Chemical Engineering July 2019 - 52

FIGURE 9. Shown here is a carbon steel corrosion
test specimen using a polyphosphate watertreatment
program
FIGURE 7. This two-pass heat exchanger that is
using a zinc-phosphate program shows corrosion
on the colder inlet pass, and fouling on the hot
outlet pass
at a large industrial complex in the
southeastern U.S., RPSI replaced
previous polyphosphate and then
zinc chemistry (Figures 9 and 10).
Carbon steel corrosion rates have
been reduced from 0.2-0.25 mm/yr
to 0.0025-0.0075 mm/yr.
On a secondary note, the change
influenced by problems with
from zinc and then to RPSI was in
part
severe algae formation in a clarifier
and recycle pond at the plant. The
removal of phosphate from the water
effectively addressed that challenge,
allowing a 70% reduction in sodium
hypochlorite (bleach) usage for microbiological
control.
In another example, also at a large
chemical plant, traditional phosphate
chemistry proved satisfactory
for corrosion control, but calcium
phosphate deposits caused fouling
in some of the plant's plate-andframe
heat exchangers. Such exchangers
are notorious for low-flow
locations and deposit accumulation.
Conversion to RPSI chemistry maintained
excellent corrosion protection
and eliminated phosphate deposition
(Figure 11).
One of the industries for which
this newer chemistry option can be
quite effective is power generation,
where most plants are no longer
base-loaded but instead operate in
a cyclical (on and off) manner as part
of normal operation. It is critical that
a protective barrier be maintained
on all cooling system metals in such
applications, otherwise severe localized
corrosion may result.
Power plants generally directly discharge
their cooling water, and consequently
face significant restrictions
on the use of phosphorous and zinc
corrosion inhibitors.
52
FIGURE 10. This carbon steel corrosion test specimen
after converstion to RPSI inhibitor
FIGURE 8. This four-pass heat exchanger that is
using the RPSI corrosion inhibitor program shows
no corrosion or fouling
Modern control methods
As with the other technologies,
chemical feed and control capabilities
have been vastly improved [2].
Automation of chemical feed and
chemistry monitoring systems can
do the following:
* Reduce chemical costs
* Reduce water usage
* Improve safety
* Improve performance and materials
protection
* Improved utilization of plant staff
* Improve diagnostics and troubleshooting
*
Achieve peace of mind
One aspect in this regard, which
is valuable for any program, has
been the development of traced
chemistry programs, which allow
solid state sensors to accurately
measure
chemical concentrations
in the circulating cooling water without
the need for reagents, or even a
laboratory. Also, the instrumentation
that is available for monitoring standard
operating parameters has improved
greatly in recent years, allowing
for accurate, continuous, online
readings of critical data, including
the following:
* pH
* Specific conductivity
* Oxidation-reduction potential
(ORP) for controlling oxidizing
biocides
* Corrosion rate
* Scaling potential
* Biofouling potential
* Chemical feed tank levels
The unit shown in Figure 12 contains
the instruments needed to
accurately monitor and control important
aspects of cooling water
chemistry. It includes corrosion coupons,
sensors, flow regulators and
other related hardware. The analytical
data can be distributed both internally
and remotely to assist both
plant personnel and external experts
with evaluation of system conditions.
Many new plants nowadays, at least
in the power industry, are minimally
staffed, with few, if any, personnel
who are rigorously trained in cooling
system chemistry.
Against this backdrop, a digital
data feed that allows outside consultants
to monitor system conditions
can be quite valuable for maintaining
system reliability. Further, ever since
Langelier produced his pivotal calcium
carbonate scale calculations in
the 1930s [3], increasingly accurate
and comprehensive computer models
have been developed to evaluate
the scale and corrosion potential for
untreated water, and to calculate
the proper chemistry, strength and
cost of products needed to control
scale and corrosion issues. Modeling
programs are available for purchase
from independent software
companies and are also offered by
water-treatment service providers,
often included with the water treatment
chemistry. The best programs
are capable of selecting and evaluating
the most appropriate and costeffective
treatment programs that
meet the plant's environmental and
operating constraints, such as water
quality, alternate water sources,
and
costs
and sewer.
A key aspect with any program
is the input of accurate historical
chemical analyses of the raw water
feed to the system. Both authors
have seen over and over again project
specifications that have only partial
raw-water data - often sampling
that is based on just a snapshot in
time. Many owners and owner's engineers
do not understand that comprehensive
water-quality data are esCHEMICAL
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Chemical Engineering July 2019

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