E1C10 09/14/2010
13:4:40 Page 450
The lower flow rate limit on vortex meters is at Reynolds numbers (Re based on d in Fig. 10.18)
near 10,000, below which the Strouhal number can vary nonlinearly with flow rate
and shedding becomes unstable. This can be a limitation in metering high-viscosity pipe flows
m > 20 centipoise
ð
Þ . The upper flow bound is limited either by the onset of cavitation in liquids or
by the onset of compressibility effects in gases at Mach numbers exceeding 0.2. Property variations
affect meter performance only indirectly. Density variations affect the strength of the shed vortex,
and this places a lower limit on fluid density, which is based on the sensitivity of the vortex-shedding
detection equipment. Viscosity affects the operating Reynolds number. Otherwise, within bounds,
the meter is insensitive to property variations.
The meter has no moving parts and relatively low pressure losses compared to obstruction
meters. A single meter can operate over a flow range of up to 20:1 above its minimum with a
linearity in K 1 of 0.5%. Because d can be replaced by a constant times d 1 in Equation 10.24, we see
that the strength of the shedding frequency sensed over the pipe area decreases as the pipe diameter
Table 10.1 Shedder Shape and Strouhal Number
Cross Section
Strouhal Number
a
0.16
0.19
0.16
0.15
0.12
a
For Reynolds number Re d ! 10
4 . Strouhal number St ¼ f d=U.
450 Chapter 10 Flow Measurements
13:4:40 Page 450
The lower flow rate limit on vortex meters is at Reynolds numbers (Re based on d in Fig. 10.18)
near 10,000, below which the Strouhal number can vary nonlinearly with flow rate
and shedding becomes unstable. This can be a limitation in metering high-viscosity pipe flows
m > 20 centipoise
ð
Þ . The upper flow bound is limited either by the onset of cavitation in liquids or
by the onset of compressibility effects in gases at Mach numbers exceeding 0.2. Property variations
affect meter performance only indirectly. Density variations affect the strength of the shed vortex,
and this places a lower limit on fluid density, which is based on the sensitivity of the vortex-shedding
detection equipment. Viscosity affects the operating Reynolds number. Otherwise, within bounds,
the meter is insensitive to property variations.
The meter has no moving parts and relatively low pressure losses compared to obstruction
meters. A single meter can operate over a flow range of up to 20:1 above its minimum with a
linearity in K 1 of 0.5%. Because d can be replaced by a constant times d 1 in Equation 10.24, we see
that the strength of the shedding frequency sensed over the pipe area decreases as the pipe diameter
Table 10.1 Shedder Shape and Strouhal Number
Cross Section
Strouhal Number
a
0.16
0.19
0.16
0.15
0.12
a
For Reynolds number Re d ! 10
4 . Strouhal number St ¼ f d=U.
450 Chapter 10 Flow Measurements