Mitsubishi Heavy Industries Technical Review Vol. 50 No. 3 (September 2013)
22

Water Quality Control Technologyfor Thermal Power Plants(Current Situation and Future Prospects)
SENICHI TSUBAKIZAKI*1 TAKAYUKI WADA*2

TAKEO TOKUMOTO*3

TARO ICHIHARA*4HARUKA KIDO*5SHUNSUKE TAKAHASHI*5

Water treatment for thermal power plants is conducted to prevent problems such as
carryover to the turbine components, as well as corrosion and scale formation/deposition in theboiler and turbine systems. Since 1959, water treatment methods have been improved to deal withequipment nonconformities. The required water quality for boilers and turbines of thermal power
plants is stipulated by the Japanese Industrial Standard (JIS) B 8223. JIS B 8223 is regularly
amended, in which Mitsubishi Heavy Industries, Ltd. (MHI) has also participated. The
improvement of water treatment methods for aging plants, higher operational efficiency, and better
environmental conservation has been high on the agenda in recent years. This paper introduces apackage of new products and technologies that we are developing.

|1. Introduction
Water treatment for thermal power plants is conducted to prevent problems such as carryoverto the turbine components, as well as corrosion and scale formation/deposition in the boiler andturbine systems.
The Japanese Industrial Standards Committee established a “specialists group for boiler
feedwater and boiler water” in the Mechanical Engineering Division to assess and determine thewater quality control criteria. In February 1961, these were stipulated by JIS B 8223 (titled “Water
conditioning for boiler feed water and boiler water”). JIS B 8223, which stipulates the required
water quality for boilers and turbines of thermal power plants, is regularly amended based on
operational results, technological innovation, equipment nonconformities, etc., to which we havealso contributed1.
In recent years, with increased focus on water treatment methods for aging plants, higher
operational efficiency and better environmental conservation, we are developing a package of newproducts and technologies that address the following problems: (1) powdered scale deposition foroxygenated treatment (combined water treatment; CWT)-applied plants, (2) flow-accelerated
corrosion (FAC) and the use of hydrazine, and (3) turbine contamination due to the leakage ofseawater.

*1

Chief Staff Manager, Nagasaki Power Systems Service Department, Power Systems Service Division, Power Systems*2Nagasaki Power Systems Service Department, Power Systems Service Division, Power Systems*3Takasago Power Systems Service Department, Power Systems Service Division, Power Systems*4

Yokohama Power Systems Service Department, Power Systems Service Division, Power Systems*5Equipment & Machinery Department, Plant Engineering Division, Engineering Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 3 (September 2013)23|2. Water systems of thermal power plants
Figure 1 gives an example of water-related systems used at thermal power plants2. In themain water system of power plants, water circulates starting as condensate, followed by boiler
feedwater, boiler water (in the boiler) and steam (in the turbine), and finally returning tocondensate. The water that is lost during the circulation is compensated for by supplying make-up
water. The other water-related systems are: (1) the make-up water treatment system to supply
highly-purified water obtained after the treatment of raw water such as industrial water, (2) thechemical dosing and water quality monitoring systems to monitor and adjust water quality forcorrosion prevention, (3) the condensate treatment system to purify the circulating-water in a plant(mainly installed for once-through boilers), and (4) the wastewater treatment system to purify
wastewater from the devices and components of thermal power plant facilities.

Figure 1

Water systems at thermal power plants|3. History of water treatment methods for thermal power plants
The history of water treatment methods for domestic thermal power plants is shown in
Figure 2. Table 1 gives identified problems and the relevant events for water treatment. Table 2 isa list of the treatment methods for boiler feedwater and boiler water3.
Table 1 Identified problems at thermal power plants and the relevant events forwater treatment
Year
1940
1950

1960

Newly identified corrosion-related problems
・ Pitting and brittleness of steam generating
tubes
・ Cracks in turbine components
・ Alkali corrosion in steam generating tubes
・ Ammonia corrosion in aluminum or brasscondenser tubes
・ Increase of pressure loss and overheating
due to steam-generating tube scaledeposition
・ Feedwater heater inlet attack
・ Detachment of deposited scale from thesuperheater or reheater・ Corrosion fatigue

Events for water treatment

・ Technological introduction from USA
・ PT/AVT application
・ Boiler chemical cleaning application
・ JIS criteria (1 Feb. 1961)
・ Oxygenatedtreatment(Germany and Russia)

application

・ Use of titanium condensers
・ Use of total condensate treatment
equipment
・ Start of commercial combined-cycle plant
operations
1980 ・ Feedwater heater drain attack
・ JIS oxygenated treatment criteria (1989)
・ Valve corrosion caused by oxygenated ・ Realization of commercial application of1990
treatment
oxygenated treatment
・ Viability assessment of High-AVT for・ FAC (flow-accelerated corrosion)
nuclear power plants (as a measure against
2000 ・ Powdered scale deposition in oxygenatedFAC)
treatment
1970

