Duan C.G., Karelin V.Y. Abrasive Erosion and Corrosion of Hydraulic machinery

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Abrasive Erosion and Corrosion of Hydraulic Machinery

(2) Process of spraying

(a) Pretreatment for the surface of component

It is quite important to clean the surface of components before spraying

in order to form the close metallurgical combination with excellent

spraying layer.

For the flow passage components of water turbine with grooves,

fish-scale pits and honeycomb structure, it is necessary at first to grind

them off with grinding wheel of other tools, and then to make the

compensation with electric or gas welding and finally to smooth the

surface by the grinding wheel. For the flow passage components of new

water turbine, the above-mentioned treatment is also needed for the

existence of rough surface in cast construction used for water turbine.

After the above treatment, another process, sand blast, is also

required, which not only can make the surface a little bit rough for

convenience of powder adhered to surface, but also can reduce the rust-

cleaning labor to some extent.

(b) Preheating for the component

The purpose of preheating is to eradicate the moisture on surface of

components, strengthening the combination between spraying layer and

component, as well as reducing the stress for avoiding the peeling off of

the spraying layer before re-melting.

The temperature for preheating is dependent on the size and

properties of components. It is usually 250° ~ 300°C for carbon steel,

and 350° ~ 400°C for stainless steel. Local symmetry preheating and

heating stress-reducing method are usually adopted for components

with large dimension, integral preheating is used for those with small

dimension. The oxy-acetylene flame and muffle furnace are valid.

There are two methods for thermal spray welding: the one-step method

and the two-step method. The one-step means that thermal spray

welding goes while the re-melting is proceeding with. Two-step means

that thermal spray welding is prior to re-melting. The process

technology is as follows:

One-step: Treatment for surface — Preheating —- Powder pre-

spraying-Powder-spraying and Re-melting — Cooling

— Post treatment;

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Two-step: Treatment for surface — Preheating — Powder-spaying

— Re-melting — Cooling — Post treatment.

(3) Deformation control method

It is difficult to correct the deformation once it has taken place because of the

complex construction in the flow passage components and the strict

assembling size and shape for the runner and guide vanes. Deformation-

controlling method is crucial to the spray welding.

The following measures have obtained good effect on controlling the

deformation in spray welding for medium and small types of water turbine.

(a) Blades of Francis runner:

The main portion for thermal spraying is located on the trailing edge of

suction side local leading edge and trailing edge of pressure side.

Because the thickness of blades has got quite difference as compared

with that of crown and lower ring, and the blade are closely ranked,

symmetry thermal spray welding should be adopted to avoid the

deformation in crown and lower ring.

(b) Upper and lower labyrinth in runner:

The runner is preheated to 150° ~ 180°C, and then is divided into four

or eight portions according to the dimension of runner as shown in

Figure 5.27. The flame should be used to maintain temperature at the

position F while the thermal spray welding is proceeding at the

position 1. The spray welding should be changed to the position F

immediately after finishing the spray at the position 1. No stop is

allowed until the job is completely finished. The runner should be

wrapped up or put into the furnace for slowly-cooling.

Figure 5.27 Process of thermal spraying welding

in upper and lower labyrinth

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The main problem is to avoid the deformation when the thermal spray

welding is going on. The guide vane is totally heated with oxy-acetylene

flame, and powder is sprayed upon the surface with certain thickness,

and finally one can start the re-melting treatment while the temperature

keeps on the level of

400°

~ 500°C on the other side with oxy-acetylene

flame. Because of the large coefficient of expansion in the spraying

layer with large contraction as cooling, the other side should be cooled

slowly at first, and then let the spraying layer cooling down slowly.

This is the reasonable process of spraying , and it can be installed

directly without shape-correcting process.

(d) Blades of axial Runner:

The portion that is readily worn out is mainly located on the leading

and trailing edge and the outer edge of runner. In order to control the

deformation, blade is preheated to 180° ~ 200°C and then thermal

spray welding is done in different regions, as shown in Figure 5.28. The

positions of

and 2 with relatively small constraints, are done at first

and the direction for remelting is illustrated as the arrows in the figure.

At this time when the integral temperature has increased to some

extent, the positions of 3 and 4 are sprayed under such a higher

temperature, getting less deformation and lower stress. The process of

spraying is demonstrated in Figure 5.29. Spraying is done at first on

the position of 1', and vice versa. After the spraying, certain level of

temperature is kept and cooled down slowly.

Figure 5.28 Process of thermal spray welding of axial runner blades

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V I 3' I 4' I 2'

Figure 5.29 Process of thermal spray welding of runner blades

(4) Application in practice

Obvious achievements have been obtained after four-year operation of 12 water

turbines from 8 hydro-electric power plants on which autolysis alloy powder

developed by China Gansu University of Technology is adopted for erosion and

cavitation resisting purpose

[5.14].

