FIELD OF THE INVENTION
The present invention in general relates to cleaning of steam generators, and in
particular cleaning of the mill scale and rust formed over carbon steel and alloy
steel surfaces during manufacturing, storage and fabrication of sub critical or
supercritical boilers. More particularly, the present invention relates to a method
of heating cleaning solvents in acidic range for removing mill scale, rust formed
during manufacturing, storage and fabrication and boiler operational stages of
sub critical, super critical, drum type / once through boilers.
BACKGROUND OF THE INVENTION
Traditionally, the purpose of preoperational cleaning of technical systems, is to
remove natural contaminants formed on the components which could cause
operating problems or even failures during initial operation. Preoperational
contaminants include mill scale, weld slag, corrosion products, oil grease, debris
and dirt, temporary proactive coatings and other contaminants left-over after
fabrication and erection of the technical installations for example, steam
generators. Mill scale, a dense iron oxide layer produced on steel surfaces during
fabrication, normally leads to develop fracture and erosion during boiler
operation. The purpose of post operational chemical cleaning of steam
generators is to remove the built-up of scale / sludge during operation which
may form due to contamination of chemical impurities and corrosion products to
the steam generating surface of the boiler. Thus, the steam generator is cleaned
periodically to prevent concentration of corrosion-causing chemical contaminants.

The art of cleaning the steam generators has progressed from the use of highly
acidic solutions of mineral acids/organic acids to alkaline chelants cleaning to
remove mill scale and rust from the metal surfaces and operational deposits to
prepare the surfaces for passivation.
During the cleaning process, the cleaning solvents (mineral acids like
Hydrochloric acid, Hydrofluoric acid and organic acids like Citric acid and di-
ammonium Ethylene Diamine Tetra Acetic acid (EDTA) is heated from ambient
temperature to higher temperature in order to enhance the chemical reaction
between the acids and iron oxides.
The heating of the solvents is conventionally carried out by heating the solvents
by steam from auxiliary boiler or steam from other boiler (if it is available) for
cleaning solvents with acidic pH as in the case of HCI / HF / Citric acid / di-
ammonium EDTA at temperature <100°C.
According to the prior art, the heating of the solvents is also carried out by
heating the solvent directly by firing the boiler for cleaning solvents with alkaline
pH as in the case of tetra-ammonium EDTA at a temperature >100°C.
Presently, without using the external steam, it is not possible to heat the
cleaning solvents working in acidic range ( Ph less than 7.0) and clean boilers.

US Patent 2011/0303247 A1 discloses a method of removing deposits and
impurities from the secondary side of a heat exchanger comprising of the steps
of: removing a volume of working fluid from the secondary side of heat
exchanger installing a temporary adaptor for direct injection of steam injection;
injecting steam through the temporary adaptor and into the secondary side of
the heat exchanger, wherein the injected steam heats the heat exchanger and
residual fluid to a target temperature range.
US Patent 4,681,705 to Baum discloses a process for cleaning deposits from the
restricted areas of a steam generator of a nuclear power plant system which
overcomes, to a large extent, the corrosion problem by increasing the
concentration of an aqueous organic cleaning agent solution in the specific areas
to be cleaned by varying the temperature and pressure of the cleaning solution.
US Patent 3,506,576 describes a process for cleaning a ferrous based metal
surface using an aqueous alkaline solution of an alkylene polyacetic agent with a
water soluble sulfide for removing copper deposits.
US Patent 4,681,705 describes a process for the removal or copper-type deposits
from the secondary side of nuclear steam generators.
US Patent 3308065 describes a method for removing hardness scale and iron
oxide from metal surfaces, for passivating ferrous metal surfaces.

US Patent 341160 teaches an improvement in the method of passivation of
ferrous metal surface, e.g. steel, especially following removal of metallic copper
from the surfaces.
US Patent 2011/0303247 A1 discloses a process for removing deposits and
impurities from the secondary side of a heat exchanger of nuclear pressurized
water reactor by installing a temporary adapter in the exposed access
penetration, the adapter being configured for direct steam injection; injecting
steam through the temporary adapter and into the secondary side of the heat
exchanger, wherein the injected steam heats the heat exchanger and residual
fluid to a target cleaning temperature range.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of heating cleaning
solvents in acidic range for removing mill scale, rust formed during
manufacturing, storage and fabrication and boiler operational stages of sub
critical, super critical, drum type once through boilers.
Another object of the invention is to propose a method of heating cleaning
solvents in acidic range for removing mill scale, rust formed during
manufacturing, storage and fabrication and boiler operational stages of sub
critical, super critical, drum type once through boilers which utilizes preheated
plain demineralized water in the steam generators under cleaning, by firing
selective number of guns.

A still another object of the invention is to propose a method of heating cleaning
solvents in acidic range for removing mill scale, rust formed during
manufacturing, storage and fabrication and boiler operational stages of sub
critical, super critical, drum type once through boilers which utilizes cleaning
solvents selected from a group consisting of HCLyHF/citric acid/de di-ammonium
EDTA.
A further object of the invention is to propose a method of heating cleaning
solvents in acidic range for removing mill scale, rust formed during
manufacturing, storage and fabrication and boiler operational stages of sub
critical, super critical, drum type once through boilers which eliminates the need
for supplying steam for heating the cleaning solvents from auxiliary boilers or
other boilers.
A still further object of the invention is to propose a method of heating cleaning
solvents in acidic range for removing mill scale, rust formed during
manufacturing, storage and fabrication and boiler operational stages of sub
critical, super critical, drum type once through boilers in which DM water is first
heated through boiler firing and then the acidic solvent is introduced into
preheated DM water.

