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Using a Jet Pump for Emergency Drainage

Emergency drainage is an important consideration for underground powerhouses. Sump pumps are commonly used for this purpose, but they cannot operate in the event of a power failure. A backup option is needed.

The backup drainage option Austrian utility Vorarlberger Illwerke AG selected for its new 450-mw Kopswerk II pumped-storage project is a specially designed jet pump.

Powerhouse setup at Kopswerk II

The new Kopswerk II facility will use the existing Kops Reservoir as the upper reservoir and the existing balancing reservoir Rifa as the lower reservoir. Total head for this arrangement is about 800 meters.

The powerhouse for Kopswerk II will be an underground cavern located near the Rifa Reservoir. The cavern is 60.5 meters tall by 30.5 meters wide by 88 meters long. The lowest point of the powerhouse lies 60 meters below the level of the water in the Rifa Reservoir.

The powerhouse will contain three 150-mw Pelton turbines. To provide emergency drainage, each turbine pit will contain its own storage (sump) pump. However, these pumps will not work during an electric power outage. In such a situation, Vorarlberger Illwerke needs a backup drainage system.

Choosing jet pumps

To provide a redundant safety feature for emergency drainage, the utility chose jet pumps. (Because of their unique mode of operation, these pumps are only an option for drainage of underground powerhouses.) Vorarlberger Illwerke uses jet pumps at its 98-mw Walgauwerk project and 276-mw Rodundwerk II pumped-storage project.

Jet pumps eject a stream of high-pressure water – taken from the headwater of the hydro project – through a nozzle, into the fluid being pumped. (See Figure 1.) The subsequent high-velocity free jet entrains the surrounding fluid, which causes secondary inflow at the suction side of the jet pump.


The jet pumps designed for emergency drainage of the turbine pits at 450-mw Kopswerk II use water from the headwater to flush fluid out of the powerhouse.
Click here to enlarge image

The primary stream and the fluid being pumped then both flow through a 1.9-meter-long mixing pipe. After covering a distance corresponding to eight times the diameter of the mixing pipe, the fluids are completely mixed. Because of the exchange in momentum resulting from this mixing, pressure in the pipe increases considerably. The liquid then enters a diffuser, designed to reduce the high kinetic energy of the mixed flow, and further increases the pressure.

From there, the mixed flow travels at high pressure through a pipe and is discharged from the facility.

Designing the jet pump for Kopswerk II

Vorarlberger Illwerke chose to design a jet pump that would meet the specific situation at Kopswerk II, for two reasons. First, the manufacturer the utility had used for these pumps was no longer in business. Second, the feeding pressure for the pumps at this hydro facility was larger than any other working facility in the world. Optimization of the jet pump design was necessary to both reduce low-pressure zones and avoid cavitation.

In May 2006, the utility commissioned HTA Lucerne in Switzerland to design the Kopswerk II jet pumps. This work was completed in six months and involved several steps, including numerical simulations to produce operational data, designing the pumps using computer-aided drafting (CAD) software, verifying the design using computational fluid dynamics (CFD) software, and performing computer simulations to minimize cavitation.

First, researchers performed numerical simulations using standard design guidelines for these types of pumps. Results indicated that the published design criteria, although rather outdated, was still valid. The simulations produced a great deal of operational data needed for Kopswerk II, including the pump level, outlet level, and flow rate in the various sections.

Taking this data from the numerical simulations, researchers produced an initial design for the jet pump using NX3 CAD software from Unigraphics. In addition to producing a three-dimensional model, using this software allowed the designers to produce detailed two-dimensional drawings for every part of the pump. They also used the software to create an assembly drawing that includes information on the forces acting on the concrete that surrounds the pump.

The next step in verifying the jet pump design involved CFD studies using CFX5.7 soft-ware from ANSYS. This allowed for optimization of various design elements “created” using the CAD software. For example, researchers were able to reduce low-pressure zones at the walls of

the injector through step-wise modifications, thus avoiding future cavitation damage to the stainless steel structure. Researchers also used the CFD software to position the nozzle within the ejector to gain maximum efficiency.

Cavitation – as a major source of noise production and pressure fluctuations – was a concern as a result of shear stresses. Due to the high velocity of the jet of water, very large shear stresses occur in the zone where the jet mixes with the surrounding fluid. To investigate local pressures within the shear layer of the jet, researchers used the Scale-Adaptive Simulation (SAS) feature of CFX5.10 software from ANSYS for additional simulation. This allowed the researchers to locate the zones with cavitation potential and move them away from the surfaces of the jet pump, thereby minimizing potential damage.

Results

Vorarlberger Illwerke manufactured the jet pump in 2006 using computerized numerical control (CNC) machining. The first unit at Kopswerk II is scheduled to be commissioned at the end of 2007. Vorarlberger Illwerke will use on-site measurements from this unit to verify data for the jet pumps. The last of the three turbine-generating units will begin operating in the first half of 2008.

– By Thomas Staubli and Otto Prugg. Thomas Staubli is professor, fluid dynamics and hydromachines, HTA Lucerne, Technikumsstrasse 21, Horw CH-6048 Switzerland; (41) 41-3493552; E-mail: tstaubli@hta.fhz.ch. Otto Prugg is with Vorarlberger Illwerke, Batloggstrasse 36, A6780 Schruns, Austria; (43) 5556-70184125.


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