optimization of a penstock
DESCRIPTION
hydropower projectTRANSCRIPT
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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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 3
THE PORLEZZA HYDROPOWER PLANT
LOCATION MAP
VAL REZZO BASIN
LUGANO LAKE
PORLEZZA POWER HOUSE
HEAD TANK
Val Rezzo Catchment: 8.24 km2
Val Riccola Catchment : 3.44 km2
VAL REZZO BASIN
LUGANO LAKE
HEAD TANK
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 4
THE PORLEZZA HYDROPOWER PLANT
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
PORLEZZA POWER HOUSE
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 5
THE PORLEZZA HYDROPOWER PLANT
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 6
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).
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 7
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 8
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.
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 9
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 10
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 11
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 12
EXPERIMENTHAL SET-UP
HEAD TANK PIT
rubber hoses were set at different
distances
from the intake in order to detect the
static pressure.
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 13
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 14
TESTS RESULTS
Maximum Water depth - No depression
occurrence at 4.5 l/s
Maximum Water depth - Depression
occurrence at 6.5 flow rate
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 15
CONTRACTION EFFECT
V
Maximum Water Level
Mimimun Water Level
POSSIBLE
DEPRESSION
DURING TANK
EMPTING AT 6.5 l/s
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 16
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.
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 17
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 18
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
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 19
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.
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Hidroenergia 2008 Conference,
Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL 20
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.