Letter to the Plant Operation and Maintenance Managers
Subject: Cavitation
Dear Operation & Maintenance Manager,
If you have problems with cavitation in the turbines of your plant, you might wish to consider
the following facts:
1) The checking of cavitation erosion on turbine parts, as it is performed during an
overhaul, shows a total accumulated effect from various turbine operating points. However, the
most important causes of erosion are effective only at some operating points, and these
are not necessarily the points with the highest load.
2) Very often, the cavitation characteristics of different turbines in one plant differ
substantially from one another. This holds true even for turbines of the same design,
operating in identical conditions.
3) The true behaviour of a prototype turbine can substantially differ from the prediction
based on model tests.
4) Repeated runners´ repairs can result in a worsening of their cavitation behaviour. This, as
well as incidents such as the passage of a solid body through the turbine tract, can result
in changes in turbine characteristics during the exploitation.
A visual check of cavitation erosion does not yield data on the dependence of cavitation
intensity on turbine operating points and model tests cannot reveal differences between
nominally identical turbines. In order to obtain such data, vibro-acoustical tests on turbines in
exploitation must be performed, and in order to follow changes in turbine cavitation behaviour in
time, repeated vibro-acoustical tests or a permanent monitoring are necessary.
The results of the
well-performed diagnostic tests and/or monitoring can be used to:
A) Determine true cavitation characteristics of a new-installed turbine or a turbine after a repair.
B) Identify causes of cavitation.
This insight can be used in an action aiming at turbine improvement. (The improvement itself is outside of our scope.)
C) Avoid operation points of a turbine which cause strong cavitation.
D) Optimise plant operation for a minimum total erosion.
In the periods when not all units of a plant are being driven at the highest load,
required total power can be obtained by having a lower load on the turbines with
stronger cavitation and a higher load on those which are less prone to cavitation.
E) Detect the deterioration effects caused by ageing and incidents.
F) Optimise the maintenance.
Schedule overhauls based on an accumulated-erosion assessed by monitoring and not according to a fixed time plan.
DIAGNOSTIC TESTS
It has been determined that simple approaches to the vibro-acoustical testing, such as those based on only one or only few sensors and not complex enough signal and data processing, do not deliver reliable cavitation assessments and do not yield the details needed for a diagnosis of causes of cavitation. Our more sophisticated multi-sensor method, Korto multidimensional method for cavitation diagnostics, yields:
a) reliable cavitation-intensity assessments, which incorporate all spatially distributed cavitation segments with equal sensitivity and deliver their operation dependence, and
b) detailed cavitation description,
which distinguishes between cavitation mechanisms, delivers data on spatial distribution of cavitation, and determines the role of turbine parts in the cavitation processes.
The multidimensional method has been used on Francis, Kaplan and bulb turbines, including
one of the largest in the world.
CAVITATION MONITORING
There is our patent-pending approach to the high-sensitivity unbiased one-sensor monitoring of cavitation in hydro turbines
which reduces the hardware of the multi-sensor multidimensional method, but preserves most of its qualities. Such a monitoring system reacts to all cavitation segments equally, and has an extremely high sensitivity.
The system is applicable on hydro turbines of all types.
ON YOUR POSSIBLE ACTIONS
If you have problems with cavitation and you wish to identify their causes, consider a diagnostic test.
If your turbines operate in a broad band of varying conditions and you have problems with cavitation, consider cavitation monitoring.
If you wish to optimise operation with respect to cavitation but you are not ready to install another auxilliary hardware on your turbine, and the band of practiced operational conditions is not epecially broad, consider performing a diagnostic test, form and obey the operation rules based on the test results, and make control tests periodically, e.g. after overhauls.
If you wish to discuss the possibilities of using the above described methods on your turbines, write us without obligation about your problems and your machinery. We will do our best to help you by advice or action.
Sincerely,
Dr. Branko Bajic, Dipl.Eng.
Owner & CEO
Korto Cavitation Services
Korto Cavitation Services
