Wide-open monitors are very common as overpressure protection in gas distribution systems but many are improperly sized due to a widely used but imprecise rule-of-thumb.
Wide-Open Monitor Sizing
Due to the tortuous path through the regulator, the monitor does create some pressure loss, decreasing the available differential pressure for the worker and consequently decreasing its flow capability. At low flows, the pressure loss will be minimal while at high flow rates, it will be larger. Impact is typically minor in applications with large differentials between inlet of the station and outlet but can be substantial in small differential applications.
Historically, monitors have been sized using a rule-of-thumb stating that installing a monitor reduces the worker’s flow capacity by 30%. In many cases this imprecise rule leads to oversized stations. There are some low pressure differential applications where this rule-of-thumb actually undersizes the station. The best way of sizing monitors is to first determine the maximum pressure drop across the monitor. Either the below formula can be used or a sizing program capable of solving for pressure drop. Once the worst-case pressure drop across the monitor is known, the worker can be sized as a stand-alone regulator using the available pressure differential to determine if the selected regulator will satisfy the flow requirement.
For an upstream monitor, use the following equation:
ΔP = pressure drop across the regulator, psi
P1 = absolute inlet pressure, psia (P1 gauge +14.7)
Q = maximum gas flow rate, SCFH
Cg = regulating gas sizing coefficient
G = specific gravity of the gas
T = absolute temperature of gas at inlet, °Rankine
C1 = flow coefficient
For downstream monitors, the same equation can be used to determine the intermediate pressure by solving via iteration. In this case, P1 is the intermediate pressure instead of inlet pressure and ΔP is the difference between intermediate pressure and outlet pressure.