Working Principles of Downstream Regulators
Performance : We can plot the performance of an ideal regulator such that no matter how the Flow consumption / demand changes, regulator will match that Flow consumption / demand (within it’s capacity limits) with no change in the downstream controlled pressure (p2). This straight line performance becomes the ideal against which one can measure the performance of an actual - real regulator.
Set Point : The constant pressure desired at regulator outlet is represented by the set point. But no regulator is ideal. The downward sloping line on the diagram represents pressure (p2) plotted as a function of flow for an actual direct operated regulator. The set point is determined by the initial compression of the regulator spring.
Flow Capacity : Flow Capacities published by regulator manufacturers are given for different amounts of droop.
Droop : Offset and proportional band are terms used to describe the phenomenon of p2 drooping below set point as flow increases. Droop is the amount of deviation from set point at a given flow, expressed as a percentage of set point. This “Droop” curve is important to a user because it indicates regulating (useful) capacity.
Accuracy : The Accuracy of a regulator is determined by the amount of flow it can pass for a given amount of droop. The closer the regulator is to the ideal regulator curve (set point), the more accurate it is.
Lock Up : Lockup is the pressure above set point that is required to shut off the regulator tight. Some extra pressure p2 is required to force the soft disc into the metallic seat to make a tight seal. The amount of extra pressure required is lockup pressure.
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