Hydrocarbon Processing - July 2022 - 61

Process Optimization
Fittings and tubing bends should be
kept to a minimum to limit pressure drop.
Also, whenever possible, long sample
runs should be avoided.
Sample panel conditioning details.
FIG. 5 outlines the common control devices
and equipment for samples when they
reach the sample panel. This diagram is
for a sample with pH, specific conductivity
and CACE online instruments.
Virtually all the coolers, valves and other
equipment shown in FIG. 5 are recommended
in various guidelines, including
those of the Electric Power Research Institute
(EPRI), ASTM International (formerly
known as the American Society for
Testing and Materials), and others.
Inlet isolation valves. An inlet isolation
valve for each sample line should be
used. Cases may arise where the entire
sample panel must be isolated; however,
much more frequent is the need to isolate
a single line for equipment repairs
and/or maintenance while the other
lines remain in service. Isolation valves
should not be used for throttling or pressure
reduction and should remain either
fully open or fully closed.
High-temperature blowdown valves.
High-temperature blowdown valves
should be periodically opened for a short
duration to flush sample lines. Longer
flushes are often necessary during startup
after an outage. The valves discharge to
piping that has been specially designed to
handle high temperatures.
Primary cooling systems. A primary
cooler is standard for lowering sample
temperatures from process conditions to
less than 122°F (50°C), with a common
guideline of 100°F (38°C).
Filters. Once standard on many
sample panels were sample filters, either
mesh pads or magnetic filters, to protect
downstream equipment. However, filters
can affect the integrity of samples,
particularly for iron monitoring, and the
co-author's company does not officially
recommend their use.
Pressure reduction. For sample
pressures over 500 psig, a variable rod-in
tube-pressure-reducing device is recommended.
Below this pressure, traditional
pressure control valves are sufficient.
Panel design should include calculations
for sample transport reduction induced
by these valves.
Thermal protection. An automatic
thermal shutoff valve on all high-temperature
samples is recommended to protect
operators and instruments in the event of
the loss of cooling water to the primary
sample coolers.
Secondary cooling systems. A standard
feature of sampling systems is a secondary
cooling circuit to moderate sample
temperatures to 77°F (25°C), +/- 2°F.
Most chemistry guidelines are based on
this temperature. Many modern instruments
have automatic temperature compensation,
but variations in the chemical
matrix and excessive temperatures can
generate
erroneous results.
Secondary
cooling systems provide uniformity
across all samples, as variations in temperature
from primary sample coolers are
common. While some primary cooling
systems may push the limit of primary
cooler capacity, these secondary systems
enable correction for these cases.
Total flow indication. A total flow
rotameter is standard. It offers a straightforward
method to observe if the sample
flowrate is correct, or if it is too low or too
high. Individual instruments also have
rotameters to ensure that each sample is
receiving the correct flow.
Grab sample pressure regulation.
A typical feature of the sample panel is
a grab sampling line for each sample.
These ports enable operators and chemists
to spot check conditions throughout
the steam-generating network. Without a
pressure-regulating valve (PRV), the low
resistance through this line would shortcircuit
flow to the instruments. Grab
sample PRVs can be adjusted in conjunction
with the primary pressure-reducing
valves to ensure that a consistent flow is
provided to the analyzers throughout a
range of conditions.
Iron monitoring. Iron monitoring continues
to emerge as a primary tool for
evaluating the efficacy of any HRSG corrosion
control chemistry program. It is
also a key tool to monitor for FAC. The
industry has seen a continuous improvement
in monitoring techniques, with
four primary methods available:
* Continuous particulate monitoring
* Corrosion product sampling
* Grab sample analysis,
using digestion to reach
low detection limits
* Online nephelometric detection.
Note: Usually more than 90% of iron
corrosion products in a steam generator
exist as iron oxide particulates. Too often,
plant personnel test for only dissolved
iron. Serious corrosion may be underway,
but when only testing for dissolved iron
is used, this corrosion can go undetected
by plant personnel.
Particulate monitoring. In the highpurity
water of high-pressure steam generators,
most particulates will be iron
oxide corrosion products. Accordingly,
FIG. 6. A corrosion product sampler.
FIG. 7. Suspended particles of magnetite
(black) and hematite (red) in water.
Photo courtesy of Hach.
Hydrocarbon Processing | JULY 2022 61
Hydrocarbon Processing - July 2022

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