(e) Test dp transmitter with dead weight pressure tester and rescale if
(f) Dress off roughness on plate.
(g) Resize orifice, if necessary, based on flow of previous year. When
orifice is resized, stamp the new diameter and coefficient on the holder.
(h) Flush all trapped sediment from unit.
(i) Reinstall orifice plate so that flow exits the bevelled side.
(j) Sensor lines should be blown down at regular intervals.
Since inspection of an orifice plate requires shutdown, scheduling may be
predicated by system supply requirements.
8. ACCURACY AND RELIABILITY. Orifice meter accuracy is up to %0.75 percent
of full scale. The major influence on accuracy is installation, where care
must be taken to ensure proper installation of the run, pressure taps, and the
tap tubing. Meter runs are highly reliable when used over the range of
calibration. If removed for cleaning or inspection, they should be
recalibrated before returning to service.
9. PRESSURE TAPS AND INDICATION DEVICES. A variety of pressure tap
configurations are available for orifice plates. Various devices are used to
quantitatively express the differential pressure. Each of the common tap
options and dp devices are described in the following paragraphs.
9.1 Pressure Taps. The common tap options are:
flange taps, corner taps,
and radius taps.
9.1.1 Flange Taps. The most commonly recommended configuration is the flange
tap (Figure 5-4). Pretapped flanges are standardized, convenient, and easily
9.1.2 Corner Taps. Carrier rings are drilled for pressure taps and insertion
between existing flanges (Figure 5-5). This type of flange should only be
used if the flange taps are unavailable. Standardization of equipment should
be the goal of a well-planned system.
9.1.3 Radius and Vena Contracta Pipe Taps. These taps are mounted directly
in the pipe (Figure 5-6). Although widely used in the past, they have been
largely replaced with standardized flange taps.
9.2 Differential Pressure Devices. Dp devices are used to provide a
quantitative display of the differential pressure; they are also called
delta P and ∆ P devices. The four most common dp devices are: manometer,
diaphragm, bellows, and electronic. All new or replacement differential
pressure installations should be electronic.