Estimation of weight loss of turbine
Introduction
Weight loss of turbine due to sand erosion is challenging task. Not only the weight
loss due to sand erosion, it is also equally difficult to measure the weight loss due to
cavitation erosion. The cavitation phenomenon and criteria are considered during
tender contract of generation equipment, whereas sand erosion is normally not
considered in performance guarantee. Sand erosion can cause dimensional change of
turbine component causing structural weakness and degradation in performance by
loss of efficiency as well as generation. The maintenance requirement also increases
due to sand erosion. Normally turbines are refurbished by rebuilding the eroded
component by welding and grinding it in to correct profile to satisfy the hydraulic
requirement for the given turbine. Material non-homogeneity and improper heating
can cause residual stress and stressed components in cyclic loading can often fail due
to fracture. Beside maintenance, erosion is also considered during design stage of the
headwork of power plants to optimize the size of sediment settling basins. Hence
estimation of erosion both in term of mass loss and dimensional change is very
essential for both operational requirement and total optimization process.
The analytical erosion models are not fully developed to incorporate material
properties and operating variables to give material loss for a given condition. Hence
normally erosion estimation is carried out based on the experimental observation of
similar environment. Several types of test rigs are used for carrying out erosion tests
and as far as the knowledge of the author is concerned, there is no standard and well
accepted method for erosion experiment for the purpose of hydraulic machinery.
Erosion test was carried out in the high velocity jet test rig at NTNU/SINTEF to
establish the relation between erosion rate and velocity, keeping all other parameters
constant. The erosion rate is believed to be proportional to (velocity)n. The erosion
rate is represented in term of weight loss of specimen per kilogram of particles
striking the specimen in a given operating condition. The validity of the experimental
results and its practical application to estimate the erosion of turbine is discussed in
this paper.
Estimation of weight loss
Jhimruk
The turbines of Jhimruk power plant in Nepal are severely eroded in its first year
monsoon operation. Turbine unit number 1 was virtually damaged to irreparable
condition after 4000 hours of operation and was replaced with spare one in 1996.
During that period, 9383 tonnes of sediment was expected to pass through this unit.
The average concentration of the sediment during this period was found to be 1500
ppm.
This turbine is made up of stainless steel 16%Cr5%Ni.
General philosophy of the design for Francis turbines velocity distribution at Jhimruk
turbines can be similar to one shown in figure with absolute velocity at the outlet from
the guide vanes 32 m/s and the relative velocity at the outlet of the runner roughly 40
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m/s. The schematic variation of velocity at different section of turbine is shown in
Figure 1, which gives indication of extent of erosion damage of different components,
considering erosion rate as function of velocity.
37 m/s
Velocity
30 m/s
12 m/s
Figure 2 shows the erosion rate as a function of velocity in above mentioned condition.
Interpolating from the relation for erosion rate and velocity gives the erosion rate at
guide vane and runner outlet. These values can be used to estimate the erosion of
material from these two components. The validation of the estimated result is
compared with the observation of erosion of actual turbine.
The interpolation of this curve gives the erosion rate at guide vane outlet 4.25 mg/kg
and that at the outlet of runner 2.22 mg/kg. Though in the turbulent flow region same
particles may strike the surface several times, the kinetics of particles is really
complex and hence it is assumed that particles strike once at the guide vane system
and then turbine blades. With this assumption, the weight loss due to 9383 tonnes of
particles gives 39.87 kg at guide vane system and 20.83 kg at blades. The validity of
this amount can compared with the observation of eroded turbine.
Erosion vs velocity
for Martensitic Steel 16-5 CrNi.
Erosion rate (mg/kg)
60
90 degree
y = 6E-05x 3.0933
R2 = 0.9728
50
40
90 degree
45 degree
y = 6E-05x 3.1264
R2 = 0.9356
30
45 degree
Pow er (90 degree)
20
Pow er (45 degree)
10
0
0
20
40
60
80
Velocity (m/s)
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60
880
70
125
125
590
125
780
880
The loss of material from the actual turbine of Jhimruk power plant is computed based
on the observation of erosion in turbine. Table gives the summary estimation of
erosion for different components and total weight loss. Erosion estimate from
experimental observation is about 6 times higher than the one estimated from
observation. The actual weight loss of eroded turbine component is now measured so
far. Weighing whole of the turbine could be difficult, but separate components life
guide vanes, top and bottom covers can be weighed and actual weight loss can be
found.
