Carving on socketweld valves until they are profiled to be butterfly weld valves sounds like a baaaad idea. 
You move the heat-affected zone much closer to the trim,
you get a valve with a non-code and probably nonuniform end-to-end dimension.
US code prohibits mahining flanges off to make weld-end valves.  THis would fall under the same justification if not specifically addresed by the code. 
And finally, if you modify the butterfly valves and there is ever an accident in the plant, the modified valves will draw a great deal of attention to you that you will not want. In the US we have lawyers.  In Russia there is the Mafiya.  Either way, you don't want the attention.  

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