FluidFlow
TROUBLESHOOTING HANDBOOK ŠFlite Software 2016
Flite Software N.I. Ltd, Block E, Balliniska Business Park, Springtown Rd, Derry, BT48 0LY, N. Ireland. T: 44 (0) 2871 279227 | E: sales@fluidflowinfo.com | www.fluidflowinfo.com
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Introduction ................................................................................................................................................... 2
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Modeling Tips................................................................................................................................................. 3
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Model Troubleshooting ................................................................................................................................. 4
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Conclusion .................................................................................................................................................... 11
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1 Introduction The purpose of this document is to provide new users with some hints and tips to help with the approach to modeling your systems. The document also discusses some typical warnings messages and offers suggestions for troubleshooting issues arising with your new models. If you cannot find any helpful information in this document regarding your specific modeling issue, please do not hesitate to contact us at: support@fluidflowinfo.com. Please attach a copy of your model together with any supporting information including drawings, design documentation and sketches. This will help us interpret your design and your query. Testimonials: “I know this software well but occasionally, I forget where to fill in the inputs and then I turn to you guys to get the support. And I am really glad that your team is so fast in response. This is just wonderful and I never experienced this with any other software providers. I truly appreciate this�. Jatinderpal Singh Mander, Process Engineer.
"Once again, I cannot begin to tell you the appreciation I have for your detailed answers and effort in my assistance in understanding FluidFlow". Gilbert Martinez, Mechanical Engineer .
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2 Modeling Tips When developing a model, it is recommended we adopt the following approach; 1. Build the model in sections using a step-by-step approach. It is prudent to develop the model in sections, testing each section as you build the model as this will help eliminate any fundamental errors. If each section converges successfully, the final model should also converge without any difficulty. If one of your smaller sub-models will not converge, it is unlikely your final larger model will converge. 2. Keep the flowsheet as simple as possible. 3. Keep connector pipes as short as possible. The more compact the model the easier to navigate. 4. Use multiple components wherever possible. For instance a pipe containing several elbows may be simplified by setting the “Quantity� of the elbow element to represent more than one elbow. You may use this approach for liquid systems, however, it is recommended that this approach is not used when modelling gas systems due to the changes in gas density which has a knock on effect on calculated pressure loss, thus effecting accuracy. 5. When creating a new model, always ensure the base Calculation Settings are correct and have not been changed by another user for a specific modelling application. It is also prudent to check the Warnings & Hints settings to ensure it matches your design criteria. 6. When developing a model which has tees with three different diameter pipe connections, there is no need to add a reducer to each connection as the software will automatically detect this scenario and include in the calculations. Note, users have the option of adjusting calculation convergence criteria. However, unless you have a valid reason for doing so it is not recommended that these settings be adjusted. If any of the above is unclear or you require any further information, please contact us at: support@fluidflowinfo.com
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3 Model Troubleshooting This section of the handbook takes us through some typical warning or error messages we may come across when we first begin to use the program. In reviewing the warning, a cause and solution approach is outlined. Element
Warning Message
Pipes
High & Low Velocity Warning: High & low velocity warnings are enunciated based on the Min. and Max. limits set under Options | Warnings & Hints (see Figure 1 below). If the calculated pipeline velocity exceeds these set limits, a warning will be enunciated to that effect. The user can change these design limits to reflect specific system design parameters.
Figure 1: Warnings & Hints.
Pipes
Closed Pipe: This warning can arise when an accumulator/vessel node has been used as the model inlet and the branch connection has been defined above the liquid level of the vessel. Since the liquid cannot flow out of the vessel, the software will enunciate a "Closed Pipe" warning. A "Closed Pipe" warning can also arise when the Status of a pipe (or component) in a large model has been set to "Off or Closed" resulting in flow being isolated from a number of downstream pipes. The software will therefore automatically close the effected pipes downstream of the node set to "Off or Closed". If you wish to isolate a specific section of a model, it is good modeling practice to highlight all nodes within the relevant part of the model and set the Status of all nodes to "Off or Closed". This will also assist model convergence.
