Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
1Development of Chemical-Cleaning Technology Effective atNeutral pH and Ambient Temperature Condition enable
Safety Improvement and Period Reduction of Maintenanceof Power Generation Equipment
YOSHINORI NOGUCHI*1 NAOTO TAGAMI*1
KAORU EGAWA*1

KOKI OHKUBO*2

YUYA YAMABE*2YUTAKA YOKOYAMA*3

In water systems in power plants, etc., scale adhesion and rust generation may cause
problems. As a countermeasure, the periodic chemical cleaning of scale and rust are carried out.
Mitsubishi Heavy Industries, Ltd. (MHI) has a developed chemical cleaning technology effective atneutral pH and ambient-temperature featuring high safety and high workability. Currently, we arecarrying out cleaning performance evaluation tests and actual equipment cleaning of adheredscale mainly on thermal power plant equipment such as electromagnetic filters, HRSG evaporator
pipes (heat recovery boiler), and main steam evaporation pipes (drum type and once-through type),
and are promoting the application of the developed technology to after-sales service to realize highsafety and the shortening of construction period.

|1. Introduction
Problems in power plants caused by water include scale adhesion to the components such asboilers. As one of the countermeasures, the removal and cleaning (chemical cleaning) of scale isperformed. Scale removal is generally performed at high-temperature（≧80℃） cleaning with acid.
This calls for the prohibition of the use of fire and an independent cleaning process for work
operations due to the explosion risk of generated hydrogen, and thus the plant has to stop for a long
period of time.
Therefore, we developed chemical cleaning technology effective at neutral pH andambient-temperature (≤40°C) condition for the purpose of improving the safety of chemicalcleaning technology and applied it to the removal and cleaning of scale adhered to plantcomponents. This paper describes the outline, application results, and future development of thisdeveloped technology.

|2. Neutral chemical cleaning agent
Table 1 compares our developed chemical cleaning method with a general chemical cleaning
method. This technology uses the Rust Vulture MK series, which is a neutral scale removal
chemical developed by MHI and Kyoeisha Chemical Co., Ltd. In 1991 in order to solve theproblems of safety and workability of acid cleaning, as the main agent and can selectively remove
just iron oxide scale (rust) by catching iron ions with the use of a chelating reaction and enhancing
the scale dissolution reaction. Since this agent has a fundamental property that reacts in the neutral
region and in the ambient temperature range, it can improve the working environment and simplify
the process and can be applied to a wide range of applications including the removal of rust on
equipment generated during storage.
Figure 1 shows the relationship between the scale removal mechanism and theoxidation-reduction potential of the developed chemical cleaning method. The iron oxide
dissolution of this technology is related to the oxidation reduction potential of the environment, and*1*2*3

Chemical Research Department, Research & Innovation Center
Materials Research Department, Research & Innovation Center
Chief Staff Manager, Chemical Research Department, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
2by controlling the environment to a reducing atmosphere (low potential), iron oxide can bedissolved even in a neutral environment.
Table 1 Comparison of our developed chemical cleaning method with general chemicalcleaning method
General chemical cleaning method
Chemical cleaning method developed by MHI
Base compound
Inorganic acid, Organic acid, Etc.
Chelating agent, Surfactant
Cleaning solution pH
AcidicNeutral
Dissolution temperature
Heating (up to 90°C)
Ambienttemperature (25 to 40°C)
Cleaning time
Up to 10 hours*
Up to 100 hours*
Hydrogen generation
Generated
Trace amount
Cleaning method
Circulating
Circulating, Pickling, Swinging*Since the time to completion of cleaning varies depending on the attached amount and properties of the scale, cleaning
time is set in advance by the dissolution test.

Figure 1 Relationship between scale removal mechanism and oxidationreduction potential of developed chemical cleaning method|3. Actual cleaning achievements and efforts
Table 2 lists application examples of the new chemical cleaning technology to rust and scale
removal. Since its development, this technology has been applied to various kinds of cleaningcases, such as the cleaning, removal, and flushing of rust generated and scale adhered mainly on thesurfaces of thermal power plant components. Currently, the technology is being developed further
aiming at application to chemical cleaning of boiler evaporation tubes on actual equipment. Case
examples of chemical cleaning of scale on electromagnetic filters and industrial drum boiler
evaporation pipes, as well as the progress of development aiming at cleaning of scale onevaporation pipes of HRSG and oxygen treatment once-through boilers, are reported below.
Table 2 Application examples of new chemical cleaning technology to rust and scale removal
TargetSpiral fin tube
Feedwater flow regulating valve VRT plate
Steam turbine oil piping
High-pressure feedwater heater
Cooling water piping
Electromagnetic filter
Drum boiler
In development

