Thanks for your reply,
I am checking a 66" and 70" pipe under vacuum condition ( design pressure -0.21 barg), internal design pressure is 0.7 barg. to prevent from buckling and ovalization, i am calculating the required minimum thickness under external pressure and the required stiffening rings, using BPVC section VIII div 1, UG28,UG29 and UG30.
My concern is the external pressure, I assume is 0.79 barg, am I correct?
thanks again for your help.
Yani