Luigi Papetti
Hydropower plantsLessons learnt, rehabilitation, failure analysis, repair
STUDIO FROSIO
Via P. F. Calvi, 9 - 25123 Brescia - I
Why hydro and small hydro?
43%
31%
55%
13%
Source: GSE
Why hydro and small hydro?
Hydroelectric output
[% total Italian hydro]
Hydroelectric production
[% total Italian hydro]
Source: GSE
Plan of the lecture
1. High head – low head: short overview of the different main problems
2. Lesson learnt and field of action in:
• New construction – high head / low head
• Rehabilitation - high head / low head
3. Difficulty in collecting information about failure
4. Failure analysis: what can go wrong in a plant.
5. Technical lesson learnt
6. Operational lesson learnt
Output = k · head · flow rate
High head – low head: short overview of the different main problems
QHP ⋅⋅⋅= γη
6
High head ⇒ small flow rate
High head – low head: short overview of the different main problems
Hg=466 m – Q=0,780 m3/s – P =2,8 MW Waterways
7
Low head ⇒ large flow rate
High head –low head: short overview of the different main problems
Hg=6,5 m – Q=60 m3/s – P =3,0 MW Waterways
8
Low head ⇒ large flow rate
High head – low head: short overview of the different main problems
Forebay and powerhouse
9
High head ⇒ small flow rate
High head – low head: short overview of the different main problems
Powerhouse
10
High head ⇒ small flow rate
High head – low head: short overview of the different main problems
Machinery
11
Low head ⇒ large flow rate
High head – low head: short overview of the different main problems
Machinery
12
High head
☺ Economically more favourable
�Far from cities and towns (access roads needed)
� Far from consumption points (electric lines needed)
Low head
☺ Wide spread all over the world
☺More suited to multi-purpose schemes
� Economically less favourable
High head – low head: short overview of the different main problems
13
1. High head – low head: short overview of the different main problems
2. Lesson learnt and field of action in:
• New construction – high head / low head
• Rehabilitation - high head / low head
3. Difficulty in collecting information about failure
4. Failure analysis: what can go wrong in a plant.
5. Technical lesson learnt
6. Operational lesson learnt
14
Lesson learnt and field of action
15
1. High head – low head: short overview of the different main problems
2. Lesson learnt in:
• New construction – high head / low head
• Rehabilitation - high head / low head
3. Difficulty in collecting information about a failure
4. Failure analysis: what can go wrong in a plant.
5. Technical lesson learnt
6. Operational lesson learnt
7. Project management of a refurbishment: control of time, costs and final results: what’s more important?
8. Guide on how to refurbish a low head small hydroelectric plant
16
Difficulty in collecting information about failure
17
SCADA: Old models
Modern tools: SCADA (Supervisory Control And Data Acquisition)
Difficulty in collecting information about failure
18
SCADA: new models
Difficulty in collecting information about failure
19
1. High head – low head: short overview of the different main problems
2. Lesson learnt in:
• New construction – high head / low head
• Rehabilitation - high head / low head
3. Difficulty in collecting information about failure
4. Failure analysis: what can go wrong in a plant
5. Technical lesson learnt
6. Operational lesson learnt
7. Project management of a refurbishment: control of time, costs and final results: what’s more important?
8. Guide on how to refurbish a low head small hydroelectric plant
20
•original design or construction sins
•degradation of the performances due to age
•incorrect operation
Failure analysis: what can go wrong in a plant
21
Typical problem:
STUDIO FROSIO
Sand/gravel deposition problems
Failure analysis: what can go wrong in a plant
22
Cause - original sin - intake in the inner side of a river bendFailure analysis: what can go wrong in a plant
23
Typical problem:
Runner blades erosion
Failure analysis: what can go wrong in a plant
24
Cause - Degradation due to age – unit in operation since 1922
Failure analysis: what can go wrong in a plant
25
Cause – incorrect operation and maintenance of the air vent at the inlet of the penstock
Typical problem:
Penstock failure due to internal depression
Failure analysis: what can go wrong in a plant
26
Incorrect operation
Degradation due to age
Original sin
THE REALITY IS:
Failure analysis: what can go wrong in a plant
Plan of the lecture
1. High head – low head: short overview of the different main problems
2. Lesson learnt in:
• New construction – high head / low head
• Rehabilitation - high head / low head
3. Difficulty in collecting information about failure
4. Failure analysis: what can go wrong in a plant.
5. Technical lessons learnt
6. Operational lesson learnt
Weirs and dams: flood managementUrago d’Oglio PlantLocation: Northern ItalyOglio River BasinQmax= 32 m3/sHg = 6,35 mRated output = 1,5 MW
•Old weir12 sliding gates 3,10 m span each37,2 m total width with 11 intermediate steel piers
Low flow situation
Medium flow situation
Flood situation
Weirs and dams: flood management
Weirs and dams: flood management
Effects of the small span of each gate
and side effects…Hang washing in boots...
Weirs and dams: flood management
Solution
Basic constraints: it must be guaranteed that:
1.Gates lower in any situation
a) flood
2.Gates raise in any situation
a) obligations connected to irrigation system upstream
3.Accurate upstream water level regulation
a) obligations connected to irrigation system upstream
b) optimisation of the plant energy production
c) compliance with the requirements of limiting the diverted water at the
maximum amount allowed by water concession rights
Weirs and dams: flood management
Solution
3 flap gates 11,50 m width Hydraulically operated
Weirs and dams: flood managementSolution
Backup diesel generator for emergency operation
Lowering guaranteed by a mechanical system even in case of complete blackout condition
Weirs and dams: flood management
Solution
Heavy flood during construction – no inundation upstream
Weirs and dams: flood management
� expensive solution
� works in the river subject to contingencies (provisional dykes
destroyed by floods three times)
� special foundations needed due to concentrated loads coming
from hydraulic cylinders
☺ high discharging efficiency during floods
☺ high safety of operation
☺ precision in water level regulation
Pros and cons
36
High head - conventional Tyrolean vs. Coanda
Maroggia plant – Northern Italy – Adda River Basin
Qmax = 0,2 m3/s
Hg = 716 m
P = 1.300 kW
Altitude of intake ~ 1.300 m.a.s.l.
Intakes
37
What’s wrong?
•Bars too wide•Diagonal layout•Void ratio too low•Screen not inclined enough
Intakes
38
Typical design of a Tyrolean intake Intakes
ψµ ⋅⋅= 01848,1
eL
e0=specific energy of the incoming flowµ=discharge coefficient~ broad crested weir ~ 0,4
ψ= ratio of opening area to the total area of the screen
L=length of the screen
Subcritical flow
Supercritical flow
39
Typical design of a Tyrolean intake Intakes
Why 1,1848?
u2= ratio of the flow rate per unit width at the end of the screen to the maximum flow rate per unit width with e0; u2= 0 in case of total withdrawal
u1= ratio of the flow rate per unit width at the beginning of the screen to the maximum flow rate per unit width with e0 ; u1= 1 in case of critical flow at the beginning of the screen
40
Solution: Coanda effect screenIntakes
Coanda effect screen: the tendency of a fluid jet to remain attached to a solid flow boundary.
