FIELD OF THE INVENTION
The present invention in general relates to cleaning of steam generators, heat
exchangers and in particular cleaning the mill scale and rust formed over carbon
steel and alloy steel surfaces during manufacturing, storage and fabrication of
supercritical boilers. More particularly, the present invention relates to a method
of cleaning and passivation of ferrous metal surfaces of supercritical boilers.
BACKGROUND OF THE INVENTION
Traditionally, the purpose of preoperational cleaning is to remove construction
contaminants which could cause operating problems or even failures during
initial operation of the boiler. Constructional contaminants include mill scale,
weld slag corrosion product, oil, grease, debris and dirt, temporary proactive
coatings and other contaminants remaining after fabrication and erection of the
steam generators. Mill scale known as a dense magnetite layer produced on steel
surfaces during fabrication, is subject to fracture and erosion during boiler
operation. Because the exposed metal is anodic to the mill scale, these surface
cracks are potential corrosion sites.
Subcritical boilers operated of pressures greater than 900 psig are generally
given hot alkaline cleaning and an acid or chelants cleaning of the boiler and the
economizer. Supercritical boilers and systems are usually given hot alkaline
cleaning along with an acid or chelants cleaning of the condensate/ feedwater
systems, boiler and economizer. Acid or chelants cleaning of the boiler and
economizer is generally performed before initial operation to prevent
contaminant damage or water chemistry upsets.
Steam generators, both nuclear powered and fired by other sources like fossil
fuels, are subject to the built-up of scale / sludge during operation which may
form concentration sites for contaminating chemical impurities adjacent to the
steam generating structures in the generator.
Consequently, the steam generator must be cleaned periodically to prevent the
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 to prepare the surfaces
for passivation.
Passivation can be defined in simple terms as the process which causes the loss
of chemical reactivity in certain chemical environments. A detailed procedure of
passivation can be found in M.G. Fontana and Greene, Corrosion Engineering, 2nd
Edition, New York, N.Y. McGraw-Hill, 1978, pp 319-324. Several important points
are provided in this reference. First, it is evident that passivation is a state where
corrosion of a metal in the environment is very low. Secondly, the passivation is
relatively unstable and is specific to the environment. It is therefore important to
understand the purpose of passivating solution applied after chemical cleaning.
After being chemically cleaned with acids or other solvents designed to remove
metal oxides, metal surfaces, especially ferrous based steels, they are usually in
a highly active state in which they are subject to rapid oxidation in the presence
of air. This is often referred to as "flash rusting" or "after rust". The normal
purpose of passivation after the chemical cleaning is to prevent this flash rusting.
The chemical cleaning passivation solution is aimed at preventing flash rusting
on steam generator surfaces after chemical cleaning.
A well-known steam generator post operational cleaning process is a two-step
chemical cleaning process based on the dissolution and chelation of iron and
copper, which are the major components in copper-bearing generator sludge,
with ethylenediaminetetraacetic acid ( EDTA). Magnetite iron, which includes
Fe+3 and Fe+2, reacts with EDTA. Copper reacts with EDTA after being oxidized
by hydrogen perioxide.
U.S. Patent No. 3,506,576 discloses a cleaning solution useful for cleaning
ferrous based metal surfaces, such as those of steam boilers, which is an
aqueous alkaline solution of strong chelating agent, for example EDTA, that
contains a water soluble sulfide capable of providing sulfide ions. The cleaning
solution additionally prevents the deposition of copper on the ferrous metal.
U.S. Patent Nos. 4,681705 to Robertson and 4,693,833 to Toshikuni et al both
disclose methods of treating radioactive liquids in the course of operating and
cleaning nuclear power facilities U.S. Patent No. 4681705 is specifically directed
to the decontamination of mixtures of water and water-immiscible organic
liquids, such as contaminated reactor lubricating oil. A water-soluble chelating
agent, such as EDTA, and, optionally, a water soluble inorganic precipitating
agent are used for this purpose.
U.S. Patent No. 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.
U.S. Patent No. 3,506,576 describes a process for cleaning a ferrous based metal
surface using an aqueous alkaline solution of an alkylenepolyacetic agent with a
water soluble sulfide for removing copper deposits.
U.S. Patent No. 4,681,705 describes a process for the removal of copper-type
deposits from the secondary side of nuclear steam generators.
Dowell Schlumherger (DS) paper on "Passivation of Steel In Ammonium EDTA
solution", presented Mar 25-29, 1985, at Corrosion 85, describes a process for
passivating steel after performance of a chemical cleaning. The invention utilizes
hydrogen peroxide as an oxidant process.
U.S. Patent 5587025 relates to a method for passivating the surfaces of a
nuclear steam generator and solution therefore includes EDTA and hydrogen
peroxide in a temperature range of 37°C to 49°C. The solution is adjusted to
have a pH of between about 9.2 and 9.5 and used on the surfaces for two hours
or more for passivating the surfaces.
U.S. Patent 3308 065 teaches a method for removing hardnessscale and iron
oxide from metal surfaces, for passivating ferrous metal surfaces.
U.S. Patent 3413160 relates to an improvement in the method of passivation of
ferrous metal surface, e.g., steel, especially following removal of metallic copper
from the surfaces.
The above prior patents pertain to cleaning of scales formed or deposition build-
up during boiler operation and passivation of the metal surfaces.
