Plants frequently rely on differential pressure created by an obstruction in a line to measure flow. Accuracy depends upon two factors: the correctness of the differential pressure measurement obtained via taps upstream and downstream, and the calculation for turning that measurement into a flow rate.
The obstruction placed in the line most often is an orifice plate — a flat plate with a machined orifice. (For more on orifice plates, see: "Remember the Old Reliable Orifice Plate," for other differential-pressure flow metering options, see: "Look Beyond Orifice Plates.") Orifice plates are cheap and reliable. Moreover, orifice plates manufactured to specific dimensions and tolerances generate known pressure drops for a given flow rate. The International Standards Organization (ISO) has summarized the dimensional criteria; all reputable orifice-plate manufacturers meet these standards.
Table 1. Required number of pipe diameters in upstream straight run generally decreases with β value.
Figure 1. Installing a short section of larger diameter pipe would create flow pattern with unknown impact on meter.
If the piping configuration doesn't meet ISO standards, accuracy will suffer. For monitoring unit trends, reduced accuracy may be an acceptable tradeoff for a cheaper meter installation. For high and reliable accuracy, always follow the ISO requirements.
ANDREW SLOLEY is a contributing editor to Chemical Processing. You can e-mail him at [email protected].