Gravity flow rates and volumes in PVC pipe.

[drainman]

In trying to determine maximum gravity flow rates for water in 4" PVC pipe, I'm coming up with some really screwy looking numbers. For years I've use tables from the University of Iowa that are based on flow at 70% of pipe volume. Unfortunately I have misplaced them in an office move.

I used three methods to compute max flow rates for 30 lf of pipe at 1% slope
1. Hazen-Williams (V=1.318*CR^.63*S^.54), Chezy coef of roughness is 150 for new PVC
2. Mannings (1.486R^.67*S^.5)/n, n = .009 for PVC and
3. a reference I found in an Engineeering Handbook GPM = (2.56*X*d^2)/sqrt Y. this last one's a bit strange and I've never seen it before...X = horizontal distance in inches, d = pipe diameter in inches, Y= vertical distance in inches.

Problem is that I got very different answers for GPH for each method and all are considerably more than the rates I remember from the tables. Can't figure out why - except that maybe the last formula is a bust - but that offers no help with different flows using Mannings and Hazen-Williams.! Can anyone help me figure this out?

 

[dicksewerrat]

Your formula for Manning's is for full pipe. I get 6150 gal. per hour. Maybe you are figuring your hyd. radius diff.

 

[KRSServices]

I used my handy dandy IPEX PVC Pipe Gravity Flow calculator. at pipe full (about 82%) i get 7,291 gph. At 70% full, I get 6,041 gph, which concurs with the previous post. KRS Services

http://www.krs-services.com

 

[gtrainor]

I always use Mannings since it seems to be referred more often than Hazen. There will be differences, but it should not be much. Have you double checked you units making sure your not using inches when it should be feet? Only the Lord knows how many times I've done that! Are you using the true diameter of the pipe. I just got the water flow calculator from Forestry Supplies:

https://www.forestry-suppliers.com/product_pages/view_catalog_page.asp?id=3400

Now I can spout off flows in a heart beat without fumbling with inches-feet conversion, hydraulic radius miscalcs, and simple math errors. I still have to produce the calcs in my Basis of Design, but now I have a very quick way to QC my work.

 

[RWF7437]

Some things to remember:

Hazen-Williams equations are NOT for gravity flow but for pressure flow and may only be used for water.

The "S" in both H-W and Mannings is the slope of the hydraulic grade line NOT the slope of the pipe.

Maximum flow in a circular pipe occurs at 0.92 of full depth.

In the H-W equations you must always be certain that you are using the correct units for diameter. The equations are sometimes written using inches and sometime written using feet. Leads to much confusion and many errors.

Using Mannings, I get Flow = 0.2749 cu ft/sec = 123.37 gpm = 7402 gph.

Russ

 

[bimr]

Suggest you locate a copy of the Handbook of PVC Pipe: Design and Construction; Fourth Edition (522 pgs) ($40.00). That is the bible for PVC pipe and should have everything that you are interested in.

http://www.uni-bell.org/unibell_order.php

 

[KRSServices]

As a side issue, why would you be interested in designing a system with 4" pipe at basically full pipe? Even in residential complexes, due to the maintenance headaches, I have gone with either a minimum 6" or 8" pipe.

In my books, 4" pipe is reserved for single family residential. Shared services automatically get a 6" pipe. Those are my thoughts, but with over 16 years of fighting with capacity problems and maintenance issues over small pipe sizes. KRS Services

http://www.krs-services.com

 

[dicksewerrat]

I have to agree with KRS. A4" like is good for a two bed, two bath single family home. two homes =6" 5 homes = 8". Maintaining a 4" like is a tough one. The biggest problem with small shared lines is the legal one. Who pays for cleaning and repairing?

 

[drainman]

Thanks for all the replies. Problem was an error in wetted perimeter. Just for info, the pipe is for a single family residence. Actually, it is for part of a yard drainage system. This is for the upper reaches of the system fed by a single 12" x 12" catch basin with a 3" discharge orifice (yeah, I know, but that's the way NDS makes them!).

Regards,
Al