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

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334

Abrasive Erosion and Corrosion

Hydraulic Machinery

cavitation-abrasive erosion within the blades of a pump impeller first

decreases and then grows as compared with the hydroabrasive erosion

process. To elucidate such an interesting phenomenon let us consider the

results obtained from a generally accepted evaluation viewpoint concerning

the cavitational development level in centrifugal pumps. It is a common

practice to distinguish an initial cavitation, a partially developed cavitation

and a fully developed cavitation

[6.1].

40[ 1—

H(m)inr

o Ah(m)

AG /G (100%)

Ah(m)

AG /G (100%)

J.2

6 a

. ... n T? ,

11.

U - ii

-. Z %4

Ah(m) *"* 6 8

Zr-

Ah(m)

1-p=l6g/l;

2-p= 10g/l; 3-p = 5g/l; 4-/?=2g/l

Figure 6.13 Erosion vs. cavitation margin at Q

1.25

opt

The initial cavitation stage is related to the conditions of emergence of

first signs of cavitation: a weak amplification of noise and the presence of a

small amount of cavitational bubbles that form an unsteady cavitational zone.

As a rule, the external behaviour of a pump at this stage is practically

unchanged.

The partially developed cavitation is specified by the presence of a steady

cavitation zone of definite size which alters the effective shape of the guiding

surface and constricts the free cross-sectional area of the flow will cause the

performance of local flow velocities accompanied by emergence of secondary

Interaction between Cavitation and Abrasive Erosion Processes

335

flow streams. The specifications of a pump to drop due to increase of

hydraulic losses.

The fully developed cavitation results in a "collapse" of the performance

of a pump, the cavitational zone is already extended not so much in width as

in length, i.e. along the blades of an impeller.

The theoretical analysis [6.7] and experimental results, obtained during a

research study of the inter-blade channels in pumps, enable to determine the

liquid flow pattern in the impeller at different operational modes of the

centrifugal pumps and also to estimate the values of

the

appropriate pressure

levels at the pump inlet in order to prevent emergence of cavitation.

At the delivery levels significantly lower than

oph

the leading edges of

blades is only partly engaged with the active flow traveling from the inlet to

the exit of the impeller. The greater part of it, adjacent to the front disk, is

filled with the flow portion traveling in the reverse direction. This back flow,

being evidently deviated in the direction of the impeller rotation, is gradually

washed out by the main stream and becomes entrained in the impeller again.

The vortex zone, being generated simultaneously, does not participate in the

flow delivery through the impeller.

As the delivery increases (with the rotational speed being constant), the

width of the active stream, at impeller inlet grows, whereas the vortex zone

correspondingly decreases. At the delivery rates approaching the optimum

level, the whole cross-sectional area of the inter-blade channel is practically

occupied by the liquid flow, and small stagnation areas may possibly arise

only nearby both sides of the blade trailing edges.

At flow rate above the optimum level, despite the fact that the active

stream occupies the inter-blade channel throughout its entire height,

separation still results from the pressure side of the blade and causes

reduction of the flow cross-sectional area at the exit, thus increasing the

average flow velocity.

With lower pressure at the inlet, the cavitation emerges within the blade

channels of the impeller throughout the operational modes of the pump. The

cavitation bubbles, originating in the depth of the active stream, enlarge in

size and compress the vortex zone and hence the intensity of the

hydroabrasive erosion at the leading edges of the blades. The further reduction

of the cavitation margin results in the formation of local cavitation zones,

which protect the impeller blade surfaces from the effect produced by the

abrasive particles. Besides, the cavitation liquid, being of non-uniform two-

336

Abrasive Erosion and Corrosion of Hydraulic Machinery

phase structure, creates within the inter-blade channel an additional resistance

to the motion of solid particles, thus decreasing the margin of their kinetic

energy. All this results, at the initial modes of cavitation (zone I in Figures

6.11 ~ 6.13), in a decrease of the erosion intensity, as compared with the

"pure"

hydroabrasive erosion process.

With the partially developed cavitation, the vortex zone disappears

completely. The steady cavitation zones reduce the open area of the active

flow, thereby increasing velocity of

the

hydromixture. Taking into account of

the cavitation erosion, the intensity of the combined erosion process grows,

thus reaching the intensity of

the

hydroabrasive erosion accomplished without

cavitation process (zone II in Figures 6.11 ~ 6.13).

