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

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324

Abrasive Erosion and Corrosion of Hydraulic Machinery

the erosion process under consideration. Consequently, the sediment particles

perform a double effect: the first is developed within the cavitation zone

limits,

whereas the second acts throughout the adjacent regions.

AG(mg)

cavitation erosion, 0=5°; 2. the same, at 6= 15°;

cavitation abrasive erosion, 6= 5°; 4. the same, with Q- 15°

Figure 6.5 Loss in mass of specimens against test duration:

The influence of concentration and grain-sizes of the sediment matter on

the cavitation-abrasive erosion behavior at a certain phase of cavitation

development is illustrated by the relationship obtained experimentally (see

Figure 6.6). Small sediment concentrations (p

5 g/1), with particle diameter

= 0.1 ~ 0.25 mm, affect the erosion process insignificantly (Figure 6.6 (a)).

When the concentration increases to 10 ~ 16 g/1, the erosion rate rises

sufficiently due mainly to the hydroabrasive failure. The particle diameter

factor (Figure 6.6 (b)) in this case is similar to its effect displayed in the

hydroabrasive failure process (see Figure 6.4). When the particle size

increases to the level above d

= 0.35 ~ 0.4 mm, practically no increase the

cavitation-abrasive erosion is observed.

In order to define the effect of the flow velocity upon the cavitation,

hydroabrasive and cavitation-abrasive erosion processes, tests were run in a

wide range of velocity values performed with the diffuser nozzle set at the

divergence angle 0=5° and with the other parameters being maintained

constant, such as /

. = 110 mm, p

5 g/1, d = 0.1 ~ 0.25 mm (Figure 6.7 a).

Interaction between Cavitation and Abrasive Erosion Processes

325

Curve 3 in Figure 6.7 corresponds to the behavior of cavitation-abrasive

erosion taking place at velocities v = 23 ~ 32 m/s, curve 2 is presented in

accordance with the hydroabrasive erosion development shown in terms of

non-cavitation streams. The different inclination angle at the initial portions of

curves 2 and 3 reveals that these curves may cross each other at lower

velocities. The point of their intersection is within the 14 ~ 16 m/s velocity

range. This fact confirms existence of certain typical flow rate values at

which the cavitation-abrasive flow intensity can be lower than that of the

hydroabrasive erosion, i.e. the cavitation zone has the function of protecting

the specimen surface from the solid particles.

5 10 15

P(kg/m

)

(a)

—

= 50 mm;

0 0.15 0.30 0.45

dh(mm)

(b)

2-l

, = 110 mm

Figure 6.6 Cavitation-abrasive erosion as a function of (a) sediment

concentration p and (b) grain size of particles d

Experiments were run, in this respect, with a model having the diffusion

angle 0=10° (Figure 6.7 b). At higher velocities the combined erosion level

exceeds the hydroabrasive one. These data are in agreement with the

experimental results obtained

[6.2],

which confirm that the protective

hydroabrasive effect of the cavitation zones in a hydroabrasive flow is only

possible in case where no material is removed by cavitation.

Figure 6.8 illustrates cavitation-abrasive erosion as a function of

cavitation development level, at a constant flow rate v = 30.5 m/s of a

suspension-conveying mixture with solid particle content p = 5 g/1. The

cavitational development level in these experiments was controlled by the

pressure set at the inlet to the nozzle H

. It is seen from the data presented

that the erosion intensity rises with decrease of the cavitational number K and

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Abrasive Erosion and Corrosion of Hydraulic Machinery

remains the same, with increase of the cavitational zone length l

. The

experimental results, however, make it impossible to arrive at any definite

conclusions on quantitative values, since variations in the pressure H

entails

changes in a number of other factors, for instance, in erosion area, pressure

gradients etc. It is hardly possible to isolate the individual effects exerted by

these factors.

