Use of belt slings for heavy pressure vessel lift

[SurreyScot]

As part of an ongoing cost reduction exercise, we are currently considering the use of polyester belt slings, for the horizontal lift of a large pressure vessel assembly. The belt slings would be used in place of lifting trunnions, which are time consuming and costly to fit.
2-off lifting slings would be used in place of the trunnions. These would be used in a straight basket lift configuration, around the belly of the column, and attached up to spreader beams via a bow shackle.
As the column has a large diameter to thickness ratio, and weighs around 225 tonnes, we are concerned about the possibility of localised buckling occuring during the lift.
I would appreciate any suggestions for a suitable classical method of analysing this type of flexible sling lift, which preferably does not involve finite element analysis.
Previously this type of lift was assessed by using a "Zick" type of saddle analysis. However, we appreciate that this method is not strictly correct for this type of belly lift.

 

[TGS4]

Because the slings are going to be "highly non-linear", I don't think that you are going to find a classical solution. I understand that you would rather not go with a FE-based solution, but in this case my recommendation would be "Hire a good FE consultant".

 

[jte]

This is unusual, but I'll disagree with TGS in this case: I think that between Zick and Roark a reasonable solution can be found. Keep in mind that Zick's effort was based on steel shells on concrete saddles. So there was a wide area of support and no direct attachment of the support to the shell or wear pad. In fact, that's why that pad is called a wear pad and not a reinforcing pad: As the vessel wiggled a bit, the concrete could wear on the shell, so a pad was put in to mitigate the wear. Thus, a sling behaves more like what Zick studied than a steel saddle with a narrow web which is welded to the "wear" pad. So I'd be comfortable applying Zick's approach to this problem. Back it up with some application of Roark's Formulas for Stress and Strain by Young, 6th edition Table 17 case 13 and you should be ok.

On a related note, I think it would be extremely interesting to see a nonlinear (due to contact, perhaps deformation) FEA of a steel shell on a concrete saddle and compare it with Zick's strain gage result based formulas. Then run the same shell with modern welded on steel saddles and compare. Note that while a shell element analysis might get you going you'd be best off modeling this with solids - which is too tedious for me... Sounds like a good paper topic. Actually considered it for my Master's thesis topic but chose another which could be handled with axisymmetric geometry instead.

jt

 

[SurreyScot]

Many thanks for the suggestions guys. I have tried analysing the problem with circular ring formulae, similar to the Roark case which you suggest (Roark 4th Edition table VIII case 24 (Tangential load transferred by tangential shear)& Blake Fig. 8.4-14 (Circular ring loaded by two tangential loads and opposed by a cosine distribution of radial load)). Both methods produce resultant stresses which are an order of magnitude greater than the Zick saddle analysis. We would have to weld heavy reinforcing bands to the shell, to keep stresses within allowable limits.
I really can't believe that there would be such a massive difference in reality. It just does not ring true.
Due to the criticality of this lift, it looks like we will be employing a consultant to carry out nonlinear FE contact analysis of the problem. I will let you know what the conclusions are.

 

[juancito]

Probably because of lenguage reasons, I did not read wether the vessel is horizontal or vertical.
How is going to be put in its operational position @ site?
May be your customer will not be happy of not finding any poitn to lift the vesel from!.

 

[SurreyScot]

My apologies, I should have given a clearer explanation. In this case the pressure vessel is a cryogenic distillation column. The column will be lifted and lowered into a Cold Box structure. The Cold Box is then shipped to site, where it will be raised from the horizontal to the vertical.
The column is therefore only ever subject to a horizontal lift. This is the main reason why the use of belt slings is so attractive in this case.