Base plate for vertical vessel

[mechpiper]

For vertical vessels with skirts, the bottom base plate is
single for short vessels say 1m or 2m in height. however for tall vessels 2 ring plates with gussets are used.

Any suggestions on the significance and guidance on when 2 ring plates are required ?

thanks and regards

 

[TomBarsh]

The single base plate for very small vessels might be built without "anchor bolt chairs". But vessels in most any sort of industrial plant will likely have anchor bolt chairs with the single base plate. The bolt chairs help to distribute the forces due to the bending moment acting on the vessel (from wind, seismic, off-center weight, etc) through the skirt and into anchor bolts.

Vessels that have especially large bending moments acting on them may require so many anchor bolts, consequently spaced close together, that the anchor bolt chairs may begin to run into each other. Or, there may be just so many anchor bolt chairs that the number of small pieces to assemble is a nuisance. In either case, placing a continuous ring as the top component of the anchor bolt chair relieves the problem.

 

[innovation2]

With any kind of significant seismic activity I like to use compression chairs. It is just a much stronger design with columns. Two rings is a little over kill except in extreme seismic conditions. The single ring should only be used for small vessels or no seismic.

This is just my $0.02

Good luck.

 

[vesselguy]

mechpiper,

I don't understand your meaning of vessel height of 1 or 2 meters. Wow, these are short vessels.

The quick run down is you chose the top compression plate type when you have so many anchor chairs such that it would be simply easier to use a single compression ring instead of welding individual compression "plate", which cost a lot of money and time. Also compression plate is slected to help to reduce the local bending stresses in the skirt caused by the anchor chairs. This type is usually selected when you have a column taller than say > 80 ft tall and slender (read large H/D ratio > 10).

Suggest you read the excellent books such as the ones from Bednar, Brownell & young, etc. They will answer your question in detail via calculations and show you how to derive the equations.

Hope this helped.

 

[TomBarsh]

Kind of on this topic: A long time ago I was walking around a shop reviewing some work a vendor was doing for us. They were also doing some work for the Army Corps of Engineers, some sort of lock or pressure-containing bulkhead. This equipment had a good-sized opening in it, which was lined with a "flange" or "leg" about 3/8" x 3". There were a bazillion little gussets welded all around the periphery of the opening. Certainly they were there to stiffen/brace this outstanding leg. But all that cutting, fitting, and welding; what an expense! I always wondered why they just didn't increase the thickness of that leg to 1/2" or 5/8". Would've saved a lot of time and money.

 

[vesselguy]

Tom,
Funny you mention this as an example. I just had a discussion with our Structural Eng. yesterday on setting rules for our standard on pipe support design. He mentioned on a project he was on, the "T" post type pipe support standard was shown with gussets to reinforce the horizontal support bar against the bending moment. Of course, in a large refinery project, there's many "T" posts. The Fabricator wants to charge double the cost to put in the gussets because it cost them time and manpower just to cut and weld on all those gussets. The vedor suggested to either pay for the gusset design or pay for a beam with thicker flange which has no cost increase on fab cost. The choice is obvious. Many Engineer and Designers in EPC and Owner side don't have shop experience and do not take fab cost into consideration when they design. I have to admit I'm guilty of this sometime. Relating this back to the subject of this post, it is cheaper to use a compression ring when you have 8 or more anchor chairs.

 

[jte]

mechpiper-

One reason you often see larger vessels have a top ring instead of chairs is that when you are erecting a vertical vessel from the horizontal position to the vertical, the rigging at the bottom is usually attached to the baseplate. In some cases - frequently for larger vessels - this step will govern the design of the baseplate. When you add a continuous top ring you get substantially more section modulus from the baseplate/(part of the) skirt / top ring acting together.

jt

 

[mechpiper]

Thanks for the responses

The single base plate I'm talking about

1.5m dia vessel and 2m in height with skirt height of 0.8m
The number of bolts 4. There are no gussets. The 1/2" thk skirt and the baseplate 15mm are checked and found ok for the vertical/lateral loads.

There is another enquiry for a vessel 5m t/t with 1m skirt...which raised the question if the vessel baseplate and bolts can be designed for the same configuration.

Technically the skirt can be checked for the buckling and the chairs would not change the design of the skirt plate (except for the small change in the height)

not sure about the impact on baseplate design

 

[mechpiper]

Correction on my last post...No. of bolts 8x used

 

[MJCronin]

mech..

A discussion of vertical tank/vessel supports, anchor bolts, loading definition etc is contained in:

"Pressure Vessel Design Handbook, Henry H. Bednar

"Pressure Vessel Handbook", 13th Edition: Eugene F. Megyesy

and other places...

Imposition of significant seismic and/or wind loadings affects the selection of anchorage and anchor bolt rings.

My opinion only..

-MJC