PRDesign™ ThermoFlash™ 2-ø Flow™


Frequently Asked Questions
About PRDesign™


Question  Answer  Discussion
Can PRDesign™ be used to design and rate single phase and two-phase relief systems? Is PRDesign™ consistent with API and DIERS methods for designing relief systems? Yes For single phase relieving fluids, PRDesign™ follows the recommended practice given in API Recommended Practice 520, Sizing, Selection, and Installation of Pressure Relieving Devices in Refineries, Part I, "Sizing and Selection", 6th edition, 1993; Part II, "Installation", 4th edition, 1994; American Petroleum Institute, Washington, D.C.
For two-phase relieving fluids, PRDesign™ uses the homogeneous equilibrium model (HEM) approach as given by Fisher, H. G. et al., Emergency Relief System Design Using DIERS Technology, The Design Institute for Emergency Relief Systems of the American Institute of Chemical Engineers, New York, 1992.
Can I override pure component physical property parameters for specific chemical components in PRDesign™? Yes PRDesign™ automatically retrieves physical property data from the DIPPR and ASPENPCD pure component property data banks. These values can be viewed and modified by the PRDesign™ user as desired.
Can I override binary interaction parameters in PRDesign™? Yes PRDesign™ performs rigorous vapor-liquid equilibrium calculations during the relief system calculations. Depending upon the selected thermodynamic methods, these computations use various binary interaction parameters. The binary interaction parameters can be edited by PRDesign™ users.
Are pressure losses due to relief system inlet and vent piping taken into account by PRDesign™? Yes Pressure drop calculations are performed for the inlet piping, vent piping and any associated fittings. Bernoulli's Equation is solved for single and two-phase fluid flow. Physical properties are rigorously computed along the pipe flow path.
Can I use proprietary physical property data in PRDesign™? Yes Pure component and binary parameters can be retrieved from proprietary corporate data banks. Contact us for implementation details.
Can I add my own thermodynamic methods to PRDesign™? Yes Custom thermodynamic methods can be easily added to PRDesign™ to model relieving fluid behavior.
Does PRDesign™ perform relief scenario analysis? No PRDesign™ performs relief system design and rating calculations. The relief system design calculations size the relief device and associated inlet and vent piping given a required relieving flow rate as determined by a relief scenario analysis. The relief system rating calculations determine the achievable flow rate through a specified relief device and associated inlet and vent piping. The relief scenario analysis used to specify the design requirements is not done by PRDesign™.
Are choke flow conditions taken into account by PRDesign™? Yes Vapor and two-phase relieving fluids can encounter choke flow conditions in the relief system. PRDesign™ calculations allow for choke flow at the orifice of a relief valve and at expansions in the relief system piping.



Frequently Asked Questions
About ThermoFlash™


Question  Answer  Discussion
Where does ThermoFlash™ obtain physical property parameters?   ThermoFlash™ automatically retrieves pure component physical property parameter data from the DIPPR and ASPENPCD pure component data banks. These data banks contain more than 1,500 chemical components. These property parameter values can be viewed and modified by the ThermoFlash™ user as desired.
Can I override pure component property parameters for specific chemical components in ThermoFlash™? Yes Pure component physical property parameter values can be viewed and edited using the Pure Component Parameters form. Note: data bank values can be restored easily.
Can I override binary pair interaction parameters in ThermoFlash™? Yes ThermoFlash™ performs rigorous vapor-liquid equilibrium calculations. Depending upon the selected thermodynamic methods, these computations use various binary interaction parameters. The binary interaction parameters can be edited by ThermoFlash™ users. The most typical case is to enter proprietary activity coefficient correlation parameters (e.g. Wilson or UNIQUAC parameters) for a two-phase fluid where vapor-liquid equilibrium is modeled with activity coefficients.
Can I use proprietary physical property data in ThermoFlash™? Yes Physical property data can be entered for individual components by entering numerical values (as noted in questions 2 and 3 above). For large amounts of data (e.g. entire data banks, etc.), contact us about incorporating your proprietary data into ThermoFlash™ automatically.
Can I add my own thermodynamic methods to ThermoFlash™? Yes Custom thermodynamic methods can be added to ThermoFlash™, contact us.
Do flash calculations ever fail? What can be done to recover from a flash failure? Yes Occasionally flash calculations fail. In the event of a flash failure, please check your input specifications to make sure you have specified physically reasonable flash conditions. If you know that your fluid is one phase, you may specify this in the Define Conditions tab. If you know that your results are in the two-phase region, you may also specify this information in the Define Conditions tab. If you still encounter flash failures, please contact P&P.
Why do some of my property results not look smooth in the graphical results?   ThermoFlash™ provides graphs for you property calculations results. Depending on your initial and final flash conditions, you may be traversing a two or three phase region during your calculations. In this case, the vapor, liquid (and possibly second liquid) phases are changing composition from the initial to the final flash conditions. Therefore, there is a composition effect as well as temperature (and possibly pressure) effect on the calculated physical properties. Also, where phases appear or disappear, there can be a discontinuous first derivative in the property graphs. If you have any questions about these situations, please contact P&P.
How are liquid phases handled in the equilibrium and property calculations in ThermoFlash™?   By default, ThermoFlash™ uses a flash algorithm that allows for the presence of a vapor and two liquid phases. This algorithm is suitable for general use and also correctly handles the less general cases of vapor only, liquid only, vapor and one liquid, and two liquids. When only one liquid is present, results for this liquid are placed in the liquid-1 phase. If two liquids are present, results for the second liquid are placed in the liquid-2 phase. If you have any questions about this, please contact P&P.
What kinds of calculation documentation does ThermoFlash™ produce?   ThermoFlash™ produces a complete record of the calculation case definition and results. This is done through several different reports: Brief Summary, Results Summary, Property Tables, and Graphs. All of the reports are available in english, metric, or SI units.
Can I generate custom reports? Yes ThermoFlash™ allows property tables and graphs to be saved to external files. These files can be used as desired to produce custom reports (e.g. from third-party packages such as Microsoft Access, Microsoft Excel, Microsoft Word, other spreadsheets, and other word processing software).



