Duan C.G., Karelin V.Y. Abrasive Erosion and Corrosion of Hydraulic machinery
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154 Abrasive Erosion and Corrosion of Hydraulic Machinery
3.23 Chen, Y.S., (1986). 'Computer code for three-dimensional
incompressible flow using nonorthoganal body-fitted coordinate
systems', NASA CR-178818.
3.24 Wu, Y.L., Dai, J., Oba, R., Ikohagi, T., (1995). 'Turbulent flow
simulation through centrifugal pump impeller at design and off-design
conditions', Proc. of2ndICPF, Tsinghua University, Beijing, 155-167.
3.25 Wu Y.L., Sun Z.X., Oba, R., Ikohagi, T., (1995). 'Study on flow
through centrifugal impeller by turbulent simulation', ASME FED-Vol.
222,
Fluid Machinery, 63-68.
3.
26 Wu, Y.L., (1996). 'Computation on turbulent dilute liquid-particle flow
through a centrifugal impeller by using the turbulence model', ASME
FED-Vol. 236, Proc. of the Fluids Eng. Division Summer Meeting,
1996,
Vol. 1, 265-270.
3.27 Chang, K.C. and Yang J.C., (1996). 'Transient effects of drag
coefficient in the Eulerian-Lagrangian calculation of two-phase flow',
ASME FED-Vol. 236, Fluid Engineering Division Conference, Vol. 1,
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3.28 Elghobashi, S.E. and Abou-Arab, T.W., (1983). 'A two-equation
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938.
Chapter 4
Design of Hydraulic Machinery
Working in Sand Laden Water
H. Brekke, Y.L. Wu and B.Y. Cai
4.1 Hydraulic Design of Turbines
H. Brekke
4.1.1 Introduction
Sand erosion in turbomachines in general may be divided into three
categories:
1.
Micro erosion which may be found on surfaces exposed to very high
velocities where silt or sand grains of magnitude 60 um and smaller are given
a high rotational velocity by the turbulence in the boundary layers and thus
causing a relatively strong abrasive erosion.
2.
Secondary flow vortex erosion, which you may find in a so-called
horseshoe vortex in corners of conduits, or other places where vortex and
secondary flow are caused by combined effect from boundary layers and/or
decelerated or accelerated flow.
3.
Accelerations normal to streamlines, which will separate the sand
grains and bring the sand grains to follow a path different from the water flow
and in collision with the walls of the conduits. Sand grains with sizes from 0.5
mm and up will normally cause severe damage in flow with acceleration
normal to the streamlines.
The conduits may be formed by blades in runners, guide vanes etc.
155
156
Abrasive Erosion and Corrosion of Hydraulic Machinery
The design of hydraulic machinery working with Reynolds numbers, Re,
from 10
6
to 10
8
will normally be exposed to flow conditions of all three
categories.
The damage will depend on the hardness of
the
sand, the shape and grain
size and naturally of the amount of sand as well as the absolute velocity and
the hydraulic radius of the regarded conduit.
Special caution must be taken in design of labyrinth seals in turbines or so
called wear rings in pumps as well as shaft seals due to very small hydraulic
radii and possible direct abrasive erosion due to direct simultaneous contact
between the sand grains and the rotational and stationary parts. It should also
be emphasized that small turbines will get a more severe damage from sand
than larger turbines of the same specific speed because of smaller hydraulic
radii and smaller radii in the curvature which brings a relatively larger
amount of sand in contact with the surface and cause a higher acceleration
normal to the stream lines.
The design of hydraulic turbines may be divided into following categories
with a significant different pattern of erosion.
- Impulse turbines
- High-head reaction turbines (Low specific speed Francis Turbines)
- Low-head reaction turbines (High specific speed Francis and
Kaplan turbines)
4.1.2 Impulse Turbines
The most commonly used impulse turbine is the Pelton turbine. The classic
type with horizontal shaft is shown in Figure 4.1 and the later developed
vertical shaft type is shown in Figure 4.2. The less known Turgo turbine
which is an axial flow machine with jets inclined relatively to the a plane
normal to the shaft is shown Figure 4.3.
Because the nature of sand erosion will be the same for both types of
these turbines and because very little information exits on sand erosion in
Turgo turbines the discussion in this chapter will deal with Pelton turbines
only. Also the low head type of impulse machine, the Ritmeyer turbines will
not be described for two reasons. At first, it is not quite clear if this turbine
type is classified as an impulse machine. Secondly, the Ritmeyer turbines are
designed for low heads and thus the sand erosion will be less severe than
machine types like Pelton turbines for high head.
Design of Hydraulic Machinery Working in Sand Laden Water
157
.8
§
1
o
N
■c
o
158
Abrasive Erosion and Corrosion of Hydraulic Machinery
Figure 4.2 Vertical Pelton trubine
Because the Pelton turbines normally are designed for heads above 600 m,
the velocity in the jets will be higher than 100 m/sec and the maximum
acceleration of
the
particles in the buckets will normally be more than 50000
m/sec depending on the size of the buckets and head of the plant.
Figure 4.3 Turgo impulse turbine
Design of Hydraulic Machinery Working in Sand Laden Water
159
In order to study the parameters or to determine which have influence on
the sand erosion, it is necessary to divide the design into following portions:
- The inlet including the inlet valve
- The nozzles
- The runner of Pelton wheel
- The wheel pit
The Inlet and Valve
For a horizontal Pelton turbine and especially for a vertical multijet turbine
the length/diameter ratio will be approximately 3 times longer than for a
Francis turbine designed for the same head.
