# Tees, Wyes and Crosses

Calculation Error/Model Fails to Converge: If you are confident in your data entry, the reason for non-convergence may be that a cross or tee is outside the experimental range used in the software. Use the List Inspector to select all tees and crosses and replace them with Connectors with No Resistance. If this is successful, then replace the Connectors one-by-one with the original junction until the culprit(s) is/are found. Usually it's one where flow is near zero or where there is symmetrical flow into or out of the branch. Flow through the Tee is unstable: This warning is enunciated due to flow instability in the SAE tee relationship in a given system. Internally, the software will revert to Crane type tee in order to get an approximate solution. It is however important to note that the Crane loss relationships do not take into consideration velocity changes or kinetic energy losses and as such, is not as accurate as the other available tee relationships, i.e., Idelchik or Miller. As a means of cross-checking your model results we would suggest the following: 1.Set the status of all tees to "Ignore Pressure Loss", calculate the model and record the flow results for a selection of pipes. We will then compare the results for the selected pipes when carrying out step 2. 2.Change all tees, firstly to SAE, then Idelchik, Crane and finally, Miller. Calculate the model after each change and record the flow results for the same selection of pipes. You will now have a clear overview of the system flow rates based on each type of tee loss relationship, including the "Ignore Pressure Loss" option. This will allow the user to establish the most accurate relationship and results. You may find when modelling a system with Idelchik or Miller tees that you have a warning indicating that the calculated K value may be outside the allowable range. It is therefore worth checking the calculated K values to ensure they are within the accepted range, i.e., -2 to 10. You can also cross-check these values by comparing to one of the other tee loss relationships. This will help verify that the calculated K values and pressure losses are similar. Note, you can multi-select and change all tees in the model in one action by selecting the "List" tab on the data palette followed by "Junctions" and using the scroll bar, scroll through all junctions until you display the tees. Simply left mouse click on the first tee and, holding the shift key, select the final tee. You will now see all tees have been selected on the flowsheet. Idelchik Tee relationship outside allowable range: This warning applies to tees which have been solved using the Idelchik pressure loss relationship. This relationship takes into consideration different sized pipe connections at the tee junction. When solving Idelchik tees, the calculated K values have a range limitation (approximately -2 to 10). The area of the branch and straight pipes must be greater than the channel pipe area or the straight and channel pipe area must be identical. A key point to note is that the assignment of the branch connection (RED DOT) for each tee junction in your model should be set correctly and in accordance with the expected flow distribution. This will ensure the highest level of accuracy. You may even notice that the warning(s) will disappear once this has been set correctly for each tee junction. There is a further explanation on the assignment of the branch connection in the Help file at: Junctions | Tee and Wye Junctions | Data Entry - Tees and Wyes. You can also view further information on this topic by selecting an Idelchik tee junction on the flowsheet followed by the Nomenclature row on the Input tab of the Data Palette. Note, even though there is a range limitation of -2 to 10 for the K values, the software extrapolates the equations to arrive at an accurate K value for the branch and straight at each tee junction. If the K value is within the expected range, we can safely ignore the warning. If the K value is outside the range, we would recommend setting the tee definition to Miller or Crane and comparing results. If the results across the different pressure loss relationships are similar, you can have confidence in the results you have developed. In general, the Miller or Idelchik relationships are considered most accurate. You may notice you develop this warning when modelling tee junctions using either the Miller or Idelchik relationship since both approaches take into account different sized pipe connections whereas Crane doesn't. So, if you set the relationship for the tee junction(s) to Crane, the warning will not appear. If any of the above is unclear or you require any further information, please contact us at: support@fluidflowinfo.com.