Here, pressure taps are located 1 pipe diameter upstream and ½ diameter downstream from the orifice plate (see figure 6.10). They are more affected by upstream disturbances and generally are less reliable than flange or vena contracta taps.
They however, are susceptible to dirt freezing and clogging with hydrates. These openings should be as close as possible to the orifice plate. In this arrangement, openings for the pressure taps are located in the upstream and downstream flanges holding the orifice plate Taps are used to sense the pressure and they are classified based on their location. The pressure drop is at its maximum at the vena contracta.
In case of orifices, the drop in pressure varies from the location of the orifice plate to the downstream side until the flow becomes fully developed. Any restriction in the flow pipe causes a drop of pressure that is, the downstream pressure lowers by an amount equal to the pressure drop due to the restriction.
