1. Is this coupler is the same with SOCKET in Thailand?
Yes, this piece is the same. The socket must be modified to fit the curve of the pipe.
2. If it is socket. How to bond it with the pipe? Since there will be a gap as attached pictures. Does the PVC glue will be able to hold it?
The coupler/socket must be cut on one side to match the curve of the pipe. The coupler and the header pipe should be sanded, cleaned and then the bonding/gluing process will hold the coupler to the header pipe.
3. K.Krit suggested that normally we use rubber material not PVC material for this kind of work.
I would like to use the rubber grommet as well, however, the smaller diameter pipe like the 12” header pipe we are using has a very sharp curve, not gradual curve like the larger diameter pipes. With the sharp curve of the 12” pipe, the rubber grommet has to bend too much and leaks from the edges.