×
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

Calculating NPSH
5

Calculating NPSH

Calculating NPSH

(OP)
Can anybody advise me how I calculate the value of NPSH?

Thanks

RE: Calculating NPSH

3
Take the absolute pressure of the fluid (NOT gauge) and subtract it's vapor pressure.  Multiply by 2.31 and divide by the specific gravity.  That is the NPSH in feet of water that you have available.

Now, this is the NPSH at the point of measurement.  If you have a run of pipe going into the pump then you need to subtract the friction loss in that pipe from your pressure gauge to the pump suction.

Technically, you are supposed to correct the pressure gauge to the centerline of the pipe and add in the velocity head term.  However, friction loss is an inexact science and you cannot expect to get accurate values (unless you are in laminar flow) to within an inch or two so trying to increase your accuracy with a small pipe and including the few inches of velocity head (since good piping practice states to keep the fluid velocity low) is not worth the trouble.  Now, if you have a 6' pipe, then go ahead and correct gauge pressure to the centerline of the pipe.

In other words, USE COMMON SENSE.

I always get a laugh at seeing specs written by "engineers" that require "15643.21 gpm @ 89.012 ft TDH with 38.922 ft TDH."  I would have thought that the P.E. process would weed out such "talent."

Tim

RE: Calculating NPSH

(OP)
Thanks for the info. I know I'm probably thick but what is the difference between gauge and absolute pressure and whats the vapour pressure of water?

Thanks again

RE: Calculating NPSH

2
Gauge pressure is the pressure measured ABOVE atmospheric pressure.

Absolute pressure is the total pressure (gauge + atmospheric.)

Atmospheric pressure at sea level is about 14.7 psia.

Vapor pressure of any liquid is a function of temperature.

Vapor pressure of water is 0.122 psia at 40°F, 0.507psia @ 80, 4.739@160, etc.

Check any general engineering reference for STEAM TABLES.

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

eBook - Rethink Your PLM
A lot has changed since the 90s. You don't surf the Web using dial-up anymore, so why are you still using a legacy PLM solution that's blocking your ability to innovate? To develop and launch products today, you need a flexible, cloud-based PLM, not a solution that's stuck in the past. Download Now
White Paper - Using Virtualization for IVI and AUTOSAR Consolidation on an ECU
Current approaches used to tackle the complexities of a vehicle’s electrical and electronics (E/E) architecture are both cost prohibitive and lacking in performance. Utilizing virtualization in automotive software architecture provides a better approach. This can be achieved by encapsulating different heterogeneous automotive platforms inside virtual machines running on the same hardware. 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