Dear all

I am a beginner in Pressure Vessel design. Here, I was studying the detailed WRC Calculations. There are 3 notations which confuses me.
Pm - General Primary memberane stress
Pl - Local primary memberane stresses
Q - Secondary stresses.

I got to understand from WRC calculations that
Pl = Algebraic sum of memberane stresses
Q = Algebraic sum of bending stresses.

Now, I am in a confused state how to calculate Pm.

Kindly help me.

Thanks to all in advance

Regards
Ethanga