Increasing Hydroturbine Operating Revenue by Challenging Cavitation Limits

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"The versatility and power of the cRIO-9035 has been pivotal to the success of the instrument. This real-time unit allows for reprogrammable setting of filters, an accurate and deterministic analysis process, and effective data acquisition through modules like the NI-9234 and NI-9205."

- Morten Kjeldsen, Flow Design Bureau AS

The Challenge:
Limits on operation of hydropower machinery are often caused by the presence of cavitation; i.e. the formation and collapse of vaporous bubbles. Cavitation can be highly detrimental to the machinery, and usually results in the machinery having to be run in conservatively set operation bands to minimize the potential effects. This banding, in turn, reduces flexibility of operation, and indirectly limits operating revenue.

The Solution:
By introducing a cavitation intensity meter the hydropower operator can detect and quantify cavitation in the turbine system. The use of CompactRIO and LabVIEW provides a flexible and effective solution for the cavitation intensity meter in its default configuration.

Author(s):
Morten Kjeldsen - Flow Design Bureau AS
Jarle V. Ekanger - Flow Design Bureau AS

Hydropower excels in its ability to rapidly change power output to meet demand. It acts as a compensator to sudden changes in power consumption, and also for changes in power production. For example, if wind speed drops, and hence wind power production also drops, hydropower can compensate for that. The power utility companies can charge for this power compensating service, for example, the market for “frequency restoration reserves” in the Nordic countries. However, some hydro power turbines cannot participate in this type of service due to restrictions in operating range, often due to cavitation imposed limits. Running the equipment outside these cavitation limits results in unacceptable levels of degradation of the hydropower unit due to the unwanted consequences of cavitation. The consequences of cavitation include erosion and induced unsteadiness resulting in fatigue ruptures. High-level cavitation can even result in substantial decrease in performance.

Figure 1. Observed cavitation erosion in turbine runner blades.

When it comes to cavitation attributes, we can broadly group turbines into two groups. The first, and most common, is to design the turbine so that the unit does not experience loss in efficiency or power production, due to cavitation, under normal operating conditions. The second type of design avoids cavitation erosion for all possible operating conditions; however, this tends to result in the turbine being ineffective for normal operating conditions. As a consequence, the power plant operators impose limits in the operation bands of the units. The cavitation intensity instrument now makes it possible for power utility companies to challenge these limits and possibly deliver additional power compensation services that can act as a new source of revenue. Since the instruments quantify the intensity of cavitation, it provides a decision tool as well. Based on a cavitation reading we can take the following actions: 1) change the operating setpoint to a cavitation safe area, 2) let cavitation occur, but for a limited period of time, or 3) after a preset accumulated cavitation intensity, take the unit out of operation for a visual inspection.  


Figure 2. Cavitation intensity as a function of unit hydraulic load. Results follows substantial amount of post processing of sensor input data.

Flow Design Bureau (FDB), an NI Alliance Partner since 2014, has extensive knowledge in industrial flow systems that goes back to the start of the millennium. With this expertise and understanding of issues within the hydropower industry, we have worked with hydropower companies to create and implement condition monitoring solutions based around this type of cavitation intensity meter.  

Figure 3. FDB surrounds the exit pipe (draft tube) of the Svorka turbine unit. FDB designed the solution for visual access by using acrylic plugs that fit the manhole and, through tree pipe stubs, fitted the draft tube for the occasion.

The cavitation instrument meter detects cavitation by using high-frequency accelerometers and an acoustic emission sensor on the air side of structures enclosing the flow of water in hydroturbines, such as the outside of pipe work or impeller casings. The structure conveys the sound of impacting bubbles on the wetted surfaces to the sensors. After detection, at rates up to 200 kS/s per channel, the signal undergoes a number of analyses; including filtering and various domain transforms. The final output relies on spectral analysis of the analytical signal, or the envelope, of the original time series. This way the instrument produces a limited number of values characterizing the cavitation intensity over, typically, 10 s intervals. For further details on the analysis, please refer to the article in the January 2015 issue of the Journal of Fluids Engineering by Escaler, Ekanger et al. We programmed all analysis in the LabVIEW environment and analysis took place on the real-time CPU on board the CompactRIO while using the CompactRIO FPGA unit exclusively for the DAQ process.

  • Feedback on the presence and the intensity of cavitation is useful in itself, but additional value comes from combining this information with other operational data. The cavitation intensity within a hydroturbine depends on the following:
  • Setpoint or load setting, which is defined by the wicket gate or guide vane opening and the pressure drop over the unit. For the typical reservoir plant, the actual pressure drop over the unit can change hugely as a result of large variations in the water level at the dam.
  • Absolute pressure downstream, which includes submersion of the unit and is usually referred to by the term net positive suction head (NPSH) in the hydropower industry.
  • Water quality, which includes gas saturation and suspended particles.

By having the cavitation intensity meter run for an extended period of time, while simultaneously combining/correlating with data related to the dependencies cited above, a map giving the cavitation limits emerges. We can use this map to predict ahead of time if cavitation will occur when changing the load setting, once we know other operating conditions.

At FDB, we have created a mobile version of the instrument that makes it possible to determine signal conditions on-site, including possible optimal use and placement of sensors before installing a permanent solution. We can also use the mobile version to create a baseline measurement to determine safe operating ranges if conditions cited above (unit pressure drop, back pressure, or submerge and water quality) remains similar for all operations.

Figure 4. Installing accelerometers at the base of the NACA0015 hydrofoil used at the University of Minnesota high-speed water tunnel, which pick up the cavitation signature and can provide a measure for cavitation intensity.

The versatility and power of the cRIO-9035 has been pivotal to the success of the instrument. This real-time unit allows for reprogrammable setting of filters, an accurate and deterministic analysis process, and effective data acquisition through modules like the NI-9234 and NI-9205. An additional benefit of using the CompactRIO platform has been the number of communication protocols available. To date, we have successfully implemented data transfer solutions based on TCP/IP, Modbus, and IEC 60870-5—all protocols relevant for the Norwegian hydropower industry.

The development of the instrument based on the CompactRIO platform was part of the PhD work of Jarle Ekanger. Trials took place at a Statkraft-operated power plant, Svorka, about two hours from Trondheim, Norway. Additional testing of the concept took place at the high-speed water tunnel at the University of Minnesota in collaboration with Professor Roger E. A. Arndt. The instrument, although a stand-alone instrument, now constitutes an important part of a larger water conduit monitoring system (HydroCord) developed by Norwegian hydroturbine operators.

Figure 5. Vapors over NACA0015 hydrofoil at the University of Minnesota high-speed water tunnel (flow from left to right).

 

Author Information:
Morten Kjeldsen
Flow Design Bureau AS
Norway
mk@fdb.no

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