Optimization of a Penstock

Hidroenergia 2008 Conference,

Bled, Slovenia, 11-13 June 2008

OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL

OPTIMIZATION OF A PENSTOCK INTAKE

BASED ON A SIMPLIFIED PHYSICAL MODEL

Emanuele BOTTAZZI Altene Ingegneri Associati, Italy

Giuseppe FLOREALE Altene Ingegneri Associati , Italy

Luigi MOLINA SOCIM s.r.l., Italy



OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 2

PENSTOCK POWER INTAKE PERFORMANCE

VAL REZZO BASIN

LUGANO LAKE

PORLEZZA POWER HOUSE

HEAD TANK

Performance and efficiency problems at hydroelectric penstock intake

are related to the critical transition from open channel flow to pressure

flow.

A optimal design should consider:

- uniform velocity distribution and accelerations;

- gradual transition from the upstream channel to a circular penstock section

In order to:

- reduce energy losses.

- prevent formation of coherent vortices, that can cause additional energy

losses and air entrainment in the penstock.

STRONG RELEVANCE IN LOW HEAD HYDROELECTRIC PLANT BUT EVEN IN

A HIGH HEAD PLANT A POOR INTAKE GEOMETRY CAN CAUSE SEVERE

OPERATION PROBLEMS

REAL CASE IS HERE PRESENTED




THE PORLEZZA HYDROPOWER PLANT

LOCATION MAP

VAL REZZO BASIN

LUGANO LAKE


HEAD TANK

Val Rezzo Catchment: 8.24 km2

Val Riccola Catchment : 3.44 km2

VAL REZZO BASIN

LUGANO LAKE

HEAD TANK





RUN OF RIVER HYDRO SCHEME

VAL REZZO BASIN

N

DIVERSION TUNNEL

PENSTOCK

PORLEZZA

POWER HOUSE

T. VAL REZZO

T. VAL

RICCOLA

T. VAL REZZO

HEAD TANK

VALVE

CHAMBER

500 m100 m

VAL RICCOLA

INTAKE

703 m asl

3.44 km2

VAL REZZO

INTAKE

705 m asl

8.24 km2 VALVE CHAMBER

SEMI-BURIED

HEAD TANK

DIVERSION TUNNEL

L = 1.7 km

slope = 0.1 %

PENSTOCK DN 700 mm

Buried

L = 740 m

PENSTOCK DN 700 mm

Blocked in bored hole

L = 125 m


1 PELTON TURBINE, 2 JETS

Max Operating Flow 1.4 m3/s

Gross Head 407 m

Turbine Capacity 4.2 MW

Generator Capacity 5 MVA





MAIN PLANT CHARACTERISTICS:

Maximum operating flow 1.4 m3/s

Average exploited flow 0.4 m3/s

Installed capacity 4,200 kW

Gross head 407 m

Energy production 10 GWh/yr

Operation Start Date: October 2006

Building start-up: May 2004

Construction time: 17 months




HEAD TANK GEOMETRY

EMERGENCY CLOSURE

BUTTERFLY VALVE

AIR ENTRY VALVE

PENSTOCK

INTAKE

VALVE

CHAMBER HEAD TANK DIVERSION

TUNNEL

Max WS

Min WS

1.90 m

1.80 m

PENSTOCK

DN 700 mm

ANCHOR

BLOCK

PEAKING OPERATION IS ALLOWED

USING THE DIVERSION TUNNEL

VOLUME (ABOUT 3000 m3).




PROBLEMS OCCURRED

SERVICE PRELIMINARY TESTS FAILURE:

Filling the tunnel with water and subsequently empting with turbine at high flow

rates (1-1.4 m3/s) in order to reproduce peaking operation.

The pressure sensor in the head tank reached low values that automatically

stopped the turbine.

Unacceptable limitation for the plant operation:

more overflows during high flow condition

limitation on peak hours operation during low flows condition




HEAD TANK INSPECTIONInspection of the head tank during tests revealed:

presence of strong turbulences and vortices within the head tank intake even at waterlevel close to maximum operation stage.

presence of two persistent structured vortices

relevant air entrainment through the air valve present downstream the penstockemergency valve.




PROBLEM ANALYSISPossible causes of the undesired phenomenon:

-flow separation and depression at intake inlet

-the reduced geometry of the head tank (great approach velocities, slightly asymmetric

approach conditions)

-an inadequate submergence above the crown of the inlet.

