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

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water pressure operated

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Figure 4.12 Spherical valve with main seal of water pressure operated rubber profile;

Maintenance seal is of the traditional steel type

Design of Hydraulic Machinery Working in Sand Laden Water

175

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176

Abrasive Erosion and Corrosion of Hydraulic Machinery

prevent direct inflow of the boundary layer water caused by the secondary

flow along the wall of the spiral casing. A modern design of stay ring for

Francis turbines is shown in Figure 4.14. The velocity profile out of the stay

vanes towards the guide vane inlet will also be more uniform with the modern

design. This effect may also be favorable for the sand erosion, but a clear

conclusion cannot be drawn. A normal paint will under normal conditions be

sufficient to protect the steel.

Figure 4.14 Modern design of stay ring for Francis turbines

For sand laden water rubber based paint in the spiral casing of high head

turbines has been used and successfully tested in Switzerland by Sulzer.

Special care should be taken of the stay ring. For ultra high head pump

turbines stainless steel may be used in the stay vanes to prevent turbulence

erosion and or sand erosion followed by cavitation damage.

The Guide Vane System

In the guide vane cascade one will find the highest acceleration and the highest

absolute velocity in a Francis turbine. The relative velocity head (C

/2g)IH„

increases from about 0.1 to approximately 0.5 from the guide vanes inlet to

the runner inlet for a high head turbine. At normal speed the pressure drop

across the guide vanes will be approximately 40% of Hn at full load and 50%

of Hn at small openings and in closed position.

Following effects create sand erosion in the guide vane cascade

Design of Hydraulic Machinery Working in Sand Laden Water

111

(1) Turbulence erosion

Turbulence erosion occurs specially in the outlet region caused by high

velocity and fine grain sand. Heavy erosion is observed on facing plates.

(2) Secondary flow

Secondary flow erosion occurs in the corners between facing plates and

guide vanes due to the horse shoe vortex along point on the inlet end.

Fine grain sand and medium size sand causes this erosion. Heavy

erosion is observed on facing plates along the guide vane contours.

(3) Leakage erosion

The leakage through the small clearance between the guide vanes and

the facing plates increases the horseshoe vortex on the suction side. In

addition this leakage flow creates also heavy erosion at the inlet

pressure side and the outlet suction side due to local separation and

turbulence. Fine grain sand creates often-severe damage caused by this

leakage flow in high head turbines.

(4) Acceleration

The acceleration of the main flow creating the rotation of the water in

front of the runner and creates an acceleration normal to the stream lines

and guide vane blade surfaces and separates sand and gravel to give

impacts of the steel surfaces. If large grain size composites exist in the

water heavy destruction of the guide vanes occurs. Secondary flow

created by the acceleration normal to the streamlines also creates

erosion by fine grain sand especially in the corners between guide vanes

and facing plates.

* Design

It is not possible to avoid erosion by means of design only. Smoothest

possible acceleration, reduction of the clearance gaps between guide vanes

and the facing plates as well as a careful choice of

the

stay vane outlet angles

bringing the guide vane in neutral position at the normal operation point could

be recommended in general. Guide vane sealing to prevent the leakage flow

will also reduce the erosion until the turbulence and secondary flow erosion

has created waviness on the facing plates. Such waviness may cause

dangerous galling between the seals and the facing plates and this guide vane

seals may cause a reduction of the reliability of the turbine.

The conclusion will be that the designer's best weapon against sand

erosion will be new material and in particular the new ceramic material that is

178

Abrasive Erosion and Corrosion of Hydraulic Machinery

now under development also for complicated geometry like the guide vane

system.

* Material development for guide vane system

For the guide vanes ceramic coating has been developed for high head

turbines. This coating is the same quality, which has been, describes for

Pelton needle and nozzles in this chapter.

* Loss in efficiency due to abrasive erosion

Abrasive erosion of the facing plates on head cover and lower cover leads

to an increased leakage between the guide vane facings and the covers.