Figure 5.30 shows the comparison of anti-

erosion property between the blades without thermal spray welding and the blades

that are sprayed with Ni37 autolysis powder of the Francis runner for Ba Jia Zui

hydro-electric power plant. In order to guarantee the conditions of water quality

and operation, the blades without spraying powder and blades with thermal spray

welding are ranked alternatively, that is to say the blades with even number are all

sprayed by autolysis powder. The big difference in erosion-resisting aspect is

found for these two sorts of blades after 15,840 hours of

operation.

On the blades

without spraying powder, one can find the serious damage and even holes on the

trailing edge of suction side, while on the blades with thermal spray welding, one

can just see the slight wear on the trailing edge of suction side without any

deformation in its shape of geometry.

Figure 5.30 Comparison of spraying and non-spraying blades after

15,840 hours operation in Ba Jia Zui Power Plant

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Figure 5.31 shows the picture of comparison experiments carried out in

the wicked gates of water turbine in Hong Ya power plant, with the rated head

is 93 m, the output is 1000 kw and the annual average density of sand is

5.3kg/m

, the maximum density of sand 203 kg/m

. Figure 5.31 (a) is the

situation of the blades without protection of spraying powder while Figure

5.31 (b) is demonstrating the situation for blades with thermal spray welding

of Ni37 alloy powder after one-year operation. One can find no damage in the

blade with Ni37 alloy powder spray.

(a) Non-spraying blade (b) Spraying blade

Figure 5.31 Erosion of blades after a period of operation in muddy water

5.6 Non-Metallic Protection Coating

E. Chen

In the past thirty years, many researches and experiences have been

accumulated worldwide in the aspect of adopting polymer coating to protect

metal construction and devices of hydraulic engineering from cavitation and

sand erosion. But only a few sorts of polymer coating material are adapted to

the protection of hydraulic machinery working under the condition of siltladen

water.

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It is reported that non-metallic coating materials of erosion-resistance for

hydraulic machinery have been developed in many countries. A lot of research

works have been implemented with great achievements. Several sorts of non-

metallic coating have obtained good results in prototype machines, some of

which are epoxy emery coating, composite nylon coating, polyurethane

coating and enamel coating.

5.6.1 Epoxy Emery Coating

It is reported that epoxy-made coating for erosion-resisting purpose was used

for blades of water turbines more than thirty years ago in the United States,

which was also used as erosion-resisting coating in hydraulic machinery and

penstocks in USSR and F.R.G. In the past ten years, erosion-resisting epoxy

emery coating has been developed and widely used in pumps, turbines and

hydraulic buildings in China.

(1) Characteristics of epoxy emery coating

Epoxy emery coating consists of epoxy, curing agent, tenacity-increasing

agent, filler and diluent, the properties of which are as follows:

(a) Epoxy:

Epoxy is a new kind of synthetic resin that is high-molecular polymer

containing oxygen-group, which possesses various sorts. Epoxy

propan-echloride is the main monomer for all sorts of epoxy resin,

which can be condensed into epoxy resin while combined with

polyhydric diphenol and alcohol, i.e. epoxy resin of diphenol propane,

phenolic resin and glycerol epoxy resin. The diphenol propane epoxy

resin, sometimes called A-diphenol epoxy resin, is mostly used, which

is also adopted in the protection of hydraulic machinery from erosion.

Ranking in order by molecular weight, A-diphenol epoxy resin has

many types, i.e. 618, 619, 6101, 634, etc., the properties of which are

as follows:

*High strength of adhesion: Strength of adhesion is dependent on

curing agent, see Table 5.21.

♦Good process for technology: At ordinary temperature, it possesses

good fluidity. After being permeated among the filler, coating

materials become plastics-like epoxy slurry, and the heated

trowel can be readily utilized to form the shape.

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♦

Small contractility: Epoxy resin has no by-products in solidifying, it

therefore produces no gas bibles while the contraction rate is

less than 2%, or even 0.1% after the emery is added, and its

coefficient of thermal-expansion is quite small.

♦

Small absorbability: The absorbability for water is less than 0.5% at

the temperature off housing, so it is impossible for coating

materials to expand when it is in water for a long time besides

its excellent steadiness and electricity-insulating ability.

Table 5.21 Adhesion strength of epoxy resin with different curing agents

Curing agent Name

_„

n.^i 2

Curing Adhesion strength

condition (kgf/cm

)

Triethylene-tetramine

Tetraethylene-pentamine

Eurelon

Dimethyl imidazole

Dicyandiaminde

Butene dicarboxylie anhydride

80°C,

100°C,3h

120°C, 4h

150°C, 4h

200°C, 2h

603

545

503

508

769

582

(b) Curing agent:

Curing agent is a sort of hardening substance that makes the linear

molecule of epoxy resin crossing in network structure. There are many

sorts of curing agent and the ammonia curing agent is mostly used for

the coating in water turbines and pumps, majority of which are

triethylene tetramine, tetrethylene pentamine and polyamide. This sort

of curing agent has the advantage of hardening at temperature of

housing, low viscosity, fast hardening and little poison. In most cases,

the main filler is emery with high hardness; its Mhos hardness reaches

11.