41
Solution: Coanda effect screenIntakes
Screen geometry and control volume Velocity vector approaching tilte-wire screen
42
Solution: Coanda effect screen
���� very expensive (6-10 times a conventional screen)���� low resistance to boulders (protection with a coarse screen)☺☺☺☺ high diversion efficiency☺☺☺☺ excellent fine sediment exclusion (up to 0,5 mm)☺☺☺☺ no maintenance or loss of water for flushing sediments���� loss of head (high inclination)
Intakes
43
Coanda screen – hydraulic computation
Reference: http://www.usbr.gov/pmts/hydraulics_lab/twahl/index.cfm
Intakes
44
Sand/gravel deposition problems
Megolo PlantLocation: Northern ItalyToce River BasinQmax= 75 m3/sHg = 12,87 mRated output = 8 MW
IntakesLow head: importance of position and shape for sediment management
Desilting gate too small
45
What’s wrong
Original sin - intake in the inner side of a river bend
Desilting gate too small:6 m over a 110 m wide weir
Intakes
Too low slope of the river
46
21 m wider desilting span
SolutionIntakes
47
Submerged longitudinal wall to concentrate flow lines
SolutionIntakes
48
Solution
Abstraction of sand and gravel deposited to reshape the intake
Before After
Intakes
49
Low head: importance of position and shape for sediment management
Original sin - intake in the inner side of a river bend
Pontey 1 PlantLocation: Northern ItalyDora Baltea River BasinQmax= 34,7 m3/sHg = 3,70 mRated output = 870 kW
Intakes
50
Problem (unsolved): sediments at intake
Possible solutions:•Modification of rules of operation increasing flushing frequency (?)•Groins?
Intakes
51
Typical problem:Leakages; hydraulic performance decrease
Channels
Headrace channels: reduction of sliding and stability problems
Prevalle-Chiese PlantLocation: Northern ItalyOglio River Basin (Chiese sub-basin)Qmax= 16 m3/sHg = 7,97mRated output = 1 MW
52
Typical problem:Leakages; hydraulic performance decrease
Channels
53
Typical problem:Leakages; hydraulic performance decrease
Channels
54
Solution: total reconstruction
Channels
55
Trapezoid cross section
Rectangular self-bearing cross section
Channels
65,1_0
_0 ≈trap
rect
Q
QIf b=h
40,1_0
_0 ≈trap
rect
Q
QIf b=2h
56
Channels - Underpressure
57
Channels - UnderpressureSolution: clapet valves
58
Channels - UnderpressureSolution: clapet valves
59
Penstocks: materials and layoutPenstocks
Allein PlantLocation: Northern ItalyDora Baltea River BasinQmax= 3,0 m3/sHg = 91 mRated output = 2,4 MW
Penstock replacement
Existing: asbestos
Interred?
Open air?
60
Steel with spherical joints☺Commercial product; easy to assembly and weld; no inner pressure limitation
� Corrosion problems (it must be protected by an efficient coating system)
�Low resistance to external loads
☺ Easy to handle, no corrosion problem
☺ Excellent hydraulic behaviour (low head losses)
� Limits to inner pressure and external loads
� Bends more than 3-5° require special parts
☺ Good resistance to corrosion; high resistance to external loads
� Difficult to handle and to adapt to local conditions (every bend requires a special
part)
Cast iron
Plastics (GRP; HDPE; PVC)
Interred?
Penstocks
61
Steel with chamfered edges☺ The whole pipe can be inssected, checked, maintained
☺ Well known technology world wide
� Corrosion problems (it must be protected by an efficient coating system)
� Visual impact
☺ Easy to handle, no corrosion problem
☺ Excellent hydraulic behaviour (low head losses)
� Expensive civil works (many saddle at small span)
� Joints critical
� Not usual
� Limitations to inner pressure because of the high thckness of the pipe wall for high
pressure
� Lower resilience at low temperatures if compared with steel
� Difficult to handle and to adapt to local conditions (every bend requires a special part)
Cast iron
Plastics (GRP)
Open air?
