Enter the external pressure, which is the pressure on the convex side of the head and the shell side pressure for heat exchanger floating heads. Normally, you may enter both the shell side and the tube side pressures and evaluate the entire head in a single analysis. However, when analyzing a type d head, the interaction between shell side and tube side pressure may result in a lower thickness than if each pressure is entered separately. Consequently, it is recommended that you run the software twice, first with the internal pressure and then the external pressure set to zero (0).