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 Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
PENSTOCK POWER INTAKE PERFORMANCE 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. HEAD TANK - prevent formation of coherent vortices, that can cause additional energy losses and air entrainment in the penstock. VAL REZZO BASIN
STRONG RELEVANCE IN LOW HEAD HYDROELECTRIC PLANT BUT EVEN IN A HIGH HEAD PLANT A POOR INTAKE GEOMETRY CAN CAUSE SEVERE PORLEZZA POWER HOUSE OPERATION PROBLEMS
LUGANO LAKE Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
REAL CASE IS HERE PRESENTED OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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THE PORLEZZA HYDROPOWER PLANT LOCATION MAP
HEAD TANK HEAD TANK
VAL VAL REZZO REZZO BASIN BASIN
Val Rezzo Catchment: 8.24 km2 PORLEZZA POWER HOUSE
Val Riccola Catchment : 3.44 km2
LUGANO LAKE LAKE LUGANO Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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THE PORLEZZA HYDROPOWER PLANT RUN OF RIVER HYDRO SCHEME
DIVERSION TUNNEL
PORLEZZA POWER HOUSE
PENSTOCK
VAL REZZO INTAKE 705 m asl 8.24 km2
VAL RICCOLA INTAKE 703 m asl 3.44 km2
T. VAL REZZO
T. VAL REZZO
VALVE CHAMBER
SEMI-BURIED HEAD TANK
T. VAL RICCOLA VAL REZZO BASIN
100 m
DIVERSION TUNNEL L = 1.7 km slope = 0.1 % PENSTOCK DN 700 mm Blocked in bored hole L = 125 m
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
HEAD TANK VALVE CHAMBER
500 m
N
PENSTOCK DN 700 mm Buried L = 740 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
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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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
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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HEAD TANK GEOMETRY PEAKING OPERATION IS ALLOWED USING THE DIVERSION TUNNEL VOLUME (ABOUT 3’000 m3).
Max WS
1.80 m DIVERSION TUNNEL
VALVE CHAMBER
HEAD TANK ANCHOR BLOCK
Min WS
1.90 m
AIR ENTRY VALVE
PENSTOCK DN 700 mm PENSTOCK INTAKE
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
EMERGENCY CLOSURE BUTTERFLY VALVE
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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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
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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HEAD TANK INSPECTION Inspection of the head tank during tests revealed: • presence of strong turbulences and vortices within the head tank intake even at water level close to maximum operation stage. • presence of two persistent structured vortices • relevant air entrainment through the air valve present downstream the penstock emergency valve.
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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PROBLEM ANALYSIS Possible 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 head tank
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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PROBLEM ANALYSIS IT 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
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
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
Then decide if a more thorough analysis was necessary (even with the support of CFD technique).
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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EXPERIMENTHAL SET-UP
head tank pit
MODEL SCALE 1:10 Channel: 1.5 m long; 24 cm by 30 cm rectangular cross section
head tank at upstream end 7 cm diameter intake plastic pipe Regulating valve
Submerged pipe
Regulating valve 7 cm diameter intake plastic pipe
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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EXPERIMENTHAL SET-UP
HEAD TANK PIT
rubber hoses were set at different distances from the intake in order to detect the static pressure.
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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TESTS PERFORMED 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
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
Swirling at intake
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TESTS RESULTS Maximum Water depth - No depression occurrence at 4.5 l/s
Maximum Water depth - Depression occurrence at 6.5 flow rate
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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CONTRACTION EFFECT
Maximum Water Level
Mimimun Water Level
V
POSSIBLE DEPRESSION DURING TANK EMPTING AT 6.5 l/s
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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WEDGE MODIFICATION TO SIMULATE THE HEAD TANK ENLARGING IN ORDER TO PROVIDE A BETTER APPROACHING CONDITION
TEST DISCHARGES: 4.5 l/s 6.5 l/s WATER LEVEL: Maximum level in the head tank
Flow path lines bump into the front wall and the they direct downward to the inlet.
RESULTS: NO DIFFERENCES WITH ORIGINAL CONFIGURATION – NO LOSSES REDUCTION (according to the experimental setup measurements accuracy) Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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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 Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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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.
Maximum Water Level
Mimimun Water Level
V
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 Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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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.
Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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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. Hidroenergia 2008 Conference, Bled, Slovenia, 11-13 June 2008
OPTIMIZATION OF A PENSTOCK INTAKE BASED ON A SIMPLIFIED PHYSICAL MODEL
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