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Pipes
Static Pressure below fluid vapor pressure or negative: This message is often indicative of a data-entry issue and every step should be taken to correct this issue in order to obtain more accurate results. This warning can occur for a number of reasons as follows; 1. When a data-entry error/omission has occurred when entering node elevations. If we have a scenario where two nodes at different elevations are connected by a single pipe which is of insufficient length, this warning will be enunciated along with Pipe Length < Elevation Difference warnings. The recommended approach is to correct the pipe length/node elevation data-entry and recalculate the model. You will notice that the Pipe Length < Elevation Difference warnings will now be eliminated and the "Static Pressure below fluid vapor pressure or negative" may also be eliminated unless there are other issues with the model. If the latter warning remains, please refer to the items outlined below. 2. When modeling piping flow systems, the continuity of energy must be upheld. As such, if the fluid velocity and associated velocity pressure (kinetic energy) in a pipeline or element has increased considerably, the static pressure may solve to a negative value. When this scenario arises, FluidFlow enunciates the warning; "Static Pressure below fluid vapor pressure or negative". This scenario is often encountered in gas systems. You may need to review the system design conditions (pipe diameters, design pressures and flow rates) in order to eliminate this warning message. 3. This warning can also occur in liquid systems where the user is attempting to develop too great a flow rate in a given system. It is recommended that the design flow rate is reduced to a level which allows the model to converge and reach an acceptable design solution. 4. This warning can occasionally be enunciated in large-scale systems which have been developed exclusively with Known Pressure nodes. When we model a system using this approach, the software will automatically determine the flow rates throughout the system. If however the design flow rate is known at any given inlet or outlet boundary in the system, try switching one Known Pressure node to a Known Flow node, enter the relevant design flow rate and recalculate to refresh the results. The system pressures and flow rates will automatically be calculated, providing more accurate results and the warning may be eliminated. Note, this methodology is more applicable to larger systems. 5. This warning may also be enunciated in a system if we have an operating scenario where a pipeline is not running full-bore with fluid, i.e. in vertical downstream lines with little or no back pressure and the fluid runs down the side walls of the pipe with vapor being
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sucked inwards to the pipe centerline (column separation). This tends to occur in systems with extreme differences in elevation or where there is discontinuity in the system elevations (see note 1 above). A starting point is to check the node elevations and pipe lengths and recalculate the model once any corrections have been made. If the warning remains, try adding an orifice plate to the end of the vertical pipeline in question, adjusting the orifice size until such times as we develop enough back pressure and the warning is eliminated. 6. This condition can also arise if we have defined pipes with a length of zero (0 M). 7. This warning may occur if you have modeled a PD pump in series with a Known Flow node. By using these two nodes in series we are over constraining the model. The calculated flow rate developed by the PD pump is a function of engineering conditions (pipe diameter, length etc.) and pump speed and cannot be manipulated by designating a Known Flow node. It is therefore best to model this scenario with known pressure nodes at the system boundaries and adjust the pump speed to reach the desired flow rate in the pipelines. Care should be taken to eliminate this warning in order to obtain accurate and meaningful results. Pipes
Flow Regime is in the critical Region: This warning message arises if the flow in the pipe is between the laminar and turbulent region, i.e. the flow is in the unstable/transition or critical region.
Pumps
Booster Affinity Law Corrected: Arises when design input for pump speed or impeller diameter has been changed. FluidFlow allows users to model the effect of changes in pump speed and impeller diameter in a given piping system (provided you have defined a Min. and Max. speed and impeller size in the database). If we model this scenario, the pump performance data is adjusted according to the affinity laws.
Pumps
It is important to note that the affinity laws only hold accuracy over a range of maybe +/- 20% to the original curve parameters. The software will enunciate a warning when pump speed or impeller size has been changed to make the engineer aware of this operating condition. It is then down to engineering judgment to accept that you are operating close enough to the original curve. If the system requires higher levels of accuracy, it is recommended that the user approaches the pump manufacturer and obtains a new capacity curve based on the exact speed and impeller size you wish to model. Pump is Viscosity Law Corrected: Performance curves for centrifugal pumps are generally based on testing with clean water. The pump performance therefore needs to be corrected if the fluid being pumped has a viscosity of 4.3 cSt or higher. The reason for this correction is the pressure loss in the impeller and diffuser channels, the impeller friction and internal leakage losses depend to a large extent on the viscosity of the pumped liquid. Consequently, characteristics ascertained for water lose their validity when pumping liquid of higher viscosities, such as oil. In
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general, the higher the viscosity of the liquid compared to water, the greater the loss of delivery capacity and head for a given power input. In general, the resulting de-rating of the pump head and flowrate may be relatively modest though the de-rating of the efficiency can be significant with consequences for motor size, switchgear and cabling if not taken into account. FluidFlow will perform viscosity correction based on the Hydraulic Institute Standards. Note, the correction applies only to centrifugal pumps delivering Newtonian fluids. This automatic correction will only occur if the option has been enabled from the Global Settings tab which can be accessed by selecting; Options | Calculation | Global Settings or alternatively, select "F2" & Global Settings (see Figure 2 below).
