Maximum Size of Skirt Openings 1

[michigander]

I have a number of references regarding column skirt designs but none of them address skirt openings very well. Are there any guidelines regarding the maximum size of skirt openings? Personally, I've limited them to about .3D.
I have a small column that the client want a very large opening in (.75D). I cannot do this, I've told them I can't do this, but it would be nice if I had a chapter and verse or some expert commentary to support my position.

Best Regards,

MIchigander

 

[waskillywabbit]

Bednar's "Pressure Vessel Design Handbook" has some comments on skirt openings that might be helpful.



Brian

 

[dtn6770]

Skirt opening limitations are purely structural (vertical loading, overturning moments, etc.). Thicker skirt material and/or reinforcement and bracing of all types can be applied. You may be to the point of impracticality with your specific case. I don't think you're going to find hard Code working but I'd hope your client would understand "fall over" without having to show 'em the math.

 

[jte]

One way to quickly evaluate the effect of the opening is to consider the moment of inertia of the cross section through the opening. Typical skirt opeining reinforcement is based on area replacement and this is reasonable when you look at the calculation for I. What is important to understand is that I is the integral of area times distance which is why you use the AD^2 part of the equation when you shift the part that you are looking at to a different neutral axis. For a relatively small opening on a relatively large diameter which has an equal amount of area put back in as reinforcement as is taken out, the AD^2 values of the part which is cut out and the reinforcement are about the same. However, as the size of the opening increases relative to the diameter of the skirt, the edges of the opening start getting closer to the centeline of the vessel. Thus, the D value for the reinforcement is less than the D value for the cut out section. With the distance being squared, this can have a substantial impact on the moment of inertia.

So… I'd recommend that you calculate the moment of inertia of the skirt without an opening, then calculate the moment of inertia with an unreinforced opening, then calculate how much reinforcement you'd need to maintain the original moment of inertia. If you do, let us know what you come up with!

jt

 

[michigander]

Thanks,

I looked in Bednad and blew right by it.

Best Regards,

Michigander.

 

[LSThill]

Stress Analysis of Skirts in COADE Mechanical Engineering News December 2004

http://www.coade.com/company_newsletter.asp

 

[dig1]

Have you looked at placing the vessel on a combination skirt and legs?

I recently did a relatively small vessel (4 m dia x 55 m OAH) with a 2:1 SE bottom head where a skirt extended down to approximately the bottom bottom nozzle in the head. From that point down the skirt was supported on four legs.

Their issue was access and removing a large piece of equipment that bolted into the bottom flange every 18-24 months.

The legs were cross braced but designed to not need the bracing with vessel empty + design wind. Placards were attached to the vessel legs that stated bracing had to be installed except for removing the outlet device and the vessel being empty. This was also in the O&M manual. This has successfully been done on vessels that have been in service for nearly 50 years.

Not the most economical option but it provided access.

Just another way of looking at it.

dig1

 

[JCAT96]

I recently designed a large silo on a skirt, we ended up with two large opennings. My first approach is always putting back the material to provide the same section properties as per jte above, however we had concerns about the anchor bolts and transfer of loads so we added a central stiffening post to the openning which was only to be removed when the silo was out of service to allow large equipment to pass through. The only danger is making sure site installs the post 10 years from now when everyones forgeotten why its there!