Can hitting or tapping your back help relieve plurisey? My partner does this for me and it helps but is it good to do?

Feels good, do it. Short answer is yes. Mechanical percussion helps to break up and mobilize mucus in the chest and "sticky" spots to reduce pleuritic pain. It is best done "posturally" -- meaning with the bottom of your lungs higher than the top. Simple way is to build wedge of pillows and lie knees-bent, head-down while getting thumped on. When done well (not too hard), it also feels good, so why not?!