| Applies To | |||
| Product(s): | AutoPIPE, | ||
| Version(s): | ALL | ||
| Area: | Analysis | ||
| Original Author: | Bentley Technical Support Group |
Problem:
What I notice in running a fluid transient analysis, as described in the Example tutorial mentioned above, that extending the rise time, these loads / displacements are reduced, however, the surge pressure remains constant. I assume that for pressure rise alone, AutoPIPE does not account for the slow valve closure. BUT, for pipe displacements / loads at anchors bends etc. AutoPIPE does account for this slow closure time?? This is fundamental in my analysis so need to be sure I am on the right track . I see extending the time on this rise input massively reduces forces, but the pressure is the same, please confirm.
Solution:
Correct, that the Joukowski Pressure (psi) and Joukowski Forces (lbs) are the same value(s) for varying Rise Times.
As mentioned in AutoPIPE's online help:
All fluid transient events such as valve closure, pump shutdown, etc. will be considered as instantaneous which produce maximum surge pressures as given by the Joukowski formula.
When the Rise Time is less than 2L/a, the maximum surge pressure is estimated from Joukowski formula and is independent of Rise Time (2L/a is the time for sound to travel from valve to tank and back, L is the distance from valve to tank and a is the speed of sound).
When the Rise Time is greater than the 2L/a time, would reduce the maximum surge pressure. Also, the calculated pressure rise in AutoPIPE might be conservative. For this special case, the use of a fluid simulation software is recommended to estimate the actual surge pressure and potential cavitation in the system if P2 case is critical. Textbooks with software are available for this purpose. "Fluid Transients in Systems, by Wylie E.B and Streeter V.L" can be used for this hydraulic analysis
As stated above, AutoPIPE only calculates the Maximum Surge pressure at a single time stamp, Rise Time = instant. This is a limitation. As mentioned above, actual surge pressure might be less for slow valve closure. In order to calculate the actual maximum surge pressure in a fluid transient with a Rise Time greater than 2L/a would require another source. However , this is a conservative approach.
Example of varying force results:
Typical procedure, select Load> Fluid Transient> enter values on the "Fluid Transient" dialog, and then press the OK button. AutoPIPE performs some calculations and displays a time history force location file (*.THL) (as seen above). Furthermore, the program creates as many force function files (*. TIH) as necessary (i.e. at the origin of the shock wave, every bend, kink (> 5deg.), pipe diameter change (>15%) and a change in segment name. All of these files are saved to the same folder location as the model.
To show the relationship between the Rise Time and force applied with the force function files(*.TIH); As the "Rise Time" decreases from 1.0 sec to 0.001 sec, notice how the peak force increases from 36.02 psi to 25870.62 psi. However, notice in each TIH file the Joukowski Pressure (psi) and Joukowski Forces (lbs) are the same value(s). This is because the "Rise Time" value is not considered in their calculations; as mentioned in the online help: "all fluid transient events such as valve closure, pump shutdown, etc. will be considered as instantaneous which produce maximum surge pressures as given by the Joukowski formula".
The following image show how the Time history force functions (*.TIH) result at the same node point change by only varying the Fluid Transient Rise Time values:
For more detailed information, recommend reviewing the "Water Hammer (Time History) Example" mentioned at the top of this WIKI page, see "Calculations: Water Hammer" document for details on the calculation performed.