flow and level measurement - Omega Engineering

Because they are very sensitive to the
velocity profile of the flowing stream,
they must be profiled at several
points across the flow path.
Insertion turbine meters can be
designed for gas applications (small,
lightweight rotor) or for liquid (larger
rotor, water-lubricated bearings).
They are often used in large diameter
pipelines where it would be costprohibitive
to install a full size meter.
They can be hot-tapped into existing
Concentric
Cone
Flow
Straightener
Alternative Flow
Straightening Vanes
Bundle of Tubes
Element
Figure 3-10: Flow Straighteners Reduce Straight Pipe Runs
pipelines (6 in or larger) through a
valving system without shutting
down the process. Typical accuracy
of an insertion turbine meter is 1% FS,
and the minimum flow velocity is
about 0.2 ft/sec.
• Turbine Meter Accuracy
Figure 3-9 shows a typical turbinemeter
calibration curve describing
the relationship between flow and
K-factor (pulses/gallon). The accuracy
of turbine meters is typically
given in percentage of actual rate (%
AR). This particular meter has a linearity
tolerance band of ±0.25%
over a 10:1 flow range and a ±0.15%
linearity in a 6:1 range. The repeata-
bility is from ±0.2% to ±0.02% over
the linear range.
Because there are minor inconsistencies
in the manufacturing
process, all turbine flowmeters are
calibrated prior to shipment. The
resulting K-factor in pulses per volume
unit will vary within the stated
linearity specification. It is possible,
however, to register several K-factors
for different portions of the flow
range and to electronically switch
10 X D
2.5 D 5 X D
D Bore
Dia.
Nominal Size
D Inches
from one to the other as the measured
flow changes. Naturally, the Kfactor
is applicable only to the fluid
for which the meter was calibrated.
Barstock turbine meters typically
are linear to ±0.25% AR over a 10:1
flow range. The linearity of larger
meters is ±0.5% AR over a 10:1 flow
range. Turbine meters have a typical
nonlinearity (the turbine meter
hump, shown in Figure 3-9) in the
lower 25-30% of their range. Keeping
the minimum flow reading above this
region will permit linearity to within
0.15% on small and 0.25% on larger
turbine meters. If the range of 10:1 is
insufficient, some turbine flowmeters
can provide up to 100:1 turn-
Coil Protection Box
3 Mechanical Flowmeters
downs if accuracy is de-rated to 1%
of full scale (FS).
• Sizing & Selection
Turbine meters should be sized so
that the expected average flow is
between 60% and 75% of the maximum
capacity of the meter. If the pipe
is oversized (with flow velocity under
1 ft/sec), one should select a Halleffect
pick-up and use a meter smaller
than the line size. Flow velocities
Concentric
Cone
under 1 ft/sec can be insufficient,
while velocities in excess of 10 ft/sec
can result in excessive wear. Most turbine
meters are designed for maximum
velocities of 30 ft/sec.
Turbine flowmeters should be
sized for between 3 and 5 psid pressure
drop at maximum flow. Because
pressure drop increases with the
square of flow rate, reducing the
meter to the next smaller size will
raise the pressure drop considerably.
Viscosity affects the accuracy and
linearity of turbine meters. It is therefore
important to calibrate the meter
for the specific fluid it is intended to
measure. Repeatability is generally not
greatly affected by changes in viscosity,
TRANSACTIONS Volume 4 43
Flow
Meter and Straightener
Connections
Radial Vane
Element
5 X D