Modeling and Troubleshooting a Back Pressure Valve
by Engineered Software, Inc.
Back Pressure Valves are installed in systems to keep a section of the network above a minimum set pressure. A BPV maintains the set pressure on the high pressure or upstream side of the valve and only allows enough flow through the valve to maintain the upstream BPV set pressure. When troubleshooting BPV installations, it is helpful to know how PIPE-FLO® internally models them.
A schematic of how PIPE-FLO® internally models BPVs is shown below: The following points about BPVs should be noted:
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 (refer to the figure). 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).
The direction of flow through the BPV, from high pressure to low pressure, is determined from the direction of the connecting pipelines.
The BPV inlet and outlet pipelines cannot have a tank or pressure demand set on the other end.
When installing a BPV in a pipeline, 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 upstream pressure to the set value by regulating the flow rate through the BPV.
(2) Fully Closed This situation occurs if the valve set pressure is greater than the pressure upstream of the valve for the case where the valve is closed. When this situation occurs in an actual piping system, the flow through the BPV reverses and the valve acts as a check valve, closing the pipeline. In PIPE-FLO®, the flow also reverses, however the BPV does not act like a check valve. The pipeline remains open and the BPV results are invalid because flow is going from the Pressure Node to the pipeline upstream node.
(3) Fully Open This situation occurs if the valve set pressure is less than the pressure at the valve outlet for the case where the valve is fully open. When this situation occurs in an actual piping system, the BPV 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 BPV 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 BPV 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, BPV 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 BPV setting should be set to fully open. For the fully closed case, the valve should be closed. The maximum pressure the BPV can be set at is equal to the pressure upstream of the valve for the fully closed case. The minimum pressure the BPV can be set at is equal to the pressure at the valve outlet for the fully open case.