Those familiar with the China Code for pressure vessels should know what I am talking about. Cost to have the document translated into English can be very expensive.
The PRC GB-150 is for the Pressure Vessel Code, and GB-151 is for the Heat Exchanger. The welded or SMLS tube requirements of different material can be found in the Material Chapter.
As for China code for S&T exchangers, the specific code is GB-151, and it is clearly specified in this code the welded tube is permitted with meeting code requirement. That is to say the code cited relevant welded tube standards shall be followed.
Back in the 70's or 80's the Chinese Government invited member of the National Board for an undetermined amount of time to their country. I don't have the itinerary and the subjects covered during that visit, however, I suspect that the Chinese Government was interested in the NB Unfired and Fired Pressure Vessel Codes, so I would not be surprised if the Chinese Unfired and Fired Pressure Vessel Codes were similar to that of the NB's.