×
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

+Where to model the discharge load - at valve or at 1st elbow?

+Where to model the discharge load - at valve or at 1st elbow?

+Where to model the discharge load - at valve or at 1st elbow?

(OP)
Dear Coleagues:
We have a little controversy at office reggarding to where to model the discharge load in a piping stress model.

In my opinion it should be modeled at elbows, points B and D (please see my sketches) with dir arrow as indicated. Some people say that should be points A and D.

Please tell me your point of view.

Regards,
Hansito

RE: +Where to model the discharge load - at valve or at 1st elbow?

Hansito,
        If you are not sure where to apply the loads how do you know what loads to apply also? There are loads at the valve initially then there are loads at the bend and then there are loads at the end of the tailpipe. The loads applied are dependant upon whether yo are doing a comprehensive time-history analysis or a simple "static" analysis with an applicable dynamic load factor.

RE: +Where to model the discharge load - at valve or at 1st elbow?

(OP)
Thanks DSB123

We are doing a simple "static" analysis with dynamic load factor 2. I'm not in the office at this moment, so I don't remember the formula exactly but is a function of Flow, molecular weight and temperature. On that conditions I think the points are B & D. I agree during the discharge time, the forces are equilibrated but not when opening or closing.

Regards,
Hansito

RE: +Where to model the discharge load - at valve or at 1st elbow?

The answer is both points will experince load.  Initially there will be a thrust load in the A direction when the relief valve opens until the pressure in the horizontal pipe equalizes.

B (in the direction you state) will be loaded when the fluid first hits the elbow and is turned up.

Once the valve reaches steady state there should just be a vertical force downward at the elbow as the pressure in the horizontal pipe will be balanced.

RE: +Where to model the discharge load - at valve or at 1st elbow?

You do not have just one force that moves around.  

Initially there IS only the end-cap force at the closed valve.  Anywhere you have a pressure differential across a valve, the net force dP * A is applied axially to the pipe.  If the valve is closed, it creates an "end cap force" at the valve with load equal to internal pressure x flow area.  When a valve is opened, the end cap load changes to the flow pressure loss (dP) across the valve x area of flow.

Anywhere you have a moving fluid mass changing direction, or escaping from a container, forces are imparted.  These dynamic forces begin when the valve opens and fluid starts moving.  

Whether the pipe can balance static end cap forces, or dynamic forces with internal tension, compression, or moments, in the case of welded joints, or if you need a brace, clamp or a thrust block in the case of bell/spiggot joints, must be decided.  

YOU must place the dP x A forces, change of momentum and thrust forces wherever they are applicable.  See attached.

Please practice free body diagrams before you try to do more pipe stress analysis.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: +Where to model the discharge load - at valve or at 1st elbow?

(OP)
Thanks BigInch.

Your answer added a very nice graphic and description to the previous valuable Thread answers. The combination Graphic-description is always very good and demands big effort, as the combination wisdom-humility.
Regards,
Hansito

RE: +Where to model the discharge load - at valve or at 1st elbow?

Humility.  I have it.  That's the name of my yacht. smile

Seriously, pay attention to FBDs.  They help you see those unbalanced moments where you might otherwise think forces simply balance.  

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.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