×
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

Applying affinity laws
2

Applying affinity laws

Applying affinity laws

(OP)
An horizontal Between Bearing, Axially Split, Single Stage Pump  @ 1000m3/h, 38m, 1480 rpm, 119 kw (impeller 368 mm). The system curve is H=26+12.10^-6.q^2
When changing duty conditions to (500 m3/h, 29m) checking online pumps charts, changing speed impeller to 1200 rpm, new power is 52,4 kw (ef.75,3%). However changing the impeller diameter to 310 mm, new power is 55,2 (ef.71,6%).
Why this difference in the efficiency? changing speed or diamater should be the same...(according to affinity laws).

In stead of using curves program on line from the supplier, how could i use the affinity laws? How can i check the values of new impeller diameter or new speed for new conditions?
Thanks in advance

RE: Applying affinity laws

You should use the curves, whenever the curves are available!

Efficiencies shouldn't change much, but they do anyway.

Affinity calculations follow the theory of geometric simularity and should ONLY be used when better information is not available.  Additionally, the range of applicability is small, generally being a maximum of +/- 10%.  You're already out of the recommended range of applicability, so stick with the curves.



http://virtualpipeline.spaces.msn.com

RE: Applying affinity laws

(OP)
BigInch, Thanks! Your information is always very helpful!

RE: Applying affinity laws

When you change the impeller speed, you aren't changing the geometry of the impeller tips or the clearances between the discharge edge of the impeller and the cutwater (or diffuser vanes, depending on construction).  Decreasing the diameter, the vanes frequently get a bit thicker, and the vane angle at the impeller discharge changes.  Also, the increased clearance between the rim of the impeller and cutwater/diffusers means that the pump isn't translating the kinetic energy coming out of the impeller into potential energy as efficiently, some of the energy is lost in recirculation and turbulence.

Affinity laws are fairly reliable for speed variation, but when you start changing diameter, there's no real way to estimate the resulting efficiency, vendor curve families for different diameters are frequently based on historical performance testing, which is why, as Biginch pointed out, applying the affinity laws to diameter changes should be kept to a small percentage trim adjustment.

RE: Applying affinity laws

Scipio, thanks for the clarification.  I made no distinction between changing speed and changing diameter in my answer.

  


http://virtualpipeline.spaces.msn.com

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