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 1.1.1 Venturi Tube/Orifice Plate Comparison. Venturi tubes can measure the
same fluids as orifice plates. Like orfice plates, they can be built to match
the size and shape of irregular piping. Venturi tubes cause less permanent
pressure drop than orifice plates and are used more often where pumping costs
are higher. Venturi tubes are less susceptible to wear from erosive fluids
than orifice plate meters. However, as the size of venturi tubes increases,
their cost increases more than that of orifice plates.
2. LIMITATIONS. The primary disadvantage of using venturi metering is their
limited range. The maximum turndown ratio is limited to 4:1, which is
inherent in the design. A marked change in flow conditions requires a new
venturi configuration. Changing venturis is more expensive than orifice
plates and requires a longer interruption of the system. Other considerations
are as follows:
q Temperature
limit is 1,000F.
q Pressure
limit is 6,000 psig.
3. INSTALLATION. The location of a venturi tube in the system is important.
Whenever possible, it is preferable to locate the venturi tube in a horizontal
line. To ensure accurate flow measurement, the fluid should enter the venturi
tube with a fully developed velocity profile, free from swirls or vortices.
Such a condition is best achieved by the use of adequate lengths of straight
pipe, both preceding and following the primary element. The minimum
recommended lengths of piping are shown in Figure 5-4. The diagram in
Figure 5-4, that corresponds closest to the actual piping arrangement for the
meter location, should be used to determine the required lengths of straight
pipe on the inlet and outlet. These lengths are those necessary to limit
errors due to piping configurations to less than +%0.5 percent. Minimum
recommended distance is dependent upon the ratio of throat-to-inlet diameter.
If minimum distances are not observed, the meter may produce inaccurate data.
4. MAINTENANCE. The following procedures are the minimum required for the
most common types of units. When developing maintenance schedules, refer to
the manufacturer's instructions. Perform the following tasks at the periods
prescribed.
4.1 Quarterly Maintenance.
(a) Flush and clean annular chamber throat and inlet.
(b) Purge trapped air from chamber and connecting pipe.
(c) Clean pressure taps.
(d) Blow down sensor lines on a regular schedule.
(e) Check pressure taps for burrs and/or debris.
5-16
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