5.2 Meter Position. The meter must be positioned so that pipe approaching
and exiting the meter are full of process fluid under all operating conditions.
5.3 Meter Location. Location of the ultrasonic flowmeter in the system is
important. Whenever possible, it is preferable to locate the primary element
in a horizontal line. To ensure accurate flow measurement, fluid should enter
the primary element with a fully developed velocity profile, free from swirls
or vortices. Such a condition is best achieved by use of adequate lengths of
straight pipe, both preceding and following the primary element. The minimum
recommended lengths of piping are shown in Figure 6-5. The diagram in Figure
6-5 that corresponds closest to the piping arrangement for the meter location
should be used to determine required lengths of straight pipe on the inlet and
outlet. These lengths are necessary to limit piping configuration errors to
less than 0.5%. If minimum distances are not observed, flow equations and
resultant flow calculations may produce inaccurate data.
6. ACCURACY AND RELIABILITY. Both types of ultrasonic flowmeters have an
accuracy of +5.0% of the upper range value. New electronics and more
efficient designs have improved meter reliability. Ultrasonic flowmeters are
available with self-diagnosing features that include the following:
Cable and circuit failures.
7. MAINTENANCE . Maintenance on these obstructionless instruments is
small. At least twice a year, remove and inspect the pressure sensors of the
secondary element. Where there is a possibility of coatings accumulating,
periodic cleaning is necessary because the coating will cause refraction
angles to change. Also, sonic energy is absorbed by the coatings and renders
the meter inoperative. Data transmission sensors and processors should be
checked and diagnosed every six months for correct input and output.