At further decrease of the pressure, the intensity of the cavitation-abrasive

erosion of the blades starts to grow sharply (zone III in Figures 6.11 -6.13).

Together with the amplification of the cavitational erosion, caused by increase

of the cavitational zone sizes, the situation mentioned above is derived from

the fact that, due to an elevated turbulence of

the

flow caused by fluctuations

of the velocity and pressure values, as well as by the flow stall taking place in

the cavitational zones, the abrasive particles, moving within the cavitational

zone and in the immediate proximity to it, obtain a considerable acceleration

and thereby increase the hydroabrasive erosion.

As seen from the plots defining the relationship between the pressure

fluctuation amplitudes and Ah (see Figure 6.14), for the pump delivery Q =

0.9

Qopt,

at the initial stage of the cavitation development (zone I in Figures

6.11 ~ 6.13), the amplitude level of the pressure fluctuation diminishes, and

as the cavitation proceeds to develop (the further decrease of

Ah,

zones II and

III in Figures 6.11 ~ 6.13), these fluctuations form a sharp rise in the flow.

The region Ah = 6 ~ 9 m in Figure 6.14 can be defined as a domain in which

the pressure fluctuation amplitude is maintained constant in the flow. Within

the indicated region the erosion intensity in the blades is reduced as well. As

indicated in the curves of the impeller blade erosion at different values of Q

(see Figures 6.11~ 6.13), there exist a zone of "optimum" cavitation margin

Ah,

which relates to the minimum rate of cavitation-abrasive failure.

The predominance of one type of erosion over the other, in the course of

the cavitation-abrasive failure, was studied by performing a specific series of

experiments carried out at different delivery Q, cavitational margin Ah and

growing concentration of the sediment. Similar relationships, on erosion

intensity versus sediment values concentrations, have been obtained in a wide

Interaction between Cavitation and Abrasive Erosion Processes

337

range of

and Ah alterations. The test results for one of operational modes

taking place in a pump are given in Figure 6.15.

H(m)

1 -

fluctuation

amplitude in

suction piping;

2 - in spiral;

3 - in delivery branch;

4 - in discharge pipe

6 a »Ah(

)

Figure 6.14 Pressure

fluctuation

flow-passage

of a centrifugal pump

at different cavitation values

Every curve I =f (e) can be divide into three typical regions. The first

region, characterized by a fairly fast increase of erosion intensity, confirms

the joint action produced by both the cavitation and sediment erosion on the

blades of the impeller (line d\d

in Figure 6.9). The second region is specified

by a smaller increase of the erosion intensity due to partial suppression of the

cavitational erosion exerted by action of the solid particles (line

in Figure

6.9). Finally, the last region in which the relation between erosion intensity

and sediment concentration relationship is linear (which indicates predomi-

nance of the hydroabrasive erosion over cavitation).

-I

<-^

"—

(

338

Abrasive Erosion and Corrosion of Hydraulic Machinery

Hence, the results of the experimental study, obtained during the survey of

the cavitation-abrasive erosion taking place in the blades of a centrifugal

pump impeller, have shown that, depending on the operational conditions of a

pump and the cavitation development level, the concentration of solid particles

does not have equal effect

the erosion process.

I (%) I (%)

Figure 6.15 Effect of sediment concentration on cavitation abrasive erosion

intensity at

h =

4.1

(1);

4.3 m

(2);

4.7 m

(3);

5.4 m

(4);

6.2 m (5);

.7,2m

(6);

dash lined - hydroabrasive erosion,

h=10.2 m

6.3 Relationship between Hydroabrasive Erosion and

Cavitation Erosion

The interrelation of hydroabrasive erosion and cavitation erosion, or certain

mechanisms of the combined cavitation-abrasive erosion, have been studied at

the V.V. Kuibyshev MIEI (under the guidance and with the participation of

the authors), as applied to the operational conditions of axial-flow pumps.

