Union and Sockets in Piping

I recommend a web search for the phrase "difference between union and socket pipe connectors".

Useful information a click away.

I think socket is welded while union is threaded.

How about this information?

A pipe union has three pieces and can be disassembled without unscrewing adjacent piping. A pipe coupling is only one piece, and can be taken apart only by unscrewing adjacent piping. Unions and couplings can be either threaded or socket-welded or mixed.

Hurrah for good answers.

one cannot join two pipes coming from opposite sides by coupling but union can join.

This is not a true statement:

"one cannot join two pipes coming from opposite sides by coupling"

As they say, the devil is in the detail!

Sockets are used to join threaded pipes (20 ft in length) and proceed length-wise. You can cut your pipe to any length, pipes come threaded both ends /you can thread it on both ends and continue.You cannot open the pipe from and socket-side unless you install a union. Only the union allows you to open and unscrew pipes threaded onto the sockets. If you want to fit a valve on a tee, you install a barrel nipple/pipe nipple then the union and then continue your pipes with sockets. Whenever you want to replace the valve you unscrew the union rest of the pipes remain socketed.