise welding.

Best regards

Morten

 

[BigInch]

Morten,

I like it. Converting from 0.05 mm to 0.001969 (big)inches that would tend to confirm what I have found - you need to reduce the recommended design roughness by 50% (see above 7 Dec 06 9:13), but that is still supposedly only roughness of the pipe wall and should be irrespective of the number of bends and fittings or if its plant piping or pipeline pipe.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[MortenA]

Yes (and no): In plat piping (especially old plat i would think) fitting meant many weldings and screwed in parts. This would be "summed up" in a higher absolute roughness - at least that what i have come to believe. I only use the lower value for pipelines. Its rare that i do calculations for plant anyway.

Best regards

Morten

 

[BigInch]

I thought I was saying that's for the pipe only, so add the equivalent lengths for valves and fittings, both for pipelines and plant piping, or are you saying the higher roughness includes the equivalent lengths?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[MortenA]

No (at least thats not what i meant) but usually i would not add a seperate equvalent length for a welding - but surely the pipe must be sligthly more irregular just here?

For plant piping there must be a fairly large number of weldings- at leat compared to a pipeline. Thats why i think that 0.05 is OK for plant but too high for pipelines.

Best regards

Morten