×
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

Backpressure calculation

Backpressure calculation

Backpressure calculation

(OP)
hello. i need to supply some back pressure to a control valve i am trying to size. currently the downstream pressure is atmosphere, so my control valve is cavitating. How can i size an orifice plate to give me the required backpressure on my valve?

RE: Backpressure calculation

You don't need to supply backpressure, you already have that.  Its driving the flow, which apparently is already too high.  What you need to do is limit the dP across the valve to pressure drops that do not cavitate, or increase the size of the valve, or use anti-cavitation trims.  

If you can't lower the upstream pressure, you need to raise the downstream pressure.  Put your orifice plate downstream, or increase downstream pressure by some other means.  

If you can't raise the downstream pressure, lower the upstream pressure hitting the valve; try putting your orifice plate there.

Be aware that putting in any orifice plate will have some flow reduction implications, so a review of overall pressure drop and flow and possibly a resize of the valve may become necessary.

 

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Backpressure calculation

"How can i size an orifice plate"?  What knowledge base are you starting from?  Did you have fluid flow in college?

Good luck,
Latexman

RE: Backpressure calculation

Latexman,

Forsizing an orifice plate would you treat the orifice plate as a sudden contraction and a sudden expansion in a pipe for calculating the pressure drop across it?

I will be honest ive never sized one, but thats the only method i can think of to account for an orifice plate, but it just seems a little too simple...  

RE: Backpressure calculation

Get a copy of Crane, it has formulas for sizing an orifice plate.  

Basically, for the dP calculation you use the orifice bore (rather than the pipe ID when calculating the piping pressure loss) with a discharge coefficient for the orifice (which is a function of the Re number in the main piping).  

For higher Re numbers and small bore to pipe diameter ratios the discharge coefficient is close to 0.6 but for it can also be significantly different for lower flow cases.  Crane has a chart of C versus Re and the beta ratio.

RE: Backpressure calculation

Most college textbooks on fluid flow cover sizing orifices too.

Good luck,
Latexman

RE: Backpressure calculation

Ok, so it is sudden expansion and contraction losses then, thanks.

RE: Backpressure calculation

jmiles,

No it's not.  You need methods specifically for an orifice.

Good luck,
Latexman

RE: Backpressure calculation

What you need is a multi orifice restrictor valve. This should solve your cavitation problem.

RE: Backpressure calculation

or, depending on your pressure drop, several multiple hole orifies in series, each with a gap between for pressure recovery such that each does not cavitate excessively.

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