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!

*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.

Jobs

Pump Vibration

Pump Vibration

(OP)
We have a pump with an ~25' shaft running at variable speed up to 229 RPM and power up to about 450 Hp. It had some initial high vibration but after two years of operation it is now vibrating to the point of tripping. Extensive vibration and modal analysis has been done and the conclusion is a bent shaft. Is it possible that the cause is imbalance on the impeller? I'm trying to get a handle on how a bent shaft would get progressively worse. Any insight would be appreciated.

RE: Pump Vibration

What's the operating temperature? Bearings? Grease?

RE: Pump Vibration

25' is not likely to be a single piece shaft, so take a look at the shaft-shaft coupling (I'm assuming it's a sleeve/key type.) A problem there could have symptoms of a bent shaft and an imbalance.

Is the 229 rpm a typo? What is the service? As far as explaining how a bent shaft gets progressively worse, would you expect it to get better?

RE: Pump Vibration

"Extensive vibration and modal analysis has been done"

I picture the pump being constructed something like this -
http://www.gouldspumps.com/getattachment/Products/...

So you are actually measuring vibration on the motor which is about all that is above ground, like this -
http://www.rainbird.com/images/products/ag/pumps/V...

Do you have more detailed information about frequency and even phase of the predominant vibration?
Is it 1X the pump rpm, or some other frequency?
Have any resonance tests been made on the pump. looking for the first bending mode / "reed" frequency?

RE: Pump Vibration

With almost no information given, this is completely guesswork. First I am guessing you are talking about a vertical pump. Then I'm guessing it is either a mixed flow or axial flow pump running at such a slow speed and at 450HP. I'm guessing this is basically a high flow, low head pump; a water mover. Probably one stage.

If my guesses are correct, then you should look at your bearings, in particular the bottom bearing (sometimes called a sand bearing). It is not at all unusual for a pump like this to experience crippling vibration with a small bit of imbalance. Typically (just guessing) you have a large weighted, high diameter impeller/propeller hanging on a shaft; once it starts to get unbalanced, things go awry quickly. If one of the lineshaft bearings gets worn, it will also degrade quickly.

Give us a little more information: Make, model, flow, head, shaft diameter, bearing arrangement, open/closed lineshaft. These details should be available in the pump's file folder, if you can find it.

RE: Pump Vibration

A bent shaft probably would not get gradually worse over time. But, if the shaft had high run-out, the resulting vibration could get gradually worse as the bushing clearances increased. You should obviously check for shaft run-out. But, you should also check for issues with the rigid coupling that could cause high run-out. These couplings are notorious. Check for clearance in the upper bushing. You should be able to do this without pulling the pump. If you are pulling the pump for a complete tear-down, you will obviously check all the bushings, any line shaft couplers, etc.

Johnny Pellin

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


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