In order to improve pipes and valves maintenance in EDF power plants by the use of reliability analysis, some wear mechanisms have been listed and their wear kinetics have been investigated. One of these wear mechanisms is cavitation erosion. Cavitation may occur in numerous pipe components such as valves, orifices, elbows and tees, in any place where low pressure and/or high velocity is reached. The effects of cavitation developments are first noise and vibrations, then erosion of the downstream pipe if the material is not resistant enough and at least loss of performance of the component (choking). A predictive model to evaluate cavitation erosion in valves and pipes was proposed in 1998 by Lecoffre . Experiments on a closed cavitation test loop for the simple geometry of two sharp-edge single hole orifices (opening factor β = d/D = 0.4 and β = d/D = 0.8) have been performed in order to validate this model. First, the cavitation developments downstream the orifice have been visualized through a transparent pipe section. Incipient and chocking pressure coefficient have been recorded and compared with data from Tullis . Then the erosion has been quantified using pitting measurements on mirror polished 316L stainless steel samples, located flush on the inner pipe surface. Cavitation pits have been measured using a laser profilometer. The plastic deformation volume measurement allows to define a pitting velocity, which can be related to a « loss of thickness » velocity, taking into account previous erosion tests on the pipe material. These pitting tests were performed for five flow conditions, characterized by three flow rate values and three pressure coefficient values. In the predictive model, these flow conditions parameters are more precisely defined by the flow velocity in the contracted section, named V1 (m/s), and by the pressure coefficient calculated with the lowest pressure in the contracted section, named σ1 (-). At least, a predictive model was adjusted using these experimental results. We found that the cavitation erosion wear velocity of a 316L stainless steel straight pipe downstream the orifice (characterized by its opening factor β) for the flow condition (σ1, V1) with cavitation σ1<1 fits well to the following formula:
with : k = f(β), V0 = g(β) where f and g are functions of the opening factor β = d/D. Tests using the same methodology have since been performed on butterfly valves and on multiholes orifices.
Volume Subject Area:Cavitation and Multiphase Flow
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