Lifting Lugs - Bit OT

[TheScullster]

Hi all

Been pondering the safe design of lifting lugs for cylindrical vertical vessels.

If you provide 4 lugs for lifting via a 4-leg sling, what is a reasonable but not ridiculously conservative assessment of the likely max load to one lug?

One approach - Judge that the 4 legs are unlikely to be sufficiently well adjusted to accept equal load down each chain leg - so design for 3 lugs carrying the load.

Another approach - As an extension of the above argument... If the vessel has a central C of G (viewed in plan) and the lugs are equispaced around the cirumference, then if one chain leg is assumed to be doing nothing, then the opposite leg will also be doing nothing (as the C of G is directly between the other 2 legs IYSWIM). So the weight must be carried on 2 legs only and the lugs designed accordingly.

Another approach - There is believed to be enough elasticity in the chain/sling legs to allow distribution of the load around any of the connected legs - therefore design of the lug for 1/4 of the lifted load is acceptable.

Any thoughts anyone?

Thanks

Phil

[dennis]

A pair of crossed beams attached to the crane hook with four straps coming down to four lugs, this helps to spread the load so its vertical at the lugs.

If any one fails it is probably going to be safe and not swing about violently like it would with three lugs. It allows easy positional adjustment by fitting an adjuster in the straps if needed.

At the worst there would be about half the load on opposite sides if one (pair?) sling is so loose it doesn't take any strain. You can't easily predict the elasticity of the sling or its actual length so I would say you have to design it for at least half the load on each lug unless you can fit load cells and adjust the slings (not something I would want to do, but I am extremely cautious of human error).

I would be tempted to say the full load on each lug but I don't know if that is economically feasible.

Are the same lugs used to tie it down for transport?

[TheScullster]

"dennis@home" wrote

Granted that eleminates the compressive component at the lug itself, but doesn't really influence the various approaches considered.

Good points

That was pretty much the conclusion I had come to. The problem with all this is that you end up applying safety factors to safety factors. This then results in a load that (whilst quite within the capacity of the lug itself) causes issues when applied to the wall of an unstiffened cylindrical vessel shell.

No, from what I've seen it wouldn't be physically feasible either in some cases.

Yes they are, and therefore the lugs are designed either on the basis of the lifted weight, or the capacity of the lashing rig (generally taken as

17.5Te).

Thanks for your thoughts Dennis.

[dennis]

You could fit two sets of lugs, one at the top one at the bottom, attach vertical steel beams and then attach the sling to the beams. This eliminates the torsion to a large extent AFAICS and may spread the load a bit.

A lot of cylindrical tanks are built into a steel frame, probably for this purpose, but I admit to never having looked at where they are supported in the frame, they may be supported across the bottom and not on the lugs. Of course its added costs.

If there are several of them you could weld/bolt together a transport cage but how you get it in and out would depend on how its mounted, I assume it has mounts that can take its full weight and should be able to lift it when empty. Its not a petrol storage tank that buried is it?

Can you weld/bolt a beam internally between the lugs?

[The Natural Philosopher]

Id make it so that any one lug is fully capable of at least twice the weight, just in case something breaks, and the thing drops and that all you have left...

That's the 'British army' mil spec approach.

OTOH if it was a nuclear reactor pressure vessel, being installed very very carefully by skilled engineers, and there were terribly good reasons to skimp on minor costs..then I'd go for about 1/4 the load

+30%, on the basis that its a reasonable margin if the operation is only being done once under ideal conditions.

[TheScullster]

"dennis@home" wrote

That might be an improvement allowing lug sizes to be reduced.

We did once have a framework built to enable lifting of a one-piece vessel - the costs were horrendous, particularly as it has to be tested to "lifting device" standards.

The vessels are usually transported in sections (usually 4 or 5) with each section being 3-4 metres tall and upto 6m diameter.

No it's a vertical cylindrical toaster with internal steam heated trays - used in the extraction process of oil from vegetable seed (rape, sunflower etc).

Yes we have been doing that on occasions, where an internal tray was not close to the top of the sections.

I wonder how they load rate these 4 leg chain slings. Looking in catalogues, load ratings are applied to 4 leg slings for different sling angles - but it is obviously not clear how many of the slings are considered load-carrying.

Thanks again

Phil

[Tabby]

Ah well, at that size you can just pop them in carrier bags :) This kind of data is very necessary to answer these sort of questions. So is the consequences of failure - if a lug breaks, do you simply tack on a new lug and repeat? Or does it fall onto 100 people's heads? I really don't think your question is answerable in any useful way without all the necessary info.

NT

[Nightjar]

IIRC, the general rule for lifting equipment is to design the safe working load for the worst case, then multiply by six to get the minimum structural strength. Proof testing should be done at 2.2 x SWL.

Also, watch that the wall of the vessel is strong enough to avoid dishing when the lug is under load.

Colin Bignell

[TheScullster]

"Nightjar wrote

Thanks Colin

But I suspect that this rule applies to lifting slings and the like which will be used and abused daily for many years, rather than lugs to be used maybe 3 times in the life history of the vessel.

Yes this seems to be one of the most significant factors - the size of attachment needed to spread the lug load into the shell sufficiently to prevent distortion.

Phil

[Nightjar]

The rule is based upon potential failure modes. One of those is the use of undersized shackle pins, which can induce a high local loading that can lead to failure above the hole. Of course, if one pin is undersize, the others will probably be too. That can lead to a domino effect as the load from one failed attachment is transferred to the remaining ones. Of course, if you have control over the lifts and can absolutely guarantee that only properly sized shackle pins will ever be used, that is not a worry.

The Lifting Operations and Lifting Equipment Regulations 1998 apply to attachment points on loads, even if they are only to be used once. The costs in fines if they fail and cause an accident are likely to be significantly more than making sure they are strong enough to begin with.

One way to overcome that is to have multiple fixing points up the side of the vessel, to which you attach a reusable over-engineered vertical lifting beam. Obviously, to do that it should be practical for a set of beams to travel with the vessels and probably for them to be returned after use. It should be possible to use lift-off hinge type attachments, to cut down on the time taken to fit and remove the beam.

Colin Bignell

[TheScullster]

"Nightjar wrote

Yes the bearing stress is deliberately minimised by adding doubler plates to prevent this local failure. Although some argue that bearing failure will to some extent be self limiting in that the pin will deform the material until the bearing stress reaches an acceptable level. This would not be the case if the pin was ridiculously undersize obviously.

Of course, if one pin is undersize,

Our approach is to issue a lifting study drawing which shows section weights, sling angles and the shackle size to be used. There are still no guarantees of course, but as we are not in control of all handling and transport aspects, it is difficult to do more.

Some useful pointers - thanks for your input Colin

Phil

[dennis]

Can you cut holes in the sides, bolt on a reinforcement (if needed) and lift in-line with the sides? There is no torsion then. You will need to fill the holes somehow.

Bolting a couple of plates (protruding above the cylinder) either side of the thin material with lifting holes at the top would allow for smaller holes (bolt sized so easier to make and fill) but you would probably need to curve the plates (milling machine job?).

[TheScullster]

Interesting idea, but I don't think our usual customers would go for it. Filling holes in new equipment tends not to go down well we've found! The vessels contain solvent laden seeds so vapour containment is critical.