Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02039944 2000-02-28
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HIGH SPEED FLAME SPRAYING PROCESS HAVING
PARTICLE VELOCITY OF AT LEAST 300 m/s
BACKGROUND OF THE INVENTION
s Field of the Invention
The present invention relates to a process for
coating blades of a rotating thermal machine.
Discussion of Background
For example in the open gas turbine process, the
~o air taken in by the compressor also contains water vapor and
solid and gaseous impurities. These have an adverse effect
due to erosion, soiling and corrosion. Some of the deposits
on the blades have a considerable concentration of
corrosively acting constituents such as NaCl and KC1. The
salts cause not only high-temperature corrosion on the
turbine blading but also increased pitting corrosion in the
compressor region and a complex chemical reduction in the
strength of the blade material. In the case of high
atmospheric humidity, there occurs in the region of the
zo compressor intake a concentration of water vapor, which
explains the greater degree to which corrosion attacks the
front rows of blades. In order to overcome this problem to
a certain extent, blades of rotating thermal machines are
often provided with protective coatings. These are used
zs both in the case of steam and gas turbine blades and in the
case of compressor blades. Thus, the main objective is to
increase the resistance to corrosion and oxidizing attacks
as well as to erosion and abrasion (wear). If, in spite of
surface treatment, the blades do exhibit damage of a degree
3o which could endanger operational reliability, the next step
is to proceed to remove the blades: either they are
replaced by new ones or reconditioned and refitted. This
removal and fitting entails relatively high costs and time
spent. Furthermore, the true condition of the blading is
35 only evident after a relatively long time, i.e. after
precleaning, and therefore the decision as to whether
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reconditioning of the blades is ~feas'~~'~b~l~ '~ or not, or
already necessary, can only be taken much later. The
disadvantages of this method are the great time losses,
the high operating costs of the installation, higher
costs for inspections and the uncertainty over the
question of the reconditionability of the blades.
Therefore, the tendency has been instead to look for
ways and means of remedying this situation. In this
context, a process has become known by which the entire
bladed rotor is lifted out of the stator in order to be
reconditioned in a separate installation. The bladed
rotors to be coated must be degreased in a suitable
way, any organic coatings which may have been applied
at an earlier time must be completely removed.
Subsequently, the areas to be coated are roughened by
dry sand blasting with aluminum oxide and the metallic
surface activated. The zones not to be coated must be
masked with suitable materials. Thereafter, the
undercoats are applied, each having to be fired. This
results in a laborious procedure: a sintering process
or firing process takes about 55 hours and, on average,
has to be carried out four times. This sintering or
firing process during coating comprises a heat
treatment at about 350° Celsius over a holding time of
about 10-12 hours. In addition, in order to carry out
the individual process steps, quite large installations
of specific geometric design have to be provided,
consider for instance that during the sintering process
the entire bladed part of the rotor has to be
surrounded by a furnace cover.
SUI~'~IARY OF THE INVENTION
Accordingly, one object of this invention is to
provide a process of the type mentioned at the
beginning with a more efficient method in terms of the
times required and costs expended for the
reconditioning of the blades. Another object of the
invention is to maximize the service life of the
coating by suitable processes and protective coatings.
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The essential advantages of the invention are
to be seen in that the bladed rotor does not have to be
lifted out of its mounting in the stator for the first
process of reconditioning: the cleaning or removal of
protective coating can be carried out before the actual
shutting-down of the machine, i.e. of the compressor,
that is to say during a final phase of operation ("on
line"). As a result, an even exposure of the blades to
be treated is attained, the consequently achieved
efficiency of this cleaning process, which ensures
extensive removal of any protective coating there may
be, permitting an immediate decision on the
reconditionability of the blades. This decision can be
made already after shutting down the machine and ,..
removing the upper part of the stator. If, after
appropriate analysis of the condition of the blades, it
is decided to recondition them, it is sufficient to
lift the bladed rotor out of the mount and place it on
blocks, where the further process steps of
reconditioning can be carried out without the aid of a
sophisticated structure. This leads to low operating
costs (overhauling costs), which means that there is
nothing to prevent this type of treatment being carried
out periodically. Consequently, the operational
reliability of the installation is increased.
A further significant advantage of the
invention is to be seen in that, by using a high-speed
flame spraying process, the blades, pretreated in the
fitted state, receive an appropriate protective
coating, preferably Si and A1 based, just where and
when needed, it being possible for this coating process
to be carried out without lengthy heat treatment and
without the aid of special additional equipment, This
simplifies the entire technical coating procedure,
while the costs are lower by about half than in the
case of the known, processes. In addition, the service
life of this type of coating is much higher than in the
case of the coatings currently used for this so-called
CA 02039944 2000-02-28
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complete coating. Since the pretreatment of the blades or
the after-treatment after spraying on the protective coating
is of great significance for the service life of the
coating, direct corrective measures can be undertaken
s specifically, as found to be required. What is obtained
after a very short time with low reconditioning costs by
means of a process which is very environmentally friendly is
blading of top quality which guarantees the operational
reliability of the installation over a prolonged period.