HRSG
Once-through boiler

PurposeRemoval of rust generated on the outer surface due to salt damage
Removal of scale adhered in the hole
Removal of foreign matter by chemical flushing, Cleaning of the inner surface
Removal of scale adhered to the heat transfer surface
Removal of rust and scale adhered in the pipes
Removal of scale collected in the filter
Removal of scale adhered to the inner surface of the evaporation tubes
Removal of scale adhered to the inner surface of the evaporation pipes(including phosphoric acid component)
Removal of scale adhered to the inner surface of the evaporation pipes(including hematite and auto-oxidation scale)Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)3

3.1

Results of cleaning of electromagnetic filter on actual equipment

When the operation of a plant includes many start-ups and shutdowns such as on weekends
and at night, the suspended substances (mainly iron oxide and hydroxide) in the system increase,causing equipment problems such as scale adhesion. Therefore, a filter is installed as a condensate
treatment device at the outlet of the condensate pump to improve the quality of the water supply.
An electromagnetic filter is one of the components of the filter, and magnetic coils are used tomagnetize the filter element in the electromagnetic filter to capture the magnetic particles containedin the fluid. Nonmagnetic particles that penetrate and adhere inside the filter element cannot be
removed by reverse cleaning with pure water at the time of plant stop. For this reason, chemicalcleaning and physical cleaning (jet cleaning) of the filter element were performed by opening theequipment during periodic inspections.
As a new cleaning method for the electromagnetic filter, cleaning with the developed agent
was carried out. Table 3 compares this cleaning method with the conventional one. Due to cleaning
with a swing blow method where the cleaning liquid is prepared in a chemical liquid container,
pumped to the filter tower for impregnating and filling, and then pumped back to the chemicalliquid container after a certain period of time, the opening of equipment, removal and reinstallationof the filter element, chemical cleaning, and jet cleaning, which are required by the conventional
method, are no longer required, and the work period has been greatly shortened from 10 to 20 daysin the past to 2 to 3 days. In addition, a cleaning cost reduction of 30% or more was achieved.
Figure 2 indicates the results of this cleaning method.
Table 3Comparison with conventional cleaning method of electromagnetic filter
Process
1. Opening of equipment (using a crane)
2. Removal of filter element

Cleaning
procedure

3. Chemical cleaning
4. Jet cleaning
5. Assembly of equipment (using a crane)
6. Washing with water
Number of working days (days)

Conventional cleaning
method●Cleaning with developedby MHI―●―●●●●
10 to 20●――●
2 to 3●: RequiredFigure 2 Results of cleaning of electromagnetic filter (opened to confirm effectiveness)

3.2

Results of cleaning of industrial drum boiler on actual equipment

The new chemical cleaning method was applied to chemical cleaning of the main boiler(evaporation amount: MCR 78 t/h) at the Yamaguchi Plant of Taiyo Oil Co., Ltd., to verify theapplication of the method to cleaning of scale adhered to the boiler evaporation pipes of actualequipment. Table 4 shows the specifications of the target unit. Since the scale component of thetarget boiler included copper scale that cannot be dissolved and removed by the new chemicalcleaning alone, the new chemical cleaning was performed for 24 hours, and acid washing with acopper dissolver added was performed for 6 hours for chemical cleaning of the actual equipment.
Figure 3 gives the results of the chemical cleaning test. Figure 4 indicates the results of hydrogenmeasurement during chemical cleaning. The appearance of the tube after the application of the newchemical cleaning method confirms that the scale on the outside of the furnace with less adhesionMitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
4of copper could be completely removed by the new chemical cleaning method alone. The
generation of hydrogen during the application of the new chemical cleaning method was lower thanthe detection limit (0.05%). On the other hand, it was confirmed that hydrogen gas with aconcentration of 2% or more was generated during acid cleaning that was subsequently performed.
As a result, it was verified that chemical cleaning can be carried out in an environment where fire isused for work operations and where other work operations are performed in parallel. The internal
inspection at the end of cleaning resulted in just several kilograms of sludge remaining in the water
drum and no sludge deposition in the steam drum. A significant reduction in the amount of sludge,
compared with cases where acid cleaning is carried out independently, was confirmed.
Table 4 Specifications of target unit
TargetBoilerspecifications

Amount ofadhered scale
Cleaning work(actual results)

Type
Evaporation amount
Steam pressure
Steam temperature
Fuel
Operation commencement
Cleaning history
Inside furnace
Inside casing
New low-temperature boiler chemical cleaning
Conventional cleaning (with hydrochloric acid)
Internal check

Yamaguchi Plant of Taiyo Oil Co., Ltd.
Main boiler
MB-EBR boiler (outdoor type)
MCR 78 t/hMaximum operating pressure 11.18MPa
Superheater outlet 500°C
Heavy oil and off gas
1993
No cleaning
88 mg/cm2
35 mg/cm2
September 14 to 15, 2016 (40°C - 24 hours)
September 16 to 17, 2016 (60°C - 6 hours)
September 18, 2016

Figure 3 Results of new low-temperature boiler chemical cleaning

Figure 4 Results of hydrogen measurement during boiler chemical cleaningMitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
53.3

