Can you explain some more about what you are trying to do?
What key and nut is it you are referring to?
OK, thanks for that.
this is something the plumber/installer has put in.
The filling loop is normally 2 valves.
You open both valves and watch the gauge until it's at the right area then close the water valves.
I can only assume the nut is one valve and the other is key operated.
Can you post up a picture of what you have? I may be able to assist further if I can see what it is you have.
It really depends on the valves used.
Lever valves can be turned 90 degrees clockwise or anticlockwise to open.
Screw valves are anti clockwise to open and clockwise to close.
Good luck with it.