AG(mg)

3 /

AG(mg)

200

150

100

-<*

24 26 28 30 v(m/e)

(a) for angle 9= 5°

30 v(m/s)

(b) for angle 6= 10°;

1 - cavitation erosion;

- hydroabrasive erosion;

3 - cavitation-hydroabrasive erosion

Figure 6.7 Interrelation of cavitation and hydroabrasive erosion processes

AG(mg)

38 « 45 Hax(m)

Figure 6. 8 Effect of development stage on cavitation-abrasive erosion

Interaction between Cavitation and Abrasive Erosion Processes

327

The research studies carried out at the V.V. Kuibyshev Moscow Civil

Engineering Institute (MIEI, USSR) have shown that, in the course of the

combined effect produced by both cavitation and sedimentation, there arise

processes which can hardly be accepted as an algebraic sum of these two

kinds of deterioration. In some cases the cavitation-abrasive erosion can be

higher than the hydroabrasive erosion, whereas in other instances it may be

lower. Further analyses are needed, both experimental and theoretical, to

study these phenomena.

An interesting trend in development of the problems connected with the

cavitation-abrasive erosion is presented in the papers written by S.P.

Kozyrev. Taking into account the influence of all the factors existing in a

suspension-conveying flow, namely: (Hj - H

), loss in the head; (P^ - P

vacuum value within a small space; v

, kinetic viscosity of the hydromixture;

v , rate of the suspension-conveying flow; r

, surface tension; Av,loss in

the material volume caused by erosion; D , characteristic diameter of the

channel; and / as the channel length, the author has established the following

functional dependence which governs the erosion process being investigated:

fDv

r .P

-P

.{P

-P

)DH

-H

(65)

'gD'

' T ' / DD

The solid particles, with the density greater than that of the liquid, will lag

behind during the motion. The functional dependence with reference to the

secondary stream participating in by-passing the solid particles will look like

dl "

(

^ -PL . fe, ~K K ", ~H

. D l)

' P

' r

X d,

(6.6)

In equations (6.5) and (6.6) one can find the following criteria established

to define the erosion involved:

—

- and -h-L. - Reynolds numbers; —

— and

- Froude numbers;

K v

gD gd

p -P

•*

Pr -

and

P~-

-PL

-r

local rarefaction coefficients of cavitational

328

Abrasive Erosion and Corrosion of Hydraulic Machinery

numbers, A" at P

=-P'

cavitation appears respectively on the

component surface and behind a solid particle); D^P

- P

)r~

- parameter

characterizing the rupture strength of the liquid; d

-P

)T~

- parameter

characterizing the liquid strength at the moment of its separation from a

particle; (//,

-//

- hydraulic gradient or loss in the head within the region

under review; (#,

-H

)d~

-resistance, while a particle is flown around;

and Dd^ - simplexes characterizing degree in lateral freedom of a

particle;

ID~

- geometric coefficient of a unit width area being worn; ld^ -

geometric coefficient taking into account the particle-to-surface mechanism of

the contact.

Although the mechanism of the influence produced by all these criteria on

the erosion process is not yet known in its summarized form, the indicated

criteria are most vital for a correct arrangement of researches to be promoted.

S.P.Kozyrev has succeeded in revealing the most typical versions in the

development of the combined cavitation-abrasive erosion

[6.2].

The line a\a

in Figure 6.9 demonstrates the hydroabrasive erosion in question. The points

b\, c\, d] and e\ are correspondingly related to the erosion intensities at

different cavitation development stages taking place in pure water. With the

emergence of sediments and increase of their concentration the cavitation-

abrasive erosion process can proceed in accordance with the patterns

described below.

Concentration

Figure 6.9 Interrelation of intensity of cavitational-abrasive erosion

and sediment matter concentration

Interaction between Cavitation and Abrasive Erosion Processes

329

The erosion rate reduces within the region b\b

as the sediment

concentration is raised. This can be attributed to the fact that the cavitation

erosion, provided it develops with a durable "incubation" period, starts with

formation of microcracks in the surface layer of

the

metal, which propagates

further into the metal depth, resulting in chipping out relatively large volumes

of metal. During the hydroabrasive erosion one can observe, however, the

formation of a thin metal "copolishing" layer, which serves to protect the

surface from cavitation cracks and indentations.

