Vessel on Two Lugs 2

[nashof]

Hello All,

I am currently designing a small vessel, which needs to be supported on lugs, and I have a doubt.
The vessel dimensions are Tl-TL 2000mm aprox 79") and diameter 600mm (24"). MAterial SA-240 304, the same as lugs.
Due to piping needs if we install it on four lugs it's really difficult to access to nozzes due to small vessel dimensions.
We're thinking about supporting it on two gussets, 240mm width, with four bolts each, placed aprox. at an elevation of 300mm from bottom tangent line.

I have calculated it with Compress, I've traied to follow rules in Denis R.Moss, and those on non mandatory appendix G in ASME VIII Div.1.
With the vessel (the shell) we don't have problems, the stress is ok (very low actually), but I'm doubting about the stability of this equipment!

On Denis R.Moss talks about vessels on two lugs as a normal thing, but I think it can be good for lugs placed above de gravity center, but what about lugs placed below gravity center?

Can someone make any suggestion?
Thanks in advance!

nashof

 

[unclesyd]

We have all manner of pressure vessels supported by gussets at 90 and 180. We have at least 10 high pressure filters supported that way lined up on two support beams. We also support similar size SS vessels in high cyclic service, temperature, that way. We then only only have two supports to repair.
Just make sure the foot is wide enough to get two bolts in as you don't want the support to become a pivot point.

 

[JoeTank]

Would you want to sit on a two-legged bar stool?

Joe Tank

 

[unclesyd]

Yes, if the footprint was big enough. I've even sat on a many a one leg bar stool.
It may be strange to some that we even support this size vessel by the piping. We even have some on 3 legs and some support all around the periphery.
All it takes to support this size vessel on two supports is not to use the vessel as a piping anchor. Support yes, anchor no.

 

[SnTMan]

I would not have any reservations about this except: If the vessel is subjected to seismic or other external loads, I would make sure that it was anchored such that it would not overturn.

Regards,

Mike

 

[zerosum]

I have seen and specified quite a few small pots with only two support lugs, off-the shelf filters are a typical example. But usually, as I remember, the CG has been below the supports.

I agree with the above consensus, just do a static calculation of all the reasonably likely overturning moments against the ability of the eight hold-down bolts and gusset arrangement to resist.

The loads that come to mind are wind, seismic, connected piping thermal expansion, and conceivably (dynamic) sloshing and momentum change reactions should the process fluid be a liquid.

 

[unclesyd]

Why this discussion as all you have to do is calculate a two lug design. Most good PV software will do this at the push of a button/key.

I just talked to a PV designer and asked how he calculated a two lug vessel. He stated that he uses an old FORTRAN program he wrote many years ago based on a set of hand calculations used the fifties. He states that his values are comparable with the results from one of the software packages he uses.

 

[chicopee]

As with one of my previous response check out ASME section I on power boilers, there is design details on supports pads for fire tube boiler steam drums.

 

[zerosum]

When one doesn't have inherent stability, it's not a bad idea to at least consider upset scenarios.

 

[chicopee]

and this is in reference to a one legged, two legged bar stool, three legged bar stoolor all of the above.

 

[jte]

Funny, I've seen, and personally pushed on, several vessels with two supports. This one's a bit larger, and I might consider three L 4x4x1/2 legs as a starting point for my design iterations, but I could see it easily passing with only two legs also. Of course wind/seismic etc loads need to be accounted for, but seriously, this isn't that big a vessel...

I've seen plenty of children's swing sets which are (gasp) only supported by two legs, yet are routinely subjected to cyclic out of plane loading. I even have personally mounted swings on a single column support structure with a cantilever beam to which the swing is attached. Actually, there are two of these in my yard, and I installed both swings. My children use these structures on a daily basis. In my non-engineering vocabulary, I call these single column structures "trees" and the cantilever beams "branches".

Run the numbers, ensure that the anchor bolts are adequate, and be done with it.

jt

 

[moltenmetal]

The thing's only 79" long! Two gusseted lugs are just fine. I'd much rather see something like this bolted with two lugs to a structure than wobbling away on three spindly legs!

To be clear, if the vessel is vertical we're talking about two flat horizontal bolting plates at 90 and 180 degrees, welded to the shell, each reinforced with one or more vertical gusset plates welded to both the shell and the bolting plate. Just make the bolting plates wide and thick enough and attach them well enough to the shell to give the bracket a little moment resistance. Unless someone runs a forklift into it, it isn't going to overturn!

If it were longer, say double or triple its projected length, then you'd also want a guide location somewhere lower (or higher) on the shell, to take wind loading to the structure directly rather than via nozzles and piping.

Support from below or above the centroid based on which way you want the thermal growth to occur. Or if that's not a concern, based on where it's most convenient to the layout and structure.