02. When should I perform a Pressure Analysis in AutoPIPE?

Applies To
Area: Analysis
Original Author:Bentley Technical Support Group


When should I perform a Pressure Analysis in AutoPIPE?


Pressure is always part of the Sustained case. In fact the definition of Sustained is Gravity + max Pressure. The reason it doesn't show in the Analysis Sets is because it is always there and cannot be removed. It relates to the component ringed in red below.

Pressure Stiffening and Pressure Extension are completely different from the pressure automatically included in the Sustained case. If you request Pressure Elongation then the P case is added so that it can be calculated. Not all code require it to be considered and even without in added, pressure is always there.

Pressure Stiffening relates to the stiffening effect on Elbows, where the SIF and flexibility will alter.
Pressure extension will add the additional axial stress ringed in blue below and also include pressure in the bending term of the equation.

Example of Code equations with / without Pcase in Sustained:

Sustained Stress includes the Hoop component of Pressure by default.  Result will be different if enabled as a different equation is used as described above. The user should decide to if this option should be enabled / disabled in their model. 

Pressure stiffening is not required by all code and can be complementary to the result.  As an example, ASME NB requires Pressure stiffening and Extension to be included and it is by default.  Later years of B31.3 require Extension but earlier years do not.  Again the user is tasked with deciding if this option should be enabled / disabled in their model. 

It s recommended that a pressure analysis should be performed when some significant strain due to pressure is applied:

a) Systems with expansion joints.

b) Low modulus piping e.g HDPE or FRP or any plastic

c) Large pressure on steel piping systems e.g >500psi

d) Large diameter e.g >=30" diameter with moderate pressure e.g. >30 psi

To include the pressure thrust then you must select “Calculate pressure extension cases” and should also include a pressure stiffening case. This will pump the system with pressure and calculated the pressure thrust and corresponding movement under pressure.

See Also

"Pressure" - AutoPIPE load case

Bentley AutoPIPE