I'm trying to install a new dishwasher, replacing the 20 year old one I currently have. I have a compression shutoff valve underneath the sink for the hot water water. It has two outputs, one going up to the faucet and the other going to the dishwasher. It's connected using rigid copper pipe. It seems I'm supposed to remove the copper pipe going to the dishwasher and install a stainless steel sheathed flexible tube instead. The problem is that I cannot seem to loosen the actual nut holding the rigid copper pipe to the dual outlet compression valve. It's green with oxidation, i'm assuming from the copper, and it seems to have some rigid white paste beneath the nut. I'm assuming this is pipe dope? I feel like if I were to turn any harder I could damage the thing. Any tips on a better way to get this loose. Am I missing something obvious?
You should be able to turn the body of the angle-stop if the nut is held firm with another wrench. if you turn the body and not the nut - It is less likely to damage the copper tube. Depending upon how tight the nut was set - will determine how deformed the copper pipe is. sometimes it is difficult to remove the ferrel.
You have so much oxidation on this valve - I would replace it.
Personally, I never install an appliance onto an old valve.
It looks like you have enough room to cut off the entire angle stop. Use a fine toothed metal blade and cut square - then deburr. A new multi angle stop can then be used with the matching outlets you need. The braided dish washer supply lines come in different configurations. I have found that many of the new dishwashers have a large threaded connector instead of the older 1/4" compression. check yours.
P.S. check inside your pipes for pitting. Often this oxidation can be a result of iron in water which destroys copper pipe.