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Figure 1 below shows a system with four nozzle lines. In the current lineup, the last two nozzles are turned off. This situation is modeled in PIPE-FLO by closing the pipeline upstream of the nozzles. As you can see, the nozzle pipelines and flow demands are colored dark gray after a calculation is performed, indicating that they are isolated from the tank set at the system inlet (there is not an open path from the tank out to this section of the system).
In some situations, areas of a system may be unintentionally isolated. Usually this occurs when controls are installed. Before continuing, we will quickly review how the program internally models Flow Control Valves (FCVs), Pressure Regulating Valves (PRVs), Back Pressure Valves (BPVs), and fixed flow pumps.
Flow Control Valves
PIPE-FLO models an FCV by creating two nodes in the system where the valve is installed (refer to Figure 2 below). A flow demand value equal to the set flow rate is taken out of the first node (the inlet node) and an equal flow demand is set entering the second node (the outlet node).
Pressure Regulating Valves
The set pressure at the outlet of the PRV is entered with the valve. For the PRV model, the program creates two nodes in the system where the PRV is installed (refer to Figure 3 below). A flow demand leaving the system is set at the first node (the inlet node), and a pressure demand equal to the PRV set pressure is set at the second node (the outlet node).
Back Pressure Valves
The set pressure at the upstream side of the BPV is entered with the valve. For the BPV model, the program creates two nodes in the system where the BPV is installed (see Figure 4 below). A pressure demand equal to the BPV set pressure is set at the first node (the inlet node), and a flow demand is set at the second node (the outlet node).
Fixed Flow Pumps
PIPE-FLO models a fixed flow pump by creating two nodes in the system where the pump is installed (refer to Figure 5). A flow demand value equal to the set flow rate is taken out of the first pump node (the suction node) and an equal flow demand is set entering the second node (the discharge node).
As you can see, these devices are modeled using a combination of flow demands and pressure demands. If parts of your system are unexpectedly isolated, you should review where the controls are installed to see if they are possibly causing the problem. To demonstrate how controls can cause unexpected isolations, we will review two situations where the use of controls leads to this problem.
PRV in series with a Fixed Flow Pump
The top system in Figure 6 below shows a PRV installed in series with a fixed flow pump. The bottom system shows how this system is modeled internally by PIPE-FLO. Note that the pipeline between the pump and the PRV is isolated from the tanks (this pipeline is circled in red).
BPV with a Flow Demand Downstream
The top system in Figure 7 below shows a BPV installed in a system with a flow demand set at the outlet. The bottom system shows how this system is modeled internally by PIPE-FLO. Note that the two pipelines downstream of the BPV are isolated from the tank (these pipelines are circled in red).
If parts of your system are isolated and you don't intend them to be, you should first look at the controls and the boundary conditions (pressures and flow demands) that are set in their vicinity. If your system is complex, it is often helpful to print out a FLO-Sheet and then sketch on top of each control how it is modeled internally by the program. You can then more easily see what parts of the system do not have an open path to a tank or pressure demand.
