**1. Introduction**

482 Mechanical Engineering

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Butterfly valves have the advantage of being very compact and simple to install compared with other types of valves, and so they are widely used in industry. However, depending on the conditions, cavitation may occur around a butterfly valve. When severe noise and vibration occur because of cavitation around a butterfly valve, the valve body and pipe wall are subjected to erosion.

Butterfly valves are sometimes used inside the piping of air-conditioning facilities and the noise and vibration caused by cavitation can, in addition to making users uncomfortable, be mistaken for mechanical trouble. The need to prevent such noise and vibration is increasing from an environmental standpoint, and the prevention or suppression of cavitation itself is very important. Accordingly, many products have been proposed to prevent or control cavitation around many types of valves (Baumann,1991; Tullis,1989). As for research on the prevention of cavitation, the characteristics of a control valve with tortuous paths and an orifice with a multi-perforated cone to prevent cavitation from occurring around the orifice were reported.(Rahmaeyer et al.,1995; Kugou,1996)

These methods have already been applied to actual products, and those products have proved very successful in reducing noise. However, tortuous path valves, for example, are applicable only to cases wherein the fluids are clean and the shapes of the piping arrangements around the valves are complicated. Moreover, the air injection method that is very effective in reducing cavitation is limited to cases wherein the effect of air can be ignored. Hence, the authors proposed the sudden enlargement of a pipe downstream of a butterfly valve (Ogawa & Uchida,2005). This method was much simpler than the conventional methods. However, the sudden enlargement of the pipe is not adequate for flows containing particles because the particles accumulate in the enlarged section of the pipe.

The author has already proposed the attachment of fins to the valve body in order to further reduce cavitation noise around the butterfly valve (Ogawa & Uchida,2005, Ogawa,2008). This method can be used for flows containing particles because of the simple shape of the valve body. Cavitation occurs intensely around the butterfly valve because of the interference of the flow from the nozzle side with the flow from the orifice side (Itoh et

Noise Reduction in Butterfly Valve Cavitation

Fig. 2. The test valves with fins.

of this noise meter was 20-8000Hz.

valve stem.

by Semicircular Fins and Visualization of Cavitation Flow 485

Figure 2 shows the test valves. The valve shown in Fig.2(a) is a normal valve without a fin

(a) Normal valve (b) TYPE-A

(c) TYPE-B (d) TYPE-C

The author clarified that the fin must be installed in the downstream of the valve in order to reduce cavitation (Ogawa & Uchida,1995). In this study, a test valve is called "TYPE-A" when a semicircular fin is attached to the downstream surface of the valve. When two semicircular fins are attached to the downstream surface of a valve, the test valve is called "TYPE-B". On the "TYPE-C" test valve, three semicircular fins are attached to the downstream surfaces of the valve. For each valve, each fin was fixed perpendicular to the

Cavitation noise measurements were performed in a closed-type cavitation tunnel, using water as the fluid. There was a pump on the downstream side of the test section. The upstream pressure was kept at atmospheric pressure and the valve opening was fixed during the experiment. The flow velocity was increased by controlling the frequency of the pump using an inverter. Cavitation noise was measured at each flow velocity and a visualization was created by a high-speed camera. Cavitation noise was measured using a noise meter placed close to the outside surface of the test section duct. The frequency range

al.,1988). To avoid this interference, semicircular fins were attached to the valve body. In this paper, it was confirmed based on the experimental results that the attachment of the fins was very effective in reducing cavitation noise.

In this study, cavitation bubbles were photographed by Hi-speed camera and the size and number were measured from those photographs and the effect of the fins and the upstream velocity distribution were investigated. In past studies, photographs focusing on the aspect of the butterfly valve cavitation have been reported in great numbers. For example, it was pointed out that the most erosive cavitation around a butterfly valve is the vortex cavitation on the orifice side by means of the pressure-sensitive films and high speed photography (Tani et al,1991). An observation of butterfly valve cavitation and the measurement of cavitation noise were performed to diagnose the cavitation condition (Kimura and Ogawa, 1986). However, since the measurements of number and size of the cavitation bubbles were not carried out, the details of the cavitation growth were not clear. In this study, close-up photographs of cavitation bubbles were taken and their number and size were analyzed. The difference between the normal valve and the valve with the fins is reported in the following.

In actual piping arrangements, straight lengths in front of butterfly valves are not sufficient to obtain normal velocity distribution in many cases. In extreme cases, more than two valves are installed in series or bends are installed just ahead of or behind those valves. Under such conditions, the upstream velocity distribution is different from the usual turbulent velocity distribution. Therefore, under the condition that the velocity distribution is biased, confirmation of whether or not the attachment of fins is useful for the noise reduction is necessary.
