What are the assumptions for a fluid transient analysis or slug flow in AutoPIPE?
1. AutoPIPE fluid transient utility is valid for pipes that are full all the time. Another words the pipe is assumed to be full of liquid. If system is empty, 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. Or, when water is filling the pipe, the load can be modeled as a long slug with assume flow velocity.
2. When modeling a fluid transient, all fluids except "STEAM" and "GAS", will be assumed to be in liquid form. The calculations require the fluid’s Bulk Modulus and Specific Gravity. They are used to calculate the speed of sound at which the pressure or shock wave travels in a liquid fluid medium (see calculation in online help). AutoPIPE provide a 9 of the most popular fluids in a library with matching data (bulk modulus and specific gravity). If a fluid is not listed, select NS and enter the respective data. Recommend contacting a manufacture for reference data, Bentley does not have any additional reference outside of the library data provided. 3. 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.
4. The maximum fluid transient loading will pass through the piping system once only and no reflections will be considered. 5. 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. 6. Example below from the AutoPIPE training given:
...Piping Code: ASME B31.4 ...Pressure : 1.034 N/MM^2 ...Temperature = 93.3 C ...Slug Length = 3.05 m ...Fluid Density = 800.92 Kg/m^3 ...Fluid Velocity = 15.24 m/sec ...Fluid Diameter = 304.8 (12 inches STD Pipe Wall) ...Fluid Area = 0.072966 m^2 ...Rise time for Elbow or 90-deg Bends will be calculated using formula = Tr = (R.theta)/V where : R = Radius of Elbow, Theta = Angle of Elbow and V = Fluid/slug velocity.
Why in that example the rise time for 90 deg bend become 0.047 sec?
...R = 1.5*NPS = 1.5*12" = 1.5 ft ...Theta = 90 deg = Pi/2 = 1.57 radians ...Velocity = 15.24 m/sec = 15.24/0.3048 = 50 ft/sec The example is based on English units and then was converted to SI, hence the confusion.
You can calculate in SI units as follows:
...R= 0.458 m ...Theta = 1.57 ...V = 15.24 m/sec ...Tr = 0.458 m *1.57/(15.24 m/s) = 0.047 sec
Dynamic "Time History" Analysis