friction factor in PVC pipe

[RV]

What do you use for the dimensionless friction factor (f) for PVC pipe when calculating frictional losses for air flow? Are there any charts out there with friction factors for different sizes of clean, new SCH 40 PVC pipe? I know there is little data for air flow in PVC pipe because it is intended for use with non-compressible fluids, but is often used in the environmental industry for air transfer as well. Maybe there is another way to calculate your losses through fittings, enlargements etc without a friction factor, but I don't know it!!

 

[pipesnpumps]

The friction factor has to be read from the Moody Diagram.

For PVC, just assume its completely smooth and this will get you there. You will also need the Reynold's number, Re = rho*V*D/mu

If you want to split hairs, the absolute roughness for PVC is 0.00000066 feet..

There is something called the Swamee-Jane formula that you can plunk in a spreadsheet which gives you the friction factor, and does not require looking it up in the Moody diagram. If you are a real go getter, you could create your own table of pressure losses per 100ft of PVC pipe. Put the different size pipes as columns, and the flowrate as rows. But this table would only be good for one certain pressure which represents the average pressure in the pipeline, since the density changes with pressure loss. You can calculate the DP according to the following formulas:

DP100=rho*(fL/D)*(V*V/Gc)

Re = rho*V*D/mu

Swamee-Jain formula for friction factor (valid only for Re>3500!!):
f=0.25/[ log{e/(3.7*D) + 5.74/Re^0.9} ]^2

e=absolute roughness in ft
D=inside diameter in ft
V=velocity in ft/sec
rho=density lb/ft3
mu=viscosity in centipoise
L=100 ft (the length you are assuming)
DP100=pressure drop per 100ft of pipe

A Crane 410 manual or a fluids textbook would do you wonders, I highly recommend getting one.

Good luck, and don't point that PVC pipe at me. There is a reason PVC is not used in gas systems. None for me thanks.

-Tom Meier

 

[pipesnpumps]

Ah, left out the old Gc=32.2

 

[pipesnpumps]

And the formula should be:
DP100=rho*(fL/D)*(V*V/2*Gc)

Note the 2 was left out of the original post

 

[RV]

Thanks very much for the information. I am familiar with using the Reynolds number to determine the friction factor off the Moody diagram. My confusion is with the difference between f and f-subscript-t. In the text I use (Motts Applied Fluid Mechanics), Darcy's formula is written as:

hl = f x L/D x v^2/2g + ft x Le/D x v^2/2g

(forgive me - I haven't figured out how to identify subscripts yet but I'm sure you get the idea)

I was taught to read f off the Moody diagram based on NR and the relative roughness (D/e) but to get ft off a chart for SCH 40 steel pipe of various sizes, with f(t) being the friction factor based on complete turbulence in fittings. Obviously I do not have a similar chart of PVC pipe so I am confused about where to read f(t) off the Moody diagram. How is it different from f?

 

[RV]

Nevermind, I think I just figured it out for myself! Since ft is the friction factor for fittings it is assumed to be in the zone of complete turbulence. And since the curve on the Moody diargram is nearly flat in this zone, ft can basically be read straight across the chart from the calculated relative roughness. Therefore you don't need the Reynolds number to calculate the friction factor in fittings. Please confirm. Thanks!

 

[RV]

BUT.......with plastic pipe you have a very large relative roughness, D/e, since e is approaching 0. This then means you have a very low friction factor in fittings (I think f=0.008 is the lowest friction factor indicated on the diagram). So should I always assume that f=0.008 when calculating the energy loss through valves and fittings for PVC pipe since diameter doesn't matter if roughness is approaching 0? Why does the friction factor decrease as flow gets more turbulent?

 

[tr6]

Chemtrol has a product called "ChemAire" which is a PVC piping system for compressed air. Go to their website: nibco.com/chemtrol, and download the "Product Guide and Technical Manual". This manual contains pressure losses for not only pipe, but also friction loss in fittings, in equivalent lengths of pipe for various fittings.

 

[pipesnpumps]

RV:

yeah, you could assume fsubt is the same for all flows. But since Re ~ rho and you are moving gas, this would might not be true since rho is small, but you can determine that easily for yourself.

But Tr6's post solves your problem anyways.

Good luck

 

[stanier]

Take care to recognise that losses at elbows and tees in uPVC is greater than for steel as the radius and local bore is smaller. They provide mor eof a flow obstruction than a long radius elboe of butt welded tee. More like socket welded fitting.