Common 12 – Back Pressure Valves
Back Pressure Valves are used to maintain the pressure in the upper parts of a system.
The pressure at node N3 in this system would be less than atmospheric pressure (about -10 psig), if the Back Pressure Valve was not included in the system.
Negative pressure at 'high' elevations in a closed pipe system can be problematic. In addition, if the Demand Flow of 1000 US gpm at node N3 needs to be at some minimum pressure, then this can be achieved by including a BPV at an appropriate point in the down pipe following this node.
A BPV has been used at the start of pipe P4 to maintain a pressure of 20 psig at node N5, which in turn maintains a higher pressure at node N3.
The BPV was not placed directly at Node 3 since this causes the BPV outlet pressure to fall below atmospheric pressure, and in some cases may require a loss that cannot be achieved, where the BPV outlet pressure needs to be below absolute zero [below 0 psi.a], which is not possible.
NOTE: The BPV cannot add pressure. If the BPV pressure setting is higher than the maximum pressure that can occur at the prior node the system will not be solved.