Watter Hammer Hobas
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Transcript of Watter Hammer Hobas
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HydropowerPressure surge
Friedrich Moser
16.02.2011
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Crash test
OPlease imagine…
oDriving a compact car (e.g. a VW Golf)…
o against a massive concrete wall…!
oKinetic energy:
1,5 Mio. kg m²/s²
OPlease imagine …
oShutting a 5 km long penstock DN 1000 (v = 2 m/s) …
o… instantaneous! j
oKinetic energy:
7,8 Mio. kg m²/s²
5 x as much !2
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Contents
O Introduction: A “crash test”
O Pressure surge – theoretical background
O Pressure surge and hydro power plants
O Measures to reduce pressure surge
O Summary
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Causes of pressure surge
OPenstocks – Changes in flow velocity
- Inertia
- Elasticity of the fluid
- Rigidity of pipe material
OPressure surge cannot be totally avoided reduction!
- Valves, pump breakdowns, HPP operation
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ODestruction of pipes and fittings
OThe penstock can burst from overpressure
OPenstock can collapse if the pressures are reduced below atmospheric – vacuum – SN 10000
Effects of pressure surge
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Pressure surge: Theoretical approach
OA special case: Instantaneous closing of a valve
- Assumption 1: tc = 0 (valve closes instantaneous)
- Assumption 2: no head losses due to friction (smooth pipe walls)
- Assumption 3: total closure of valve
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Pressure surge: Theoretical approach
reservoir penstock
valve
(e.g. gate valve)
Where will the kinetic energy of the water column end up ?
Velocity of the water column v0
tc
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Pressure surge: sequencereservoir
valve overpressure
Negative pressure
TR (or tc): reflection time or critical time8
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Pressure surge: sequence
OWhat happens?
- Valve closes, water collides with the valve ( momentum is physically stopped)
- Slight compression of the water pressure wave is formed, which travels back the pipe
- At the reservoir, p = 0 Pressure cannot rise
- Pressure wave is reflected and “turns around” in direction of the valve
a
LtR
tR … runtime of wave
L… penstock length
a… wave velocity
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O Instantaneous Valve Closure
- Definition: closure time is less than or equal to 2L/a.
- Joukowski formula
- Assumption for maximum pressure surge, extreme case*)
0vap Jou
With
a … wave velocity
ρ… density of fluid
v0… flow velocity at t=0
*) be aware of multiple opening/ closing or very long pipelines
Pressure surge: estimation
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Pressure surge: causesOWave velocity
o Wave velocity has large impact on magnitude of pressure surge
o Wave velocity depends on:
- modulus of elasticity of pipe material
- bulk modulus of water
- Fixation and bedding of pipe
o The lower the E-modulus The slower the pressure wave
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OE-modulus / bulk modulus:
o “Deformability” of a material
oEven water is deformable under pressure
OPressure wave velocity in a pipe
oBulk modulus fluid
oStiffness of the pipe
oPipe zone bedding and backfill have an influence
With
a … Wave velocity in a pipe
ρ … density of fluid
EF… Bulk modulus fluid
ER… E-modulus pipe
di… inside diameter pipe
s… wall thickness
μ... Poisson number
Wave velocity
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0
200
400
600
800
1000
1200
1400
1600
Vollkommen starres Rohr Rohre aus Guss, Stahl Faserzement Rohre aus Kunststoff und GFK
Rohrtyp [-]
Dru
ckw
ell
en
ge
sc
hw
ind
igke
it a
[m
/s]
Min [m/s]
Max [m/s]
Wave velocityP
ress
ure
wav
e ve
loci
ty a
[m
/s]
Completely rigid pipe Steel, ductile iron,
fibre cement
GRP, plastic13
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OAdvantage of GRP pipes:
- Pressure wave travels more slowly less pressure surge
Behaviour of different pipe materials
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OComparative analysis: Joukowski- formula
Characteristics Penstock 1 Penstock 2
Pipe material Ductile iron GRP
Discharge Q 3 m³/s 3 m³/s
Nominal diameter DN
DN 1000 DN 1000
Operating pressure p0
10 bar 10 bar
Wall thickness t13,5 mm
21,2 mm (SN 10000)
E-modulus Ep170.000 N/mm²
7.000 – 15.000 N/mm² (11.000)
Pressure wave velocity a
741,1 m/s 363,3 m/s
Pressure surge ∆p 22,6 bar 11,2 bar
Behaviour of different pipe materials
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Linear variation of flow
OClosure time tc
OCase 1: Closure time tc = 0
- not possible in practice; a mechanical valve requires some time for total closure
OCase 2: Closure time tc > tR
- Gate valves, pumps, turbines, butterfly and ball valves …
