Vertical Vessel Skirt/Base Ring on Shims 1

[PPVE]

In my study of the design of vertical vessels on skirts, base ring sizing appears to be dealt with adequately well, except for one practical consideration: My Civils colleagues want to place shims under the base ring for levelling purposes before flooding with grout and tightening the anchor bolts.
My questions are:
1) how strong is the grout?
2) if the grout is weak, how is load transferred between the base ring and the main concrete slab?
3) should the shims be placed near the bolts or mid-span?
4) how does this affect the base ring design sizing
5) why is this not addressed in the well known design routines?

All constructive comments welcomed.

 

[gr2vessels]

ppve,

The grout is not part of the base ring design, nor is the shimming, hence the skirt design assumes full support under the base ring. The rest is the civil engineer's job to ensure the rigid grouting under the base ring for proper support of the vertical vessel. Apart of that, the shims can be removed if located suitably or can be embedded in the grout permanently, at civil's discretion.

cheers,

gr2vessels

 

[PPVE]

Thanks for your reply: It sounds plausible. However, as I have an investigative nature I'm off to see if my local Civils man understands his part in the scheme. I'll post the outcome for the benefit of others reading this post.

 

[RossABQ]

On the vertical equipment I have been involved with (LN2 tanks which actually have short legs, stacks supported on base rings, chimneys, etc) the structural engineer was not counting on a uniform load around the base rings. They specified shims right at bolts, if needed. The grout was packed after the ring was supported by a nut on the anchor bolts, under the ring, and was solely to weatherproof the ring and anchor bolts (in fact they were OK with leaving the grout out completely). Their analysis of the loads under seismic conditions was based on discrete load distribution among the anchor bolts. Analysis of a uniform loading on the ring is apparently very difficult to model reliably, and very difficult to achieve in the field on large equipment (they aren't set by machinists). As a practical matter, all the equipment I was involved with was anchored with drilled-in-place epoxy anchors -- contractors have been bitten too many times by poured in place anchors that were a little off.

 

[TomBarsh]

In my long-ago work on heavy mechanical equipment in paper mills it was customary to use steel shims to level equipment before placement of grout. Critical machinery even had installation manuals with the procedure listed for placement and size of shims to prevent damage to the equipment.

In fact, I cannot imagine how it would be possible to set a pressure vessel or other equipment in place without using shims. What will support the equipment the 1" or so off the top of the concrete foundation while grout is poured or packed, then sets up? This is just not possible without a "sky hook".

Most likely, such problems as this are generally left to the people who install the equipment. If the owner is lucky, these people will have some engineers on staff who will assist in developing an erection procedure for large, heavy, costly equipment such as a shop built vessel.

But the question raised is a good one. In the design procedure for skirt base rings on pressure vessels as outlined in numerous texts there is no mention of erection or construction condition where there is no uniform support under the base ring; instead the vessel and base ring likely will be supported at discrete points, likely by steel shims, and likely these shims will be located near the anchor bolt chairs or other gussets to prevent any local buckling of the skirt due to the concentrated load.

It would be a good idea to check the skirt and base ring for such temporary concentrated loads as this. Design guidance for this can be found in the AISC Steel Manual for concentrated loads for beams and plate girders, or in a textbook on structural steel design addressing same issues.

 

[JStephen]

This does affect the base ring design somewhat, as you need to size the base ring so that it's wide enough to distribute the load- usually not a big problem, though.

Non-shrink grout usually has a higher compressive strength than the concrete. Not sure about a cement-sand grout- it quite likely would.

 

[RossABQ]

As I stated above, all the ones I have seen set were set down (still "on the hook"), the anchor bolt locations drilled thru the mounting ring, the vessel lifted, the anchor bolts set (epoxy) with a nut just above the foundation, then the vessel set on the anchor bolts (supported by the nuts). The nuts are used to level the vessel. I am not talking about precision machinery (lathes, mills, etc). I have never seen shims used except when there was some problem with achieving level with the nuts, and then they were like 1/4"-thick plates, not .050" shim material.

 

[dig1]

Most of the vessel I deal with are field erected and the base plate is supported by steel shims on each side of the anchor bolt until the base plate is grouted.

Grouting is specified when the vessel has reached a certain height so as not to overstress the base plate and skirt in the shim supported condition.

Shop erected vessels are set on shims as well and then grouted.

Normally, the empty vessel weight (without insulation, platforms, piping, contents) is small enough so that the base plate & skirt are unaffected by this temporary loading condition. Often, this empty weight may only be 15-20% (often less) of the flooded (operating) weight so in the cases I normally work with it isn't an issue.

Typical grout in either case is 5000 psi non-shrink.