1.1 General
This part of GB/T 15469 specifies the measurement and evaluation of the amount of cavitation pitting on certain specified flow passage component for given conditions, which are defined in the Contract by power, specific hydraulic energy (water head), rotation speed, material, operation condition and duration of operation etc.
The Contract shall state whether cavitation pitting on the reaction turbine should or should not be anticipated in all or in some operating range. Moreover, the cavitation pitting guarantee value shall be explicitly stated.
1.2 Excluded Content
This part does not cover other effects which cavitation may possibly have on the hydraulic turbine such as:
- The power, efficiency, vibration, mechanical integrity and noise;
- The material defect revealed during operation.
Material defect revealed by abrasion on the flow passage component during operation are not included in a cavitation pitting guarantee.
1.3 Consideration Concerning Chemical Composition of Water
It is assumed in this part that the water is not chemically aggressive. In cases where the water is considered to be chemically aggressive, the cavitation pitting guarantee shall be given on the basis of an agreed water analysis. Since chemical aggressiveness is dependent upon so many possible chemical compositions and the material of the machine, it is beyond the scope of this part to set particular limit.
If it is indicated in the course of further analysis that the water is in fact more aggressive than the agreed analysis indicated, this must be taken into consideration where evaluating whether the cavitation pitting guarantees have been met. If the increased cavitation pitting zone where damage can be proven to be caused by chemical or electrochemical effect, then such zone shall be excluded from the evaluation of cavitation pitting.
1.4 Consideration Concerning Water Containing Solids
The cavitation pitting evaluation is based on the ordinary water quality which means there are a small amount of solids in water or the abrasion is slight.
Abrasion due to water contaminated with solids (for example, sand) cannot be considered as cavitation pitting. Where a turbine operates in water containing a significant amount of solids, the loss of material will be intensified due to the joint action of both cavitation (if it exists) and abrasion. Moreover, when the surface of material is altered by abrasion, cavitation can be initiated and accelerated because of the hydraulic shape change. With respect to measurement of damage, it is important to realize that, for cavitation pitting and abrasion damage, the appearance, position and mechanism of destruction is not similar. If cavitation pitting occurs in zone where damage can be separately attributed to abrasion, such damage shall be excluded from the evaluation of cavitation pitting guarantee.
The cavitation pitting guarantee value and measurement method provided by this part is only applicable to cavitation pitting. For some turbines operate in water that contains significant amount of solids for at least some portion of the year, the abrasion damage degree depends on many aspects such as solid concentration, mineral composition, size grading of solids, impact parameter (like relative speed and impact angle), material characteristic of flow passage surface as well as operation condition of turbine which are beyond the scope of this part.
