Lifting lug design with corrsion allowance?

[Spoonful]

Hi All,

One concern just popped up in my head, when you design a lifting lug, do you normally included a corrosion allowances(for carbon steel)?

Normally carbon steel lugs will be painted, but no matter how good the paint it is, after couple lifting (eg, installation), the contact area paint will be scratched off, and the structural will sit there and the lug will starts to rust, in few years time, when the structure will need to lifted again, it lifting won't have the same meat on it as it was new.

Any comments?

 

[dhengr]

Spoonful:
Normally lifting lugs are designed with a pretty big factor of safety (3, 4 or 5), so they should be able to tolerate a little corrosion over time. They should also be, at least, visually inspected prior to reuse after a number of years. Why not save a quart of the paint and hide it someplace on the skid, with instructions that all scratches be cleaned and touch-up painted after the skid is installed. Alternatively, if it is just the lifting lugs, why not have them rub some grease on any exposed steel when they are done with the lifting. There’s half a chance that the grease will already be there.

 

[Spoonful]

Hi dhengr,

Thank for the reply,

Just reading from ASME BTH 3-3.3.3, pinhole should be drilled, reamed to surface finish of min 12.5micron, ones it starts rush on the pinhole, there is no guarantee of the surface finish finish for next use.

re design safety factor, I tend to use one safety factor for on aspect only, and I don't think using the structural load safety factor to cover corrosion issue is an good engineering practice.

re procedure/instruction, should be given for installation/operation/maintenance, but implementing of them along the line is another story.

 

[KENAT]

What treatment do you put under the paint?

Aerospace stores lugs were traditionally Cad plated, though we were starting to use zinc on some.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[SnTMan]

I would not normally apply a corrosion allowance to any externals, unless known to be in a corrosive environment. It is up to the owner to maintain paint, etc.

Regards,

Mike

 

[Cockroach]

Yeah, I agree. You're greatly over thinking the problem. Usually oxidation leaves a passification layer which prevents additional rusting. If the layer is damaged, a new layer forms over the fresh metal. Hooks on cranes outdoors are not treated.

I would suggest to liquid nitrate the piece, thus removing free iron from the surface. Then no rust at all and you get a good increase in hardness to mitigate metal-to-metal transfer or galling.

Regards,
Cockroach

 

[johnchrc]

I agree, corrosion will slow as rust forms and you have plenty of material so it isn't and issue. Plaster it with grease before lifting. If you are offshore or on the beach then maybe you consider switch material. A coating will rub off and then corrosion will increase where the coating is rubbed off. If you are really concerned and want them to look good, dip the lot in QPQ bath.

I wouldn't worry about it. If lug shows signs of deterioration, replace it. it will be less expensive than using a SS.

- CJ

 

[Cockroach]

If a guy is completely bummed out over this, why not buy a new hook and compare it to one of exact make and model from field usage? Simply measure the weight loss. I bet you would find negligible mass loss, hence very low corrosion rates per year. That would give you an idea of how trivial your concern is.

I mean a sky hook doesn't rust away to thin air over it's lifetime of usage! So I'm not sure where that concern comes from. There is a mental illness about being too safe, unfortunately all around us. I'm not talking about professional, well seasoned or schooled engineers, but the tradesmen themselves. Safety also acknowledges practicality and reasoning, so I think you're just being over cautious.

Regards,
Cockroach

 

[johnchrc]

[highlight #73D216][/highlight]Mnay new engineers overthinking everything. They are terrified of making a mistake. Job market isn't great. If you own a home it is in the toilet. I assume job security isn't peaking for another 5-6 years

Wouldn't have been nice to have the resouces today 20 years ago? You just had to use common sense and engineering judgement. If you did make a mistake, you learned lessson. He should jsut go to his Sr. and tell him what he wwants to do and why.


- CJ

 

[oldrunner]

IEEE C57.12.10 Factor of safety for a lifting a transformer is 5 of Fu. This applies to lifting lugs and jacking pads. The real issue would not be the lifting lug itself but the analysis of the welds and what the lug is welded to. Some companies like to weld a 2 inch thick lug to 3/8" plate with no regard to any kind of stress to the tank shell. We like to see a FEM analysis of both the lug and what ever it's connected to and a separate FEM run of just the welds themselves.

We've also seen lifting tabs for tank covers orientated incorrectly putting the tab in a bending attitude along with pure tension.

Also we've seen four lifting points with the total load divided by 4 and no regard for the center of gravity of the load - where sometimes one lug will be taking a much greater load than some of the others.

 

[Cockroach]

Yeah, true enough but those are equipment design issues. Spoonful was specific in lug corrosion allowance rather than design related issues. And weld-on lugs is an assumption, if I choose to thread-on, an eye for example, then your point is completely mute. We chose to answer the question, not make assumptions and address those probable issues.

Regards,
Cockroach

 

[Qshake]

In the heavy civil industry we'd typically use 1/16" to 1/8" of corrosion loss for carbon steel subject to benign environments to aggressive (salt water) environments respectively. This is over a design life of 75 years. That is without any protective coating on the steel. There are other recommendations for bridge steel which need to resist aggressive de-icing chemicals but the amounts listed above are for steel that is away from those de-icing chemicals. The design application for the values cited above includes axial, shear and moments.

Regards,
Qshake

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