It is NOT possible to have a complete theoretical analysis of the problem the theoretical values of minimum submergence are totally general and, especially in

presence of peculiar intake geometry or high approaching velocities, as in this

particular case, the submergence could not be the only parameter to predict critical

conditions.

improving the submergence level would basically involve the re-building of the headtank




PROBLEM ANALYSISIT WAS CHOSEN TO USE A LABORATORY TESTS ON SCALED MODEL IN ORDER TO:

1) totally understand the phenomenon

2) verify if a least-cost (in terms of time and economics) remediation solution could

be found.

Perform a preliminary and fast evaluation with a rudimental physical

model in order to obtain a first qualitative understanding of the

phenomena

Then decide if a

more thorough

analysis was

necessary (even

with the support

of CFD

technique).

Investigate the possibility of improving approaching

condition basically enlarging the head tank volume

and/or modify the head tank geometry in order to

guide a gradual contraction of the flow




EXPERIMENTHAL SET-UP

MODEL SCALE 1:10

Channel: 1.5 m long;

24 cm by 30 cm rectangular cross section

head tank at upstream end

Submerged pipe

Regulating valve

head tank pit

7 cm diameter intake plastic pipe

7 cm diameter

intake plastic pipe

Regulating valve




EXPERIMENTHAL SET-UP

HEAD TANK PIT

rubber hoses were set at different

distances

from the intake in order to detect the

static pressure.




WATER LEVEL:

Constant maximum level in the head tank

TEST DISCHARGES:

4.5 l/s

6.5 l/s

RESULTS:

Swirling at intake even at maximum level in the head tank

Development of unstable vortex formations

Pulling a small amount of air bubbles to intake

Contraction effect (vena contracta) detected at intake Swirlingat intake

TESTS PERFORMED




TESTS RESULTS

Maximum Water depth - No depression

occurrence at 4.5 l/s

Maximum Water depth - Depression

occurrence at 6.5 flow rate




CONTRACTION EFFECT

V

Maximum Water Level

Mimimun Water Level

POSSIBLE

DEPRESSION

DURING TANK

EMPTING AT 6.5 l/s




TO SIMULATE THE HEAD TANK ENLARGING IN ORDER TO PROVIDE A

BETTER APPROACHING CONDITION

WEDGE MODIFICATION

TEST DISCHARGES:

4.5 l/s

6.5 l/s

WATER LEVEL:

Maximum level in the head tank

RESULTS:

NO DIFFERENCES WITH ORIGINAL CONFIGURATION NO LOSSESREDUCTION

(according to the experimental setup measurements accuracy)

Flow path lines bump into the front wall and

the they direct downward to the inlet.




INLET IMPROVEMENTS - FUNNEL

The results suggested to improve the transition profile at inlet in order to approximate the flow

trajectory moving downwards and inletting in the intake, a first configuration was conceived.

FUNNEL CONFIGURATION

RESULTS:

Precence of Swirling at intake

Weak unstable vortex formations

NO Contraction effect (vena contracta) detected at intake

Intake head losses reduction




INLET IMPROVEMENTS - DONUT

The good results obtained with funnel configuration suggested, with a special attention to possible

construction difficulties and costs related to the intervention on an existing building with a

complicated accessibility, the donut configuration.

RESULTS:

Swirling at intake

Weak unstable vortex formations

NO Contraction effect (vena contracta) detected at intake

Intake head losses reduction comparable with funnel model

The wedge modification was added at both funnel and

donut configurations and the model results showed no

significant differences

V

Maximum Water Level

Mimimun Water Level




CONCLUSIONS

Both the configurations tested (funnel and donut) showed good enhancement at intake contraction

Both the configurations tested showed a head losses reduction

Donut solution resulted easy to install as it could been achieved by simply fixing, at the penstock

intake, a half-donut shape shield.




CONCLUSIONS

The results obtained with a possible simple and economical intervention suggested to

continuing the study directly on the prototype and then evaluate the possibility of further

improvements.

The use of two steel commercial curves of 90

sectioned longitudinally and subsequently welded,

have allowed to achieve the necessary half-donut.

The tests performed on the modified plant operation have showed the efficiency of the intervention

and no further actions were necessary. The plant operation, that schedules daily emptying of the

tunnel during peak hours up to minimum level at maximum flow, encountered no further problems.

LESSONS LEARNED:

the support of a physical model, although rudimental, could be decisive and cost effective

modest expedient can solve problems related on a poor intake geometry that can cause

severe limitations on hydropower plant operation.

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Optimization of a Penstock

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