SUMMARY OF THE INVENTION
Accordingly, there is provided a method of heating cleaning solvents to acidic
range for removing mill scale rust formed during manufacturing, storage and
fabrication and boiler operational stages of sub critical, super critical, drum type
once through boilers, comprising the steps of filling the boiler with only plain
demineralized (DM) water; raising the temperature of the water to a desired
level by boiler firing; injecting a concentrated inhibited acidic solvent into the
boiler; and mixing the solvent in heated DM water by a boiler circulation pump,
wherein the solvents include one of HCI, HF, Citric acid, and di ammonium EDTA
and wherein the temperature of desired temperature range is between 45°C to
95°C.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 shows a process flow-chart according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, the mill scale and rust formed during manufacturing,
storage and fabrication of carbon steel and alloy steel components of a new
boilers including accumulated deposit over the steam generating surfaces of old
boilers are solubilized by a mineral acid for example HCI, HF organic acid such as
citric and acetic acid based organic chelating agent solution containing corrosion
inhibitor at higher temperature.

Conventionally the higher temperature requirement of the solvent for effective
cleaning is achieved by 1) by the use of external steam from auxiliary boiler/
other boiler in case of acidic solvents 2) by firing the selective number of oil
guns of the steam generators in the case of alkaline solvents.
As per the present invention, and as shown in figure 1, the boiler is filled with
plain demineralized water up to normal drum level in the case of drum type
boiler and up to normal Separator Storage Tank (SST) level in the case of once
through super critical boiler. The boiler is lighted up with one or two oil guns
with LDO. The Boiler circulating pump remains in operation. The Boiler water
temperature is slowly raised to 70 to 95°C depending on the temperature
requirement. Once stabilized temperature of specific interest is reached
throughout the boiler - evaporator circuit, the boiler is tripped and boxed up.
Adequate quantity of heated DM water from the boiler is drained keeping a close
watch on the drum level / SST level in order to accommodate the concentrated
cleaning solvent solution.
Required quantity of inhibitor is added in a separate external mixing tank
containing concentrated solvent HCI/ HF / Citric acid/di ammonium EDTA. The
temporary pumps are connected to the mixing tank, and the pumps are run on
recirculation for optimum mixing of the inhibitor with cleaning solvent.

This concentrated inhibited cleaning solution is injected into the boiler so as to
achieve desired concentration of the acid in the boiler for example, 4-6% in the
case of HCI, 1-2% in the case of HF, 3-5% in the case of citric acid and 3-5% in
the case of di ammonium EDTA.
Depending on the volume of hot DM water drained, desired concentration of
cleaning solvent in the boiler and concentration of cleaning solvent is prepared in
the mixing tank. One or more batchers of injection of concentrated solvent is
used. The operation of the circulating pump ensures uniform concentration of
the cleaning solvent in the boiler. Since the boiler is boxed up, the temperature
of the cleaning solvent is maintained in the desired range 55°C - 65°C in the
case of HCI, 45-60°C in the case of HF, 80°C - 90°C in the case of citric acid/di
ammonium EDTA.
Since the cleaning solvents are in the acidic range, the boiler cannot be fired to
raise the temperature of the cleaning solvent from the ambient to desired
temperature. According to the prior art, use of external steam either from
auxiliary boiler or from other or from other boiler to raise the temperature of the
cleaning solvent from ambient to desired temperature is resorted to.
The inventive method achieves the desired temperature of the cleaning solvent
by heating the plain DM water till achieving the temperature by boiler firing and
boxing up the boiler and injecting the concentrated cleaning solvent into the
boiler and mixing up using boiler circulating pump and maintaining the desired
temperature of the solvent.

WE CLAIM :
1. A method of heating cleaning solvents to acidic range for removing mill
scale rust formed during manufacturing, storage and fabrication and boiler
operational stages of sub critical, super critical, drum type once through
boilers comprising the steps of:
• filling the boiler with only demineralized (DM) water;
• raising the temperature of the water to a desired level by boiler firing;
• injecting a concentrated inhibited acidic solvent into the boiler; and
• mixing the solvent in heated DM water by a boiler circulation pump;
wherein the solvents include one of HCI, HF, Citric acid, and di
ammonium EDTA, and wherein the desired temperature range is between
45°C to 95°C.
2. The method as claimed in claim 1, wherein the boilers may be a new
boiler or an operating boiler.
3. The method as claimed in claim 2, wherein the boiler includes a sub
critical boiler or a once through super critical boiler.

4. The method as claimed in claim 3, wherein the sub critical boiler includes
a drum type boiler or a once through boiler.
5. The method as claimed in claim 1, wherein the boiler firing is achieved
by operating 'n' number of oil guns where 'n' may vary between 1 to 4.
6. The method as claimed in claim 1, wherein the pH acidic solvent varies
from < 1 to less than 7.

ABSTRACT

The invention relates to a method of heating cleaning solvents to acidic range
for removing mill scale rust formed during manufacturing, storage and
fabrication and boiler operational stages of sub critical, super critical, drum type
once through boilers, comprising the steps of filling the boiler with only plain
demineralized (DM) water; raising the temperature of the water to a desired
level by boiler firing; injecting a concentrated inhibited acidic solvent into the
boiler; and mixing the solvent in heated DM water by a boiler circulation pump,
wherein the solvents include one of HCI, HF, Citric acid, and di ammonium EDTA
and wherein the temperature of desired temperature range is between 45°C to
95°C.