Table 2

Characteristic Simple cavitation monitor Multidimensional cavitation monitor
Information content Only delivers data equivalent to that from the global cavitation characteristic, i.e. total cavitation intensity, and other signal signatures like modulation depth etc. as well Delivers this data and also data equivalent to that contained in the set of other turbine cavitation characteristics. This yields diagnostic data on turbine parts and cavitation mechanisms
Bias error By using only one or a few sensors, the simple monitor does not cover the entire volume in a turbine and thus may have a high bias error in intensity estimates. Correctly covers the entire turbine volume and all the turbine parts and thus yields unbiased, representative estimates of all cavitation components.
Monitoring sensitivity Detects the onset of a detrimental effect by looking for a change in the total cavitation intensity. In early phases of development, detrimental effects are usually well localised. Thus, the multidimensional monitor, which resolves turbine parts and parts of the turbine volume, detects them by looking for a change in the cavitation intensity in a resolution cell. This makes the sensitivity of the multidimensional monitor compared to that of the simple monitor higher by a factor equal to the number of runner blades, the number of guide vanes, or the product of these two numbers, depending on which of the cavitation characteristics is included in the monitoring algorithm.
Installation Adjustment of few amplifications usually suffice for a simple monitor to be ready for use. An introductory diagnostic test is needed in order to design the monitor for a given turbine. However, the test delivers detailed turbine cavitation characteristics, reveals turbine weak points and, if necessary, shows what improvement steps can be made. Thus, the test delivers results which have their own practical value.
Costs Minimum preparatory work in the plant, and minimum investment in the sensors. Higher costs for the work in the plant and for the sensors. However, quick payback through improved turbine maintenance is ensured as is much safer operation.