Chemical Engineering March 2023 - 35

10Ca2+ + 6PO4
3Ca3(PO4)2
.Ca(OH)2
3- + 2OH- 
(9)
Magnesium and silica react with
hydroxide alkalinity to form the nonadherent
sludge serpentine:
3Mg2+ + 2SiO3
3MgO.2SiO2
.2H2O
- + 2OH- 
(10)
These two compounds exist as
soft sludges that are much easier
to remove than the hard scale that
would otherwise form. The materials
typically settle in the mud drum
or lower headers and are removed
by blowdown. Careful analyses
and system chemistry evaluation
are necessary to design and operate
the chemical feed systems to
maintain proper conditions. Various
synthetic polymers have been developed
to assist with hardness and
iron oxide deposition control. These
compounds cannot be introduced
arbitrarily to boilers, but must be
selected and dosed based on rigorous
chemistry evaluations. Some
polymers are better at sequestering
hardness ions, while others are more
effective in controlling particulates.
No " one size fits all " exists.
How plant personnel handle boiler
chemistry upsets may generate additional
problems if incorrect procedures
are followed. For instance,
the reaction shown in Equation (6)
illustrates that some chemistry excursions
can lower boiler pH significantly
and quickly. As pH drops
below 8.0, steel corrosion rapidly
increases. The best procedure in
such cases is to kill the boiler fires,
increase blowdown, drain the boiler
when safe, and eventually refill it with
fresh, treated water. Before restart,
it is imperative to find and correct
the contamination source. However,
these procedures may curtail
plant production, which can cause
management angst. A " knee-jerk "
alternative is to keep the boiler in
operation and inject caustic to raise
the pH. Unfortunately, the rapid pH
elevation induces precipitation of the
recently-generated iron corrosion
products, which settle on the boiler
tubes and add many more locations
for future underdeposit corrosion.
A final note
All systems are different, and, as
with all other technologies, due diligence
is necessary to determine the
feasibility of utilizing these methods.
Always consult your equipment
manuals and guides. Also, reputable
chemical vendors can offer the products
to achieve your goals.
n
Edited by Gerald Ondrey
References
1. " Consensus on Operating Practices for the Control of
Feedwater and Boiler Water Chemistry in Industrial and
Institutional Boilers, " The American Society of Mechanical
Engineers, 2 Park Avenue, New York, N.Y. 10016, 2021.
2. Buecker, B., and K. Perryman, High-purity water treatment:
Using membranes to protect membranes, Water
Technology, May/June 2022.
3. Guidelines for Control of Flow-Accelerated Corrosion in
Fossil and Combined Cycle Power Plants, EPRI Technical
Report 3002011569, the Electric Power Research
Institute, Palo Alto, California, 2017.
4. Buecker, B., Monitoring of Water and Steam Chemistry
for Steam Generators, Chem. Eng., September 2019. pp.
33-42.
Acknowledgment
Special thanks to Richard Tribble of ChemTreat for supplying
the photos in this article.
Authors
Brad Buecker is president of
Buecker & Associates (Lawrence,
Kan.; beakertoo@aol.com), specializing
in technical writing and
consulting.
He most recently
served as senior technical publicist
with ChemTreat, Inc. He has
over four decades of experience in
or affiliated with the power industry,
much of it in steam generation
chemistry, water treatment, air quality control, and results
engineering positions with City Water, Light &
Power (Springfield, Illinois) and Kansas City Power &
Light Company's (now Evergy) La Cygne, Kansas station.
He also spent two years as acting water/wastewater
supervisor at a chemical plant, and eleven years with
two engineering firms, Burns & McDonnell and Kiewit.
Buecker has a B.S. in chemistry from Iowa State University
with additional course work in fluid mechanics, energy
and materials balances, and advanced inorganic
chemistry. He has authored or co-authored over 250
articles for various technical trade magazines, and has
written three books on power plant chemistry and air
pollution control. He is a member of the ACS, AIChE,
AIST, ASME, NACE (now AMPP), and the Electric Utility
(and
Co-Generation)
committee.
Wendy McKinnon is vice president,
Applied Technologies for
ChemTreat, Inc. (Glen Allen, Virginia
23060; Phone: 804-9352000;
Fax:
804-965-0154;
Email: wendym@chemtreat.com).
She has been with ChemTreat for
20 years. Prior to becoming vice
president, she was in a technical
consultant role specializing in
water and wastewater clarification, reverse osmosis and
deionization. She has a B.S. in chemistry from the University
of Mary Washington.
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