~o According to a broad aspect of the present
invention there is provided a process for coating blades of
a rotating thermal machine having a compressor. The blades
undergo a first cleaning process, in the operating state of
the thermal machine, by means of an agent mixed with the air
15 Stream to the compressor for a predetermined time through a
centrally placed three-jet nozzle which acts in an intake
channel of the compressor. The blades undergo a preparation
process for the subsequent treatment stage wherein the
blades are treated in an erosive bath after opening the
2o machine in a stationary state. The blades are coated in the
stationary state with an aluminium-based protective coating
by means of a high-speed flame spraying process which sprays
protective coating particles onto the surface of a base
material at at least a velocity of 300 m/s. The porosity of
2s the protective coating is below 0.5% and the composition of
the coating is one of the following: a) 6 to 15o by weight
Si, the remainder being aluminium; b) pure aluminium, c)
80% by weight A1, 5 to 15o by weight Si and the remainder
being Cu, Mn, Mg, Ni.
3o Advantageous and expedient further developments of
the way in which the object is achieved according to the
invention are identified in the further claims.
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BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and
many of the attendant advantages thereof will be readily
obtained as the same becomes better understood by reference
s to the following detailed description when considered in
connection with the accompanying drawings, wherein all
elements not required for direct understanding of the
invention have been omitted and the direction of flow of the
various media is indicated by arrows and wherein:
Fig. 1 shows a turbo group with units for a
pretreatment stage;
Fig. 2 shows a view of Fig. 1 in the plane II-II;
Fig. 3 shows a cleaning stage or removal of
protective coating in a vibrating, erosive bath;
15 Fig. 4 shows a view of the rotor according to Fig.
3 along the plane IV-IV;
Fig. 5 shows a final cleaning process with jet
nozzles; and
Fig. 6 shows a coating of the blades by a high-
2o speed flame spraying process.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like
reference numerals designate identical or corresponding
parts throughout the several views, in Figure 1 a
conventional gas turbo group 11 is shown, essentially
comprising a compressor part lla, a combustion chamber
llb and a turbine part llc. In the pretreatment of the
blades, a distinction must be drawn between whether, in
the original state, they were uncoated or coated.
Irrespective of this precondition, a first cleaning of
the blades takes place before shutting down the
machine, i.e. the compressor. In the case of coated
blades, this cleaning is based preferably on erosion °
abrasion by soft-blasting granules. Of course, the
cleaning of uncoated blades can be performed simply by
means of an aqueous solvent, for example
trichloroethylene. The cleaning agent (soft-blast
granules, aqueous solution etc.) is injected into the
air stream to the compressor for a certain time through
a centrally placed three-jet nozzle 1 (also see Fig. 2
in this respect), which acts in the intake channel of
the compressor. The even and intensive exposure 12 of
the compressor blades produces an efficient cleaning
process in the case of uncoated blades or an extensive
removal of the old protective coating in the case of
coated blades. The cleaning process is repeated several
times, as required. Since the soft-blasting granules
burn at temperatures of about 300 degrees Celsius,
there are no problems in this respect with regard to
disposal. If an aqueous solution is used, consideration
likewise has to be given to these aspects. The control
arrangement for the multi-jet nozzle 1 consists of a
ball cock 2, which is connected downstream, in the
direction of flow of the cleaning agent, of a mixing
chamber 3 and serves for controlling the rate of flow.
The pressure in this mixing chamber 3 is displayed by a
pressure gauge 7. Provided upstream of the mixing
chamber 3 is a tank 4, in which for example granules
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are stored, a screen 5 and an inlet valve 6 ensuring
that the mixing chamber 3 is supplied with homogeneous
material. The required pressure in the tank 4 is
provided by means of an air feed line 10, a pressure
reducing valve 8 and a main valve 9 in the air line
being further auxiliary means of the control
arrangement. By corresponding provisions, the turbine
blading can be similarly treated.
If necessary, the blades undergo further
cleaning or removal of protective coating. As Figs. 3
and 4 show, this takes place by means of a vibrating,
erosive bath 14. For this purpose, the bladed rotor lla
and llc is taken from the stator and placed on blocks _
13a and 13b in such a way that a certain part of the
blading is immersed in the bath 14. The individual
erosive components of the bath 14 are made to vibrate
by a vibration generator 15, causing removal of the
residual soiling or residual protective coating on the
blades. In principle, all types of blades of a rotor of
a gas turbo group can be treated in this way.
