POWER September 2011 - 59

WATER TREATMENT
manufacturer will specify the maximum
distance between the probe and the monitor
to minimize that interference, particularly
with conductivity meters.
With an integrated sensor, all the signal
processing is done on a microprocessor
located inside the probe. The signal leaving
the probe is a digital signal that can be
sent as far as required without significant
signal loss or interference. Furthermore,
because the calibration information is also
on the chip, the probe can be calibrated in
the controlled setting of a laboratory or
maintenance shop, disconnected, and then
installed in the field. Once installed, the
probe is ready to go.
Besides pH, instrument manufacturers
are also making integrated conductivity,
oxidation reduction potential, and
dissolved oxygen probes (see " How to
Measure Corrosion Processes Faster and
More Accurately, " May 2009). Some of
these sensors are " smart " enough to monitor
themselves and predict the need for
maintenance. For example, Mettler-Toledo
Thornton's Intelligent Sensor Management
product line stores the calibration
data collected in the lab on a chip in the
sensor. This approach ensures that the latest
calibration data for the field-mounted
instrument is always used.
Another advantage of integrated sensors
has been an expanded range of linear
operation. A single model of an integrated
conductivity probe can be used for everything
from the condensate to the cooling
tower. The user no longer has to stock different
probes for different applications nor
has to make sure that the cell constant and
conductivity range match that application.
Automatic Calibration. More online
instruments are incorporating a completely
automatic calibration sequence using vendor-supplied
standards. This is especially
worthwhile where the calibration process
requires an extended period for equilibration
to low concentrations such as with
sodium and silica measurements. The calibration
sequence can be initiated by an internal
timer. The analyzer then stores this
information for later retrieval. For these
instruments, service consists mainly of
replenishing calibration standard solutions
and reagents.
Wireless Data Transmission. Wireless
capability is available in all manner
of devices, so why not a pH meter or sodium
analyzer? As the reliability of signal
transmission and security improves, and
battery life and sensor costs decrease, expect
to see more of your online analytical
data available on your smart phone or other
wireless device. Wireless data transfer
58
is already being used at plants to monitor
data taken at remote locations, such as
plant water outfalls.
Analyzer Improvements
Beyond the general trends mentioned
above, a number of analyzers have been
significantly improved or replaced with
better technology. Some of these have
been around for a few years, but they may
not yet be common in the North American
power plant.
High-Purity pH: Measured, Calculated,
or Both. Determining the pH of
high-purity condensate or deionized water
can be frustrating to near impossible
with a conventional pH probe or a benchtop
meter. The primary reason is the extremely
low number of ions in the sample,
which are necessary to " complete the circuit "
across the reference junction in the
probe. Add to this stray currents and gas
(ammonia and carbon dioxide) exchange
between the sample and the atmosphere,
and what you get is a pH reading that
never settles out but can drift over time.
This is a situation where the pH as measured
by the benchtop meter is rarely as
accurate as a properly configured online
pH analyzer.
The manufacturers of online pH meters
for the power industry have developed specialized
pH probes for high-purity water
samples, particularly when measuring condensate,
feedwater, and boiler pH. These
specialized probes share a number of characteristics.
If you are revamping an old sample
panel, or building a new plant, you will
Best Practices for Sample Line and Sample
Conditioning Panels
■ Use a heavy-walled stainless steel
sample line from the source on the
equipment to the sample panel. Most
sample lines are 3/8-inch inside diameter
(ID) tubing, though some plants
can get by with 1/4-inch tubing.
■ Isokinetic sampling nozzles are
recommended for steam samples.
Remember that the nozzle is only
isokinetic if the sample is flowing at
the designed velocity.
■ The sample run should be as short
and straight as possible and always
run downhill. This is particularly important
for steam samples.
■ The sample velocity in the line
should be >6 ft/sec (2 m/s). This
translates into about 3.3 L/min of
sample for a water sample and 1.2 L/
min of a steam sample for a 3/8-inch
ID sample line. This velocity should
minimize settling of particulates in
the sample line.
■ The sample line design must include
provisions to safely drain a sample
line that is not in service, to prevent
freezing.
■ For high-pressure samples (>500 psig)
an adjustable rod-in-tube type of
pressure-reducing valve is the standard
(such as Sentry Equipment's variable
pressure reducing element or VREL).
■ Design your sample cooler to maintain
a constant 77F (25C) sample
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temperature. This almost always requires
two sample conditioning chiller
loops, a primary and a secondary
one. The primary or roughing cooler
reduces the sample temperature
close to that of the cooling water
(the typical heat exchange source).
Remember that the cooling water
temperature varies seasonally, so
pick a reasonable design point. The
secondary chiller uses closed-loop
cooling water, where the temperature
is maintained at a constant 77F that
is usually controlled with a refrigeration
unit.
■ Use a thermal shut-off valve to protect
the downstream analyzers if the
cooling water is accidentally isolated
while the panel is in service.
■ Install a back-pressure regulating
valve to maintain a constant pressure
in the sample line so as to provide a
constant water flow to the analyzers.
This system also sends most of the
sample volume to drain, as the analyzers
and grab sample will typically
use far less sample flow to maintain
their desired flow.
■ Install temperature and pressure and
flow indicators as a quick visual way
to confirm that all systems are operating
properly.
■ Install a sample flush line located
after the pressure reduction valve.
POWER | September 2011
http://www.powermag.com
POWER September 2011

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