How to support a heavy upper part vertical vessel during a PWHT 1

[lganga]

Dear colleagues

I have to deal with the job of joining the upper part of a heavy vessel by welding and PWHT.
I have reclaim the construction company, the procedure to suport this upper part during heat treatment.
This upper part is 60 mm- SA 516 70 carbon steel plate
Their answer, was to hold it with a crane (600 tons.). I have refused this procedure for holding the part, due to lack o plumbness and relative movements that induce cracks during the process.
I have already done this job in shorter and lighter columns. But this is a 100 tons part and 52 meters high column (both parts)-
As always, I will appreciate your comments.

Regards
Luis

 

[TGS4]

First, have you confirmed that additional support is required? How?

 

[lganga]

TGS4

The action of temperaure expansion gives a very high tension not able to with stand yield point at 650°C vs weight and wind compression load.

 

[weldstan]

We have used cranes to provide support for similar PWHTs.

 

[sharmass]

For such a thick and heavy column, it is suggested to use C shape clamps welded to top and bottom part in addition to the support provided by crane

 

[TGS4]

lganga - what you have written doesn't make any sense. The thermal expansion of the vessel in the upwards direction should be free. In other words, when you perform the PWHT, you should be removing all of the attachments sch as piping. Nevertheless, the thermal expansion (if somehow restrained) will generate a compressive load. However, since the thermal expansion should not be restrained, there should be no load (and hence no stress) due to thermal expansion.

Therefore, all that you are left with is the self weight of the portion of the vessel above the PWHT band (you are doing a full-encirclement band, per WRC 452?) and the resulting net over-turning moment. The important stress that these will cause is the compression, because the likely failure mode for this type of work is creep-buckling. And this failure mode has absolutely nothing to do with the yield stress.

I suggest that you find an engineer experienced in performing these types of evaluations and have hem check - just to be sure - whether or not you need additional support. As welstan has said, there has been the successful use of canes for support, but only when and if such support is required. I would estimate, based on my experience, that less than 5% of such PWHTs actually require additional support.

sharmass - any such c-clamps that would extend from below the PWHT zone to above the PWHT zone would tend to artificially restrain the thermal expansion of he PWHT. In addition to causing problems during the PWHT, such an approach is likely to induce substantial residual stresses into the heat treated zone - and that is completely the opposite of what you are trying to accomplish with a PWHT.

 

[weldstan]

TGS4,
Agreed. For the most part, we have placed no added support for in-situ PWHT of circumferential seams in vertical vessels.

 

[lganga]

TGS$
appreciate your comments
i think it merits to be studied by an specialist or the vendor.
Regards