×
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

Insufficient NPSH and Airbound

Insufficient NPSH and Airbound

Insufficient NPSH and Airbound

(OP)
Application is water at 85C in an open vessel (14.7 psia) with 1 ft of static suction head between pump centerline and liquid level.  A centrifugal pump is currently in use.  At the specified duty point, I've calculated that there is only 4 ft. NPSHA due to significant friction losses in suction pipe.  NPSHR for the centrifugal is about 5 ft.  In addition to the above, this pump is normally started "dry" with the vessel and suction pipe empty.  After 1 minute or so, liquid enters the vessel and then travels down the suction pipe.

The problem is that sometimes the centrifugal pump does not transfer any liquid out of this particular vessel.  My guess is that the centrifugal pump is becoming airbound upon startup and that there is not enough NPSH to push the air out.

The exact same pump is used successfully on other vessels with the only difference being that these vessels are elevated about 10 feet higher.  I'm guessing for these vessels that the pump probably becomes airbound initially when started dry but eventually there is sufficient NPSH to push the air out?

To summarize, here are my questions:

1. Do you think the pump is becoming airbound because it is being started "dry"?  Would the problem be solved just by waiting for suction to become fully primed before starting the pump?  My guess is that even if the pump is fully primed it will still become airbound due to insufficient NPSHA.

2.  It is being considered to replace this pump with a Liquid Ring design.  I spoke to the Liquid Ring pump manufacturer and they said that their pump requires 9 ft. of NPSH - higher than the existing centrifugal.  If a Liquid Ring pump is placed into service, and the software is changed to start the pump after the suction is fully primed, then how do you think the Liquid Ring pump would operate if the NPSHA is lower than NPSHR?

Thanks for reviewing this.

RE: Insufficient NPSH and Airbound

Disregarding the problems due to temperature, the 1ft static head difference on the pump inlet, and the apparent discrepancy between NPSHr and NPSHa - all of which are a problem in their own right, it would be a fair bet to say that the pump is "airbound".

Phitsanulok
Thailand

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