Frictional Loss - Black steel vs Galvanized Iron Pipes 2

[PS]

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.

 

[PEDARRIN2]

Per the charts I have, galvanized pipe has a roughness of 0.0005 ft, whereas steel or wrought iron pipe has a roughness of 0.00015 ft. Use the values in Darcy-Weisbach to determine the difference.

 

[EnergyProfessional]

I would use the more conservative of any source or the one that indicates rough surface. Over time the surface will corrode and get rougher. Yes also galvanized corrodes over time.
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.

 

[PS]

Thank you @PEDARRIN2 @EnergyProfessional .

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!

 

[Artisi]

seem to be splitting hairs over nothing, as you have pointed out the difference is not worth the consideration when you factor in all the other considerations, aging of the pipe, bends, elbows, connections, over or under capacity of the pump selected etc etc.

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.)

 

[EnergyProfessional]

Take whichever is the more conservative. Also consider different ID for schedule 40, 80, copper L, K, M etc.

 

[MintJulep]

You can read a Moody diagram with 6-decimal precision?

 

[ChasBean1]

It's sort of academic. Friction loss based on relative roughness takes a distant back seat to pipe sizing and velocity in the piping. If you have a 2" pipe you want to squeeze 100 gpm through, your losses are huge. If you want to put 25 gpm through the same pipe, no problem. The relative roughness has a lesser role. Both galvy and black iron are imperfect and have rough interior surfaces which will become less smooth over time.

 

[PS]

Thanks @Artisi, @EnergyProfessional, and ChasBean1.

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.

 

[katmar]

You have mixed up your roughness values. Your calculation for GI should be with a roughness of 0.15 mm. This gives a pressure drop increase of about 18%, which is probably still negligible.

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"

 

[PS]

@katmar: Thanks a lot pointing it out! That 18% was the one which I was looking for. I misread the roughness value while calculating. So I need to add 20% as correction for G.I. pipes when I use Black steel chart for calculating pressure drops. Am I right?

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!

 

[katmar]

The 18% difference isn't a fixed value. It will vary according to where you are on the Moody chart. Rather calculate for GI specifically than use black pipe and apply a correction factor. I don't know what it is about the galvanizing process that increases the roughness.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[PS]

Thanks @katmar.