A final cleaning is carried out according to
Fig. 5 with an industrial glass blasting agent 18. This
final cleaning is based on erosion removal by said
agent, which may consist of glass. A certain part of
the blading is covered by a special capsule 16; with
simultaneous suction extraction 19 of the injected
agent, the cleaning is accomplished by means of one or
more jet nozzles 17.
Further process steps can he n,-w;r;Pr; a~
required:
- Grinding away of the still existing pitting at the
most corroded points.
- A crack testing of the blades.
- A dimensional check of the blades, if they have
undergone a grinding process.
- Roughening of the surface by sand blasting.
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- Before actual coating, it is recommendable to
preheat the blades to about 80 degrees Celsius,
for example by means of radiator.
Fig. 6 shows a possible way in which the high
s speed flame spraying process can proceed. For this
purpose, a shrouding 16 is provided, which is
accessible from the side and encloses a number of ,.
prepared blades. By means of a spray nozzle 20, the
protective coating is applied to the blades, it being .
readily possible to perform the guidance of the jet
nozzle 20 manually. A suction extraction 19 ensures
that excess agent can be removed immediately from the
vicinity of the blades.
An aftertreatment of the sprayed blades
generally comprises the following process steps:
- To reduce the surface roughness, light rubbing
over with an emery cloth and/or blasting, for
example with glass beads.
- To protect the undercoating and further reduce the
surface roughness, a top lacquer coating can be
applied with a paint-spray gun. This is on
condition that this lacquer does not require a
high sustained firing temperature (no furnace
construction). A two-component lacquer may be used
for this, at least for the first rows of the
compressor, where still relatively low tempera-
tures prevail in operation.
A plastic-based polyurethane reaction lacquer may
be an example of such a top coating.
Regarding the quality of the protective
coating, it should be said that conventional compressor
coatings very often exhibit low erosion resistance.
Since such galvanic protective coatings axe only
effective if they are present in the metal-coating-
electrolyte system, a locally eroded coating is reduced
in its protective effect.
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The aluminum-based protective coating employed
here is an active corrosion-protected coating, the
composition of which is preferably as follows:
1. One protective coating consists of 6 to 15~ by
weight Si, the remainder aluminum;
2. Another protective coating consists of pure
aluminum;
3. Another protective coating consists of 80$ by
weight A1, 5 to 15~ by weight Si, the remainder Cu, Mn,
Mg, Ni.
The chemical structure of the abovementioned
protective coatings and the application process
likewise described above (high-speed flame spraying
process) provide a less erosion-sensitive "sacrificial
anode" coating, which actively protects the base
material against corrosion. The application process,
which is a high-energy coating process, provides
erasion-resistant protective coatings which have good
adhesion and the desired electrical connection to the
base material without further protective coating-
specific aftertreatment. The proposed protective
coatings may be additionally provided with a top
coating. This dirt-repelling top coating may, for
example, be black. Such a dirt-repelling top coating
makes it easier to detect icing on the blades by means
of ice detectors. The high-speed flame spraying
process, which opexates at a particle velocity of at
least 300 m/s, represents an optimum bonding of the
coating to the base material of the blades. Even in the
case of a relatively thick protective coating, it is
ensured that the coating does not peal off. The
explanation for this is that, upon impact of the powder
particles, the high kinetic energy produces residual
compressive stresses in the coating respectively
sprayed befare. The maximised resistance against
corrosion is attributable to the fact that the coatings
used here have a very high hardness. The process
proposed here has the effect that the oxide content of
the coating is lower than in the case of protective
coatings sprayed in air. That means that the coating is
more pure, for which reason it oxidizes less quickly, ,.
any oxidation their may be occurring only on the
surface. Due to the fact that the protective coatings .
axe very dense, there porosity is below 0.5~. There is
virtually no chance of them being destroyed by
corrosion: in the salt spray test specified in
DIN 50021, a commercially available ceramic aluminum
coating was compared with a protective coating
according to the above composition and with the above
process. The results have fully confirmed the above
statement. In a fatigue process, an analogous
comparison was carried out: it was found that the
loading until the first fatigue crack in the blade was
20$ higher in the case of blades coated according to
the above composition and the above process than in the
case of the comparison blades. This means that the
immunity of the blading to fatigue failure could be
increased.
On the basis of the stated advantages and the
results after several thousand operating hours in a
compressor of a coastal installation, an improvement in
the service life of the active protective coating of
50~ resulted.
Obviously, numerous modifications and
variations of the present invention are possible in
light of the above teachings. It is therefore to be
understood that, within the scope of the appended
claims, the invention may be practiced otherwise than
as specifically described herein.
