Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)63

Advanced Water Treatment Technologies (High-AVT)
for HRSG Corrosion Control corresponding to
New JIS B8223 (Japanese Industrial Standards)

SENICHI TSUBAKIZAKI*1

TAKAYUKI WADA*2

TAKAYOSHI IWATO*3

TSUYOSHI NAKAHARA*4

MITSURU NAKAMOTO*5YOSHINORI NOGUCHI*6

In October 2015, JIS B8223 (Boiler feedwater and boiler water quality standards) wasrevised and issued, and for the Heat Recovery Steam Generator (HRSG) of combined cycle plant,
the operational standards for High-AVT (high pH) water treatment that Mitsubishi Hitachi PowerSystems, Ltd. (MHPS) has applied at actual plants since 2001 were adopted. High-AVT water
treatment has been increasingly applied as a measure against Flow Accelerated Corrosion andphosphate corrosion problems both inside and outside Japan and is useful as an alternativetechnology to hydrazine, the carcinogenicity of which has been pointed out. In addition, from an
economic point of view, the advantages of the adoption of High-AVT water treatment, such as theshortening of plant start-up time and the reduction of operating costs, have been verified.

|1. Introduction
HRSG installed at combined cycle plants uses the flue gas from the gas turbine as a heatsource. Therefore, the heat load is small and the water quality control was considered to be easy. At
some plants, however, Flow Accelerated Corrosion (FAC) and phosphate corrosion events haveoccurred. As countermeasures, MHPS set the pH of feedwater at a high pH exceeding theconventional JIS standards (upper limit of 9.7) and has applied phosphate-free High-AVT water
treatment to actual plants since 2001. The adoption of High-AVT water treatment has beenpromoted at domestic commercial thermal power plants and its operational performance has beenrecognized. As a result, it was adopted as a standard in JIS B8223, which was revised and issued inOctober 2015.

|2. Revision of JIS B8223-2015 (Boiler feedwater and boiler water
quality standards)
The corrosion-related problems at domestic thermal power plants and the history of water
treatment are shown in Table 1. In the 1950s, subcritical pressure drum type boilers (17 MPa class)were constructed, and operations started with the use of alkali treatment. About half a year after thestart of operations, alkali corrosion problems of water-wall tubes frequently occurred, andimprovement of the feedwater/boiler water treatment technology was urgently required. The water
treatment technology and the boiler chemical cleaning technology were introduced from overseas,
and the "Technical Committee for Boiler Feedwater and Boiler Water Quality," which was set up
in the Japanese Industrial Standards Committee, deliberated about the water quality standards. As aresult, JIS B8223 "Boiler feedwater and boiler water quality standards" was established in February
1961. The water quality required for boilers/turbines at thermal power plants (JIS B8223) has been
periodically revised with operational performances, new technologies, equipment nonconformities,etc., being reflected, and MHPS has also taken part in the revision committee.
*1 Chief Staff Manager, Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*2 Nagasaki Power Systems Service Department, Power Systems Service Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*3 Chief Staff Manager, Equipment & Machinery Department, Plant Engineering Division, Engineering Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*4 Chief Staff Manager, Boiler Engineering Department, Boiler Technology Integration Division, Engineering Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*5 Chief Staff Manager, Plant Engineering Department, Plant Engineering Division, Engineering Headquarters, Mitsubishi Hitachi Power Systems, Ltd.
*6 Chemical Research Department, Research and Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)64

Table 1 Identified problems at thermal power plants and the history of water treatment
Year
1940
1950
1960
1970
1980Major 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 brass condenser
tubes- Increase of pressure loss and overheating due tosteam-generating tube scale deposition- Feedwater heater inlet attack- Detachment of deposited scale from the superheateror reheater- Corrosion fatigue- Feedwater heater drain attack- Increase of boiler pressure loss due to rippled scalegeneration

19902000- FAC (Flow Accelerated Corrosion)

Events for water treatment- Technological introduction from USA- PT/AVT application- Boiler chemical cleaning application- JIS criteria (Feb. 1, 1961)- Oxygenated treatment application (Germany andRussia)- Adoption of condensate demineralizer- Start of commercial operation of the combined cycleplant- The oxygenation treatment criteria set in JIS (1989)- Realization of commercial application of oxygenatedtreatment- Viability assessment of High-AVT for nuclear power
plants (as a measure against FAC)In JIS B8223: 2015, the following revisions were mainly made:(1) The upper limit value of the pH of feedwater based on the performance at actual plants
was reviewed. (Table 2) (pH9.7⇒10.3)(2) The hydrazine-free water treatment method was set. (Table 3) (Treatment of volatile
substances in oxidized form, low-oxidized form)(3) The lower limit value for phosphate ion concentration was eliminated. (As acountermeasure against excessive injection, the performance at actual plants wasreflected.)(4) The iron concentration during the operation of oxygenated treatment (OT) (Measureagainst deposition of powder scale)(5) The conformity to international standards was considered. (Oxygenated treatment: CWT⇒OT, the name of acid electrical conductivity adopted, etc.)
Table 2 Control items and control values of water conditioning for feedwater of heatrecovery boiler for power plants (excerpt)
Division