Frequently Asked Questions
About 2-ø Flow™

  Two-Phase Flow

Question Answer
How does 2-ø Flow™ handle fluid physical properties? Currently, 2-ø Flow™ uses fluid physical property values as input by the user. These values are assumed to be constant for the calculations of a given pipe. Long pipes should be divided into smaller lengths so that accurate fluid properties can be used for each pipe segment. As a rough guide, the total pressure drop for a given pipe should not be greater than 10% of the inlet pressure.
How are pipe diameters specified? 2-ø Flow™ uses pipe table information relating nominal diameter, schedule, and pipe inside diameter for commercial steel pipe. If this table is not applicable for your application, choose "Not Specified" for the pipe schedule and then enter the actual pipe inside diameter in the diameter input field for a pipe.
How does 2-ø Flow™ treat losses due to fittings? 2-ø Flow™ uses standard equivalent lengths for valves and fittings from Crane Technical Paper 410. For other fittings or losses not included shown in the input fields, you can specify either (or both) an equivalent length (L/D) for additional losses or a number of velocity heads (fL/D) for additional losses.
Are the boundaries between flow types on flow maps sharp or spread out? The boundaries between flow types shown on flow type maps are actually not as sharp as a simple graphic line would indicate. This is because of two reasons: (1) for empirically based flow map correlations, there is scatter in the data used to construct the flow map; and (2) some flow transitions are gradual, e.g. for the boundary between Annular Dispersed flow and Intermittent flow, it is difficult to distinguish between highly aerated slug flow (Intermittent) and annular flow with large roll waves.
What information does 2-ø Flow™ provide to help understand how close to a transition boundary the flow type is for a pipe? 2-ø Flow™ computes the flow pattern based on flow rates, physical properties, etc. using the user specified flow regime method. However, since the transition between flow types may be gradual (not sharp, see immediately preceding FAQ question and answer), the computed flow pattern should be compared with the location of the flow pattern transition boundary on a flow map. 2-ø Flow™ gives the transition boundary points in the "Calculation Messages" report. This information should be considered to gain insight into how close to your flow is to a transition boundary.
In some cases, 2-ø Flow™ changes the user-specified flow regime correlation or the pressure drop method selection. Why? 2-ø Flow™ does not allow a flow regime correlation or pressure drop method to be used for situations where the method is not applicable. If you inadvertently specify a flow regime correlation or pressure drop method that does not apply for a given pipe orientation, 2-ø Flow™ overrides the user-specified method. For example, consider the case where you specify that the Mandhane et al. flow regime map be used for a pipe that is inclined at an angle of 5 degrees upward. Since the Mandhane flow map is not applicable for pipes with this much inclination, the program overrides this selection and actually uses the Taitel and Dukler flow regime correlation for the calculations. Please examine the results reports to see the methods actually used during calculations.
In some cases, 2-ø Flow™ changes the user-specified pipe length (and possibly elevation, for inclined pipes). Why? If the available pressure drop is not enough to force the specified flow rate through the specified length of pipe, 2-ø Flow™ resets the pipe length to a small value and continues the calculations. This is done to try to obtain some useful information about pressure drop per unit length for the specifed flow rate. In these cases, the "Calculation Results" report sets the Warning level to "High" and prints a message. The "Line Summary" report prints the length used in the calculations. The "Calculation Messages" report also gives messages. Please check your results carefully. The typical solution for this situation in rating calculations is to specify a larger diameter pipe or a smaller flow rate.
Are there any iterative calculations in 2-ø Flow™? If so, can these iterative calculations fail? What can be done in case of failure? There are iterative calculations in 2-ø Flow™. The most noticeable iterative calculation is performed during Design calculations that determine a pipe diameter that will pass a specified flow rate through a given length of pipe that meets a specified pressure drop criterion. The most common type of convergence failure for this situation is encountered when the assumed pipe diameter during iterative calculations becomes too small. To remedy this, simply perform Rating calculations with a couple of trial pipe diameters. Then, use one of these diameters as the initial estimate of pipe diameter in the Design calculation case.


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