INTRODUCTION

The orifice plate flowmeter, the most common of the differential pressure (DP) flowmeter family, is also the most common industrial flowmeter. It is apparently simple to construct, being made of a metal plate with an orifice that is inserted between flanges with pressure tappings formed in the wall of the pipe. It has a great weight of experience to confirm its operation. However, it is far more difficult to construct than appears at first sight, and the flow through the instrument is complex.

Some key features of the geometry of the orifice and of the flow through it are shown in Figure 5.1. The behavior of the orifice plate may be predicted, but the predictions derive from experimental observation and data. The inlet flow will usually be turbulent and will approach the orifice plate where an upstream pressure tapping (one diameter before the orifice plate) will measure the pressure at the wall, that is in this case the static pressure. (Flange and corner tappings will be discussed later). The pressure across the pipe will have a constant time-mean value because the only velocity across the pipe is due to turbulent eddies. The flow close to the orifice plate will converge toward the orifice hole, possibly causing a recirculation vortex around the outside corner of the wall and orifice plate.