Platform clips on heads

[codeeng]

Placing platform clips in the knuckle region on 2:1 heads has often been avoided in the past. Does anyone have any comparison between clips in the crown vs knuckle ? I recall seeing a paper on this some time ago, perhaps someone has a copy ?

 

[JoeTank]

I have never heard that one should avoid platform clips on a knuckle. Whether placed on the dished portion or on the knuckle, both situations require that the local stresses be determined and thier effect considered in the head design. These are stress-related issues that can be easily handled.

For the case of a shop fabricated vertical vessel, I could see a shipping-related advantage in keep the clips out of the knuckle region. the vessel would have a smaller shipping envelope.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[jte]

There are two reasons why I would avoid placing clips on the knuckle:

* The knuckle tends to be the thinnest part of the head.
* 2:1 heads under internal pressure fail by buckling (wrinkling) at the knuckle. Buckling is a failure mode which is very susceptible to geometrical imperfections. Clips will result in some local imperfection due to the transmitted load, welding, and local stiffening effects.

On the other hand, I have seen it done, and have ordered clips on a knuckle recently. This was a case where a vessel with a 2:1 head and top platform was replaced in kind *very* rapidly. The old ladder and platform were used, so I had the same old clip locations on the knuckle to match the existing platform.

Depending on platform size, the best place for platform clips is on the shell. The load is transmitted primarily through shear with virtually no shell stress to worry about.

jt

 

[deanc]

The problem I have seen at the knuckle is thinning and sometimes light surface cracking from the stretch. It sometimes depends on which way the grain was running when they formed(bumped) the head.

I would avoid welding to this area if I could. You could place a 90 degree or angle iron bracket and weld to the dome and the skirt.

 

[bvi]

The knuckle can be subject to significant strain when the head is subject to internal pressure. So, adding a stress riser in the knuckle region is a fatigue consideration. Not a requirement not to do so, but a consideration.