In addition, small amount of alumna is added to increase the

adhesion and some nylon yarn and nylon clothing are put into it to

increase the integrity.

The prescription for coating materials is dependent on their

purpose, condition of increasing temperature on site and resource of

materials. The general prescription is demonstrated in Table 5.22.

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Table 5.22 Prescription of epoxy emery coating materials in

erosion-resisting composite layer for blades of hydraulic machinery

Material and its

proportion

Epoxy 618

Epoxy 6101

Amilan 200

Amilan 300

Trierhylene-tetramine

Dimethyl imidazale

Ethyl butyl ether (501)

Polyvinyl butyral

Paracril

Emery

Porcelain powder

Aluminum oxide

Moly-sulflde

Nylon wire

Acetone

Prescription

100

375

100

375

100

375

100

Note: BS=bottom sheet, FS=fundamental sheet, SS=surface sheet

(2) Technology of operation

(a) Treatment for surface

Rust and oil residue on the surface of components can be erased by the

method of sand blast until the original color appears on the surface.

(b) Heating the component

The component should be preheated to 50°C approximately before

putting the coating material upon the surface.

Prescription of coating material is usually made up of

~ 10 materials,

some of which will have the chemical reaction as encountering. So that,

certain process must be followed in the allocation. The allocation for

epoxy emery coating material is usually divided into three sheets,

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bottom sheet, fundamental sheet (slurry sheet) and surface sheet. The

function of bottom sheet is to increase the adhesion with the component

and slurry sheet. The slurry sheet is mainly in charge of erosion-

resisting function, and the surface sheet will make the slurry sheet

smoothing. The steps of allocation are as follows:

To put epoxy resin and tenacity-increasing agent, diluent and

curing agent, filler into three different containers. If the temperature is

satisfactory with specific requirements, the materials in the two

containers forwards are mixed together with fast stirring until it

becomes homogeneity, and then the material in the third container is

added into with stirring at once. Then coating jobs should start as soon

as possible to avoid the solidification phenomenon.

(d) Making coating

*Bottom sheet: To paint the bottom sheet with the requirements of

being thin and homogeneous, and the slurry sheet is then

painted when the bottom sheet has been glued (after 10 minutes

approximately).

* Slurry sheet: To make this sheet keeping the temperature at 80°C or

so.

* Surface sheet: The slurry sheet is to be completely examined at its

initial level of solidification, and then to paint the surface. For a

small component, it is valid to paint for once or twice. For a

large component, spray painting is available, but 60 percent of

acetone must be added into the painting liquid for the purpose

of dilution. The surface sheet should be painted twice, and the

thickness in first spray painting should be very thin, otherwise

flow of painting liquid appears, making the surface rough.

(e) Solidification and maintenance of coating

At the completion of epoxy emery coating , the full solidification is

required to guarantee the adaptive strength and adhesion. In ordinary

temperature the cringe agent will be cured completely at 25 °C or so

after one week, and will cured after four hours if the temperature is

raised to 60°C. The latter is better in quality. For medium-temperature

curing agent, three hours are enough to cure completely at the

temperature of 120°C.

(3) Application

practice

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303

Protection of epoxy emery was used in the runner blades of the turbine at San

Men Xia Power Plant on the Yellow River, obtaining great achievements.

After operation of 23,000 hours, the coating maintained more than 95% on

the pressure surface, and 30% of coating was stripped away on the suction

surface, mainly concentrated in the region with strong cavitation damage.

With such methods of protection, the overhaul period is three times as much

as that without protective coating. The similar test was done at the Tian Qiao

Power Plant on the Yellow River, and after the accumulating operation time

of 15,560 hours for three years, 99% of coating was not eroded on the non-

cavitation region. This method was extended into other power plants and

pump stations with serious erosion, and good achievement was also obtained.

According to our experience in application of epoxy emery resin coating,

we have the conclusion that this sort of coating is good in erosion-resisting

function but not superior in the cavitation-resisting aspect, and we find the

coating on the cavitation region is all damaged which should be further

improved in the future. So that, this sort of coating is mainly used in erosion-

resisting aspect.

5.6.2 Composite Nylon Coating

(1) Prescription of nylon coating and its technical Specification

The prescription and physical-chemical properties of composite nylon are

demonstrated in Table 5.23

Table 5.23 Prescription and properties of composite nylon

Name

Ternary system

171

Transformatio

1010-171

Ternary system

H18-1

Transformation

1010-H18-1

Adhesive Shearing Tearing

strength strength strength

(kgf/cm

) (kgf/cm

)

637-716 428-468 582-606

612 398 481

683 434

691 424

Hygroscopicity (%)

1 day 30 days 180 days

0.62 2.91 3.31

0.42 1.30 1.56

0.52 2.84 3.56

0.30 1.30 1.93