Results of application of developed cleaning method to HRSG chemicalcleaning

Since HRSG has a smaller heat load than a conventional drum boiler, it was thought thatchemical cleaning is not required. However, because leakage problems due to water treatment
incompatibility (phosphate corrosion) occurred, the removal of corrosive scale by chemicalcleaning is considered to be one of the countermeasures. Figure 5 shows an example case ofcorrosion caused by a boiler compound considered to remain in scale observed in an overseas plant.
In this case, the phosphate hide-out phenomenon (phosphate is not detected even though injected)
was confirmed during trial operation, so the injection of phosphate was stopped. However, leakage
occurred 10 years or more after the injections were stopped. The cause is thought to be thedevelopment of phosphate corrosion due to the phosphorus component remaining in the scale. This
led to the determination that the removal of corrosive scale is necessary.

Figure 5 Example of problem with HRSG
(phosphate corrosion)

The fin tubes used for HRSG have a larger heat dissipation effect than general tubes, so it isexpected that maintaining the cleaning temperature is difficult. For this reason, a chemical cleaning
test under a non-heating condition was carried out in a laboratory to confirm the phosphate scale
removal capability, which is the cause of corrosion. Figure 6 gives the results of the scaledissolution test. Figure 7 shows the schematic system and process of chemical cleaning for theremoval of scale adhered to the inner surface of HRSG, which was studied based on these test
results. Since there is no need for a heating source and the risk of hydrogen gas generation during
cleaning is low, it is considered that safety can be ensured, and cleaning can be performed inparallel with other work in the case of actual equipment.

Figure 6

Results of scale dissolution test of HRSG on actual sample equipment

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
6Figure 7 Schematic of system and process of new low-temperature boiler
chemical cleaning method for HRSG

3.4Efforts toward application of developed cleaning method to once-throughboiler cleaning for oxygen treatment

For oxygen treatment (OT), which is the main water treatment of once-through boilers,
cases of poorly soluble powder scale (the chief component of which is hematite) adhered andcreated on the scale surface were reported. Therefore, in conventional chemical cleaning based onan acid solution, magnetite scale on the inner surface is melted and peeled off, and then discharged
by conveying with liquid flow.
Figure 8 depicts the residual sludge of a once-through boiler after chemical cleaning(organic acid is applied). Most of the hematite (over 90%) remains un-melted as sludge and cannot
be sufficiently discharged by cleaning with water. It has been reported that such sludge clogged apipe after the boiler started and caused a leak. For this reason, it is necessary to collect sludge by
installing a filter in the system during cleaning, and to cut an inspection hole in the header andjet-clean the remaining sludge in the boiler.
Development toward the application of the new chemical cleaning technology – which ishighly capable of dissolving hematite – to actual equipment is being promoted. Figure 9 shows theresults of the scale dissolution test using hematite reagent. Figure 10 is a schematic representationof the periodic inspection period shortening effect expected by the application of this technology.
While a reduction of plant downtime is desired, the inspection period can be shortened by about
one week by establishing this technology.

Figure 8 Residual sludge of once-through boiler
after chemical cleaning (organic acid is applied)Figure 9 Results of scale dissolution test using
hematite reagent

Mitsubishi Heavy Industries Technical Review Vol. 55 No. 2 (June 2018)
7

Figure 10 Periodic inspection period shortening effect due to application of new chemicalcleaning technology|4. Conclusion
In order to improve safety, shorten the work operation period, and reduce the cost ofchemical cleaning by optimizing processes, which is implemented as one of the measures against
water-related problems in plants, etc., we developed an ambient-temperature and neutral chemicalcleaning technology that can be substituted for general high-temperature and acidic cleaning, andwe are working on its application to cleaning work. This paper presented the results of theapplication of the new chemical cleaning technology to plant components, HRSG and industrial
drum boilers, and explained the application situation to once-through boilers. In the future, we will
expand the application of this technology to actual equipment and further advance it. Thetechnology introduced above is applicable to not only power generation equipment, but also theiron piping of chemical plants, marine boilers, etc., as a technique to remove the scale and rust
produced and adhered therein. We intend to expand the application of this technology.

References
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2. Uchida, Y. et al.，Introduction to Electromagnetic Filter Cleaning by Neutral Rust Removal Chemical，Thethermal and nuclear power，Vol. 63 No.4 (2012)
3. Shimoda, S. et al.，Application of Neutral and Non-Heating Scale Removal Method in Combined Cycle
Plant，The thermal and nuclear power，Vol. 66 No.2 (2015)
4. 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)
5. Tsubakizaki, S. et al., Advanced Water Treatment Technologies (High-AVT) for HRSG Corrosion Control
corresponding to New JIS B8223 (Japanese Industrial Standards), Mitsubishi Heavy Industries TechnicalReview Vol. 54 No. 3 (2017)