Re: Changing Duty of an API Tank

From: <Christopher>
Date: Fri Jul 13 2007 - 00:04:00 EDT

On Jul 12, 2007, at 5:37 PM, Paul Bowers wrote:

> "Low pressure storage tanks can be lethal and dangerous vessels – even
> in the pressure ranges of 0 to 15 psig and especially in the ranges of
> negative pressure (partial vacuum or 'external pressure')."
>
> See the link below for the rest:
>
> <a href="http://www.cheresources.com/chexpress/chexpress07_14.shtml#technically">http://www.cheresources.com/chexpress/chexpress07_14.shtml#technically</a>
A fine piece of work. The fact is that 5 psi is a dangerous pressure when exerted over enough of an area. 5 psi applied to a an 18 inch manway exerts a force of just under 1300 lb. If you're standing in front that manway trying to undog it, you're dead meat.

I got an e-mail once from a guy asking about a pneumatic test he was about to do on a gravity tank some 20 feet in diameter. The tank was to be leak checked prior to being reused for something or other. Turns out he and a partner planned to put just a little air in the tank while they went up on the roof to look for leaks. His boss had told him that it was safe because the liquid head on the tank was a lot more than the air pressure they wanted to use. Anybody see the fatal flaw yet?

Because the tank only held liquid, there was no pressure load acting on the roof in service, nor any axial load applied to the tank wall. The weight of the contents was carried by the slab and the anchoring only designed to resist wind loading. The 5 psi air pressure inside the tank put a force of about 225000 lb on the roof acting vertically upward. That force was also carried by the anchorage. When the roof separated or the anchorage failed it would have been all over for anyone on the roof. It would have come to attention like a the seat on a 20 foot toilet and kicked my correspondent and his pal a long long way.

The lesson is never to think of pressure without thinking about the area that's carrying the pressure and the force that's being generated. The pressure may seem trivial, but if you have enough area the force can easily be more than enough to ruin your whole afternoon.

Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=HTLOCKKB2V3_2U_LPg8RQlXjY2gV3zBVYiEL6OhdgVZf0-hpNm0Uuh-P11e2tZe18vEAiBdmYrg4GiNyUQ">chrisw@skypoint.com</a> | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania

1864)
<a href="http://www.skypoint.com/~chrisw/">http://www.skypoint.com/~chrisw/</a> Received on Fri Jul 13 00:04:00 2007