General site construction, air conditioning water system less than 100 with galvanized steel pipe, is not welded. Because once welded, the galvanized layer is destroyed, need to do a second galvanized treatment, the second galvanized treatment is troublesome and ineffective. Another if it is a key project, in the quality supervision station inspection welding was found to be under the rectification notice to the project, requiring rectification, but also to rectify the complete photo reply. Of course, if you just consider to connect the pipes together, you can weld, but does not meet the construction acceptance specifications. DN less than 100 generally use galvanized pipe, wire connection, that is, you said the threaded connection, the difficulty is that the seamless pipe to galvanized pipe in that section of the connection method, the construction unit will often use direct welding, which is not reasonable.