Luigi Papetti - unibs.it

Luigi Papetti

Hydropower plantsLessons learnt, rehabilitation, failure analysis, repair

STUDIO FROSIO

Via P. F. Calvi, 9 - 25123 Brescia - I

[email protected]

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


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



Forebay and powerhouse

9



Powerhouse

10



Machinery

11



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


13


2. Lesson learnt and field of action in:







14

Lesson learnt and field of action

15


2. Lesson learnt in:



3. Difficulty in collecting information about a failure




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)


18

SCADA: new models


19






4. Failure analysis: what can go wrong in a plant



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


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


24

Cause - Degradation due to age – unit in operation since 1922


25

Cause – incorrect operation and maintenance of the air vent at the inlet of the penstock

Typical problem:

Penstock failure due to internal depression


26

Incorrect operation

Degradation due to age

Original sin

THE REALITY IS:


Plan of the lecture







5. Technical lessons 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


Effects of the small span of each gate

and side effects…Hang washing in boots...


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


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


Solution

Heavy flood during construction – no inundation upstream


� 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


Channels

53


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









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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