XTS Special Conditions
(XTS Module Only) Some junctions have Special Conditions that alter the normal state of the junction.
The junctions that have special conditions set are shown using a special symbol before the ID number (an “X” by default). This symbol can be customized in the User Options window.
Special Conditions with No Transient Data
If a Special Condition is applied to a junction without any transient data, then the transient solution will retain the Special Condition. For instance, if a Special Condition is set for a valve junction, the steady- state solution will solve with the valve closed. In addition, the transient solution will run the entire simulation with the valve closed as well because there is no transient data entered which implies the valve state is not intended to change.
Special Conditions with Transient Data
If a Special Condition is applied to a junction that has transient data, then the transient solution will ignore the Special Condition. It is then incumbent upon the engineer to relate the Special Condition to the initial transient data.
For instance, assume the user wants to simulate the transient that occurs during a valve opening. The steady-state solution will have the valve closed, and this is modeled by using a Special Condition for the valve junction. In such a case, the Cv data entered on the valve junction's Loss Model tab will be ignored.
On the Transient tab, a transient can be entered. The first data in the table should then be Cv = 0, which corresponds to a closed valve. From there the Cv can be increased above zero as desired to open the valve.
If the user were to keep the Cv equal to zero, then that would be equivalent to the previous case where the user set the Special Condition but did not enter any transient data.
Pump Special Conditions
Pump Special Conditions are slightly more complicated than for valves. In fact, pump junctions have two types of Special Conditions. The first is to turn the pump off and have no flow through it. This is called “Pump Off No Flow”. The second type of Special Condition turns the pump off but allows flow to go through the pump. This is called “Pump Off With Flow Through”.
Why would one want to use one Special Condition rather than the other? The simple answer is that the first Special Condition is more appropriate for positive displacement type pumps and the second for centrifugal pumps.
Here's why. When a positive displacement pump is turned off and has a pressure difference across it, it will usually act like a closed valve and not allow flow to go through it. Thus the first Special Condition would be most appropriate. For instance, assume one wants to model the transient that occurs during the startup of a positive displacement pump. The best way to do this would be to use the first Special Condition with no flow, and then input a flowrate transient in which the first data point is zero flow.
On the other hand, when a centrifugal pump is turned off and has a pressure difference across it, in the absence of other valving which prevents flow the pump will usually allow flow to go through it. Thus the Special Condition that allows through flow would be most appropriate. For example, assume one wants to model the transient during the startup of a centrifugal pump. One would use the second Special Condition, and, in conjunction with a pump curve entered at 100% speed, input a speed transient with the initial speed as zero.
Transient Special Conditions
Junctions which accept transient data also support Transient Special Conditions. The default for all junctions is None. The other choice is Ignore Transient Data. This allows one to have no transient initiation at that junction without having to delete the transient data.
When a Transient Special Condition is set to Ignore, a # symbol is displayed adjacent to the junction number on the Workspace. This symbol can be customized in the User Options window.
