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) :
Answer:
Yes, see the following WIKI here, #4 Jacketed Piping
Item #2:
Is AutoPIPE able to analyze a piping system under -ve (negative) pressure?
Answer:
Most piping design codes take into account gauge pressure rather than absolute pressure. The same applies to AutoPIPE, which requires the input of gauge pressure. Compliance with the code pertains to mechanical loads in addition to positive internal pressure; that is, pipe code stress calculations do not factor in negative pressures:
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- 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.
- 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.
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For vacuum piping systems, external pressure effects on the pipe must be evaluated separately in accordance with the applicable pressure vessel code (for example, ASME Section VIII, Division 1). AutoPIPE does not currently perform external pressure or buckling checks for vacuum conditions.
For piping code compliance within AutoPIPE, the vacuum system is typically assigned a small positive internal design pressure (gauge). This pressure is used in the pipe stress calculations to satisfy the piping code requirements.
In addition, add another temperature/pressure case (ex. T2/P2) may be defined with the vacuum pressure entered as a -ve value. Provided that "Calculate pressure extension cases, e.g. P1, P2, etc" is enabled in AutoPIPE’s Analysis Sets, this case will generate displacements, reactions, and load effects. These results can be used for the design and evaluation of pipe supports, restraints, and connected equipment. However, this vacuum pressure case is excluded from stress code compliance checks and should not contribute to code stress calculations.
In summary, pipe wall thickness adequacy and any required stiffening for vacuum (external pressure) conditions must be evaluated separately in accordance with the governing design code. AutoPIPE cannot perform this type of external pressure analysis at this time, although enhancement requests have been logged for future versions. Nevertheless, AutoPIPE remains a useful tool for evaluating other loading effects in vacuum systems, including pressure-induced contraction, thermal expansion, weight, and occasional loads.
Item #3:
How does AutoPIPE handle -ve pressure?
Answer:
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, -ve pressure may affect axial forces and anchor reactions due to pressure expansion/contraction effects. This is reflected in P cases if "Calculate pressure extension cases, e.g. P1, P2, etc" options is enabled.
Here is an example of applying a second pressure case in AutoPIPE's example model - WALKTHRU: P1=300 psi and P2=-300 psi
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- Notice Forces/Moments for P2 case is opposite to P1 case.
- Notice that HOOP check gives zero stress for -ve pressure P2
- Sustained stress for -ve pressure is reduced in this case due to reduction in Axial force.
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See Also
Modeling Approaches in AutoPIPE