# 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. It is recommended that the default settings are retained however, you can change these design limits to reflect specific system design parameters. Note, there are different velocity limits for different fluid phase states, i.e., liquid, gas and two-phase. You can view the settings by selecting the appropriate tab at the top of the "Warnings & Hints" form (Figure 1). Figure 1: Liquid Limits. 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 affected pipes downstream of the node set to "Off or Closed". If you wish to isolate a specific section of a model, it is good modelling 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. 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 modelling 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 displays 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 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. | Finally, 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. 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. Incorrect use of Shell-MIT: This warning message arises if the "Friction Model" for the pipe has been set to Shell-MIT. The reason for this is that this pressure loss relationship is specific for heavy crude oil and heated liquid pipelines and should not be applied to any other flowing conditions. If you are modelling a different fluid type, it is recommended that the "Friction Model" for the pipe is set to "Moody". Incorrect use of FIXED friction factor model: This warning message arises if the "Friction Model" for the pipe has been set to "Fixed Friction Factor (Darcy)". In general, it is not a recommended modelling approach as the friction factor in most cases needs to be calculated. However, if you have a scenario where the friction factor is known based on plant measurements or has been back-calculated, you can use this option and directly define the known value for friction factor. Pressure/Head Units: You can change the pressure/head result units by selecting F9. Note, results can be shown in pressure units (Pa, kPa, etc) or head units such as "m fluid", "m water", "in water", "in fluid", "ft fluid" etc. If you are modeling a system and you have a pressure loss result for example of 70.91 m fluid, the conversion to "m water" is based on the fluid density. So, if the modeled fluid density is 1303 kg/m3 and for simplicity assuming a water density of 1000 kg/m3, the conversion ratio is 1303 kg/m3/1000 kg/m3 = 1.303. Therefore, the result for the modeled fluid in "m water" would be 70.91 * 1.303 = 92.39 m water. If any of the above is unclear or you require any further information, please contact us at: support@fluidflowinfo.com.