HydropowerPressure surge

Friedrich Moser

16.02.2011

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

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

3

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

4

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

5

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

6

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

7

Pressure surge: sequencereservoir

valve overpressure

Negative pressure

TR (or tc): reflection time or critical time8

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

9

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

10

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

11

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

12

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

OAdvantage of GRP pipes:

- Pressure wave travels more slowly less pressure surge

Behaviour of different pipe materials

14

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

15

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 …

16

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

17

Why thinking on pressure surge

OHigher pressure in pipe

OHigher PN classes necessary

OCosts safety

18

Reduction of Pressure surge turbine

OAdaption of closure time,

Ochoice of the right valve closing law

19

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

20

OVar A: linear closure in 10 s

OVar B: linear closure in 60 s

Example: closure times of valve

21

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]

22

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

23

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]

24

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

25

Reduction of Pressure surge pipe

OBigger penstock diameter

26

Reduction of Pressure surge option

OSurge tank

27

Surge tank

28

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

29

How a surge tower works

Pressure oscillation

mass oscillation (water)

Close

Open

Headrace tunnelHeadrace tunnelHeadrace tunnel

30

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

31

Reduction of Pressure surge turbine

OPelton deflectors, (bypass)

OFrancis bypass, flywheel

OKaplan heavy flywheel

32

Pressure surge in hydro power plants (HPP)

OFlow regulation

OLoad removal and plant start

OEmergency shutoff

OPower breakdown

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

34

How a surge tower works

Pressure oscillation

mass oscillation (water)

Close

Open

Headrace tunnelHeadrace tunnelHeadrace tunnel

35