Loads on legs support

[fjodor]

Hi,

Maybe this was answered before but I don't have luxury of time to search entire forum, so hopefully I can get some help.
We got small, relatively standard vessel to design. We later received forces and moments on nozzles, one moment, circumferential one, was 5.5 kNm and that forced us to thicken shell and add reinforcement. There are also circumferentail shear forces of 8.5 kN acting on both nozzles in same direction...there is the problem, that creates big forces on legs trying to bend the vessel, if you want to calculate it, which creates from small standard vessel a fighting tank.
What would you suggest to do in this case?

 

[JStephen]

First thing to do is to see where those loads come from.
Quite often, when unreasonable nozzle loads are specified, they have either just been pulled out of thin air or were derived assuming that the nozzle to which they were attached was entirely rigid. Where the nozzle is fairly flexible, or, in your case, where the entire vessel can flex some, those loads may be completely wrong.
And if required, consider going with a skirt or with cross-bracing on the legs. Or using legs consisting of wide flange beams with flanges radial, webs tangential.

 

[fjodor]

THX,
That is the main problem, thoose numbers we got, basically 17kN when you combine vector in one direction, look very strange, almost impossible looking at entire pipework. Trouble is we are only hired to do vessel, and price is going up for huge legs or skirt.
I managed to found this:

http://www.eng-tips.com/viewthread.cfm?qid=324660

Nice topic

 

[racookpe1978]

You are NOT working alone: Your company (your boss) is NOT going to succeed (or get future contracts!) if your company fails to challenge these loads and the envelope your tank is sitting it. The project then is threatened by either a tank too expensive, or a tank design too restrictive to be affordable or on-time.

But, having said that, "some one" (some person or some company) IS saving a lot of THEIR time (or THEIR money) by assuming too high a load so YOUR costs go up, or not buying an adequate pipe support or by not calculating any real load but just picking maximum-ever-worst-case-loads-and-multiplying them by some arbitrary constant so YOUR costs go up. Or that other company or person simply doesn't know and doesn't realize they are threatening their installation by requiring too heavily built a small tank.

YOUR copany is the one threatened (with not getting paid for a widely-over-built a tank, or by poor performance of a tank built with too thin a wall for the nozzle stresses). So, you have to push those spec's back at the customer and the integrating (design) company.