how to do hot tap on 30" existing line. branch=20"

Hossein,
Please remember by definition the term "Hot Tat' means that a connection is made while the line is in service and under pressure. So cutting into the line as you suggest would not be possible.

Zahra,
My mistake, I missed your country location when I first made my suggestion.
Now that I understand your location and the realities involved I suggest the following.
Your proposed method for making the "Hot Tap" tie-in should work with the following modification. I feel you must weld all the seams and all edge joints of the reinforcing pad to the pipe. If you are concerned with the trapped space under the pad then drill (and tap) a very small vent hole which you should plug later.
You are not quite correct in that the pad is not a pressure containing part. The reinforcement is just that, reinforcement to the pressure containment.

It also offers structural strength to the branch while the "Hot Tap" machine is mounted and making the cut.

chanock,
It is never a good idea to cut or weld on the long seam of (rolled and welded) pipe.
You have two choices:
1. You can move the connection around the pipe to a different orientation thus avoiding the weld seam.
2. You can move the connection point upstream or downstream until you are clear of the weld seam in your present orientation.

Both of these will add extra cost due to the requirement for additional fittings, pipe, welding and supports. However this must be weighed against the potential of a seam failure under full operation and the lives that may be lost.

You are not making a weld with this type of (clamp-on) branch fitting. However I would try to stay out of the originat seam weld "heat effected zone." I would suggest that two to three times the wall thickness of the header pipe would suffice.