There are two alternative processing versions which are easier to organise:
... No trial is made to identify the rotor/stator interaction sources; the J(Ο,Φ) graph is displayed above the simple Φ-axis.
... No cutting of J(Ο,Φ) into intervals is made, and thus original versions of J(Ο,Φ) are shown.
The version One
is illustrated in the graph above. The data related to the runner blades, C
, and guide vanes, E
, are not available; only the mean global intensity, J(Ο), is known.
The version Two
- the graph below. Instead of comparing two sets of rectangles, red and green ones, which shows clearly the difference between the running cavitation and that in the reference, the direct comparison of the running and reference J(Ο,Φ) makes the difference less visible. In the shown case, the match of the Φ-positions of the running and reference traces is perfect; this reduces the difference to the values close to the peak, which is good. In most cases, however, the two Φ-positions are not as good as that. This may make the related running and reference traces only partially overlap or even lie next to each other. With the display with the rectangles such mis-match, which is not essential but attracts attention, would be hidden. The full processing as presented above in the Description is thus highly recommended. It requires an additional work in the calibration test.
High resolution with respect to turbine elements has its direct diagnostic value, and it also guarantees high sensitivity in cavitation detection. The second graph in the Description above and the three graphs below demonstrate these effects. They show the resulting displays for four different situations with respect to cavitation spatial spreading - with running cavitation intensity passing over the reference by a rather small amount of 10 %:
- on only one runner blade behind only one guide vane;
- on all runner blades behind only one guide vane;
- on one runner blade behind all guide vanes;
- on all runner blades behind all guide vanes.
Comparison of the mean-intensity displays and the big graph with the instantaneous cavitation intensity demonstrates everything clearly. A small rise of cavitation in one isolated point of the machinery (e.g. on one runner blade behind one guide vane) is clearly visible in the big graph. There, the rise is shown in its full scope. In the global mean intensity display the rise is hardly noticeable; in reality it might be hidden in the random fluctuations of the result. It is a bit better in the other mean-intensity displays. In them, however, such a small and well-localised cavitation is visible via 1/4 or 1/7 of its intensity value. With rise of cavitation which involves more turbine elements the visibility in the mean-intensity displays is improved but it is still the best presented in the detailed display on the right.
All the displays shown in this illustration are a part of what one has on the cavitation-monitor screen, directly or in the cavitation channel of the central plant monitoring system. Further displays include time log over a selected past time interval, running display of the cavitation-intensity dependence on operation parameters, an option for focusing on the past details, and the accumulated erosion display.
On the use of the monitoring results
Early detection of damage
even if well-localised is guaranteed by the high spatial selectivity and the resulting high sensitivity.
and other systematic changes in turbine cavitation characteristics are visible in all the displays. The differences between different turbine elements are also visible. If calibration was fully made, the elements can also be identified.
Optimisation of turbine operation
can be organised via plant SCADA in a way to avoid or suppress cavitation in a turbine, or to minimize the total cavitation intensity in turbines of a plant provided all the turbines are equipped for cavitation monitoring.
Optimisation of turbine maintenance
can be organised via the log of the accumulated erosion. As noted in the Optimisation of turbine maintenance in the Tests, the erosion rate can be estimated by J
, where J is the mean global cavitation intensity, J in J(Ο). The same holds true for the intensities related to the runner blades or guide vanes. A vector consisted of all these erosion-rate estimates should be read in equidistant moments while the turbine is operated and added to the running sum. The resulting vector of the accumulated erosion estimates should be calibrated as described in the Tests. Once calibrated for J(Ο), the same proportionality constant should be used for the estimates for the runner blades and guide vanes. Such final estimates of the accumulated erosion reached till the actual moment can then be used to decide on the overhauls. Here, the role of each turbine element can also be reliably assessed.
Services offered to the monitoring-system manufacturers