Some orifice plates employ non-square-edged holes for the purpose of improving performance at low Reynolds number values, where the effects of fluid viscosity are more apparent. These orifice plate types employ rounded- or conical-entrance holes to minimize the effects of fluid viscosity. Experiments have shown that decreased Reynolds number causes the flow stream to not contract as much when traveling through an orifice, thus limiting fluid acceleration, and decreasing the amount of differential pressure produced by the orifice plate. However, experiments have also shown that decreased Reynolds number in a venturi-type flow element causes an increase in differential pressure because of friction against the entrance cone walls. By manufacturing an orifice plate in such a way that the hole exhibits “venturi-like” properties (i.e., a dull edge where the fast-moving fluid stream has more contact with the plate), these two effects tend to cancel each other, resulting in an orifice plate that maintains consistent accuracy at lower flow rates and/or higher viscosities than the simple square-edged orifice.
Two common non-square-edge orifice plate designs are the quadrant-edge and conical-entrance orifices. The quadrant-edge is shown first:
The conical-entrance orifice plate looks like a beveled square-edge orifice plate installed backward, with flow entering the conical side and exiting the square-edged side:
Here, is it vitally important to pay attention to the paddle’s text label! This is the only sure indication of which direction an orifice plate needs to be installed. One can easily imagine an instrument technician mistaking a conical-entrance orifice plate for a square-edged, beveled orifice plate and installing it backward!
Instrumentation Basics 