Enter the system internal design pressure (P). WRC 107/537 only analyzes internal pressure and the value must be positive. The pressure stress equations used are:
Longitudinal Stress = Pressure * ri2 / (ro2 - ri2)
Hoop Stress = 2.0 * Longitudinal Stress
For the spherical case, the membrane stress due to internal pressure uses the Lamé equation to calculate the stress at both the upper and lower surfaces of the vessel at the edge of the attachment.