Figure 2: Global Settings â&#x20AC;&#x201C; Viscosity Correction.
Unexpected Pump Results: If we have not defined the capacity curve for a centrifugal pump across the full quadrant and the calculated duty point for the pump is beyond the curve data available, we may achieve unexpected operating results for the pump. Figure 3 shows the capacity curve for a sample centrifugal pump. As we can see, the capacity curve is not defined across the full quadrant, i.e. the maximum curve co-ordinate is 2816.8 usgpm at 96.14 m fluid. If we were to model this pump in a system and the duty point was beyond this maximum co-ordinate value (e.g. 3100 usgpm), we may get unexpected results. This is simply due to the fact that the software needs to extrapolate the curve relationship data and provide an estimated value.
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Figure 3: Pump Capacity Curve (Not Defined Across the Full Quadrant).
End suction centrifugal pump curves supplied by manufacturers rarely extend far to the right hand side of the quadrant. It is therefore recommended that users make an estimate for the final curve co-ordinate. The estimate you make can be guessed (this is OK because we will never operate the pump anywhere near to this condition). When you make the guess at head of zero, ensure that you do not "upset" the shape of the curve for the valid data points by selecting a value that lies on a smooth curve. If you do not make this guess you run the risk of using the pump in a system that is difficult to solve (converge). Figure 4 shows how an additional row has been added where we have made an estimate of the flow at 0 head. As you can see, the curve still intersects the coordinates and is an accurate representation of the manufacturer's data. The Max Limit is retained as 2816.8 usgpm. If we solve this pump in a system and the duty point is beyond the Max Limit of 2816.8 usgpm, we will get sensible results as the curve is fully defined, however, we will also receive a warning message to that effect. This informs us that we are operating outside the range for the pump based on available manufacturer's data. We would therefore need to investigate the pump performance further with the vendor or select an alternative pump model. Further information on defining pump curves in the Database is available in the Helpfile at: Databases | Boosters.
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Figure 4: Pump Capacity Curve Defined Across Full Quadrant.
Pumps
Booster is operating in reverse flow direction: This warning arises if the pressure downstream of a pump is greater than the maximum pressure the pump can deliver. Select a larger pump or alternatively, reduce the downstream pressure, increase the pump speed or fit a larger impeller in an attempt to increase the pump capacity.
Pumps
Static Pressure below fluid vapor pressure or negative: This warning may occur if you have modeled a PD pump in series with a Known Flow node. By using these two nodes in series we are over constraining the model. The calculated flow rate developed by the PD pump is a function of engineering conditions (pipe diameter, length etc) and pump speed and cannot be manipulated by designating a Known Flow node. It is therefore best to model this scenario with known pressure nodes at the system boundaries and adjust the pump speed to reach the desired flow rate. Calculation Error/Model Fails to Converge: Modeling multiple flow control valves in series can occasionally cause convergence errors due to too the system becoming over-constrained.
Flow Control Valves
It is recommended that users give careful consideration to the positioning of flow controllers and the controlled flow rate. Design Note: It is recommended that flow control valves are not modeled in series with known/fixed flow boundary nodes or centrifugal pumps which are set to Automatically Size as this leads to duplication of defined system flow rates which may or over-constrain the model.
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Known or Assigned Flow
Design flow rate set to Zero on Input Tab: If you have a Known Flow node defined in a system with a flow rate of zero, it is recommended that the status of this node is set to "Off or Closed" as this will help model convergence. It is also recommended that a Known Flow node is not modeled directly in series with a relief valve, flow control valve or a centrifugal pump which is set to Auto Size on design flow rate, as this will potentially over-constrain the system or duplicate an already defined flow rate. Try switching the Known Flow node for a Known Pressure boundary node.
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4 Conclusion This document attempts to address some of the warning messages which we may obtain when we build a model for the first time. The steps outlined herein should help new users troubleshoot models and refine the approach to take to modeling your systems. If any points documented are unclear or if you have a specific issue which you would like to discuss with us, we would welcome the opportunity to review your model in detail. Please forward a copy of your FluidFlow model file to our office together with any supporting information and/or drawings. We are always available to help. Our contact details are outlined below:
support@fluidflowinfo.com Flite Software NI Ltd Block E Balliniska Business Park Springtown Road Derry Northern Ireland BT48 0LY T: +44 2871 279 227 F: +44 2871 279 806 www.fluidflowinfo.com
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