A decrease in erosion may take place while cavitation erosion rate is

somewhat greater than that of abrasive erosion. The moment the abrasive

erosion rate starts to exceed that of cavitation erosion, due to increase in

concentration of the suspended matter, there appears a rise of general failure

intensity in the region

b-$.

Here the abrasive erosion acts ahead of the

cavitational erosion and thereby as though suppresses the latter.

The line c\c

characterizes the erosion taking place in the events the

cavitation erosion rate significantly surpasses the rate of the hydroabrasive

erosion. In this rage of fluctuation, the intensity of the sediments is

insufficient to alter the course of the cavitation erosion. It is only later, as the

sediment content proceeds to rise, that the hydroabrasive erosion starts to be

superimposed on the cavitation erosion process and, consequently, to increase

the combined erosion level (region c

The line

{

specifies the terms in which the cavitation erosion intensity

is very great and the metals being worn out are of high plasticity (copper,

aluminum, lead etc.). In this case the metal surface affected by cavitation is

covered at random by material squeezed out.

The presence of sediments in the water serves to remove the metal chips,

thus causing a sharp rise of the erosion process (line

d\d

With the further

increase of the sediment concentration this effect will become weaker (line

The line e\e

corresponds to the highest intensity of the cavitation erosion.

The presence of the suspended sediments results only in superimposing the

hydroabrasive erosion over the cavitation erosion.

Within the operating components of pumps the cavitation erosion

proceeds, as a rule, with a fairly long incubation period; even with the

sediment content in the water equaling 0.1 ~ 0.5%, the hydroabrasive erosion

starts to prevail over the cavitation erosion. Therefore, the pump components,

which have been subjected to cavitation abrasive erosion, are identified

330

Abrasive Erosion and Corrosion of Hydraulic Machinery

mainly by smooth worn-out regions, i.e. the regions with hydroabrasive

erosion only. But on the operating pump components there are also other

areas where cavitation erosion takes the lead over hydroabrasive erosion.

Then the surface under study develops, as a result of cavitation, a sponge-like

structure.

6.2. Effect of Cavitation on Hydroabrasive Erosion

The cavitation phenomena influence on the nature of hydroabrasive erosion,

within the flow-passages of hydraulic machines, will further be considered

with reference to a research study carried out with centrifugal pumps

operating in laboratory conditions.

The analyses accomplished with the use of the simplest equipment, such

as a Venturi nozzle, have demonstrated a complicated dependence of the

cavitation-abrasive erosion process on the hydrodynamic features of the flow

under review, the surface shape of the components as well as the quantity and

composition of the sediments. Although the results available give a general

idea about the basic mechanism of the material failure effected by the

combined action of both the abrasive particles and cavitation, they do not

provide, for the time being, definite conclusions concerning any specific

qualitative recommendations.

It is most probable that things may become even more difficult in an

attempt to establish feasible combinations of hydroabrasive and cavitation

erosion processes taking place in such complicated hydraulic systems as in

pumps. The first experiments in this direction were carried out in the V.V.

Kuibyshev MIEI [6.5, 6.6]. The results obtained have revealed that the

cavitation development level evaluated by, for instance, the size of cavitation

margin, affects significantly the erosion of pumps operating with water which

includes abrasive erosion particles.

To test centrifugal pumps, an experimental jig was used (see Figure 6.1).

The pressure at the inlet to a pump (and, hence, the cavitational margin Ah)

was controlled with a latch fixed in the suction pipe of

the

pump. The content

the

sediment matter contained in the water varied for different experiments

in the range of 2 to 16 g/1 (e = 0.13 ~

1.02%).

