I would like to share a story, purely for comic effect and a good lesson to beginner techs.
Got a call for no cooling (last July). Get there. AC's running. Put my gauges on and it is showing an "equalized" pressure of about 5 psig.
I put a trace charge in and find two leaks. I can hear the compressor trying to run, but doing nothing.
Recover all the refrigerant, repair the leaks, install a new filter-drier, evacuate the system, and start weighing in the charge (r-22). Once my gauges show about 90 psig equalized, I kick the AC on and figure I will add the rest to superheat as vapor.
Only problem is pressures are equalized. Oh crap, what is wrong with this compressor? Check the capacitor - good. Try a hermetic analyzer to unlock it - no luck. OK, the compressor is shot. I think...probably from running with no refrigerant, you know? Call the developer and he says "just go ahead and install a new one".
Wait for the OEM part to come in and go back. Recover all of the refrigerant. Open up the condensing unit. Me and my buddy (a more experienced tech) are working together. This was my first actual compressor changeout. So, I start cutting into the discharge line...face right down inside.
What's that sound? fsss...fsss...fsss...? Weird. I am almost all the way through and I have this sensation that says "stop" and let it bleed of. Just a little deminimus. My buddy leans over, with his face right in the condensor, and hits my cutters with a friggin' pipe wrench.
Line blows wide open and erupts a ten to fifteen foot tall volcano-like explosion of saturated refrigerant and oil into his face and the surrounding area. He jumped back and barely got hit. But we had to stand there and go "damn" and watch this happen for about five minutes. Neighbors pointing. Customer watching out the window.
The point of this story "always check to make sure the service valves are open (backseated). The original installers never opened them. When we cut into the line we released the entire (original) factory charge.