Preventive Maintenance of Hydraulic Installations Based on Monitoring Cavitation-Induced Vibrations Andrei Dragomirescu, Carmen-Anca Safta, Lucian Mândrea, Department of Hydraulics, Hydraulic Machinery and Environmental Engineering University Politehnica of Bucharest Bucharest, Romania andrei.dragomirescu@gmail.com; safta.carmenanca@gmail.com Nicolae Oranu, Ioan Maghei Department of Mechanics University Politehnica of Bucharest Bucharest, Romania ioan_magheti@yahoo.com; norasanu62@gmail.com Abstract—This paper presents results of experimental investigations on the frequency domains of cavitation-induced vibrations at an orifice plate and at a butterfly valve. These are two components that are often encountered in hydraulic installations. Orifice plates are usually used for measuring the flow rate, but they can also be used for restricting the flow. Butterfly valves are normally used to quickly shut off the flow and, to some extent, for throttling the flow (i.e. for adjusting the flow rate). Under certain operating conditions, cavitation and the accompanying damage can occur both at orifice plates and at butterfly valves. The results presented in this paper suggest that cavitation caused both by butterfly valves and by orifice plates seems to be indicated by high frequency vibrations having local maxima at roughly 18 kHz. From a practical point of view, these results could be used for preventive maintenance in industrial hydraulic installations. Keywords—hydraulic installations, industrial fittings, cavitation, vibration monitoring, vibration analysis, preventive maintenance I. INTRODUCTION Hydraulic piping systems are often subjected to flow- induced vibrations due to various phenomena, such as pressure fluctuations, turbulence, water hammer, or cavitation. Regardless of their nature, vibrations can cause fatigue failure in process pipelines. As a result, the pipes are broken by cracking and all the hydraulic equipment – including pumps, valves, branch connectors, etc. – can suffer damages. For this reason, in hydraulic systems working with water, hydraulic oil, or other Newtonian liquids, the monitoring of vibration and noise levels is an important tool in preventive maintenance [1] that aims at avoiding or, at least, mitigating the consequences of equipment failure. This paper presents results of experimental investigations on the frequency domains of cavitation-induced vibrations in water piping systems. Two different installations are investigated, each of them having an orifice plate mounted on the suction pipe of a centrifugal pump that supplies the system. In addition, one of the installations has a butterfly valve mounted on the pressure pipe. In both installations cavitation can occur under certain flow conditions. Orifice plates and butterfly valves are often encountered in hydraulic installations. Orifice plates are usually used for measuring the flow rate, but they can also be used for reducing the pressure or for restricting the flow. Butterfly valves are normally used to quickly shut off the flow, but they can also be used for throttling the flow (i.e. for adjusting the flow rate). The common characteristic of orifice plates and butterfly valves is that they cause a local contraction of the pipe cross- section, that leads to a local increase in flow velocity and, consequently, to a local decrease in static pressure. Cavitation is a phenomenon that consists in the appearance of vapor-filled bubbles inside a flowing liquid when the static pressure inside the liquid drops locally below the vapor pressure. The bubbles are then transported by the liquid flow and, when regions of pressure higher than the vapor pressure are reached, they implode (collapse) abruptly. The implosion is due to micro-jets that penetrate the boundaries of the bubbles at extremely high velocities. Cavitation is accompanied locally by different phenomena, such as very high local pressures and temperatures that can trigger chemical and electrochemical processes. When cavitation is not or cannot be controlled in hydraulic installations, the accompanying phenomena lead globally to excessive levels of vibrations and noise, to sudden decreases in the performance of hydraulic machines, and to the destruction by erosion and corrosion of the sensitive components of an installation, such as valves or turbomachinery runners and impellers. Different experimental studies approached the hydrodynamic behavior of valves and orifices under cavitation conditions. Rudolf et al. monitored pressure fluctuations caused by cavitation for different types of orifice plates, including a single-hole orifice, and observed that the frequency spectra of the pressure signals became noisy under cavitation conditions [2]. Nagaya et al. performed an acoustic study of the cavitating flow at an orifice plate and noted that cavitation caused an increase in noise levels at frequencies that are generally higher than 1 kHz and particularly higher than 6.3 kHz [3]. Testuda et al. observed that the acoustical power spectra downstream of a cavitating orifice exhibit a hump form in the upper frequency range (above 200 Hz) [4]. 978-1-5386-5062-2/18/$31.00 ©2018 European Union 0883 10th International Conference and Exposition on Electrical and Power Engineering (EPE2018)