Hydrocarbon Processing - July 2022 - 60

Process Optimization
that chloride and sulfate may be well
above 2 ppb at the recommended CACE
limit. A relatively new instrument offered
by an instrumentation providera
allows
analyses of these two impurities down
to a 0.1-ppb level.8
The instrument separates
ions via the process of capillary electrophoresis.
The ions are then measured
by a conductivity analyzer.
It has long been known that silica in
steam will precipitate on turbine blades.
While the compound is not corrosive, it
can influence turbine aerodynamics and
reduce efficiency. Therefore, 10 ppb is
the recommended limit.
Sampling with saturated, main and
reheat steam is normally recommended
for power boilers. Main and reheat steam
are the most important, as they provide
data on impurities directly entering the
turbine. Besides drum carryover, impurities
can come from contaminated attemperation
water. Such contamination
should also be detected by feedwater instruments.
Conducting saturated steam
sodium measurements on a periodic basis
is typically recommended to evaluate
mechanical carryover from steam drums.
Failed or damaged steam separators in
evaporator drums are common causes of
mechanical carryover. Rapid load swings
can also induce carryover.
The criticality of proper sampling
and conditioning. The quality of data
from the sample analysis system is dependent
on the integrity of the samples
provided to the sensors and analyzers.
It is imperative to consider all aspects of
the sample path from point of extraction,
sample transport, sample conditioning,
and analyzer care and maintenance.
The objectives of any good sampling
system are to provide a statistically significant
sample to the measurement tool
that is representative of the process. In
the case of high-pressure, high-temperature
steam and water sampling, this includes
controlling the temperature, pressure
and velocity of the sample from the
point of extraction to the sensor.
Sample extraction. Liquid samples
can be effectively extracted from
a process line via a side of a pipe entry.
Three o'clock or nine o'clock positions
are best. The sample tap should never be
placed at the bottom of the pipe.
For years, isokinetic nozzles were
recommended for all steam samples
(FIG. 3), as these nozzles are designed to
extract steam at the same velocity as the
flow in the pipe to ensure representative
samples. In recent years, however, some
experts have questioned the need for isokinetic
sampling on superheated steam
lines, while recognizing that it is needed
for saturated steam.
Another design detail that is important
for high-temperature and/or highpressure
sample lines is a double isolation/root
valve (FIG. 4). The double
valves provide added protection for isolating
a live sample. Some guidelines also
call for the primary steam sample cooler
to be placed remotely at the sample tap
to convert the steam to condensate for
its journey to the sample panel. However,
these remote coolers are often difficult to
access and to maintain with high ambient
air temperatures. In most applications, superheated
steam is brought to the sample
panel as a vapor and is then condensed.
Sample transport. Depending on
velocity, flowing samples, and especially
liquids, can deposit or pick up solids.
This effect is minimized when the linear
velocity is 6 ft/sec. Additionally, high
velocities could induce some erosion
of the tube walls-although this would
probably be minor, given that the recommended
material is usually either 304 or
316 stainless steel.
For a standard 0.25-in. outside-diamFIG.
3. Diagram of an isokinetic steam sampling nozzle. Illustration courtesy of Jonas Inc.
eter tube with a wall thickness of 0.049
in., a 6-ft/sec linear flowrate equates
to 1,200 cc/min. More often selected
is 0.375-in. outside-diameter tubing,
where a 6-ft/sec linear flowrate corresponds
to 3,300 cc/min, which can be
much greater than the common requirement
of 1,800 cc/min-2,200 cc/min. As
a compromise, a linear flowrate range of
3 ft/sec to 6 ft/sec is standard, as this still
ensures accurate analyses.
FIG. 4. Diagram of a steam sample tap with a double isolation valve. Illustration courtesy
of Jonas Inc.
FIG. 5. Sample conditioning diagram.
60 JULY 2022 | HydrocarbonProcessing.com

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Hydrocarbon Processing - July 2022

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