×
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

Orifice plate thickness to achieve critical flow

Orifice plate thickness to achieve critical flow

Orifice plate thickness to achieve critical flow

(OP)
When checking my orifice calculation method (after reading denniskb's thread) I came across a statement saying that choked/critical flow does not occur accross thin, sharp edged, orifice plates. For chocking to occur the thickness of the orifice plate must exceed the orifice diameter.

Is this true and why?

RE: Orifice plate thickness to achieve critical flow



you are correct, neither thin or thick plates exhibit critical flow characteristics because of the entrance losses.

Thick plates do offer a discharge coefficient that is nearly independent of reynolds number and tend to approach critical flow behavior better than a thin plate.  You only approach critical flow in really thick plates i.e. T/d>>>D, because you achieve pipe flow prior to the final expansion, but you still have to deal with the entrance losses.

There are corrections for the "critical flow" behavior of thin/thick plates (T/d~1) in Millers Flow Handbook, but they only increase the complexity of the calcualtion rather than improve performance.

A flow nozzle does achieve true critical flow. Some of the meter manf. can supply them.

RE: Orifice plate thickness to achieve critical flow

Joel,

For thin plates it only occurs as a limiting condition as acknowledged in the web-site you've referenced.

The issue raised by Mark is that critical flow does not generally occur with plates that fall in the thin-plate category. True critical flow conditions can occur only for true nozzles with a contoured inlet.

RE: Orifice plate thickness to achieve critical flow

To Hacksaw, if I'm not mistaken the "0.5-0.6 pressure ratio" ROT doesn't apply to flow through nozzles which may reach critical (sonic) flow already at a 5-10% pressure drop. Right or wrong ? Please comment.

RE: Orifice plate thickness to achieve critical flow

A good discussion on this topic is Thread798-51260, or get a copy of the following thechnical paper on choked flow thru square edged orifices:
author A.J. Ward-Smith , Int J Heat Fluid V1 no 3 pp123-132 1979.

Basically the discharge coeficient is related to the ratio of the plate thickness divided by the orifice diameter. For a truly sharp edge t/d=0, the Cd=1.0 . For a t/d in the range 1<t/d<3, Cd=0.81 . For very thick plates t/d >7 the Cd is calculated using the Fanno relationships for frictionally choked flow in piping.

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! Already a Member? Login


Resources

White Paper – Choosing the Right Spring Loaded Connector
In today’s cost-sensitive world, designers are often driven to specify the lowest cost solution for every aspect of their designs to ensure that their solution is competitively priced and their company remains profitable. However, specifying a low-cost, low-quality connector solution can result in premature failure, considerable re-work costs and damage to reputations. Download Now
eBook – Own the Lifecycle: Sustainable Business Transformation
Increasingly, product and services companies are seeking more information and control in the operational lifecycle of their products, including service and use. Better information about the operational lifecycle, and the ability to use that information, requires more than just unstructured data flowing back from products in the field. 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