Frictional Loss - Black steel vs Galvanized Iron Pipes
Frictional Loss - Black steel vs Galvanized Iron Pipes
(OP)
Hi Professionals,
I am working on an HVAC system that requires hydronic piping. The tender specification calls me to use Sch 40 - Galvanized Iron pipe for the application. I need to size the pump. I am having flow rates for all pipe segments. The problem which I face is when using the friction chart. ASHRAE's friction chart mentions that the chart is applicable for Sch 40-BLACK STEEL. Even I referred McQuay Pipe Sizer and found that it is a calculator version of the ASHRAE's chart.
Now, is it acceptable to use the ASHRAE's friction chart for Black steel (Sch 40) in the case where G.I. Pipes are used? What will be difference in friction between Black Steel and G.I. Pipes? If there are any considerable differences, do I need to consider any multipliers?
Please do help me in getting a clarity.
Thanks in advance for the help.
I am working on an HVAC system that requires hydronic piping. The tender specification calls me to use Sch 40 - Galvanized Iron pipe for the application. I need to size the pump. I am having flow rates for all pipe segments. The problem which I face is when using the friction chart. ASHRAE's friction chart mentions that the chart is applicable for Sch 40-BLACK STEEL. Even I referred McQuay Pipe Sizer and found that it is a calculator version of the ASHRAE's chart.
Now, is it acceptable to use the ASHRAE's friction chart for Black steel (Sch 40) in the case where G.I. Pipes are used? What will be difference in friction between Black Steel and G.I. Pipes? If there are any considerable differences, do I need to consider any multipliers?
Please do help me in getting a clarity.
Thanks in advance for the help.





RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
I'm too lazy to look it up, but think ASHRAE has tables with roughness factors and they use smooth and rough version of each type of pipe. It isn't like every piece of pipe in the world is the same with exact same roughness.
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
I have a doubt here. It may look childish, but even then I'm putting it up to get a clarity. Even I've seen the roughness values for both materials as 0.05mm and 0.015mm for GI and Black Steel, respectively.
Take for example a sch 40, 100 mm dia pipe in which the flow is 208 USGPM. Let's calculate the friction loss for both materials.
Case (i): Black Steel
From ASHRAE tables/any other calculator, the friction loss is found to be 244 Pa/m = 2.49 ft WC/100 ft pipe..
Case (ii): G.I.
Given Data:
Inside Diameter (D) = 102.26mm = 0.10226 m
Velocity (V) = 1.6 m/s (I'm using the same value as that of Black Steel Pipe)
Density (r) = 998.576 kg/m3
Dynamic Viscosity (m) = 0.00131 Pa.s
Calculations:
This puts my Reynolds Number to Re = (rVD/m) = 124720 (=1.25 * 10^5).
Now, the relative roughness of GI pipe is e/D = 0.05/0.10226 = 0.000489 (= 5*10^-4).
Then from Moody Chart, my Darcy-Weisbach friction factor (f) = 0.01973 (which was 0.01954 for Black Steel, almost the same as GI).
This is getting me a value of 247 Pa/m = 2.51 ft WC/100 ft pipe.
The difference here is 3 Pa/m (1.22%) which is approximately 0.03 ft WC/100 ft pipe. This is nothing when compared to other losses in pipes. So, is it justifiable to use the same values as that of Black Steel in calculation and add some 5% as a correction factor for GI?
Is my justification right?
Please do advice on this regard. Thanks, again!
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
Attention @MIntJulep - I did NOT read out friction factor from Moody Graph. Spreadsheet Calculators are available online for Moody Chart. When I input relative roughness and Reynolds number, I will get Moody friction factor as output.
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
Sometimes it is a good idea to plot your e/d and Re numbers on a paper Moody chart to see what impact the difference has. This makes it clear if you are in an area where the lines are all bunched up, or if they are widely spaced.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
Another thing which I was wondering is when we do galvanizing I believed that the roughness of steel will decrease and I wondered to see that it actually increases while galvanizing. I would request you to address this point as well.
Thanks!
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Frictional Loss - Black steel vs Galvanized Iron Pipes