The increased loss in efficiency caused by abrasive erosion of 2 mm in a

high head turbine operating at 540-m net head is shown in Figure 4.15. An

eroded facing plate is shown in Figure 4.16.

100

ORIVA POWER PLANT.

EFFICIENCY OF SANDERODED AND REPAIRED TURBINE

FRANCIS TURBINE P = 71.5MW, H„= 540m. n = 600RPM

100%

Contribution from increased

labyrinth leakage.

0 MEASURING POINTS BEFORE REPAIR (JULY 1978)

O MEASURING POINTS AFTER REPAIR (FEB. 1979)

50 60

MW(atH

=540)

Figure 4.15 Efficiency loss corresponding to the erosion

shown in Figure 4.16

Design of Hydraulic Machinery Working in Sand Laden Water

179

Figure 4.16 Eroded facing plate for Driva Francis turbine,

average depth = 2 mm

It should however be emphasized that for a new turbine the clearance in a

pressurized turbine will be approximately 0.5 mm on each side. The dry

clearance (without pressure) will be 0.05 mm to 0.1 mm on each side.

Because of the small clearance one must be careful when coating the

facing plates in order to obtain the necessary tolerances and surface finish.

The Runner

In the runner the highest relative velocities occur at the outlet regions while

the highest accelerations may be found at or close to the blade inlet. At off

design operation very high local accelerations may occur at the inlet edge of

the blades.

Turbulence erosion from fine grain sand or silt may then be the reason for

outlet erosion together with erosion from coarse sand while the acceleration at

the inlet is mainly caused by coarse sand which may cause severe erosion

specially at off design operation.

In any case the inlet region will be sensitive to incorrect pressure

distribution and large difference in pressure between pressure and suction

180

Abrasive Erosion and Corrosion of Hydraulic Machinery

sides.

Splitter blades will reduce the pressure difference from pressure side to

suction side and thus reduce the damage of flow around the inlet edge in off

design point operation, which may cause severe local sand erosion.

Cross flow from hub to shroud caused by incorrect blade leaning will also

increase the so-called horseshoe vortex in the blade roots. At the inlet of the

blade horseshoe vortices may create sand erosion groves in a similar way as

around the guide vanes.

Labyrinth Seals

The velocity of the labyrinth seals on the runner is in the order of

m/sec in

a high head turbine. Turbulence erosion from silt laden water and sand

erosion from coarse sand will always to some extent occur in this region. The

clearance between the stationary and rotating parts is between 0.5 ~ 1.5 mm

depending on the size of the turbine. A strong turbulence occurs in the flow

making the sand erosion severe in this region. However, the efficiency loss

caused by the increasing leakage will be small. The efficiency loss due to

erosion of labyrinth seals was only 0.5% or approximately 10% of the

efficiency loss shown in Figure 4.15.

The Draft Tube

Except for the portion of the draft tube closest to the runner outlet where the

highest absolute velocities occurring sand erosion is normally not a problem

in this part of the turbine. The section closest to the runner will normally be

made of stainless steel, which is more resistant to abrasive erosion than

carbon steel.

The Shaft Seal

Mechanical shaft seals with carbon rings may be damaged by silt and sand if

the water running through the turbine reaches this part.

Clean pressurized water injected to the seal with overpressure higher than

the water from the turbine flow may save this part from damage. Labyrinth

seals,

which allow for an arrangement to let the shaft seal from any sand

erosion under operation at normal speed. Such arrangement has been used

successfully in a large number of high head turbines operating in sand-laden

water (Lubuge Power Plant in China is one example of this type).

Concluding Remarks

Sand erosion cannot be avoided in turbines because sedimentation of the silt

with grain size less than 0.1 or 0.06 mm is not possible in majority of cases.

Design of Hydraulic Machinery Working in Sand Laden Water 181

By careful design of the blade loading and by using dry running shaft seals of

labyrinth type a safe running may be obtained.