Accordingly, the prior art fail to provide a cleaning process to remove mill scale
and rust forming over carbon steel and alloy steel during manufacturing and
fabrication of a once through supercritical boilers. The prior art further fail to
provide a passivation process for the above carbon steel and alloy steel surfaces
of the once through steam generators.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a cleaning method
for removing mill scale and rust formed during manufacturing, storage and
fabrication of carbon steel and alloy steel components of once through
supercritical boilers which eliminates the disadvantages of prior art.
Another object of the present invention is to propose a method of passivation of
the cleaned surfaces of the carbon steel and alloy steel components of the once
through super critical boilers.
A further object of the present invention is to propose a method for cleaning and
passivation of the carbon steel and alloy steel surfaces of once through super
critical boilers passification in a single solvent by altering the temperature and
chemical addition.
DETAIL DESCRIPTION OF THE INVENTION
According to the invention, the millscale and rust formed during manufacturing,
storage and fabrication of carbon steel and alloy steel components of a once
through supercritical boilers are solubilized by an alkaline, acetic acid based
organic chelating agent solution containing corrosion inhibitor at higher
temperature. The required alkaline condition is achieved by injection of ammonia
to get pH between 9.0 and 10.0. The corrosion inhibitor containing organic
nitrogen and sulphur compounds at a concentration between 0.01 and 3.0
weight percent is used.
A higher temperature, more than boiling point of water and less than the
dissociation temperature of the salt of the acid of organic chelants is envisaged.
The required temperature of the solvent is achieved by firing the selective
number of oil guns of the steam generators.
The acetic acid based organic chelants is of the formula :
( HOOCCH2)2-N(CH2)n-N(CH2COOH)2
wherein 'n" vary from 2 to 5.
The alkaline condition by ammonia converts the acid into salt of the formula
(NH400CCH2)2-N-(CH2)n-N(CH2COONH4)2
The concentration of the salt can be maintained between 1.0 to 10.0 weight
percent for effective removal of mill scale and rust.
Once the removal of mill scale and rust is achieved which is based on
stabilization of concentration of iron in the cleaning medium, firing of oil gun is
put off, and the temperature of the solvent is reduced to less than 60°C by
operating the forced draft fan and the induced draft fans of the steam
generators. The sodium nitrite solution is injected into the cleaning solvent so as
to get the concentration between 0.2 and 2% by weight in the cleaning solvent.
During the entire cleaning/ passivation process, the solvent is circulated by boiler
recirculation pump.
After a time period of 3 to 5 hours subsequent to sodium nitrite addition, the
cleaning solvent is drained out from the steam generators.
At the end of the cleaning cum passivation process, all the solvent touched
carbon steel and alloy steel surfaces of the supercritical steam generator in
contact with the solvent are cleaned and become passive and are resistant to
corrosion.
WE CLAIM:
1. A method for cleaning milt scale and rust formed over the surfaces of
carbon steel and alloy steel components of new once through super
critical steam generators and passivating the cleaned surfaces, comprising
the steps of:
• contacting the surfaces of the generators with aqueous, alkaline
solution of a polyamide poly acetic acid chelating agent with corrosion
inhibitor,
• increasing temperature of the solution from ambient to higher
temperature by firing selective number of oil guns of the steam
generator;
• maintaining the circulation of the cleaning solvent by a circulation
pump for a predetermined time period;
• cooling the solvent to below 60°C by putting off the oil guns after a
specified time and admitting ambient air flow into the combustor;
• injecting oxidant sodium nitrite into the cleaning solvent and
maintaining the circulation for a time period of about 4 hours; and
• draining of the solvent from the steam generators.
2. The method as claimed in claim 1, in which the cleaning solution
comprises: from about 1 percent by weight to about 10% by weight of
alkaline polyamine polyacetic acid and corrosion inhibitor containing
sulphur and nitrogen compounds from about 0.1% by weight to about 3%
by weight of cleaning solvent.
3. The method as claimed in claim 1, in which the carbon steel comprise
SA210 Gr A, SA210 Gr B, SA210 Gr C and the alloy steels comprise SA213
T12,SA213 T22, SA213 T23.
4. The method as claimed in claim 1, in which the steam generators
comprise one of once through super critical, once through ultra-
supercritical steam generators and advanced ultra- super critical boilers.
5. The method as claimed in claim 1, in which the temperature of the
cleaning solvent is raised either by firing one or more oil guns of the
steam generators or by external heating by gas burners, wherein the
temperature is above the boiling point of water or less than the
dissociation of the salt of acid chelating agent.
6. The method as claimed in claim 1, in which the solvent is cooled from the
raised temperature to below 60°C by natural cooling or by admitting
ambient air through operation of both forced draft fan and induced draft
fan of steam generators.
7. The method as claimed in claim 1, in which the concentration of sodium
nitrite in the cleaning solvent may be 0.2 to 2.0% by weight.

ABSTRACT

The invention relates to method for cleaning mill scale and rust formed over the
surfaces of carbon steel and alloy steel components of new once through super
critical steam generators and passivating the cleaned surfaces, comprising the
steps of contacting the surfaces of the generators with aqueous, alkaline solution
of a polyamide poly acetic acid chelating agent with corrosion inhibitor,increasing
temperature of the solution from ambient to higher temperature by firing
selective number of oil guns of the steam generator; maintaining the circulation
of the cleaning solvent by a circulation pump for a predetermined time period;
cooling the solvent to below 60°C by putting off the oil guns after a specified
time and admitting ambient air flow into the combustor; injecting oxidant sodium
nitrite into the cleaning solvent and maintaining the circulation for a time period
of about 4 hours; and draining of the solvent from the steam generators.