It seems there is small confusion. Pipe is sh 80 or pipe size is 80 NB? Class 300# is applicable for valves and fittings, not for pipe.
If you have 15 NB valve which is socket welded type, then necessarily you have to weld it. But if you want to go for threading, then you should have valve with threaded ends and first you should weld a threaded socket to the pipe. Both ways are acceptable for this application.
What is difference between 150# pipe and 300#pipe of same pipe class?
does not have a class like a flange. There basically is no 150# pipe.
The pressure rating for pipe is determined by calculation taking into
consideration its material, size, wall thickness, temperature and the code to which you are designing. In some cases the limitation of the pipe design is structural (such as support) rather than pressure.
dear prashant, following things you need to check wheather you should go for flanged end connection pressure gauge or threaded pg:
1) for connecting any pressure gauge in the pipe first thing which comes is one isolation valve which gives either 3/4 in threaded or 2 in flanged ( normally) for this you have to ask your piping department which type of valve they are providing for isolation.
2) if they can provide threaded valve for your service of cold water then wiithout any doubt you can go for threaded connection pg. But make one thing sure that you purchhase that pressure gauge should be with solid front. If your vendor is wika then model number should be 232:30.
3) if your isolation valve is of flange type, typically flanged end connection is used when diaphragm seal is required, but in this you can ask your vendor to provide flange end connection without diaphragm seal they can easily provide you.
4) 300 pound or 150 pound is not a problem if your pressure gauge can withstand your design pressure so select that range of pg which covers your design pressure.