There is quite a variation of erosion estimated from the actual observation of eroded
turbine and erosion estimated based on experimental finding.
Estimation of weight loss of Francis turbine
Jhimruk turbine
Area (mm 2) Volume (mm 3) Quantity Total Volume (mm 3) Weight loss (kg)
Part Name Assumption
Sketch
Blade
Erosion of 1mm uniform thickness all around
78125
78125
16
1250000
9,75
Guide vane One of the following eroded components of guide vane system
Face plate Face plate erosion 1mm
130310
130310
(0.5mm top + 0.5mm bottom
Horse shoe L*W*D
L= (70+60)*2times
W=12
D=4
Guidevane 125X125
face
Only on suction side
1mm erosion
Guidevane
top-bottom 1mm in total
1
130310
1,02
3120
12480
20
249600
1,95
15625
15625
20
312500
2,44
5156,25
5156,25
20
103125
0,80
86350
0,67
125X2
125X2.5
55
125/2
125
10
Grooves
groov length
dia of shaft
39,25
2158,75 2X20
16,63
55
Sum
Discussion
The erosion experiment is carried in the laboratory in the closed condition and it is
expected that most of the particles strike the surface and cause erosion. But the
sediment transport in actual turbine is complex. It is not clear what proportion of the
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sediment passing thorough the turbine actually strikes the turbine surface and take
part in material removal. From the practical example of flow of particle through
straight pipe in laminar condition, we can guess that all the particles will not strike the
wall, but only the particles at the perimeter will have erosion effect. Though the actual
flow condition inside the turbine is complex and the distribution of concentration is
not uniform, the condition can be assumed to be simple and the portion of the
particles striking the surface can be estimated.
All the particles will not strike the wall, because the particles can be assumed to travel
in the packets of particles of small unit volume.
Let’s assume that the concentration is uniformly distributed in to unit volume and
total number of particle in that unit volume of water is N
If these particles are considered equally divided in to cubical space, we get
L*L*L = N,
where L is length of cubical volume.
Hence no. of particle in each edge is (N)1/3 and each face is (N)2/3, but the total
number of particles in the outer surface in the cube is given by
Nouter = 6N2-2*8-(N-2)*12
This equation gives
1200
no of particles
1000
800
Total particles
600
Part. In outer surface
Series4
400
200
0
2
3
4
5
6
7
8
9
10
Particles in edge
When we consider this, unit volume is passing through pipe as shown in figure, only
particles in the six sides will have effect in erosion.
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N
Flow
Direction
Figure: Particles uniformly distributed in unit volume
Considering the particles between 1mm to 0.1 mm for the concentration of 1500 ppm,
the ratio of the total particle to the particles at the outer surfaces which will have
erosion effect is shown in fig below. It shows that, as the number of particles
increases, there will be smaller ratio of effective particles.
Now if we follow this hypothesis, the erosion estimated from experimental result if
very less.
ratio with outer particles
0,06
0,05
0,04
0,03
Series1
0,02
0,01
0
0
2E+08 4E+08 6E+08 8E+08 1E+09 1,2E+0
9
no of particles
I could not find any logical statement to support this hypothesis.
Other thing we can look upon is ratio of wetted surface to volume of water inside the
water. I have not found any information regarding this yet. Trying to get some relation
in term of number of particles (so that we can co-relate in term of ppm), but no
breakthrough till now.
I discussed this with Tjørborn but not been able to discuss this with Hermod. Any way
I have to find out some logical explanation for this.
Some of the expression presented here is very vague, I am just leaving them like that
thinking may be something will evolve by this.
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Main thing I am looking for is: How much or what percentage of the particles passing
through turbine will actually take part in turbine erosion? This is needed because, in
the result of lab test, it is assumed almost all the particles strike the surface.
Other results in difference in two results: particle shape and size, impact angle etc ???
Hence how much variation between estimation from experimental result and actual
erosion should be accepted?
More realistic result can be obtained once we have actual weight loss in guide vane
(hope they have weighed before installation and will be weighed after dismantalling).
We have more realistic velocity data for guide vane.
-------------- ----------------
Hydraulic radius
= area / perimeter
L = d X n1
B = d X n2
perimeter = (d X n1) X 2 + (d X (n2-2)) X 2
Area = L X B
(d X n1)(d X n2)
Hydraulic Radius = -----------------------------------------------(d X n1) X 2 + (d X (n2-2)) X 2
d X n1 X n2
Hydraulic Radius = -----------------------------------------------n1 X 2 + ((n2-2)) X 2
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