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Common Assumptions Made By Other Fluid Flow Programs and The DFS Approach

Many fluid flow analysis programs make questionable or clearly incorrect assumptions when solving problems. Most of these assumptions remain undocumented, or documented where they will not be read by the average users. When questioned, most of these companies will downplay any of these issues by stating that they just don’t matter. We think you should decide for yourself.

Below are some of the common assumptions that can result in significant errors in calculated results, as well as a simple problem for each assumption that one can use to demonstrate the error. In no case has ABZ provided the worst case situation in an attempt to bias the results. All of these errors can be worse or better, depending on the specific details of the problem being analyzed.

 Assumption: Fluid velocity is constant throughout any system. This assumption is mandated by the fact that most other programs ignore the velocity terms of Bernoulli’s equation, thus ignoring conservation of energy. One such program even goes so far as to state that if the user wants to conserve energy, that the velocity component must be manually factored into the system pressures! Impact: The impact of this assumption is that any system which contains tees or crosses (with flow in more than two legs) or reducers or enlargers will be analyzed incorrectly! How to check: Draw a pipeline consisting of two different sizes of pipe (with the appropriate reducer or enlarger between the pipes). Make the sizes very different and the lengths of pipe a reasonable length to make the error larger (such as 36 inch pipe and 1 inch pipe with 10 feet of each size of pipe). Determine the flow rate with a 20 psid with flow in both directions. The flow rates should be very different because of the change in velocity (increases in one case, decreased in the other). DFS Approach: An energy balance is automatically performed across each and every component in the system. The user does not need to perform any separate steps to include the effect of changing velocities.

 Assumption: Component resistances do not depend on flow direction. Most programs do not correctly calculate component resistances unless the flow direction is properly chosen when the problem is specified. Thus, in situations where the flow direction is not known or changes for different conditions, the component resistances will be calculated incorrectly. Impact: The impact of this assumption is that all flowpaths must be drawn in the correct direction initially (and this direction must be known), and anytime a different configuration of running pumps/open valves or known pressures and flows is to be analyzed, any flowpaths where flow direction can change may require the user to respecify those flowpaths. How to check: Draw a pipeline with a size change (reducer or enlarger). Define flow in one direction. Look at the calculated flow resistance of the size change. Now define flow in the other direction. Look at the calculated flow resistance of the size change again. The values should be different. DFS Approach: Each time flow direction is changed (whether during the analysis of a problem or due to the user specifying known flow information) the resistance of each item is reviewed and recalculated if required.

 Assumption: Tees and Crosses have no flow resistance. Most programs use tees and crosses to combine and split flow only, and include no resistance for the tee or cross. If desired by the user, such resistance must be added separately. Of course, the resistance depends on the total flow, the direction of flow for each leg, and the amount of flow in each leg. All of these values must be known prior to determining the resistance of the fitting. Impact: The impact of this assumption is that any problem or flowpath which includes tees will calculate incorrect flow rates or differential pressures unless the user adds the correct resistance for the tee or cross. This generally requires that the user know the flow rates prior to analyzing the problem. How to check: Draw a flow network consisting of a tee with 10 feet of 2 inch pipe connected to each leg. Define the flow at the end of one of the pipes to be zero. Define pressures at the ends of the other two pipes to specify a differential pressure of 20 psid. The differential pressures across the two pipes should not add up to the total differential pressure (since the tee has resistance) if the program includes resistance for the tee. DFS Approach: The resistance of a tee or cross is determined automatically based on the flow direction and flow rate for each leg of the tee or cross. The user does not need to separately add any additional resistance nor to know the flow rates prior to adding a tee or a cross.

 Assumption: Compressible flow analysis can be performed using the Darcy-Weisbach Equation. The Darcy-Weisbach equation for liquid flow assumes that the density of the fluid flowing in the system is constant. By definition, properties (such as density) change for a compressible fluid as pressure changes. For example, a 40 percent change in pressure for air at 200 psia and natural gas at 200 psia results in about a 40 percent change in density. This change results in a calculated flow that is too high if differential pressure is specified, or a calculated differential pressure that is too high if flow is specified. Further, for any differential pressure greater than 10 percent, these programs require that average fluid properties (calculated from both the inlet and outlet properties) be used throughout. This, of course, requires that the properties at both ends be known before the problem is solved, or that the problem be solved multiple times (changing the fluid properties each time), until this condition is matched. For a larger network, this would require that separate fluid properties be calculated and specified at numerous locations throughout the network. Impact: On any single flow path, this assumption can result in a significant error (which is worse for smaller component resistances). In fact, problems with small resistances can even reach sonic conditions prior to a reaching a pressure change of 40 percent. This assumption, together with a larger flow network, can result in significant errors and even errors in flow direction. How to check: Analyze a piece of pipe at a flow rate which results in a pressure drop of 40 percent of the inlet pressure. Compare the inlet and outlet volumetric flow rates. If they are the same, the program is not performing a correct compressible analysis. Since the density changes from inlet to outlet, the volumetric flow rates must change as well. DFS Approach: DFS uses a true compressible analysis, and allows for two heat transfer assumptions (adiabatic and isothermal). Further, DFS is the only program to provide for conservation of energy across tees, reducers and enlargers, and changes in elevation.

