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

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Abrasive Erosion and Corrosion of Hydraulic Machinery

1.4 Abrasive Erosion of Pump

1.4.1 Examples of Hydraulic Abrasion Taking Place in Pumps

Let us consider, as illustrative examples of hydraulic abrasion taking place in

pumps, certain results obtained during the study carried out on some pump-

houses in Azerbaijan (USSR), operation with water which contained a great

amount of sediment matter (above 2.5 kg/m

When analyzing operational conditions of pumping plants in the

Azerbeijan SSR, it can be ascertained that deterioration of the flow-passage

portion components in a pump (in impellers, walls of cases etc.) takes place

not only due to cavitation but also owing to hydroabrasive effect produced by

suspended matter and abrasive-cavitation phenomena, remarkably enhanced

when the pumps operate in those points of

the

curve "Q-H" which differ from

the optimum.

Visual inspection of the centrifugal pump impellers with 1.5-m diameter

has shown that peripheral regions in these impellers were mainly the regions

subjected to the greatest erosion. Here one can reveal areas of uniform

hydraulic abrasion as well as regions with deep channels and grooves. Certain

blades and impeller rims include through holes. The erosion is increased in the

direction faced towards the periphery, reaching its maximum level on the

working surfaces of blades, at points of separation of the stream from the

surface being flown around.

In axial pumps cavitational-abrasive erosion takes place on the rear faces

of the blades included in impellers with 0.8-m diameter The worn-out surfaces

in these regions are of sponge-shaped structure and such erosion sometimes

occurs at the leading and trailing edges of the working blades.

These components, being made of carbon steel, were significantly worn

out after 1 to 1.5 year of operation. Visual inspection of these blades has

revealed that their surfaces are covered with multiple cavities measuring

10x20 mm in area and up to 6 mm in depth, formed as a result of the effect

exerted by cavitation and sediment matter. In addition to the damage of

impellers the motionless components in the hydraulic machines were

deteriorated as well. An illustrative example of this is found in the surface

failure, which took place in the impeller case of a large propeller-type pump.

The case under review, fabricated from case iron, is designed to include two

Fundamentals ofHydroabrasive Erosion Theory 35

compartments (the lower and upper ones). The metal deterioration pattern

formed a strip of

~ 25 mm in size and 2-3 mm in depth. The case surface

of the impeller has a complete porous structure in the failure region.

An example of erosion taking place in centrifugal pumps under the

influence of suspended sediment matter is available from the results obtained

during the survey carried out on the units of a large irrigation system pump-

house situated on the Kure river. The pumping plant is equipped with eight

pumps of the 24 Z4 NDN model. This pumping station operates 6 months a

year. The pump impellers having 0.6-m diameter were subjected to the

erosion.

A conventional factory-made pump impeller is worn-out for a period of

one to one and a half season of operation, and its blades are the components

subjected to the greatest erosion.

Visual inspection of the damages emerged on the impeller blade surfaces

in the 24 Z4 NDN pump has revealed the following. Traces of deterioration

caused by abrasive erosion have shapes resembling large-size grooves with

smooth surface. The inner and outer surfaces in the blades are covered with

deep scaly erosion with the trailing edges of the impeller blades being notched.

The erosion is characterized by availability of distinctive separation edges

and peculiar-shaped grooves and cuts stretched in the direction of the flow

stream. Individual grooves are 30 ~ 50 mm long, 5-6 mm wide and 1 ~ 3

mm deep. The entire working surface areas in these blades are covered with

grooves and cuts of the indicated shape. This results, probably, from mutual

action of cavitation being formed by the surface unevenness and suspended

matter present in the area of the pulsation cavity.

The joint effort exerted by both the sediment matter and cavitation

resulted in deterioration of entire blade areas. At the completion of

one

year of

operation, in the water including suspended sediment particles, there are cases

of complete absence of edges in the pump blades within the area reaching up

to 5 cm in width.

The pump is provided with protective seal rings: two of them are

stationary and the other two rings are mounted on the external rim of the

impeller. The stationary seal rings, being fabricated from case iron, are

replaced generally twice a season. The seal rings of the impeller are usually

fabricated from cast iron as well. These rings get worn out during one month.

Therefore they are replaced with rings fabricated from a steel strip 3 mm

Abrasive Erosion and Corrosion of Hydraulic Machinery

thick and 15 mm wide; the rings are mounted in hot state and then machined

to shape. These rings prove operable in the course of three months.

To prevent the pumps from hydraulic abrasion, the trailing edges of the

impeller blades, specifically, are built up in thickness. The operation of

impellers with thickened edges has demonstrated that such edges maintain

serviceability in the course of

one

to one and half year of operation.