Penstocks
62
Reasons
☺ Uncertainties in the final profile (probably need for
adaptation on site during works)
☺ No visual impact
� Need for cathodic protection (interference with an oil
pipeline
The winner is…Interred steel penstock with spherical joints
Penstocks
63
Final resultPenstocks
64
When shifting from horizontal to vertical shaft is not only a matter of a 90° turning
Tombetta 1 Plant
Location: Northern Italy
Adige River
Hg = 10,5 m
Qmax = 4 x 15 m3/s
P = 4 x 1.450 kW
Past situation: 4 Francis open flume in operation since 1922
Turbine pit
Machines hall before replacement
Units replacement
65
Units replacement
Francis open flume Kaplan conventional single regulated
Alternatives
66
Final choice: Vertical EcoBulb® turbinesUnits replacement
Double regulated vertical bulb turbines
67
Final choice: Vertical EcoBulb® turbines
Units replacement
☺ High efficiency even at partial loads (double regulated
turbine)
☺ Few civil works to fit the new units to the existing
powerhouse
� Expensive
� No vertical units already installed (prototype) – Only
horizontal units installed
� Draft tube replacement required (works below tailwater
level)
68
Units replacement
Units during erection
Pressurised bulbPermanent Magnets Generator
Powerhouse now
HPUs and compressors only
69
Units replacement
Rendement pales bloquées de 0° à 13,2°
0,800
0,850
0,900
0,950
1,000
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600
puissance (kW)
rend
emen
t
8,00
9,00
10,00
11,00
12,00
Rendement total chute nette m
Results of efficiency tests
70
Units replacementLESSON LEARNT
1. Problems with shaft seals (partially solved)2. Problems with compressors (solved)3. Accurate hydraulic modelling required for flow in the pit with formation
of stationary vortexes (solved)
LESSON: even for primary manufacturers, shifting from horizontal to vertical shaft is not only a matter of a 90° turning . Mechanical and hydraulic problems must be expected, faced in advance (possibly) and, in any case, solved, at the end.
71
Transient phenomena
Prato Mele Plant
Location: Northern Italy
Serio River (Adda RB)
Hg = 10,5 m
Qmax = 3 x 4,3 m3/s
Refurbishment:
1. Replacement of 3 old Francis open flume with 3 sub-vertical bulb turbines
2. Headrace channel repairs
3. Substitution of intake gates
4. New TRCM
72
Units replacement
Alternative 1ηw = 82,1 %
Existing
73
Units replacement
ηw = 85,55 %
Alternative 2
ηw = 78,2 %
Alternative 3
74
Units replacement
Final choice: alternative 3
☺ Minimum impact on the existing building
☺Minimum works below tailwater level
☺Best fitting to actual powerhouse layout
☺ Minimum risk of contingencies
� Low efficiency of units
75
Transient phenomena
Units trip at rated discharge!!
LESSON LEARNT
76
Transient phenomena
QLzA
zLz
L
AgV m
m
m
∆=⋅⋅
⋅⋅+⋅+⋅−
0
20
0 22
3
∆Q = flow rate variation in the channel
A0 = wetted area before the wave
V0 = flow velocity before the wave
Lm = channel width
Height z of the wave surge
77
Transient phenomena
( ) ( ) ( ) ( ) ( ) ( )
( ) ( ) ( )( )
( ) ( ) ( ) ( ) ( ) ( )0
2
11
4
3111
8
11
*28
1*2
2
18
11
8
11
4
311
2
2
222
=+⋅⋅+⋅+
−⋅+⋅−⋅++⋅−⋅++
⋅−++⋅⋅−+⋅+⋅⋅⋅−
−+⋅−⋅−+⋅−+
−⋅+⋅−⋅
ififiiffif
ififififm
sf
ffiiffiiif
yyyyAyyAyyAyy
cyyyycyyyyAL
L
yyyyyyyyyy
µ
Action to reduce the height of the wave: lateral spillway at theforebay in front of the turbines
yi = ratio of wave height (measured from the bottom of the channel) at the beginning of the spillway to the depth h0 before transient
yf = ratio of wave height (measured from the bottom of the channel) at the end of the spillway to the depth h0 before transient
c* = ratio of the spillway height to the depth h0 before transient
A = Fr-1
Lsf = length of the spillway
µ = discharge coefficient of the spillway = 0,4
( ) ( ) ( ) ( )
−+⋅−⋅+−
+−⋅+⋅= 1
4
3111
8
112
0 iiifffsf yyAyyyAyQQ
Flow spilled over the crest of the lateral spillway during transient