With the aid of a test jig (Figure 6.16), the influence of the cavitation

development level on the intensity of

the

hydroabrasive erosion in axial-flow

pumps is studied. In the experiments carried out with use of

the

experimental

pump iir 35-MA, the blades of its impeller (with 350 mm diameter) were set

at angles

<p =

+2°,

0° and

-3°,

with the other parameters presented as n = 960

rev/min, p

= 10

g/1,

the particle dia. being 0.34 mm.

Interaction between Cavitation

and

Abrasive Erosion Processes

339

Four bladed impellers for

this

pump (two of them were experimental ones)

were fabricated from silumin

A/1-

9B.

The erosion of the impeller was evaluated by loss in mass AG and by the

position of and change in the geometric size on the silumin blades after 5

hours of

operation.

Cylinder-shaped specimens having 12 mm dia. and made

of silumin were used to evaluate the erosion of the casing of the impeller

chamber; the specimens were mounted flush with the surface of the casing.

The leading parts of the blades were made to accommodate flat specimens

fabricated from silumin, 100x80x3 mm in size. The working surfaces in the

blades, used as the cylindrical and flat specimens imbedded in the pump parts

were treated to reach the same level of surface finish.

1 - electric motor; 2 - pressure regulator; 3 - axial-flow pump;

4 - suction

piping;

5 - latch;

- reservoir;

- control latch; 8 - pressure piping

Figure 6.16 Experimental stand diagram layout for study of erosion in the

flow-passage portion of an axial-flow pump

As an abrasive material chosen to be used was river sand. To maintain the

concentration of the sediment constant in the system, special samples were

selected during each experiment, from different points on the outlet area of the

pressure piping, every 0.5, 1.5, 3 and 4.5 hours; the selected samples were

checked for concentration of solid particles in all the six samples selected.

340 Abrasive Erosion and Corrosion of Hydraulic Machinery

The absence of cavitation on the impeller blades was defined, in the

course of the hydroabrasive erosion study, using pure water and easily

removable paint coating. The research study performed at three different

impeller blade angles, i.e.

= +2°, 0° and -3°, and with various rotational

speeds. The paint coating remained undamaged (not effected by cavitational)

up to zl/z > 6.8 ~ 7.2 m. The mechanism of the hydroabrasive erosion was

analyzed at Ah being above 7.5 m.

To determine the forces acting on the walls of the impeller case, the

pressure fluctuations were measured by use of oscillograms, using

transducers mounted throughout the whole length of the case.

Figure 6.17 illustrates changes in the erosion in flow-passages of the

pump at different values of

Ah;

the results are obtained for

= +2° and the

delivery Q = 0.356 m

/s >

opt

(Figure 6.17 a) and Q = 0.271 m

/s <

opt

(Figure 6.17 b). As seen from the figure, the erosion intensity of

the

impeller

and its case drops noticeably with the decrease of Ah to a certain level, which

is due to the protective effect exerted by the cavitational zones. The

development of this effect was ascertained by experiments carried out using a

Venturi tube and by the use of a centrifugal pump. However, the same

operational conditions give a gradual rise in the erosion of the leveling device,

which results, most probably, from the fluctuating cavitational vortexes

generated by the impeller. The chart results illustrate that the region of the

minimum erosion intensity in the impeller, as presented by the characteristic

curve of the pump, may be displaced to one side of the curve or the other,

depending the modes of operation.

In the experiments performed with the blade angle

- 0°, at flow rate Q

> 0.93

oph

the dependencies obtained and related to Ah, are similar to those

presented in Figure 6.2, 6.3 and 6.4. But at flow rate level Q < 0.9

opt

the

characteristic curves change (Figure 6.18). The reduction of Ah resulted in a

gradual growth of erosion intensity in the blades and the impeller case. At this

stage of the study it may be explained only by the differences in motion of the

suspension-conveying flow traveling through the inter-blade channels, and

specifically, at low delivery levels by the formation, of cavitation zones on the

blade surfaces not subjected to hydroabrasive erosion.

In the first series of experiments performed with

- -3° and at all flow

rates of the pump, no protective effects of cavitation zones were detected at n

= 960 rev/min, therefore additional experiments were run for

= -3° at n =

Interaction between Cavitation and Abrasive Erosion Processes 341

900 and 1020 rev/min, and also with a wider range of sediment concentra-

tions.