1.4.2 Silt Erosion in Pumps

The results obtained during the methodical observations over different types

of pumps subjected to intensive hydroabrasive erosion and to cavitational-

abrasive action, along with numerous data given in some publications, allow

to define, in a fairly precise manner, those components within the flow-

passage portion of pumps, which are most susceptible to erosion. In this

respect, when considering the erosion elements, associated with the flow-

passage portion of a pump, it is hardly possible to avoid mentioning the

cavitation phenomena, which accompany often the hydraulic abrasion process

in question.

It should be noted with reference to the cavitation that the general

pressure drop at the inlet to a pump results in an upset of

the

pump operation,

or bring about such a significant distortion of its performance which creates

the operational conditions considered to be unacceptable; moreover, they are

not characteristic of the situation enabling to evaluate the cavitation erosion

taking place within the working components of a pump. The greatest interest,

in this regard, refers to the local pressure fall, which causes emergence of

local cavitation zones. Although the influence level of these zones on the

energy quality and vibration state of a pump assembly is insignificant, they

may become, nevertheless, a source of intensive erosion.

The impellers in the impeller pumps of all types are the elements subjected

to the cavitational erosion most greatly, due to the hydrodynamic processes

which develop in them. It is estimated that for the optimum behavior, with due

regard for the meridian and circumferential constituents, the relative flow

velocity, at the inlet to the interblade channels of

the

impeller, is equal to 20 ~

45 m/s, depending on the standard size of a device. As the flow travels around

the blade leading edges with velocity so high and, in accordance with the

leading edge shape and the shape of the front region in the blade, the local

pressure drop can reach the level of 0.2 ~ 0.3 MPa. Consequently, the

Fundamentals ofHydroabrasive Erosion Theory 37

emergence of cavitation in these areas is possible even at a significant

excessive pressure. Steady cavitational zones within the areas adjacent to the

leading edge can be found practically in all the modes differing from the

optimum operation. It was established also that in operations with a partial

delivery (Q <

opt

)

the cavitational zones are formed at the rear face of a

blade (zone 1 in Figure 1.11 a, b); in speed-up operations (Q >

opt

)

they are

generated at the working surface side (zone 2). As a result of the flow

separation from the rear surface, the cavitational erosion also affects those

regions of the blades, which are immediately adjacent to the trailing edges

(zone 3). The erosion intensity in these regions in dependent on the edge

shape, and it increases, as a rule, with a rise in the flow delivery.

(

) (b)

(a) centrifugal pump (b) axial-flow pump

Figure 1.11 Map of the erosion arising incentrifugal (a)

and axial-flow pumps (b)

As a result of the dynamic interaction between the in-flowing steam with

the components forming the inter-blade channels of the impellers in

centrifugal pumps, cavitational zones can emerge in the regions adjacent to

the adjoin line of the impeller blades with the disk surfaces (Figure 1.11 a,

zone 4) and on the inner surfaces of the impeller outer disks within the one-

way entrance (Figure 1.11 a, zone 5); they can also arise on both disks of

impellers in the two-way entrance, due to deviation of the stream flow line at

38 Abrasive Erosion and Corrosion of Hydraulic Machinery

the bend. In a similar way erosive destruction took place within the surface of

the impeller hubs in axial-flow pumps, at the places where the blade inner

faces adjoined (Figure 1.11 b, zone 6), as well as within the cylindrical

regions of the hub, under the blade journals (zone 7).

In axial-flow and mixed-flow pumps the end faces in impellers (Figure

1.11 b, zone 8) and the impeller case walls (zone 9) are the places which are

damaged most frequently, as a result of the slit-shaped cavitation so

characteristic of the hydraulic machine type under review. The presence of a

gap (a slit) between the impeller end faces and the case walls, which provide

free rotation of the impeller, allows the water to flow from the area positioned

above the impeller to the region located beneath the impeller; the great

pressure differential at the both sides of a blade creates extremely high flow

velocities of this secondary stream. As a result a local pressure drop occurs,

thus forming on the blade suction surface a cavitational zone traveling within

the case inner surface with the velocity equal to the circumferential rotational

speed of the impeller. In addition to the hard operational conditions affecting

the material of the impeller case, its surface is subjected to high fluctuation

loads.

The case surface is affected by alternating loads generating in its

material fatigue stresses which contribute to accelerate its failure.

In axial-flow and mixed-flow pumps with a spiral casing, the latter's

walls are subjected to cavitation in the places adjacent to impeller case. In a

number of instances the cavitational failure affects the blades of the leveling

device (Figure 1.11 b, zone 10) which, probably, results from discrepancy

between the angle of the flow in leakage and that of the blade positioning.

