| Applies To | |||
| Product(s): | AutoPIPE | ||
| Version(s): | 2004, XM, V8i | ||
| Environment: | N/A | ||
| Area: | Analysis | ||
| Original Author: | Bentley Technical Support Group | ||
| Date Logged & Current Version | Sept. 2015 09.06.02.06 | ||
Problem:
What are the Fluid Transient load case assumptions in AutoPIPE?
Solution:
1. The pipe is assumed to be full of liquid. If system is empty, then as a valve opens / pump starts to introduce fluid into the piping system; adjust the bulk modulus so that the "Speed of sound" calculation equals the speed of fluid.
2. 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:
Surge Pressure = (Fluid Density) * (Fluid Velocity) * (Speed of Sound)
Actual surge pressure might be less for slow valve closure.
3. The maximum fluid transient loading will pass through the piping system once only and no reflections will be considered.
4. When a pump is shutdown, there are two shock waves generated. A positive pressure wave on the suction end and a negative pressure wave on the discharge end are generated. The maximum negative pressure wave is equal in magnitude to the pump static discharge pressure (Ps) less the liquid vapor pressure (Pv). This maximum surge pressure (Ps-Pv) is produced during cavitation (i.e. when the discharge pressure becomes less than the liquid vapor pressure and a vapor cavity is formed). The sudden pressure drop on the discharge end may also cause a backflow, which will create its own water hammer effect when it slams against the idle pump as the cavity collapses. Cavitation and backflow should be avoided by limiting the surge pressure to less than Ps-Pv otherwise AutoPIPE results will become invalid.