API 620 Question - Why Tension?

[crosby84]

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

http://www.eng-tips.com/viewthread.cfm?qid=140665

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?

 

[JStephen]

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

 

[Compositepro]

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.

 

[crosby84]

Thank you both for the replies.