Chemical Engineering February 2014 - 50

Feature Report
conditioner and no shield on the flow
sensor likewise have low pressure
drops. These two configurations are
why TD mass flowmeters are considered
to be in the class of flowmeters
with low pressure drops.
As with most kinds of flowmeters,
the performance of TD mass
flowmeters can be degraded if the
flowmeter is installed where flow
conditions are different than those
for which it was flow-calibrated.
Improper installation is the single
biggest cause of measurement inaccuracy
for any kind of flowmeter.
Components in the piping system
upstream and, to a far lesser extent,
downstream of the flowmeter
can create non-uniformities in the
flow profile, swirls and turbulence.
All of these phenomena degrade
performance. Such flow-disturbing
components include single and
multiple elbows, expansions, contractions,
tees, valves and pumps.
Fortunately, viscous forces in a sufficiently
long length of straight pipe
upstream and downstream of the
flowmeter reduce swirl and drive
the flow toward a fully developed
velocity profile.
Table 2 shows the straight pipe
length requirements for both inline
and insertion TD mass flowmeters
and, for purposes of comparison,
an orifice-plate flowmeter
with a 0.7 beta ratio (ratio of the
orifice diameter to the pipe internal
diameter). The in-line flowmeter in
Table 2 has a built-in flow conditioner
consisting of two upstream
separated perforated plates. Figure
7 shows the upstream straightpipe
requirements downstream
of a single elbow for three kinds
of flowmeters. This shows the
marked contrast between the one
pipe diameter length required for
the TD mass flowmeter with the
built-in flow conditioner versus
the ten and twenty-eight diameter
lengths required for typical vortex
and orifice-plate flowmeters, respectively.
Since piping systems in
the industrial process-control field
seldom have suitably long straight
piping runs preceding the desired
location for flowmeter installation,
Table 2 and Figure 7 reveal the
Thermal
D
1 x D
Vortex
D
10 x D
Orifice plate,
beta = 0.7
D
28 x D
Figure 7. Shown here is a comparison of
the upstream straight pipe requirements for
three kinds of flowmeters located downstream
of a single elbow. The thermal dispersion mass
flowmeter has a built-in flow conditioner consisting
of two separated perforated plates
7
3
2
4
3
T P
5
4
11
P T
10
9
T P
1
6
5
8
Figure 8. A setup for a pressurized closed-loop gas-flow calibration system is
presented here. The numbered components are as follows: 1 is the gas charging
source; 2 is the flow source; 3 is the flow control element; 4 is a flow-conditioning
section for a flow calibration standard (two shown); 5 is an in-line flow calibration
standard (two shown); 6 is the pressure relief element; 7 is the vent to the outside
environment or to a scrubber or other gas purifying device; 8 is the vacuum pump
for evacuating the gas charge; 9 is the heating section; 10 is the flow-conditioning
section for the flowmeter under test; and 11 is the flowmeter under test
installation advantage in-line TD
mass flowmeters with the built-in
flow conditioner have over alternative
flowmeters. In essence, in-line
flowmeters with the built-in flow
conditioner trade the advantage
of greater accuracy for a small
amount of pressure drop.
Normally, the transmitter is
mounted directly on the flow body
or probe. In cases where the ambient
temperature at the pipe line
exceeds the specified limit for the
transmitter (usually, approximately
60°C), then the transmitter
must be located remotely. Additionally,
in some cases the application
requires that the transmitter be
located remotely for easier access.
Since the wires leading to the sen50
ChemiCal engineering www.Che.Com february 2014
sors are part of each sensor's electrical
circuit, remote location can
cause measurement errors if the
cable length is altered in the field
from that for which it was flow calibrated.
The four-temperature microprocessor-based
system avoids
this problem by incorporating highimpedance
voltage sensing wires in
the flowmeter's cable (see Figure
3) that essentially make remote
transmitter location independent
of cable length.
Some applications require that
the flow in the process line not be
interrupted. This case is solved by
employing an insertion flowmeter
installed in the pipe with hottap
hardware. The hot-tap method
and assembly provides an isolation
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Chemical Engineering February 2014

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