Treatment method
Reducing agent

yes
Reducing[AVT (R)]
8.5 to 10.3 *2

Redox properties

All-volatile treatment
no*1
Low oxidizing *1[AVT (LO)]
8.5 to 10.3 *2

Oxidizing *1[AVT (O)]
8.5 to 10.3 *2pH (at 25°C)
Acid electrical conductivity (at 25°C)
mS/m
0.05 max.
0.05 max.
0.02 max.
(μS/cm)(0.5 max.)(0.5 max.)(0.2 max.)
Feedwater
Dissolved oxygen
O：μg/l7 or under
Under 55 to 20Iron
Fe：μg/l
20 max.
20 max.
10 max.
Copper
Cu：μg/l
5 max.
5 max.
5 max.
Hydrazine
N2H4：μg/l
10 min.
――*1 Hydrazine (deoxidizer)-free all-volatile treatment method is set.
*2 The upper limit value of the pH of feedwater is changed 9.7⇒10.3. (High-AVT is applicable for water treatment.)

Table 3Comparison of AVT (R), AVT (LO), AVT (O) and oxygenated treatment (OT)Water treatment

Waterconditions

Reducing
agent

AVT (R)

Reducing

Used

AVT (LO)

Lowoxidizing

AVT (O)

OxidizingNot usedNot used

Dissolved
oxygenμg/L
7 or underUnder 5

5 to under 20

Informative OT(oxygenatedOxidizing Not used
20 to 200treatment)
R: Reducing, LO: Low Oxidizing, O: Oxidizing

Form of iron oxides

NoteFe3O4
When pH value and the concentrationof dissolved oxygen are low, only Fe3O4
film is formed in many cases. When pH
value and the concentration ofdissolved oxygen are high, very thin
Fe2O3 film is formed on Fe3O4 film.

―Thin Fe2O3 film is formed on Fe3O4film.

If hydrazine injection issuspended in power
plants in Japan, such
water conditioning stateoccurs.
If hydrazine injection issuspended in power
plants in other countries,
such water conditioning
state occurs.

In feedwater and boiler whole systems,
Oxygen injection control
stable and dense Fe2O3 film is formed on
facility is required.
Fe3O4 film.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)65

In the conventional all-volatile treatment, dissolved oxygen is chemically removed using a
deoxidizer such as hydrazine to inhibit corrosion in the reducing atmosphere. As shown in Table 3,by this revision of JIS, instead of the term AVT, AVT (R) (Reducing condition) was adopted in
keeping with the overseas standards such as Germany’s VGB.
In addition, as a hydrazine-free water treatment by which a low concentration of dissolved
oxygen can be maintained, AVT (O) (Oxidizing condition), which is in conformity with overseas
standards, was newly added. In many cases at thermal power plants overseas where the injection of
hydrazine has stopped, the concentration of dissolved oxygen in feedwater becomes 5 μg/l or more.
Therefore, the water condition of AVT (O) is set at the concentration of dissolved oxygen of 5 tounder 20 μg/l. It has been reported that in AVT (O), hematite (Fe2O3) film is formed, which has aneffect in inhibiting FAC.
It has been verified that the water condition in Japan is such that the concentration ofdissolved oxygen in feedwater is 5μg/l or lower in many cases depending on the difference inperformance of the condenser, deaerator, etc., and hematite (Fe2O3) film is not formed, which isdifferent from the water condition of AVT (O) under overseas standards. Therefore, AVT (LO)(Low oxidizing condition) is set as a new water condition.

|3. Application of High-AVT (LO) water treatment to HRSG
Table 4 shows a comparison of the conventional phosphate treatment and the High-AVT
water treatment. It has been verified that at a high pH, the tube thinning rate by FAC is reduced,and the inhibition effect has been expected. Figure 1 presents an example of FAC, and Figure 2
provides the relationship between pH and the thinning rate of FAC.
Table 4Comparison of phosphate treatment (PT) and High-AVT (LO) water treatment
Chemical used
Feedwater
Ammonia
Hydrazine

Boiler water
Disodium phosphate

pH of feedwater

Feed water: AVT (R)
Used
Used
Used
8.5 to 9.7
Boiler water: PT
Feed water/Boiler water:
Used
Not used
Need not be used
9.8 to 10.3
High-AVT (LO)* In cases where PT is replaced with High-AVT, it is recommended that chemical cleaning should be performedat the time of replacement to remove risks due to phosphorus remaining in scale.

