| dc.description.abstract | The control valve is one of the important issues in the piping system. When controlling the flow, if the control valve is not carefully selected, it would often lead to cavitations, causing noise in the pipe-work, vibration and erosions, and thus diminishing the efficiency of the system. Over the years, the industry often used globe valves as the main control valve. However, due to the high-cost of the globe valves and its bulky size, nowadays many factories have switched to the ball valve for flow controls, which lower the cost and the weight of their products. Ball valves use different shields to control the flow, and can be programmed with computer for the angle of turning angle, therefore accurately controlling the flow. The use of different shields holds many advantages, such as the minimum flow resistance, rapid responding time, easy to control, good sealing capability, longer life-span, accurate controllability, and high reliability.
Ball valves often operate to meet the requirement of high precision flow control yet in bad-condition environment, many issues are encountered, such as cavitation, noise and vibration problems. This study uses computational fluid dynamic software to simulate flow over the ball valve, and then calculate the flow coefficient, Kv. Flow coefficient is one of the main parameters for selecting the control valve, one can compare the computed Kv with the experiment, so as to design a better ball valve.
This study uses the software FloEFD to simulate the flow over the ball valves with different V-shaped openings, and other custom-made opening shapes. The focus of this study is to simulate orifice, pure ball valve, and ball valve with different shields, which allow one to find the best performing shield shapes which can suppress or eliminate cavitation. Simulation results show that the orifice creates contraction region in the downstream, thus accelerating the flow, and as the flow pressure lowers than the absolute saturated vapor pressure, cavitations will occur. The smaller pressure difference is, the higher cavitations index is, and cavitation is less likely to occur. Simulation of standard ball valves (38mm and 50mm in diameter) reveal that as the opening decreases, the pressure difference across the valve increases, and the pressure difference is increasing with increment of Reynolds number. When the opening is enlarged, the flow motion is smoother, creating a smaller vortex circulation. At the same opening, the smaller angle of the V-Port shield, the faster the flow at downstream. In addition, the corresponding pressure is lower, and the cavitations and noise are more likely to occur. Choosing appropriate ball valve shields with different pipe diameters and flow speeds can accurately control the flow motion and prevent cavitation.
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