 Assumption: Component order within a pipeline has no effect on the calculated results. Most programs allow the user to enter the number of specific valves and fittings that are contained within a pipeline, as well as the presence of a size change or pump. None allow the user to define the order of components within the pipeline. Impact: For liquid systems, the order of components is important when looking at pressures along the pipeline, and to ensure that the proper size component has been specified when the pipeline contains a size change. Ignoring pressures along the pipeline can result in incorrect flow rates being calculated when cavitation occurs (which would not be foreseen if component order is ignored). For compressible systems, in addition to these two reasons, the calculation may be incorrect if an incorrect component order is assumed (depending on the specific hardware being analyzed) since the flow velocity changes as the fluid pressure changes, and thus the head loss across each component is different depending on its location in the pipeline. How to check: Build two pipelines in series, each with 1 foot of NPS 2, schedule 40 pipe. Add an additional resistance with a K factor of 10 to the first pipeline. Analyze the system with a differential pressure of 40 percent of the inlet pressure. Note the differential pressure across the pipe without the added resistance. Now remove the additional resistance from the first pipeline and add it to the second pipeline. Analyze the system again for the same differential pressure. Again note the differential pressure across the pipe without the added resistance. The two noted values should be different. If they are the same, then the program does not consider component order and all calculated values may be incorrect. Alternatively, build a pipeline with 100 feet of NPS 1, schedule 40 pipe oriented vertically with two inline globe valves at the top of the pipe. Analyze this pipeline with atmospheric pressure at both ends. If a flow rate is calculated and no error about cavitation is provided, then the program does not consider component order. DFS Approach: DFS allows the user to input components in their correct order, and analyzes systems on a component by component basis. Thus, component order and its effect on the calculations is always considered automatically.

 Assumption: The user is not interested in differential pressures across individual components and flow rates or velocities within a pipeline. Most programs require that valves and fittings be specified as contained in a given pipeline, but they do not show calculated values across individual components; rather they provide values for the entire pipeline as a single item. In some programs, the user may instead choose to add each fitting or valve as a separate item (independent of any given pipeline). This approach, however, quickly exceeds the capabilities of such programs to display a network with even a normal amount of valves and fittings. Impact: While not generally a calculational issue, the inability to view calculated values across each valve or fitting may make obtaining desired information difficult if not impossible. How to check: Build a pipeline with several valves and fittings. Analyze this pipeline for a given flow rate. Attempt to view the differential pressure across each valve and fitting. DFS Approach: DFS allows the user to view flow rate and pressure information before, after, and across each component within a pipeline. In addition, available printouts include “big picture” graphical printouts which illustrate information on a flowpath level, and “detailed” graphical printouts which provide flow and pressure information for each component within every pipeline.

 Assumption: Negative absolute pressures are a valid result of a flow calculation. May programs show calculated results with pressures less than absolute zero. They may provide a warning, but even if such a warning is provided, it is typically difficult to see. Impact: Calculations can provide nonsensical results that the user is not made aware off (either due to the lack of a warning or error message, or due to the lack of a visual flag that such an error exists). How to check: Build a system with two pipelines in series. Make the first pipeline NPS 2, schedule 40, 100 feet long, with an increase in elevation of 50 feet (from inlet to outlet). Make the second pipe NPS 2, schedule 40, 100 feet long, with a decrease in elevation of 100 feet (from inlet to outlet). Specify atmospheric pressure at both the inlet and outlet. The program should indicate that flashing has occurred (if the program is designed to perform two-phase flow calculations), or should provide a clear indication that an error exists (since the fluid will flash in the middle of the two pipelines and flow will not be single-phase). DFS Approach: For many problems, DFS simply does not consider flows which result in cavitation within liquid systems. Where this is not possible (such as when flow rates have been specified by the user), DFS provides a clear indication of the resulting error condition.

 Assumption: The user always knows whatever flow or pressure information the program is designed to need. Most programs are designed to accept only one type of flow information, such as mass flow rate (e.g., lbm/hr). This makes the job of the programmer easier at the cost of increased difficulty when using the program. Further, certain types of known flow information can rarely be specified by the user, such as volumetric flow rate relative to standard conditions (e.g., scfm) and velocity (e.g., fps). Impact: When information is known by the user in units other than that accepted by the program, the user must convert the known information into whatever the program demands. In the case of a compressible problem, this is difficult if not impossible (since these conversions generally depend on the fluid state which is not known until the problem has been solved). How to check: Add a simple pipeline with any hardware and attempt to specify flow information. Observe what types of information the program allows (most demand mass flow rate; a few also allow volumetric flow rate). Look specifically for standard volumetric flow rate (scfm) and velocity (fps). DFS Approach: DFS allows the user to specify flow information as mass flow rate, volumetric flow rate (relative to flowing or standard conditions), or velocity. This information can be entered in any of the unit types supported by the program (54 types for flow information alone; more can be added by the user).

 Assumption: Flow velocities higher than sonic velocity can be calculated with any geometry fitting. With the exception of some very specific fittings designed to obtain supersonic flows, the flow rate of all fluids is limited to the velocity of sound of that fluid at that temperature. This limit can change throughout a system as the fluid temperature changes. Impact: Calculating flows that are physically not possible provides no useful information to the user; rather, unless he somehow figures out that a physical limit has been violated he may unknowingly use the incorrect flow rates. How to check: Add a pipeline with 5 feet of NPS 4, schedule 40 pipe. Define the fluid at the inlet of this pipe to be Air at 60 degrees Fahrenheit and 200 psia. Define the outlet pressure to be 120 psia (a 40 percent drop in pressure). The program should indicate clearly that this pressure drop cannot be obtained due to sonic flow limitations. DFS Approach: DFS checks for sonic flow limitations for each and every component. DFS also addresses the limitations in flow rate associated with component reduced flow areas. Further, DFS considers limitations associated with heat transfer when analyzing compressible isothermal flow.

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