Many assume that the lockup of their regulating station will be slightly above the setpoint of their worker regulator (at the worker’s lockup). This is true for stations that use relief valves or slam-shuts as the only form of overpressure protection and it is also true for many stations that use monitors as overpressure protection; however, some monitor configurations lockup at the monitor’s lockup which can surprise operators who were unaware.
Lockup at Worker
Lockup at Monitor
Wide-open Monitor
Direct-operated Regulator
Unloading-style Pilot-op
Upstream monitor
When the monitor pilot bleeds to the intermediate
When the monitor pilot bleeds to the downstream
Downstream monitor
Loading-style (Two-path) Pilot-op
Exceptions exist
Working monitor
Unloading and Loading Style Pilot-op
Besides confusing operators when outlet pressure continues climbing above the expected lockup, this elevated lockup can interfere with alarms for worker failure. Using token reliefs, SCADA, and whistle reliefs as alarms (set between the worker and monitor setpoint) will result in false alerts if the station has zero flow demand. Only travel sensing devices such as proximity switches and travel position monitors can be used in these applications without causing nuisance call-outs.
Why do some monitor configurations lockup above the monitor setpoint? Because the monitor pilot is set at a pressure higher than the worker, it will stay open even after the worker is locked up. If the monitor pilot’s pilot supply comes from upstream and the bleed is piped downstream, it will fill downstream, until the outlet pressure rises high enough for it to lockup.
The schematic below is of unloading-style pilot-ops in an upstream wide-open monitor setup. At zero downstream demand, the downstream pilot closes which closes its main valve. No pressure can now escape from the intermediate to downstream because the pathway through the downstream pilot and main valve are now closed. Still, a path exists going from the upstream pressure, through the upstream pilot, and through the sense/bleed piping to downstream. The upstream pilot set at 450 psi will continue feeding downstream until the outlet pressure rises high enough for it to lockup.
At Lockup:
The below schematic is the same as above except the upstream pilot has separate sense and bleed piping and the bleed line is piped to the intermediate. The upstream pilot will stay open, hoping to get outlet pressure up to its 450 psi setpoint but instead of feeding the downstream, it feeds the intermediate which is isolated from downstream by the closed downstream pilot and main valve.