the silver connector in the pic not able to unassemble, it just turns together with the bottom pipe, I have no idea how can I remove it. Any idea? Thanks a lot!
That is interesting. Your plumber got creative. That is schedule 40 PVC and it was not meant to have a nut on it. Yet, there it is. One nut evidently firmly attached.
If I am wrong, which I highly doubt, and that nut is supposed to be there, then it will unscrew with a big pair of channel locks. I assume you already tried that.
Why was a nut used? What is holding the nut in place? What magic glue? Epoxy? Two part or single? A bucket of superglue? Old polyurethane? Heat to soften the PVC and create a perfect shape?
And now you want to know how to get it off...
If it's not leaking, then leave it on. It it is leaking, then its time to invest in a small grinder. Not that I like Dremel, but they do make the tool which will accurately cut that nut off.
PVC is harder than its glue is strong and slotting the outside so that a screwdriver can be used to pry the PVC off will usually take a joint apart.
Whenever I do plumbing, before I begin, I always assure the customer that plumbing is the 'surprise' trade. I never know what I am going to find, and I never know what is going to happen.