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 3 (September 2013)24

As shown in Figure 2, a subcritical pressure drum-type boiler of 17 MPa class was built in1959 and started operations using alkali treatment. However, problems due to alkali corrosion ofwater-wall tubes frequently occurred about 6 months after the commencement of operations. The
subsequent demand for urgent improvement of feedwater and boiler water treatment technologies
led to the introduction of all-volatile treatment (AVT) using ammonia and hydrazine, which had
been under development in Europe. AVT was adopted by subcritical pressure drum-type boilers in1960 and by once-through boilers in 1961, both producing good results. Thereafter, with the rapid
popularity of its application, AVT was used for most of the new large-scale thermal power plants.
In the middle of 1965, the inspection results of scale deposition on the inner surface of steam
generating tubes over the years necessitated the re-assessment of phosphate treatment (PT)application. As a result, drum-type high pressure boilers adopted low-phosphate treatment(Low-pHT), in which the phosphate ion level was maintained low (3 mg/L or less) and the Na/PO4
molar ratio in sodium phosphate was adjusted to be between 2.5 and 2.8 in terms of the preventionof alkali corrosion. Favorable effects, such as a reduced build-up rate of scale deposition in steam
generating tubes, more stable scale properties, and less frequent chemical cleaning of boilers, weredemonstrated.

Figure 2 History of water treatment methods for domestic thermal power plants
Table 2Treatment
CTPT

Phosphatetreatment
Low-pHphosphatetreatment
AVT

Treatment methods for boiler feedwater and boiler water
Water quality control
Feedwater
Boiler water
Ammonia
Caustic soda (NaOH)
Hydrazine
Trisodium phosphate
Ammonia
Trisodium phosphate
Hydrazine
Ammonia
Hydrazine

Disodium phosphate
Trisodium phosphate

Ammonia, hydrazine[AVT(O): only for ammonia]

NWT

OxygenCWT*1Ammonia
OxygenOT

ApplicationJISB8223

LP or IP drum-typeboilers
IP drum-type boilers
IP or HP drum-typeboilers
HP drum-type boilers
Once-through boilers
HP drum-type boilers(with condensatedemineralizer)

Once-through boilers
CT : Caustic Treatment
OT : Oxygen Treatment
PT : Phosphate Treatment
NWT : Neutral Water Treatment
AVT : All Volatile Treatment
CWT : Combined Water Treatment*1 : In Japan, OT generally indicates CWT.

Published in
1961

Published in1989

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 3 (September 2013)25

In both AVT and Low-pHT, scale deposition and corrosion of materials used for the systemcomponents are suppressed by maintaining the dissolved oxygen level as low as possible, thereby
preventing the occurrence of water-related problems.
In CWT, on the other hand, a high anti-corrosion effect can be achieved through the use ofhighly-purified water containing an appropriate level of dissolved oxygen. Following the success inother countries such as Germany and Russia, Japan also conducted verification tests for CWT andin 1989 published its first water quality control criteria in JIS B 8223. The first domestic CWTapplication was to a large once-through utility boiler in 1990. As of the end of March 2013, CWTis operational at 53 plants.

|4. Package of new products and technologies regarding water
treatment for thermal power plants
4.1

Measures against powdered scale deposition in CWTFigure 3 shows the difference of scale on the inner surface of steam generating tubes
between AVT and CWT. The advantages resulting from CWT application include (1) reduced
frequency of chemical cleaning owing to lowered build-up rate of scale, (2) the prevention ofrippled scale formation leading to the alleviated increase of boiler pressure loss, and (3) decreased
scale deposition on equipment surfaces.

Figure 3 Difference of scale on the inner surface of furnace steam-generatingtubes between AVT and CWTIn recent years, some of the CWT-applied plants experienced increased amounts of irontransferred into the boiler, and the deposition of hematite scale on the inside surface of boiler
furnace wall tubes, causing an increase in the temperature of metal furnace wall tubes. Thedeposited hematite scale, known as “powdered scale,” has low thermal conductivity and a porous
structure consisting of small particles.
For the prevention of powdered scale deposition, the iron levels transferred into the boiler
need to be lowered. As stipulated by the water quality control (recommended) criteria (Table 3),
the iron level at the inlet of boiler (economizer inlet) should be 2 μg/L or less (targeting 1 μg/L orless). According to the material balance of iron in the plant system, the examination results indicate
the low-pressure feedwater heater drain system as the primary source of iron, thus making it
essential to reduce iron levels in the drain system. Suspended iron removal equipment with ahigh-temperature filter has been developed and installed at the actual plants. The results weresatisfactory4.

