04f. Model Vacuum (Negative Pressure) Piping in AutoPIPE

Comment, Questions, and Answers about modeling Vacuum Piping in AutoPIPE

Item #1:

Can AutoPIPE model pipe inside of pipe (i.e. Vacuum Piping or Jacketed Piping) :


See the following WIKI here, #4 Jacketed Piping

Item #2: 

Is AutoPIPE able to analyse a piping system under negative pressure? 


The piping design codes always work in gauge not absolute pressure and so does Autopipe. The code compliance is for mechanical loads plus positive internal pressure i.e. Pipe code stress calculations do not consider negative pressures.

B31.1-2004 Section 104.1.3 refers to ASME BPVC code for such calculations. Pipes can carry very little external pressure compared to internal pressure due to collapse or buckling of pipe wall. Sometimes stiffening rings are added to increase resistance to external pressure. You can still use AutoPIPE for other type of loads.

B31.1: 104.1.3 Straight Pipe Under External Pressure. For determining wall thickness and stiffening requirements for straight pipe under external pressure, the procedures outlined in UG-28, UG-29, and UG-30 of Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code shall be followed. 

B31.3-2006 refers to the same ASME code but with some other limitations under section 304.1.3.

Typically for vacuum systems - the external pressure loading on the piping system needs to be checked independently to a pressure vessel code (e.g  for ASME section VIII div I). But for code compliance - the vacuum system will also have specified some positive internal design pressure (gauge) which would be used in the pipe stress calculations. Then a 2nd temperature case T2 defined with the vacuum pressure (-ve) entered and provided the pressure analysis is turned on under AutoPIPE's Analysis Sets, the P2 case will show displacements and loadings for this case for use in design on supports and equipment. But this case is ignored for stresses.

In conclusion, . Vacuum pipe wall thickness and stiffening stress requirement should be done separately following the applicable code. AutoPIPE cannot perform this analysis nor can it handle external pressure at this time some enhancements have been logged to included these features in a future version of the program. However, AutoPIPE can be used to check other loading conditions in a vacuum system such as pressure contraction, thermal, and occasional loads.

 Item #3:

Would like to know how AutoPIPE handles negative pressure.


Negative pressure is included in the pressure contraction of the pipe which is very important for vacuum pipe in particular FRP piping

However for the sustained stress calculations (long press stress) only maximum +ve pressure from all temperature cases are included.

In AutoPIPE no checks are done for wall buckling due to external pressure or vacuum condition. As mentioned above, negative pressure may affect axial forces and anchor reactions due to pressure expansion/contraction effects. This is reflected in P cases if pressure extension is enabled.

Here is an example of applying a second pressure case in example model WALKTHRU using P1=300 psi and P2=-300 psi

Notice Forces/Moments for P2 case is opposite to P1 case.
Notice that HOOP check gives zero stress for negative pressure P2
Sustained stress for negative pressure is reduced in this case due to reduction in Axial force.

See Also

Modeling Approaches in AutoPIPE

Bentley AutoPIPE