Pressure loss - miter cut pipe or tube

[1952mike]

Hi there!
I am at loss, need helpful hints.
I need to calculate pressure loss thru a 30" ID formed tubing miter cut and welded together to form a 45° & a 90° elbows. 90° has two miter cuts. 30" leg length.

Looking at Crane data book I see that for alpha=45° =15fT, and for alpha=90° K=60fT.
OK, fine, what do I need to do with this data.
I can't find fT for 30" pipe or tube.
Do I use "Head Loss" = K * v^2/2g?
When I do, I get "HL" = 33' for flow rate of 34785 cubic-ft per minute & fT of 0.01 (I grabbed 0.01 from thin air). So what am I doing wrong. How can a 45° miter cut 30" tube have 33' hl?
Please can someone helpppp!!!

 

[katmar]

Mike, you are basically on the right track. You can extend the table at the top of Pg A-26 of Crane 410 as long as you use the roughness for commercial steel pipe (i.e. Don't use the actual roughness for the pipe you are using). With a roughness of say 0.002" and an ID of 30" you have an e/d of 0.000067. From the graph on Pg A-24 you can estimate the friction factor for fully developed turbulent flow. I get about 0.011 for f_T.

This would give you a K value for the 45[°] miter of 0.011 x 15 = 0.165

My calculation agrees closely with your loss of 33 ft for the 45[°] miter, but don't forget that these a feet of the pumped fluid which I guess is a gas from the velocity of 118 ft/s.

The Crane value of 60f_T for the 90[°] bend is for a single cut bend. You want to take twice the 45[°] K value for your bend because it is made up of two 45[°] cuts.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[1952mike]

Thanks katmar for replying.
You are correct. This is for exhaust gas at around 650 °F.
Reason I didn't say it was because i was interested in methodoloy, not accuracy at this time.

So, I guess i need to get exh gas density involved in 33 feet to get its pressure loss in PSI?
Am I right?
Actually I need to get it in "inches of water column".

And thank you again for answering.
Mike

 

[BigInch]

Might be easier for you "replace" the 45º with the equivalent length of straight pipe and figure your expected pressure drop for the total equivalent directly using your pressure drop formula of choice.

http://virtualpipeline.spaces.msn.com

 

[1952mike]

Thanks "BigInch"!
Any hint on how to get "equivalent length"?
I have forgotten most of these that I knew once.

Mike

 

[BigInch]

Crane chart.

Which depends a bit on diameter, but most sizes fall closely to 1.25 x the pipe diameter (") = feet of Le for a 45º Ell. And for a 90º ell use 1.5 x diam" = feet Le.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Whoa! sorry, The 1.5 factor is for medium sweep 90º ells
Should be a 2.5 for standard 90º ells.

http://virtualpipeline.spaces.msn.com

 

[1952mike]

Many Thanks!
Mike

 

[katmar]

You can convert between equivalent length and K values by looking at the Darcy Weisbach formula, i.e.
Head = ([ƒ]_ML/D + K) x v²/2g

where [ƒ]_M is the Moody friction factor. This is equivalent to the [ƒ]_T used by Crane 410.

The L/D value of a fitting can therefore be expressed as K/[ƒ]_M.

In your example [ƒ]_M is about 0.011 and the K value for 2 off 45[°] miters will be 2 x 0.165 = 0.33. This makes L/D = 0.33/0.011 = 30. Your ID of 2.5 ft (30") makes the equivalent length 30 x 2.5 = 75 ft.

This is identical to the value given by BigInch. The K value using the method of Hooper (Chem Eng, Aug 24, 1981) is 0.34 so I would be fairly confident of this value.


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[katmar]

Mike, I just realized that what I posted above, while correct, is a very long way around to do a simple calculation. Provided that you are working with pipe that has a roughness close to that of commercial steel pipe, the value of 15 in the formula given by Crane 410 (i.e. K = 15[ƒ]_T) is the L/D ratio.

If you were working with pipe of a significantly different roughness you would have to calculate the L/D using the method in my previous post, i.e first calculate the K value using the [ƒ]_T for commercial steel pipe in the Crane formula, and then convert to equivalent length of your pipe by dividing by the [ƒ]_M for your particular conditions.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[1952mike]

Many thanks katmar!

Mike