Hydrocarbon Processing - July 2022 - 58

Process Optimization
pressure circuit essentially serves as a
feedwater heater for the intermediateand
high-pressure circuits. Although
ments to monitor system performance,
including pressure, temperature, flow and
specific conductivity. The recommended
Many HRSGs operate at high temperatures and
pressures, where the harsh conditions can transform
seemingly minor chemistry upsets into major
problems. Online water/steam chemistry monitoring
is critical not only for normal chemistry control,
but also to detect upsets.
once-through HRSG designs exist, most
HRSGs are drum-type units. The common
term for HRSG boilers is " evaporators, "
which will be used throughout the
remainder of this article.
While coal ash issues are non-existent
in HRSGs, some HRSG steam-generating
tubes may have fins, which can collect
particulates that impede heat transfer.
While various methods have been
developed (and continue to evolve) for
external tube cleaning, the close packing
of tubes can still make cleaning a complicated
task.
The following sections outline recommended
HRSG water/steam chemistry
monitoring parameters, and why
these choices are critical for reliable and
safe operation.
HRSG water/steam chemistry monitoring
recommendations. The sampling
locations of primary importance
throughout the steam-generating network
are:
* Makeup treatment system
* Condensate pump
discharge (CPD)
* Feedwater/economizer inlet
* Boiler water
* Saturated and main/reheat steam.
Makeup treatment system. No system
is completely closed, and, even in the
tightest steam generators, a small amount
of process water/steam is lost via controlled
blowdown or at leaking valves and
other fittings. Losses must be made up
with high-purity water. The most common
makeup process is reverse osmosis
(RO) followed by either mixed-bed ion
exchange (MBIX) or electrodeionization
(EDI) to polish the RO effluent. RO units
are normally equipped with several instru58
JULY 2022 | HydrocarbonProcessing.com
instrumentation and normal limits of the
ion exchange polisher effluent are:
* Specific conductivity: ≤ 0.1 µS/cm
* Silica: ≤ 10 parts per billion (ppb)
* Sodium: ≤ 2 ppb.
Continuous online measurement
ensures consistent high-purity makeup
water. An increase in any of the values
indicates that either the MBIX resin has
reached exhaustion or that a problem
has occurred in the EDI unit; therefore,
prompt corrective action is necessary.
CPD. In steam-generating power
units, the primary spot for potential contaminant
ingress is the condenser (if it is
water-cooled), where a tube leak allows
cooling water to infiltrate the high-purity
condensate. Cooling water in-leakage
will introduce a variety of impurities, including
hardness ions, sodium, chloride,
sulfate and silica, which, when subjected
to the harsh environment in the steam
generator, can cause serious corrosion or
scaling problems. A condensate polisher
can provide a buffer against contaminant
ingress; however, polishers are often not
installed on drum units, primarily to reduce
project capital cost.
The following are recommended with
online CPD analyses:
* Cation conductivity: ≤ 0.2 µS/cm
* Specific conductivity: Consistent
with pH (as generated by ammonia
or ammonia/amine feed)
* Sodium: ≤ 2 ppb
* Dissolved oxygen: ≤ 20 ppb
* pH: 9.6-10 (this is the pH
range for the CPD/feedwater of
the HRSG design in FIG. 1; the
range may be lower for some
other HRSG designs).
Sodium monitoring is very effective
for detecting condenser tube leaks.
With a tight condenser, sodium levels
in the condensate are normally very low
(< 2 ppb) and, in many cases, are less
than 1 ppb. An increase in sodium
provides the earliest indication of a
condenser tube leak.
Cation conductivity is now often
referred to as " conductivity
after cation exchange (CACE) " to
represent that the sample is routed
through a cation exchange column
to replace the cations (e.g., ammonium,
sodium, calcium) with
hydrogen ions. This creates a very
diluted acid solution of primarily
trace amounts of chloride and
sulfate ions, whose conductivity is then
measured. CACE eliminates the artificial
influence of ammonia on conductivity
and is more sensitive than specific conductivity.
As with sodium, an increase
in CACE indicates impurity in-leakage,
although this measurement is influenced
by carbon dioxide (CO2
) ingress (e.g.,
from increased air in-leakage at the condenser).
Therefore, degasified CACE is
becoming increasingly popular, which
utilizes
either
a
reboiler
or
nitrogen
.
sparging compartment to remove CO2
A low CACE value is a requirement for
proper control of all-volatile oxidizing
treatment [AVT(O)] chemistry, which
is recommended for condensate/feedwater
treatment to minimize FAC.2,3
An
issue that can influence CACE accuracy
appears in units where a neutralizing
amine is utilized as a supplement to, or
in place of, ammonia. These compounds
decompose to small-chain organics and
CO2
in high-temperature steam. The organic
compounds are not removed in a
degasifier, thus artificially increasing conductivity.
CACE readings may become
worthless in those cases.
Dissolved oxygen analyses are important
for monitoring condenser air
in-leakage. A sudden dissolved oxygen
increase may indicate a mechanical failure
at or near the condenser, which allows
excess air to enter the system. However,
with modern AVT(O) chemistry,
some dissolved oxygen is required for the
chemistry to be effective.2,3
Regarding specific conductivity and
pH, ammonia (or sometimes an ammonia/amine
blend) is the pH-conditioning
agent for condensate/feedwater. However,
the direct pH measurement of highpurity
water can be tricky. Algorithms
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