The particle grain sizes were

within 0.1 ~ 0.25 mm. The experiments were carried out at six different

delivery modes of the pump model 4K-8, ranging from 0.54

opt

to 1.25

opt

Interaction between Cavitation and Abrasive Erosion Processes 331

with various values of Ah (see Figure 6.10). At first the chipping-off

cavitational pump characteristics were obtained with the pure water delivered

at a constant flow Q. Then a certain amount of abrasive material was added

to the jig tank and the experiments were continued with a certain number of

typical values, from non-cavitation to chipping-off patterns. After each

measured point a number of experimental points were "scanned" (normally

from 9 to 12). The intervals between Ah values were made shorter when

dealing with well-developed cavitation modes. In the normal pump operation

range such intervals are of no significance since

values in this range do not

affect energy parameters. The experiment for every Ah value lasted 4 hours.

The erosion was evaluated according to the loss in mass AG of two test-check

silumin blades of an experimental impeller.

" p 5 10 15

25 Q (1/s)

0 20 tO SO SO /MQ(m

/h)

dashed line - specification provided by manufacturer

solid line - specification obtained by test

Figure 6.10 Specification of experimental centrifugal pump 4K-8

The results of

these

experiments, carried out at different pump discharge

and with various concentrations of sediment are presented in Figures 6.11 ~

6.13.

The corresponding chipping-off cavitational characteristics of pumps

332

Abrasive Erosion and Corrosion of Hydraulic Machinery

are also illustrated. With alteration of the cavitational margin Ah, the erosion

rate within the blades of an impeller does not remain constant.

In centrifugal pumps it is an adopted practice to distinguish, as applied to

their energy parameters, two critical values in the cavitational margin, i.e. the

first and the second. The first critical of Ah\ corresponds to the moment of

external cavitation symptom of the pump. With reference to the cavitation

curves this mode is specified by reduction of the pump head or efficiency by 1

~ 2%. The second critical value Ah

refers to the moment of the pump

operation failure.

H(m)

SEE

SfcrD

Q(n/c)

Ab/b(100%)

2.0 r

1.2

0.4

AG/G(100%)

0.72

0.56

0.40

0.24

0.08

■— II !■

— '

1-p=l0g/l;

4 6 8 Ah(m)

2-p = 5g/l; 3-/?=2g/l

Figure 6.11 Erosion against cavitation margin at Q = 0.7

opt

For the pump operation points Q = 0.7

opt

( see Figure 6.11) and Q =

0.9

(Figure 6.12) the pressure drop at the inlet of the pump, to the level

corresponding to cavitation margin Ah

shows an appreciable decrease in the

rate of cavitation-abrasive erosion in the blades, as compared to a

conventional hydroabrasive erosion process. For instance, at Q = 0.9

opt

Interaction between Cavitation

and

Abrasive Erosion Processes

333

with Ah = 6 ~ 8 m, the relative erosion of

blades

AG/G is reduced 1.5 times

against

the

erosion level obtained without cavitation

{Ah =

10.2 ~ 10.4 m).

AG/G (100%)

4 6 8 Ah(m)

1-/>=l6g/l;

2-/7=

lOg/1;

o=5g^; 4-/? = 2g/l

Figure 6.12 Erosion versus cavitation margin at Q

0.9

opt

At a higher delivery, such as Q

1.25 g

^ (Figure 6.13), a sharp decline

of erosion rate occurs as the cavitation margin proceeds to decrease to the

level up to Ah = 8.5 ~ 9.5 m. At further decrease of pressure in the suction

pipe the erosion rate increases rapidly again. Consequently, the minimum

intensity of the cavitation-abrasive erosion in these conditions is 1.5 ~ 2 time

lower than the intensity of a pure hydroabrasive erosion occurring in the

absence of cavitation.

Thus,

with successive decrease of cavitation margin in a centrifugal pump

operating with water containing a certain amount of solid particles,