Figure 1

Example of FAC

Figure 2 Relationship between pH andFAC rate

If a change in electrical conductivity of a boiler due to a change in the concentration ofphosphate (hide-out/hide-out return phenomena) shown in Figure 3 is observed, phosphate may be
excessively injected and corrosion problems such as the thinning of pipe walls or leakage arehighly likely to occur.
Figure 4 shows one example of the labelling on a hydrazine tank under GHS (GloballyHarmonized System of Classification and Labelling of Chemicals). It has been pointed out thathydrazine is carcinogenetic, and the Global Plan of Action in the Strategic Approach toMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)
66

International Chemical Management indicates “Toward 2016-2020, alternative substances tocarcinogenic or mutagenic substances (hydrazine, etc.) shall be identified and used.”
Hydrazine-free low oxidizing water treatment High-AVT (LO) is an effective option foreliminating the use of hydrazine.

Figure 3

Example of corrosion caused by phosphate hide-out/hide-out return phenomenaFigure 4

GHS labelling for hydrazine tank

Currently, for long-term storage, high-concentration hydrazine water is added after the boiler
water is blown off, and the hydrazine water is blown off before start-up. In High-AVT water
treatment, occurrence of corrosion during storage at a high pH can be inhibited. Therefore, as
shown in Table 5, the boiler water can be stored under the same water condition as in operation,and the start-up time can be shortened and the usage of pure water used and the drain treatment cost
can be reduced.
Table 5Water
treatmentAVT

Shortening of stop/start-up process by introduction of High-AVT (LO)Processes

Blow down

Filling water for
layup

Preservation

Blow down of
layup waterBoiler

Hydrazine

High-AVTBoiler

To be maintained as under the conditions of water
quality during operation(i.e., no replacement of water)

Comparative
operation time
Water filling for and volume ofstart-uppure water used
￭Operationtime: About
420 min￭Volume ofpure water
used: About
500 ton￭Operationtime: 0 min￭Volume ofpure water
used: 0 ton* For one unit of 501F-class HRSG
* In High-AVT operation, the boiler water can be stored under the same water condition as in operation, and the simplificationof stop/start-up process and the reduction of the amount of pure water used can be expected.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)67|4. Steps for introducing High-AVT (LO) water treatment to HRSG
Figure 5 presents the steps for introducing High-AVT (LO) water treatment. The watercondition of feedwater is pH9.8 or higher, and therefore, after the steps of (1) prior study about thewater control procedure, (2) hydrazine stop test, (3) pH change test, and (4) modification of agent
injection equipment/control logic based on (3) pH change test, the operation of High-AVT (LO)water treatment can be started. For the investigation of the thinning of pipe wall due to corrosion
caused by phosphate treatment in the past, the tube-inserted ECT (Eddy Current Testing) shown in
Figure 6 is recommended.
Even if thinning is minor, it is considered to be necessary to remove scale including
phosphate on the tube inner surface at the time of conversion to High-AVT (LO) water treatment,to avoid the development of corrosion in the future. Figure 7 indicates one example of the resultsof the scale dissolving test by neutral/unheated (low-temperature) chemical cleaning which wasdeveloped as a scale removal method for HRSG fin tubes, which dissipate a large amount of heat(and therefore, make it difficult to maintain the cleaning temperature).

Figure 5

Steps for introducing High-AVT (LO) water treatment

Figure 6

Tube-inserted ECT (Eddy Current Testing) system

Figure 7 Results of the scale dissolving test of HRSG phosphate corrosion partMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)
68|5. Conclusion
The actual performance of High-AVT water treatment, which is a water treatment method for
HRSG at combined cycle plants, has been well-received, and High-AVT water treatment wasadopted as the standard in JIS B8223 which was revised and issued in October 2015. It has beenverified that its high-pH operation has the effect of reducing FAC and phosphate corrosion and also
allows hydrazine-free operation/storage. In addition, from economic points of view such as theshortening of start-up time and the reduction of pure water/amount of waste water, advantages inadopting High-AVT water treatment can also be expected, and the rate of adoption is increasing atdomestic thermal power plants.
In introducing High-AVT water treatment, it is recommended that the states of theoccurrence of corrosion thinning caused by phosphate treatment in the past should be investigated,and scale on the inner surface of tubes, which will cause the development of corrosion in the future,should be removed (by chemical cleaning).

References
1.
2.
3.
4.

Japanese Industrial Standards “JIS B8223-2015”
Suzuki, T. et al., The Application of High AVT (O) in Gas Turbine Combined Cycle Plants, Power Plant
Chemistry, 2011, 13 (9), pp. 288-297.
Tsubakizaki, S. et al., Advantages and New Technologies of High-AVT Water Treatment in Combined
Cycle Plants, Mitsubishi Heavy Industries Technical Review Vol. 52 No. 2 (2015)
Urata, M. et al., Non Destructive Inspection Technology for Thermal Power Plants, Mitsubishi HeavyIndustries Technical Review Vol. 52 No. 4 (2015)