Because of this the velocity will normally be kept moderate and following
formula may be used for normal turbines(except minihydro turbines).
C = K,(2gHe)
05
, (0.08 <K,<0.1) (4.1)
In the bifurcation sections the flow is accelerated towards the inlet in front
of the nozzles after a local deceleration.
A typical design of a manifoil of a Pelton turbine is shown in Figure 4.4.
The velocities in the inlet will then be from 10 m/sec to 15 m/sec for
heads from 600 ~ 1500 m, which is the normal range of head for Pelton
turbines.
Because of the moderate relative velocities in the inlet section the sand
erosion will be moderate and normal modern high resistant paint (epoxy based
or others) will meet the requirement of erosion resistance with normal periodic
maintenance.
Special heavy content of gravel or sand may require special maintenance
with special paint soft rubber based coating or equivalent. In such cases a low
velocity should be chosen in the inlet portion.
The velocity in the inlet valve is for economical and constructional
reasons (maximum weight limitation) normally chosen somewhat higher than
in the inlet manifoil or distributor. After the inlet valve a conical section with
decelerated flow must then be installed.
The velocity in the inlet valve may be expressed by following formula
C = K
v
(2gHe)
05
, (0.095 < K
v
< 0.12) (4.2)
160
Abrasive Erosion and Corrosion of Hydraulic Machinery
Figure 4.4 Manifoil for
5
jet turbine
In the by pass pipe the velocity in the bends should be chosen below 25
m/sec if sand laden water is expected. (The operational time for filling of the
distributor through the by pass pipe normally is relatively short and thus the
erosion will be reduced and a higher velocity than in the inlet has been chosen
for this reason. Inspection for erosion should however be made as part of the
maintenance program.)
The valve seals in the by pass system are exposed to heavy erosion during
the closing period in the same way as the seals on the main valve during
emergency closure with open needles. For Pelton turbines rubber seal has not
sufficient strength to withstand an emergency closing sequence without being
damaged by touring when the steel is approaching the rubber during the
closure with full pressure difference across the seal.
Also seals completely made of steel are exposed to damage during
emergency closure, but also during normal closure because of sand jammed
between the movable and stationary part during closure.
Design of Hydraulic Machinery Working in Sand Laden Water 161
Because of the high pressure any leakage caused by sand will cause
serious damage of a closed seal due to cavitation when the turbine is stopped
if also the nozzles are leaking and the valve is exposed to full pressure
difference.
Because of this the inlet valve and by pass system should be regularly
inspected for leakage if the turbine is operating in sand laden water. The valve
system should also be closed before long stopping periods in order to avoid
cavitation damage.
The main purpose for a maintenance seal however is to allow for repair of
the main seals if leakage has occurred.
The Nozzle System
Through the nozzles of a Pelton turbine the flow is normally accelerated from
0.12 to 0.15 x
{2gH„f
5
to 0.99 x (2gH„)
05
which will be the jet velocity.
Because of the extreme high velocity on the nozzle outlet and the needle
surface a strong turbulent effect occurs on the needle surface. We are here
talking about velocities up to 150 m/sec (nozzle diameters are normally
between 170-300 mm). In order to obtain highest possible efficiency and to
reduce damage by cavitation the nozzle is normally made with a relatively
sharp or very short (less than 1 mm) contact against the needle tip in closed
position.
From the outlet of the fins for the needle guidance the flow as described
above is exposed to a very strong acceleration towards the nozzle converting
all energy into "velocity energy". It is important that no rotational velocity
component occurs in order to obtain a uniform velocity profile with minimum
loss in
the
jet and avoid splitting of the jet.
However, the boundary layer on the needle tip creates a lower velocity in
the center of
the
jet.
This is clearly illustrated in Figure 4.5 showing measured energy over the
cross section in a jet.
The high velocity creates a strong turbulence in the boundary layer close
to the surface of the needle tip. Observations of needle tips operating in water
with hard fine grain sand or silt with grain size less than 60 urn for the
majority of the content show a severe sand erosion after short time operation.
In Figure 4.6 is shown a typical erosion pattern from the start of the fine
grain turbulence erosion on a needle tip.
Inside bend Outside bend
-—
/
Inside bend Outside bend IIS
fly/
I mSa
\ '3^L
^?|s9ro/
0-9925
X^^-^
^
aaeOyVVyy
ifc!5
W'
HSZh^
\
/
J
Nozzle with 2 fins
Nozzle with 6 fins
Figure 4.5 Velocity distribution in a jet of a Pelton turbine
13-
2
5'
a
§
Si-
1
5'
3
I
s
S'
I
S-
S'
as
<3
Design of Hydraulic Machinery Working in Sand Laden Water
163
Figure 4.6 Beginning of fine grain sand erosion of
a needle tip for a Pelton turbine
In Figure 4.7 is shown a severe erosion followed by cavitation for a high
head 5 jets vertical Pelton turbine after 600 hours of operating at 920 m net
head. The material of
needle
and nozzle were of the 13 % Cr 4% Ni type with
hardness BH « 300. The sand or silt was in this case very fine grain size with
composition as shown in Table 4.1.
Table 4.1 (a) Grain size
Grain size
0.500
0.250
0.125
0.063
%
Larger than
0.00
0.07
1.01
23.03
% Smaller than
100.00
99.93
98.99
76.97
This table shows that 76.97% of grains were smaller than
diameter 0.063 mm.