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OExample: Clousure of a valve
- Approximation by Micheaud - Allievi
- Assumption: linear closure With
a … Pressure wave velocity
∆v… Change in flow velocity
tR… Reflection time (= 2*tL)
tc… Closure time
C
RAM t
t
g
vap
Linear variation of flow
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Why thinking on pressure surge
OHigher pressure in pipe
OHigher PN classes necessary
OCosts safety
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Reduction of Pressure surge turbine
OAdaption of closure time,
Ochoice of the right valve closing law
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Example: closure times of valve
OGross Head H = 80 m
ORated discharge QA = 2,5 m³/s
OPenstock, type DN 1000 GRP (SN 10.000, PN10)
OLenght: LR = 3.000 m
OFlow velocity v = 3,18 m³/s
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OVar A: linear closure in 10 s
OVar B: linear closure in 60 s
Example: closure times of valve
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Var. A: tc = 10s
0
50
100
150
200
250
0 200 400 600 800 1000 1200
Zeit t [s]
Dru
ckh
öh
e H
[m
]
-10,00
-8,00
-6,00
-4,00
-2,00
0,00
2,00
4,00
6,00
8,00
10,00
Du
rch
flu
ss Q
[m
3/s]
H20 Q20
Pressure and discharge vs. time
Pre
ssur
e H
[m
]
Dis
char
ge Q
[m
³/s]
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80,00
90,03
99,88
109,53
118,91
127,99
136,73
145,11
153,09
160,66
167,81 170,26 170,74 171,22 171,70 172,18 172,66 173,14 173,62 174,10 174,58
80,00 79,01 78,03 77,04 76,05 75,07 74,08 73,09 72,11 71,12 70,13 69,14 68,16 67,17 66,18 65,20 63,22 62,24 61,25 60,2664,21
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000Stationierung [m]
En
erg
ieh
öh
e [m
]
Hmax, c Hstat Hdyn
Was
serf
assu
ng
Tu
rbin
e
Erdverlegte Druckrohrleitung DN1000, SN 10.000 PN10
Pressure and discharge vs. positionVar. A: tc = 10s
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Example B
Druckhöhen- und Durchflussverlauf am Knoten K20
0
50
100
150
200
250
0 200 400 600 800 1000 1200
Zeit t [s]
Dru
ckh
öh
e H
[m
]
-10,00
-8,00
-6,00
-4,00
-2,00
0,00
2,00
4,00
6,00
8,00
10,00
Du
rch
flu
ss Q
[m
3/s]
H20 Q20
O Closure time 60 s
Pressure and discharge vs. time
Pre
ssur
e H
[m
]
Dis
char
ge Q
[m
³/s]
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Energiehöhendiagramm und Bemessungsdrucklinien
80,00 81,45 82,89 84,30 85,69 87,06 88,41 89,73 91,03 92,30 93,55 94,77 95,97 97,14 98,29 99,41 100,50 101,57 102,61 103,63 104,62
80,00 79,01 78,03 77,04 76,05 75,07 74,08 73,09 72,11 71,12 70,13 69,14 68,16 67,17 66,18 65,20 63,22 62,24 61,25 60,2664,21
0
50
100
150
200
250
0 500 1000 1500 2000 2500 3000Stationierung [m]
En
erg
ieh
öh
e [m
]
Hmax, c Hstat Hdyn
Was
serf
assu
ng
Tu
rbin
e
Erdverlegte Druckrohrleitung DN1000, SN 10.000 PN10
O Closure time 60 s
Pressure and discharge vs. position
Var. B: tc = 60s
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Reduction of Pressure surge pipe
OBigger penstock diameter
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Reduction of Pressure surge option
OSurge tank
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Surge tank
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OMedium or high head plants wirth long headrace pipelines
OHPP producing peak time energy
OSurge tank shortens the distance between turbine and open surface (hydraulic separation)
faster opening/closure possible
Less water hammer
OReliable system (take care of serial opening/closure operations when dimensioning)
Surge tank
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How a surge tower works
Pressure oscillation
mass oscillation (water)
Close
Open
Headrace tunnelHeadrace tunnelHeadrace tunnel
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Worst case: Power breakdown
OBehaviour of Pelton turbines
- In general unproblematic (deflectors will be activated)
OBehaviour of Francis turbines
- Flow-passing capability decreases with increasing rotational speed pressure surge
OBehaviour of Kaplan turbines
- Flow-passing capability increases with increasing rotational speed pressure surge
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Reduction of Pressure surge turbine
OPelton deflectors, (bypass)
OFrancis bypass, flywheel
OKaplan heavy flywheel
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Pressure surge in hydro power plants (HPP)
OFlow regulation
OLoad removal and plant start
OEmergency shutoff
OPower breakdown
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Discussion pressure surge
O Cannot be avoided during HPP control operations
O Depends on equipment (valves, etc…) and turbine type
O Various options to reduce pressure surge
O In general, HOBAS - GRP pipes are able to reduce pressure surge to a certain degree
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How a surge tower works
Pressure oscillation
mass oscillation (water)
Close
Open
Headrace tunnelHeadrace tunnelHeadrace tunnel
35