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Table 3Once-through boilers: water quality control criteria during
CWT operation (our recommended values)pH
Dissolved oxygen
Iron
Electrical conductivity

4.2(μg/l)(μg/l)(mS/m)[Recommended value] Economizer inlet
8.5-9.3 (9.0)
20-200 (50 ± 30)
2 or less (1 or less)0.02 or less( ): Target value

Measures against FAC and use of hydrazine

Figure 4 is an example of FAC-caused thinning of pipe walls in a heat recovery steam
generator (HRSG) of an overseas combined-cycle plant. In this case, after three years of operation,
1.2-mm thinning of the walls was identified at the ventilation part of the low-pressure evaporator.
As a countermeasure, we recommend the adoption of High-AVT, which is an all-volatile treatmentusing feedwater with a pH level higher than the feedwater quality control criteria (upper limit: pH9.7) stipulated by JIS B 8223:2006. It has been used mainly in nuclear power plants. As shown inFigure 5, higher pH levels decrease the rate of FAC, and the thinning of pipe walls caused by FAC
is expected to be suppressed.
High-AVT water treatment was introduced to the Tuxpan No. 2 combined-cycle power plantin Mexico5. Since the start of commercial operations in 2001, no abnormalities have been foundduring equipment inspections over more than 10 years, thus producing good operational results. It
has also been demonstrated that higher pH levels cause no rust, even after long operational
interruptions. Switching to hydrazine-free water treatment becomes possible.

Figure 44.3

FAC example

Figure 5 Relationship between pH levels and
FAC rates(6)Measures against turbine contamination due to the leakage of seawater

Figure 6 shows the detrimental effects caused by the leakage of seawater (in the boiler). The
seawater leaking into the plant system is concentrated in the boiler and produces hydrochloric acid,leading to equipment damage from acid corrosion. The seawater was considered to leak because ofdeterioration over time such as corrosion of copper-alloy condenser tubes and therefore to bepreventable through the adoption of titanium condenser tubes. However, as indicated in Figure 7and based on our experience over the past 30 years, more than half of the seawater leakage
incidents occurred as a result of mechanical damage to the titanium condenser tubes.
Figure 8 is based on a case in which the plant had to be shut down immediately after theoccurrence of seawater leakage. Even in the process of shutting-down, the condensate supplied tothe spray attemperator of the turbine gland was contaminated by seawater, precipitating salt inamounts sufficient to cause the formation of rust in the gland. Application to actual plants is being
examined and carried out by commercializing the anti-contamination method for this cause(purification system).

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 3 (September 2013)
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Figure 6

Detrimental effects caused by the leakage of seawater (in the boiler)

Figure 7 Incidents of the leakage of seawater (1981-2010; based on ourreported cases)

Figure 8 Detrimental effects caused by the leakage of seawater (degree ofturbine system contamination)

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 3 (September 2013)28|5. Conclusion
Water treatment technologies for thermal power plants have been examined and improved asa countermeasure against equipment damage due to factors such as corrosion and scale deposition.
As shown in Figure 9, abnormalities in water quality can be a precursor of problems and therefore
serious problems can be prevented by analyzing the data and taking necessary measures.
The use of a package of new products and technologies we are developing can be expected toproduce a beneficial effect on aging plants, operational efficiency and environmental conservation.

Figure 9

Expected problems from abnormal water quality data

References
1.
2.

Water conditioning for Boiler feed Water and Boiler Water JIS B8223-2006
Tsubakizaki.S, Ichihara.T, Kido.H，The water treatment system and water quality control in thermal
power plants，Separation Process Engineers, Vol. 41 No. 3, 2011
3. Tsubakizaki.S, Learn from the Experts “Cycle Chemistry” Part 1, The Journal "The Thermal andNuclear Power", Vol. 58 No. 11, 2007
4. Takada. M, Nanjo. S, Asada. H, Consideration of Countermeasures for Powder Scale Deposit Problems
in CWT Operation Plants, Conference Proceedings of Thermal and Nuclear Power, 2011
5. Uchida.Y et al.， Improved Reliability of High-AVT (High-pH Water Treatment) Application toCombined Cycle Plants, The Journal "The Thermal and Nuclear Power" Vol.63 No.12, 2012
6. Murata et al., Evaluation of Flow Accelerated Corrosion by Rotating Disc Test Method., Proceedings ofJSCE materials and environments, 2006