The best weapon against sand erosion however, may be the development

of the ceramic coating which increase the time between necessary repair by

one or two decades compared with the surfaces of hardened stainless steel.

4.2 Effects of Silt-Laden Flow on Cavitation

Performances and Geometric Parameters of Hydraulic

Turbines

Y. L. Wu

4.2.1 Effects of Silt-Laden Flow on Cavitation Performances of

Hydrauliv Turbines

A large number of hydraulic turbines working in silt-laden rivers encounters

serious problems of silt abrasion and aggravated cavitaion. In lighter cases

machine performances are impaired and in serious cases structural damages

may result. In recent years, many hydraulic engineers are faced with the

serious challenge of solving the problems of silt-laden flow in the many hydro

projects on the silt-laden rivers.

Effect of Sand Content on the Cavitation Process

* Incipient cavitation number

The existence of silt in water flow has a definite influence on the inception

of cavitation and its behavior in bubble growth. Liu, Y.X. (1983) did the first

tests to study the incipient cavitation number in silt-laden flow in China [4.1,

4.2].

His experiment was carried out in a water tunnel with a test section of 8

x 20 mm

and the flow velocity of 12 to 23 rn/s to measure the cavitation

bubbles following a cavitation generator (a cylinder of diameter D = 15 mm).

Tests were made with sand from the Huanghe River with grain size of 0.014

-0.018 mm and sand content of 187 kg/m

. The incipient cavitation number

is defined as

182

Abrasive Erosion and Corrosion of Hydraulic Machinery

= 2{P

- P

)l(pUl) (4.4)

where P

and U

are the reference pressure and velocity at the inlet of the

water tunnel. Figure 4.17 shows the experimental results of the variation of

incipient cavitation number obtained by observing the bubble inception and

the experimental results of the status cavitation number K

for maximum

sound intensity of cavitation noise. The inception cavitation was determined

by measurement of the cavity length, here X = 0.5 mm. Another group of

experimental results were obtained by using a NASA 4412 airfoil in another

water tunnel with plastic sand of d

= 0.165 mm and density p = 1070

kg/m

, as shown in Figure 4.18. From these two experimental results, it is

very clear that there exists a critical concentration of S„ = 10 ~ 13 kg/m

which the incipient cavitation number

AT,

reaches a maximum value. For the

consentration, S, less than 10 ~ 13 kg/m

the sand plays a role of inducing

cavitation whereas for S greater than 10-13 kg/m

it has a retarding effect.

fl 3

O 2

C/5

>..

Kiin ;lear wate

Ki (observation U>3%)

At maxin urn intens ty of nois;

20 40 @0 80

100 120

Sand content S(kg/m

)

Figure 4.17 Effects of solid content on incipient cavitation number

for a cylinder

Design of Hydraulic Machinery Working in Sand Laden Water

183

* Cavity length

In clear water, the cavity length does not change for a given cavitation

number. But for the test with silt-laden flow, the status cavitation number K

for a certain cavity length changes with variation of sand concentration, as

shown in Figure 4.19. Figure 4.20 shows the results of variation of cavity

length with sand concentration, S, at the fixed value of

K„,

which are taken in

the values from 2.2 to 3.0. Figure 4.21 is the results of the same experiment

as that for Figure 4.18. Figure 4.21 shows the variation of of cavity length

with sand concentration at a fixed value of several cavitation numbers, K

For a definite cavity length, the critical concentration S

(for which the

cavitation number is highest) may be 5 ~ 8 kg/m

for Figure 4.19 and 10-13

kg/m

for Figure 4.21.

* Cavitation Fluctuation

There is a critical concentration value S

for the variation of cavity

fluctuation length AL

max

as shown in Figure 4.22, where the S

is 10 ~ 13

kg/m

for NASA 4412 airfoil.

Sand content S(kg/m

)

Figure 4.18 Relation between sand contents and Ki for NACA4412