PAOLO CARETTI – Voith Simens Hydro S.p.A. Milano

NINO FROSIO – Studio Frosio

WATERHAMMER PROBLEMS AND SOLUTIONS IN A LONG PENSTOCK UNDER A SMALL TOWN

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The Gardone Val Trompia small hydroelectric plant

GARDONE VAL TROMPIA PLANT

River MellaDrainage area 205 km2

Maximum flow rate 4,5 m3/sMinimum flow rate 1,2 m3/sAverage flow rate 3,0 m3/sGross head 27,30 mMaximum capacity (installed) 980 kWFunctioning annual times 8.000Annual production 4.000.000 kWh

3,80 s895 m/s1.701,1 mWHOLE PENSTOCK

0,67 s1.070 m/s357,1 m6 mm/25 cm∅ 1.800 mmSteel and concrete coating

2,05 s750 m/s768,4 m36 mm∅ 1.800 mmGRP

0,54 s940 m/s254,5 m21 mm∅ 1.800 mmCast iron

0,54 s1.180 m/s321,1 m0,70 m2,50 x 1,50 mConcrete tunnel

202,86,85 x 1,50Open air channel

Reflection timeWave speedLengthThicknessDimensionsType

Supply channel and penstocks

Main characteristicsPlant location

Penstocks

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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

Schematic profile of the plant

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Weir

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The intake structures Studio Frosio

Supply channel: open air breach

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Siphon intake

Siphon intake

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Existing open channel changed in depressurised channel

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Existing open channel changed in depressurised channel

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Existing open channel changed in depressurised channel

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Cast iron installation phase

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Glass reinforced (GRP) penstock installation

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Connection joint between the GRP and the steel penstock

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Steel penstock-concrete: before the concrete casting

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Steel penstock-concrete: after the concrete casting

Kaplan unit installation drawing

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

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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

Surge tank: foundation basement

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Surge tank: diaphragm

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Surge tank: assembly phase

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Surge tank

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Surge tank

Tower net height 23,60 mDiameter: 4,00 mMaterial: steel S275JRThickness : 11 mm

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CONCLUSIONS1. Nowadays long penstocks are very often preferred to the traditional scheme “open

channel/penstock”, because those are cheaper, quickly installed, and easier to maintain 2. Lot of care is needed when long penstocks are connected to low inertia units as this causes

significant waterhammer phenomenon, most of all where supply open channels don’t exist3. Waterhammer phenomenon 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 for the pipes

4. Typically the waterhammer phenomenon causes negative pressure with low inertia Kaplan turbines or dissipation valves at the end of the penstock

5. If faced in the preliminary designing phase, the waterhammer phenomenon isn’t a dramatic problem

6. Good simulation model is necessary to find out the worst working situations of the plant and to design the best solutions: the turbine manufacturer’s contribution is essential to get suitable results

7. Accurate field tests are essential as well, in order to obtain the actual penstock and units parameters, mainly the wave reflection time and the flow rate gradient during the overspeed time (closing transient)

8. The most suitable solution is the surge tank, where possible9. The total plant investment was 3.600.000 €; the cost of the mathematical simulation and of

the field tests was 20.000 € (0,6%); the surge tank cost 130.000 € (3,6%)

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