What is PRV Offset (Droop) and Why Does it Occur?

All pressure reducing valves exhibit a drop in set point pressure (Offset, Droop) as flow through the valve increases. Offset is described by valve manufacturers as a % change in set point pressure as flow changes. This change is illustrated by the test curves in the graph for two different regulator types. Offset is caused a number of physical forces that act on the diaphragm, spring and plug as flow changes.

The amount of set point offset (the degree of slope) is dependent upon a number of factors. Some of these factors are intrinsic to all regulator designs, and cannot be controlled by the user. However, other factors can be controlled by the user, either by regulation selection or installation and operation practices.