×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Question About Air (Air Release Valve) Losses through Sudden Reduction Tap

Question About Air (Air Release Valve) Losses through Sudden Reduction Tap

Question About Air (Air Release Valve) Losses through Sudden Reduction Tap

(OP)
We're considering allowing a 2" tapping saddle for a 3" diameter air release line into a new pipe (contractor 'forgot' to install tee). Assuming air flow works similar to water, the losses due to the sudden 3" to 2" reduction are negligible at the flow rates we're looking at. The flow of air would need to be 1300 GPM in the 3" line, which would then go through the 2" tap and into a 6" waterline. Any ideas?

RE: Question About Air (Air Release Valve) Losses through Sudden Reduction Tap

Don't waste any more time thinking about this. Losses are negligible.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Question About Air (Air Release Valve) Losses through Sudden Reduction Tap

(OP)
Well, I need to justify it if I'm going to ignore it. As far as I can tell, the losses are pretty significant. First off, air is technically compressible, but at velocities slower than Mach 0.3, I don't think we need to consider that. Technically it could be travelling at Mach 0.37 through the 2" saddle, but I wouldn't expect that to throw off my number by anything significant.

Minor Headloss = K*V^2/(2g).
The volumetric flow of water leaving the pipe (and thus the volumetric flow of air that must enter) is 1300gpm (about 2.9cfs).
The air is therefore travelling through the 3" line at 59.0ft/s.
K for a sudden contraction equals 0.5(1-β^2)*sqrt(sin(θ/2))/β^4, where β = 3" diameter / 2" diameter; K = 1.41. (θ here is 180 degrees; see source)
Therefore the headlosses equal 1.41*(59ft/s)^2/(64.4ft/s^2) = 76 ft of head = 33 psi...

Since the air starts at ambient pressure of 14.7psi, this tells me we'd end up with a vacuum. This isn't obviously something I'm very familiar with, but any guidance would be very much appreciated; thanks!

Source: https://www.roymech.co.uk/Related/Fluids/Fluids_Pi...

RE: Question About Air (Air Release Valve) Losses through Sudden Reduction Tap

Fair enough. The air though is whistling in. Sort of suggests your 3 inch is too small to start with.

I think though you need to use the density of air in your last conversion from feet to pressure. How did you get 33 psi?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Question About Air (Air Release Valve) Losses through Sudden Reduction Tap

where are you getting 1300 gpm? that is quite high for a 6 inch diameter waterline and results in a velocity of nearly 15 feet/sec.

RE: Question About Air (Air Release Valve) Losses through Sudden Reduction Tap

(OP)
Sorry CVG, it is actually an 8" pipe, and this is for fire flow so the v is pretty high.

LittleInch, I think that was exactly the problem. I was using feet of water instead of air. Confirmed this with manufacturer as well. Losses in this case are 76ft of air = 0.04psi. So we're good.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources

eBook - Mastering Tolerances for Machined Parts
When making CNC machined parts, mastering tolerances can be challenging. Are general tolerances good enough? When does it make sense to call out for tighter tolerances? Do you need a better understanding of fits, datums, or GD&T? Learn about these topics and more in Xometry's new e-book. Download Now
eBook – How to Choose the Correct Corrosion Testing Method
When designing a metal component, engineers have to consider how susceptible certain alloys are to corrosion in the final product’s operating environment. In a recent study by NACE (National Association of Corrosion Engineers), it was estimated that the direct and indirect costs of corrosion in the United States is approximately 6.2% of the GDP. In 2016, that cost exceeded $1 trillion dollars for the first time. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close