Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: Dynamic Analysis Subarea: Original Author: Bentley Technical Support Group Comments, Questions, and Answers on Dynamic Analysis Time History tab: Note: See additional information on Time history analysis on the following WIKI pages: "Fluid Transient" - AutoPIPE Load Case Water Hammer (Time History) Example - Modeling Approach in AutoPIPE "Time History" Results in AutoPIPE Item #1: Question 1: What are the assumptions for a fluid transient analysis or slug flow in AutoPIPE? Answer: 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 Item #2: Question: After entering data into Load Time History Location dialog screen, and press the apply button the Scale column automatically updates to 90.000 for all points, pressing apply again, all values for DX are updated to 90.000, pressing apply more times eventually updates all the values to 90.000 in the dialog. Why? Answer: Close the model and confirm that both the Time history load name (*.THL) and Time History file (*.TIH) meet the file naming requirements. Copied from the online help: The following file types which typically do not have the system name as a prefix, will continue to be limited to 8 characters. Load data files (*.SPC, *.THL, *.TIH, *.HMF) Unit files (*.UNT, *.UNI) Library files (*.LIB) The following set of characters are not allowed in the model file name 1. / \ : " * ? | 2. Non-Ascii characters (ANSI character codes less than or equal to 31). In addition, blank spaces are NOT allowed in these file names. After fixing the file names, r eopen the model and reassign the time history file in Load Time history location dialog screen. After making these changes, notice that hitting the " Apply " button does not change the values in the dialog screen as had happened before. Item #3 Question: What information is available on modeling Slug Flow in AutoPIPE? Answer: select the following hyper link: Modeling Slug flow in AutoPIPE Item #4: Question, what value would I enter into the Time history dialog screen for "Duration"? Answer: Please see the following AutoPIPE help section: Help Contents Contents Tab Modeling Approaches Modeling Approaches Water Hammer (Time History) Example Executing Water Hammer Example (apham1). Highly suggested as one of your first time history analysis that you read the entire section on this example and actually perform the example as explained in the online help, the file is located at C:\Bentley\AutoPIPE V8i SELECTseries\ Example\Apham1.dat. After performing this example you will understand clearly how to calculate the time history duration. For those time sensitive people, as mentioned in step #11 of the Executing Water hammer (time history) example document, "Duration" = 0.443 (First period, 0.384 sec, plus water hammer duration 0.059 sec), These values can be found : a: Water hammer duration value 0.059 sec comes from the THL file: B. First period 0.384 sec comes from the Frequency output report Under some circumstances, this duration time may need to be increased to ensure to capture all the time needed for the transient and 1st period to occur. The results should not change if a longer duration than needed is specified. Item #5: For the Time History dynamic analysis, is there anyway to run more than 4 file names at a time? I have broken the analysis into segments and have more than 4. Answer: I apologize for the inconvenience, however this is a limitation of this dialog that only 4 time history files can be run at one time. Add additional cases as needed for a separate analysis with 4 other files. Please keep in mind that a time history path can span more that one segment: Example: Walkthru.dat, time history from A00 to B04: : Or on a straight line with multiple segments: A. Insert Fluid transient as shown below: B. Press OK button to see Fluid transient routing highlighted on the model, shown below: Item #6: Given the above scenario where I have 4 transient files specified for 1 case, what values should I use to calculate the duration in the time history analysis? If I am only able to run the first four events after the valve closure, should I use the entire duration + mode 1 period or the duration that has occurred in the 4 events? Answer: Yes, the duration should include the total time of the transient + first mode. Should easily located by the highest value found in 1 of 4 *.THL files; under "TIME AT END OF FLUID TRANSIENT :" + "time of the first period from Frequency report" as outlined in Item #4 above. See Also Bentley AutoPIPE External Links Bentley Technical Support KnowledgeBase Bentley LEARN Server Comments or Corrections? Bentley's Technical Support Group requests that you please submit any comments you have on this Wiki article to the "Comments" area below. THANK YOU!
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