G(mg)

6.8 7.2 7.6 8.0 8.4 Ah (m)

(a)

G(mg)

3500

2500

1400

1200

1000

800

6.8 7.2 7.6 8.0 8.4 Ah(m)

(b)

1 - impeller; 2 - impeller case; 3 - leveling device

Figure 6.17 Hydroabrasive erosion of flow-passages of

an axial-flow pump versus cavitation margin

G(mg)

• i

■«-

sot

Ann

300

»x

. ^

or-

(A S.t 7.2 IS 8.0 «*Ah(m)

Figure 6.18 Variation of cavitation-abrasive erosion as a function of

cavitation at delivery Q < 0.9

opt

and

- 0

(1,2,3 - the same as in Figure 6.17)

342

Abrasive Erosion and Corrosion of Hydraulic Machinery

The experiments have shown have shown that for

q> =

-3° the cavitational

zone protective effect appears at n = 1020 rev/min, only at the Q

opt

One

such

relation AG =f(Ah)

opt

atQ

0.297

/s is presented in Figure 6.19

(curve 1). At different solid particle contents p, reducing the Ah to a certain

value, a fall of

the

erosion intensity was seen. But at further reduction of the

former, a fast rise developed in the cavitation-abrasive erosion of blades

(curves 1), of

the

case (curves 2), of the impeller and of the leveling device

(curves 3). The experimental results obtained have shown that, with increase

in the content of the sediment

the minimum erosion zone is displaced to the

left, i.e. the higher the concentration of

the

sediment in the flow is, the later

the cavitational zone effect will appear.

The similar case was noticed earlier, during the research studies carried

out on hydroabrasive erosion mechanisms. It was found that, within a wide

range of changes in sediment concentration and grain size, the erosion rate at

Ah = 7.67 m is less intensive than atAh

8.42 m.

As can be seen from Figure 6.19, the curves related to the pattern of

cavitation-abrasive erosion, as appeared on the experimental blades and on

the reference blades (additionally prepared for

this

experiment from CHI2 - 28

gray cast iron) remains unchanged at p = 10 g/1. In relation to this, the

silumin-to-gray cast iron erosion resistance ratio is equal to 0.81.

In accordance with the relationship presented in Figures 6.11 ~ 6.13, 6.17

and 6.19, it is possible to carry out a qualitative comparison specifying the

development of the cavitational-abrasive erosion processes related to the

impellers of centrifugal and axial-flow pumps.

The experiments [6.5, 6.6, 6.7] have show that for centrifugal pumps with

= 80~100 using enclosed impellers, the erosion intensity I alters gradually

for a wide range of alterations

values (see Figures 6.11-6.13).

In axial-flow pumps, the blade channels are connected to each other via a

gap at the impeller tip. The cavitation zones being formed at the peripheral

areas of blades, especially at the initial stages of cavitation development,

serves the purpose of protecting the blade surfaces against the action of the

solid particle, as in the case of centrifugal pumps, and also reducing the

consumption of

the

hydroabrasive mixture flowing through the end face gap.

Therefore, the erosion weight loss AG alteration in the impeller of an axial-

flow pump, depending on Ah, is more intensive way than it does in a

centrifugal pump.

Interaction between Cavitation and Abrasive Erosion Processes 343

Q 0/s)

300

280

260

AG (mg)

8000

6000

4000

2000

AG (mg)

AG (nag)

800'

600

400

200

6.8 7.2 7.6 8.0 Ah (m)

Figure 6.19 Effect of cavitational zones on cavitational-abrasive erosion in

flow-passage portion of an axial-flow pump

(1,

2, 3 see Figure 6.17)

The experiments have shown that, as cavitation-abrasive erosion took

place in an axial-flow pump, one significant factor affecting this process was

the incidence angle

<p,

as determined by the angle of the impeller blades.

Figure 6.20 illustrates the influence of

upon the mentioned erosion AhG/Q

for three angles determining the position of the blades at n = 960 rev/min at

delivery Q >

opt

The smaller the position angle

<p,

the later the cavitational

protective effect takes place (sections I, II and III). The greatest protective

effect of the cavitation zones at the given rotational speed develops at

=35SaLa=:E

»«-

Ufc—

■o

f—

-=^-

rf: