球閥使用不同特性擋板的流體控制及孔蝕問題探討; Investigation of flow control and cavitations for ball valve using different shields

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題名:	球閥使用不同特性擋板的流體控制及孔蝕問題探討;Investigation of flow control and cavitations for ball valve using different shields
作者:	徐志誠;Chih-cheng Hsu
貢獻者:	機械工程研究所碩士在職專班
關鍵詞:	孔口板;球閥;球型閥;孔蝕;流量係數;Ball Valve;Globe Valve;Cavitation;Flow Coefficient;Orifice
日期:	2010-07-16
上傳時間:	2010-12-08 13:42:56 (UTC+8)
出版者:	國立中央大學
摘要:	管路系統中，控制閥為重要探討的議題之一，在管流操作過程中，若不當的選配控制閥，往往會產生孔蝕現象(cavitation)，造成管路的噪音、振動及孔蝕腐蝕，進而降低管路系統效能。多年以來，工業界常使用球型閥(globe valve)來當作主要的控制閥，但因球型閥製造成本高及體積較為笨重，現在許多廠商已改用球閥(ball valve)來作流量控制，使得成本及重量也隨之降低。而球閥利用不同特性的擋板來控制流量，同時還可以利用電腦自動控制球閥的轉動角度，達到精確的控制流量。球閥使用不同特性擋板具有許多優點，例如最低的流阻、開關迅速及方便控制、密封性能好、使用壽命長、控制精確及可靠性高。球閥常使用於要求精密流量控制但卻惡劣的環境下運作，其中遭遇的問題包括孔蝕腐蝕、噪音及震動。本文將利用計算流體力學軟體來進行數值模擬，計算出流量係數(flow coefficient, Kv)，而流量係數是調節閥選型的主要參數之一，吾人可將數值預測的Kv與實驗數據比對，進而設計出較好的球閥。本研究以FloEFD軟體模擬球閥在不同V形開口及客製化形狀開口中的流場的現象。研究重點放在孔口板、純球閥和球閥使用不同特性擋板的流場模擬，以決定最佳特性擋板造型，能抑止或消除孔蝕現象。數值模擬結果顯示孔口在下游產生收縮截面，造成流體加速，當流體壓力下降至絕對飽和蒸汽壓力時即形成孔蝕。當壓差愈小時，孔蝕指數也愈大，也不容易發生孔蝕現象；標準球閥（直徑38 mm及50 mm）的數值解顯示，開度愈小時球閥上下游的壓差增加，雷諾數愈大時對應的壓差愈大。開度愈大時流體流動較為平順，而產生渦流現象也隨之減小。在同樣的開度時，V Port的擋板角度愈小，使得下游縮流部流速愈快；此外對應的壓力也降低，此時較容易產生孔蝕現象及產生噪音。在不同管路直徑及不同的流速，選擇適當的球閥擋板，可以精確的控制流量及預防孔蝕現象的發生。 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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