AXIAL FLOW VALVES

Pressure reduction with monitor. Duringnormal operation, a single AVF, the worker,performs the pressure cut. The monitoring pilot isset at an output pressure slightly higher than theworking pilot. Since the monitoring pilot is alwaysopen, the monitor AFV is held in the full openposition. If a malfunction occurs on the workerside, the output pressure rises to the monitorsetpoint and it assumes control. The roles ofworker and monitor can be reversed by simplyresetting the pilots.Two-stage regulators with monitor override.Under normal conditions, both AFV’s areperforming pressure cuts. However, they aresized so either one can handle the entire cut in theevent of a malfunction. If a problem occurs in the1st stage, P2 increases and the 2nd stage makesa correspondingly larger cut. If the 2nd stagemalfunctions, P3 increases and the override pilotassumes control of the 1st stage AFV, causing itto take a larger single stage pressure cut. Themaximum interstage pressure, P2, is limited to themaximum spring adjustment of the 1st stage pilot–325 psi for the SZC or 600 psi for the ZSC-320.