A high velocity of the flow relative motion, in the pockets formed by the

impeller disks and the case in centrifugal pumps, can bring about formation of

cavitational zones on the outer surfaces of the disks and inner sides of the case

(Figure 1.11 a, zone 11). Among the other components forming the flow-

passage portion in pumps, where the emergence of local cavitaional zones is

possible, it is necessary to distinguish the gaps and slits foreseen by the design

and located between the rotational and stationary components of the device:

inner surface of the inlet immediately adjacent to the impeller; guide fins

mounted in the inlet pipes of certain designs, in order to reduce the spinning of

the flow prior to its entrance into the impeller; tug of the spiral casing.

The hydraulic abrasion in pumps, as compared with the cavitational

erosion, is of more general nature, since the presence of sediment solid

particles, suspended in the water being transferred, contribute to the failure of

Fundamentals ofHydroabrasive Erosion Theory

practically all the surfaces in contact with the water stream. The pump

components subjected to the abrasive erosion, as related to the effect produced

by them on the energy quality of the unit, can be divided into five basic

groups.

The first group of components subjected to hydraulic abrasion includes

impellers of the impeller pumps which are intensively destructed when the

water contains suspended abrasive particles.

In centrifugal and mixed-flow pumps incorporating closed-type impellers

the erosion process occurs within the following components:

* blade of an impellerthe process develops most intensively at the

working side and, especially, heavy damage is distributed at the

leading edges, on the both sides, and at the trilling edges on the

working face side (see Figure 1.11 a, zone 2). In this respect, the

areas adjacent to the rear disk are subjected to the greatest erosion;

* inner faces of the rear disk (see Figure 1.11 a, zone 14), most

intensively at the area of the impeller hub, i.e. in the region of the

flow turn;

* inner face of the front disk (it is worn more uniformly and

significantly less than the front disk).

The components being worn in axial-flow and mixed-flow pumps, with

closed-type impellers, are as follows:

* working surfaces of

blades,

in which the erosion intensity increases in

the direction towards the trailing edges;

* end face edges of bladesas a result of the erosion caused by a slit-

shape cavitation, with the sharply increased intensity when the water

contains abrasive particles;

* rear surfaces of blades in the areas adjacent to the leading edges, due

to vortex-type streams flowing around the blade in non-specified

operations.

The second group of components subjected to an intensive hydraulic

abrasion is compiled from components included in guide and leveling devices

used in some types of pumps.

To the third group of components being worn, it is necessary to ascribe

those which constrict the rotating impeller, i.e. the impeller case walls in

axial-flow and mixed-flow pumps and the walls in casings of centrifugal

pumps.

Abrasive Erosion and Corrosion of Hydraulic Machinery

The fourth group comprises sealing components and bearings included in

a pump shaft. The solid particles getting into these seals contribute to a rapid

erosion of their elements. The destruction of seals, in addition to volumetric

losses, can be caused, in a number of

cases,

by solid particles getting into the

bearings, which is quite intolerable.

The fifth group includes inlet and outlet devices of pumps. According to

the data obtained in a number of analyses, it is possible to conclude that the

hydraulic abrasion rate developed in the pump outlets, particularly in the

centrifugal pumps, which is presented by losses in mass per unit of time,

exceeds considerably the erosion intensity in the impellers. However, the

component which is decisive in determining the inter-repair time duration,

under the conditions of hydroabrasive effect, is an impeller, since the outlets

and casings in a pump are greater in their size and mass. Moreover, the

partial erosion in the latter affects the performance of a pump in a lesser

degree.

With reference to the combined effect produced on the pump components

by both the cavitation process and the solid particles contained in the liquid

transferred, it should be noted that to ascertain the exact boundaries of the

areas subjected to the cavitational-abrasive erosion is rather difficult in

practice, since in operational conditions one of the two erosion patterns is

predominate, as a rule, over the other, therefore, the erosion subjected surface

possesses distinctive features characteristic of

one

of them.

However, as to the intensity of the cavitational-abrasive erosion, it cannot,

by any means, be evaluated by an ordinary summation of individual patterns

of failure and is defined, in accordance with the specific operational

conditions, by a set of various factors.

In order to solve practical problems in connection with, specifically, the

operational reliability of the pump units, it is necessary to use quantitative

parameters capable of providing the evaluation of the erosion process under

review. They are, first of

all,

as follows:

Area S and depth b for a certain period of the pump operation T, S and b

values can be presented both in absolute units (cm

, m

, mm or cm) and in

shares (%) of the total area or initial thickness of the component being worn.

The loss in volume V of

the

material used to fabricate a component being

worn for time T. To determine the meanings of V (mm

, cm

) in practice, the

dependencies used are as follows:

Fundamentals of Hydroabrasive Erosion Theory

V=Y

]

-S

-b

)

(1.19)

kY(S

-b

)

(1.20)

where

and

are correspondingly area and depth

individual regions

erosion,

or k

are coefficients with allowances made for the erosion taking

place within the surface and depth of a component.

