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About batch control
Vortex meters are not usually recommended for batching or other intermittent flow applications. This is because the dribble flow-rate
setting of the batching station can fall below the meter's minimum Reynolds number limit. The smaller the total batch, the more significant
the resulting error is likely to be.
Low-pressure, low-velocity Gas Applications & Pressure Loss
Low pressure (low density) gases do not produce a strong enough pressure pulse, especially if fluid
velocities are low. Therefore, it is likely that in such services the rangeability of the meter will be
poor and low flows will not be measurable. On the other hand, if reduced rangeability is acceptable and
the meter is correctly sized for normal flow, the vortex flowmeter can still be considered.
The permanent pressure loss through a vortex meter is about half that of an orifice plate, roughly two velocity
heads. (A velocity head is defined as V2/g, where V is the flow velocity and g is the gravitational constant in
consistent units.) If the pipe and meter are properly sized and of the same size, the pressure drop is likely to
be only a few psi. However, downsizing (installing a smaller-than-line-size meter) in order to increase the
Reynolds can increase the head loss to more than 10 psi. One should also make sure that the vena contracta pressure
does not drop below the vapor pressure of the process fluid, because that would cause cavitation. Naturally, if the back-pressure on the meter is below the vapor pressure, the process fluid will flash and the meter reading will not be meaningful.
Measuring Slurries and Sludges
If the process fluid tends to coat or build-up on the bluff body, as in sludge and slurry service, this will eventually change the meter's K factor. Vortex-shedding flowmeters are not recommended for such applications. If, however, a dirty fluid has only moderate amounts of non-coating solids, the application is likely to be acceptable. This was demonstrated by a 2-year test on a limestone slurry. At the end of the test, the K factor was found to have changed only 0.3% from the original factory calibration, although the bluff body and flowtube were badly scarred and pitted.
Multiphase Flow Measurement
When measuring multi-phase flow (solid particles in gas or liquid; gas bubbles in liquid; liquid droplets in gas), vortex meter accuracy will drop because of the meter's inability to differentiate between the phases. Wet, low-quality steam is one such application: the liquid phase should be homogeneously dispersed within the steam, and vertical flow lines should be avoided to prevent slugging. When the pipe is horizontal, the liquid phase is likely to travel on the bottom of the pipe, and therefore the inner area of the pipe should be kept open at the bottom. This can be achieved by installing the bluff body horizontally. Measurement inaccuracy in such applications is about 5% of actual flow, but with good repeatability.
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Determine the Right Size
When installing a vortex flowmeter in an existing process where the flow range is not known, it is recommended
to first make some approximate measurements (using portable pitot or clamp-on ultrasonic devices). Otherwise,
there is no guarantee that a line-size vortex meter will work at all.
Straight Pipe Requirement
The vortex meter requires a well-developed and symmetrical flow velocity profile, free from any distortions or
swirls. This necessitates the use of straight up- and downstream piping to condition the flow. The straight
length of pipe must be the same size as the meter and its length should be about the same as required for an
orifice installation with a beta ratio of 0.7. Most vortex flowmeter manufacturers recommend a minimum of 30
pipe diameters downstream of control valves, and 3 to 4 pipe diameters between the meter and downstream pressure
taps. Temperature elements should be small and located 5 to 6 diameters downstream.
About half of all vortex meter installations require the "necking down" of oversized process piping by
concentric reducers and expanders. Even if flow straighteners are installed, some straight (relaxation)
piping will still be required.
Full-Pipe Requirement
Vortex meters can be installed vertically, horizontally, or at any angle, as long as they are kept flooded.
The meter can be kept flooded by installing it in a vertical upward flow line. When installing the flowmeter
in a downward or horizontal flow, the downstream piping should be kept elevated. Check valves can be used to
keep the piping full of liquid when there is no flow. Block and bypass valves are required if the replacement
of the sensor in the particular design requires the stopping of the flow and the opening up of the process.
Precautions on the Flange Connection
Mating flanges (on the schedule 40 or schedule 80 mating piping) must have the same diameter and smooth bore
as the flowmeter. Weld neck flanges are preferred, and reducing flanges should not be used. The inner surface
of the mating pipe should be free from mill scale, pits, holes, reaming scores and bumps for a distance of 4
diameters upstream and 2 diameters downstream of the meter. The bores of the meter, the gaskets and the adjacent
piping must be carefully aligned to eliminate any obstructions or steps.
Pipe Vibration
Excessive pipe vibration can be eliminated by supporting the piping on both sides of the meter, or by rotating
the meter so that the sensor is moved out of the plane of the vibration. Process noise due to valve chattering,
steam traps, or pumps can result in high readings or non-zero readings under zero-flow conditions. Most meter
electronics allow for increasing the noise filter settings, but increased noise reduction usually also decreases
the low-flow sensitivity of the meter. One option is to relocate the meter to a less noisy part of the process.
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