Modeling and Troubleshooting Pressure Regulating Valves
The function of a pressure regulating valve is to lower the pressure in a section of the system to a specific downstream set pressure, regardless of the flow through the connecting piping. When troubleshooting PRV installations, it is helpful to know how PIPE-FLO internally models them.
A schematic of how PIPE-FLO internally models PRVs is shown below:
The following points about PRVs should be noted:
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 the figure). 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).
The direction of flow through the PRV, from high pressure to low pressure, is determined from the direction of the connecting pipelines.
The PRV inlet and outlet pipelines cannot have a tank or pressure demand set on the other end.
When installing a PRV, there are three different conditions under which it can operate: (1) regulating, (2) fully closed, and (3) fully open. The manner in which the valve operates depends on the value entered for the set pressure. The fully closed and fully open positions represent the extreme valve positions. Each valve position is discussed below:
(1) Regulating The valve maintains the downstream pressure to the set value by throttling the flow rate through the PRV.
(2) Fully Closed This situation occurs if the valve set pressure is less than the pressure downstream of the valve for the case where the valve is closed. When this situation occurs in an actual piping system, the flow through the PRV reverses and the valve acts as a check valve, closing the pipeline. In PIPE-FLO, the flow also reverses, however the PRV does not act like a check valve. The pipeline remains open and the PRV results are invalid because the flow is going into the Pressure Node from the downstream pipeline node in order to maintain the set pressure value.
(3) Fully Open This situation occurs if the valve set pressure is greater than the pressure at the valve inlet for the case where the valve is fully open. When this situation occurs in an actual piping system, the PRV maintains a fully open position and it has no effect on the pipeline flow conditions. In PIPE-FLO, the differential pressure across the valve will correspond to a pressure gain rather than a pressure drop. The PRV results are invalid because it is acting as a pump rather than as a pressure control.
If the valve status is listed as invalid, the flow direction should be checked to determine if condition 2 or 3 is occurring. If the flow is opposite the assumed direction, the valve should be closed and the calculations run again. If the flow is in the assumed direction, the PRV setting should be set to fully open and the calculations run again.
NOTE: You can close a valve by clicking on it with the Open/Close tool. This closes both the inlet and outlet pipelines.
In most cases, PRV operation problems can be avoided by first determining the valve's pressure regulating range. This is done by running two lineup calculations which simulate the fully open and fully closed valve positions. These lineups provide the pressure range over which the valve will regulate the flow. For the fully open case, the PRV setting should be set to fully open. For the fully closed case, the valve should be closed. The maximum pressure the PRV can be set at is equal to the pressure at the valve inlet for the fully open case. The minimum pressure the PRV can be set at is equal to the pressure downstream of the valve for the fully closed case.