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


API 620 Question - Why Tension?

API 620 Question - Why Tension?

API 620 Question - Why Tension?

I've read this thread (thanks to JStephen):


My concern is T1. I understand the concept of an atmospheric cylindrical tank having the static pressure (acting up) equaling the weight/area (acting down) and thus cancelling, which would leave the shell in compression due to the weight of the metal.

My question is about tanks with non-cylindrical geometry. Say we're talking about the same level of consideration (still in a cylinder) but somewhere higher in the tank there's a transition to smaller diameter. Therefore in the equation, the weight term would be less (because less volume of fluid above the level of consideration), and therefore Pstatic and Wf would NOT cancel out, and the net force would be acting up, and T1 comes out positive, indicating tension. I cannot wrap my head around this - I understand the math, but how could the shell be in tension longitudinally?

RE: API 620 Question - Why Tension?

Free body diagram below illustrates the effect- not sure what else to say about it.

RE: API 620 Question - Why Tension?

This diagram is not correct. Hydrostatic pressure is the same in all directions at any given point, but changes with elevation. So in the above diagram the red arrows pointed up would be in balance with red arrows pointed down.

Move these arrows up to where the small diameter meets the larger diameter. There are no counter-balancing red arrows pointed down on the shoulders. This is what causes tension in the walls.

To help visualize this, imagine cutting the tank wall just under the shoulder. Water pressure would lift the top part of the tank.

RE: API 620 Question - Why Tension?

Thank you both for the replies.

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


White Paper - A Guide to 3D Printing Materials
When it comes to using an FDM 3D printer effectively and efficiently, choosing the right material at the right time is essential. This 3D Printing Materials Guide will help give you and your team a basic understanding of some FDM 3D printing polymers and composites, their strengths and weaknesses, and when to use them. 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