The loss

mass

AG for a

certain time period

can

expressed

absolute value (mg, g and kg)

AG =

V-p

(1.21)

where

is material density of a component being worn, or

-G

(1.22)

where

and

Gf,„

are

respectively

the

initial

and

final masses

of the

component being worn.

The relative values

G' corresponding to loss

mass are also used

practice, which looks like

»-

.23)

In the field conditions of operation, when dealing with hydraulic machines

which have great geometric size

and

weight,

it is

essential that,

for

approximate evaluation of

the

erosion, and in accordance with the regulations

laid down

the I.E.C. standard

[1.15],

the mass

electrodes M

i be used,

required to rebuild the surface destructed. The M

value

defined by actual

consumption or approximately by the expression

=(\

-^)-p

-V

(1.24)

Abrasive Erosion and Corrosion of Hydraulic Machinery

where p

is material density of the electrodes, b

max

is maximum depth of the

destruction. Equation (1.24) takes into account the treatment of the surface

being rebuilt in a component, after the building up.

In number of cases the erosion is estimated by the operational time of a

machine which was used period to a certain level of failure in a check

component, or by alteration arising in the energy quality of a device, for

instance, by a drop in its efficiency by a certain value. The erosion indicator

to be chosen is selected due to operational conditions and the problems that

should be solved by making use of field measurements.

The erosion rate is generally defined by estimation of

the

erosion per unit

time.

In accordance with the erosion index chosen, the expressions used in

practice are as follows:

J >' Iy

J ; I

— (125)

The limits of failure to be developed within the flow-passage portion of,

for instance, pumps and caused by cavitation, sediment matter or cavitational-

abrasive effect are functions of:

1) the standard size of pumps (parameters n

, Q);

2) the materials used to fabricate the components composing the flow-

passage portion of a device;

3) the specification of water-power stations and a water flow (chart of

water delivery, availability and composition of pumps);

4) the duration of the interrepair period of operation.

1.5 Technical and Economic Effect Caused by the

Erosion Arising in Hydraulic Turbines and Pumps

Technical and economic results of the erosion, being developed in hydro-

turbines due to attrition process caused by suspended sediment matter and

cavitation, manifest themselves in two ways. First, the development of the

process leads to a decline in energy properties of turbines (a decrease in

efficiency and, in certain cases, in power capacity), with the associated fall of

the electric energy output. Secondly, the process necessitates repair jobs

Fundamentals ofHydroabrasive Erosion Theory

aimed at removal of erosion aftereffect in components composing in hydro-

turbines the flow-passage portion; such maintenance service requires a

considerable amount of labour and material expenses. As this takes place, the

total extra costs rise so high as to acquire an independent technical and

economic value.

The impairment of energy properties in turbines due to cavitational-

abrasive erosion taking place in the components, associated with the flow-

passage portion of the devices, has been ascertained during the full-scale

testing performed. A drop in the efficiency of a heavily worn-out turbine, as

compared with a repaired turbine with a newly installed impeller, amounts to

12 ~ 14% within the entire operational range of the power variation limits.

The decrease of efficiency and reduction of electric power output in

turnaround time, brought about by this, can be amounted, with a sufficient

level of accuracy, to 6 ~ 7% value of the total output. Assuming that the total

electric energy generated by the hydroelectric unit in the inter-repair time

(above 12,000 h) is equal to about 75 ~ 80 million kwh, it can be estimated

that the energy underproduction, through deterioration of the turbine quality,

amounts to about 5 million kwh.

In assessing this aspect of the problem, special attention should be given

to the fact that the greatest drop of the turbine efficiency, caused by the

cavitational-abrasive erosion, appears, approximately by the end of high

waters taking place in mountain rivers, therefore the worsening of

hydroelectric station performances are particularly susceptible to changes

arising in this winter period being most scarce both in energy and water

discharge volume. Besides, while a turbine efficiency drop, in the time of a

summer flash flood, may fail to be attended, in certain occasions, with a fall

in its capacity and output (due to a speed-up flow rate formed by a redundant

run-off volume), in winter and spring times, however, the mentioned

efficiency drop results in decreasing both the guaranteed power and the

amount of the electric energy generated, owing to a deficiency in the water

flow. The real cost of this under-produced energy is, of course, much higher

than in the other seasons of the year.

Rebuilding of heavily worn-out turbine impellers entails certain

difficulties associated with configuration intricacies in the surface shape of

impeller blades, as well as with difficulties preventing a fee access to them.

At the same time the reclamation of relatively small damages presents no

special problems and helps to practically restore the energy properties of the