pipe under both internal and external pressure

hi folks,

I have to design a pipe carrying 4000 psi having an circular opening into another pipe covering/surrounding the internal pipe..

i referred Machine Design data book by K. Lingaiah and got this formula

tangential stress : =F3=E8 =3D a + b/r2

radial stress : =F3r =3D a - b/r2

where,

a =3D ( pi * di^2 - po * do^2) / ( do^2 - di^2)

b =3D { di^2 * do^2( pi - po) }/ { 4r^2( do^2 - di^2)}

pi =3D internal pressure po =3D external pr. di =3D inner dia do =3D external dia. r =3D any radial distance

now, if external and internal pressure are equal.. tangential and radial stress will be equal to -ve pressure value..

is that ok? .. that i can have any inner dia and thickness (if material's permissible yield strength is more than applied pr.)

pls help me with that..

regards, Pranav