78
Transient phenomena
Surge tanks in low head plants
Plant Gardone
Maximum flow rate 4,5 m3/s
Minimum flow rate 1,2 m3/s
Average flow rate 3,0 m3/sGross head 27,30 mMaximum capacity (installed) 980 kW Annual hours of operation 8.000Annual production 4.000.000 kWhStart production 30/05/2002
79
Transient phenomena
Surge tanks in low head plants
Section A: open channel (202,8 m)
Section A: siphoned channel (321,1 m)
Section A: cast iron penstock (254,5 m)
Section B,C,D1: GRP penstock (768,4 m)Section D2,E: steel & concrete penstock (357,1 m)Section E: tail race (358 m)
Surge tank
80
Transient phenomena
Schematic profile of the plant
81
Transient phenomena
Lesson learning: a brief history of THE PROBLEMS
Vacuum bubbles risk
Negative pressure stresses too
Positive pressure stresses
Action turbine
Action turbine
Notes
Sophisticated calculation model and field tests
Preliminary mathematical model implementation
First waterhammer evaluation
None
None
Consequences
DramaticSiphoned intakeConstruction project
Significant Kaplan turbine 750 rpm
Construction project
Not significantKaplan turbine 600 rpm
Second bid
NoneCross-flow turbine confirmed
First bid
NoneCross-flow turbine
Concept project
WATERHAMMER PROBLEMS
ITEMSPHASES
82
Transient phenomena
Lesson learning: a brief history of THE SOLUTIONS
• Checking theoretical calculation
• Setting-up the hydraulic operating systems (wicket gates, blades and dissipation valve)
• Removing every plant limitation
Final field survey
• Dramatically cutting off the negative pressure waves
• Lowering the positive pressure waves
• Getting the plant full capacity
Surge tank erection
• Most dangerous operation situations taking into account the penstocks and the Kaplan unit together
• Best closing law of wicket gates and runner
• Geometric parameters of the surge tank
• Diaphragm optimum size to fulfil the boundary constrains
Sophisticate mathematical model
• Actual penstocks and Kaplan unit critical parameters (wave reflection time, flow rate gradient during the transients)
• Waterhammer effect on the penstock without the surge tank
• Set-up of the hydraulic system (wicket gates, runner blades, dissipation valve) to operate the plant in safe condition
First field survey
• Worst operating situations
• Maximum stresses in the penstock
• Plant operation limits to keep the stresses of the penstocks within safety range
Preliminary simulations
(without surge tank)
ISSUESITEMS
83
Transient phenomena
84
Transient phenomena
85
Transient phenomena
Surge tank: assembly phase
86
Transient phenomena
Surge tank
Tower net height 23,60 m
Diameter: 4,00 m
Material: steel S275JR
Thickness : 11 mm
87
Transient phenomena
Waterhammer doesn’t mean only overpressure but negative pressure too, caused by the very quick increase of the flow rate during the shutoff transients, which could be more dangerous than positive pressure waves for the pipes
Transient phenomena must be duly investigated even for small low head plants where penstock have replaced conventional open-channel headrace channels
88
Transient phenomena
89
Transient phenomena
90
Transient phenomena
Tr =0,45 s = 2*L/c < <Tc = 23 s
91
Transient phenomena
92
1. High head – low head: short overview of the different main problems
2. Lesson learnt in:
• New construction – high head / low head
• Rehabilitation - high head / low head
3. Difficulty in collecting information about failure
4. Failure analysis: what can go wrong in a plant.
5. Technical lesson learnt
6. Operational lesson learnt
93
An automated and unattended plant doesn’t mean an abandoned plant!
94
Don’t play with water!!! Maximum care in manual operation of hydraulic devices as valves, distributors….
0Vcp ⋅⋅=∆ ρ
Sayano-Sushenskaya accident2009-08-1775 people died
95
g
VcHTT c
0⋅=∆⇒<
Penstock diameter 7 m
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