| Abstract |
Nowadays, the phenomenon of cavitation erosion has become a critical issue in control valve. Many researchers strive their effort to provide any possibility to solute this problem. Numerous literatures are available this issue with emphasis on orifice and butterfly valve, since they are very popular to use in the industry. Due to simple structure, cheaper price and various options in size. Unfortunately, Orifices and butterfly valve are not suitable for the high pressure environment. On the contrary, although the globe valve is more expensive than the orifice and butterfly valve, it can be used to control the flow rate efficiently under the high pressure. Therefore, globe valve is the best choice for those valve users who need to tackle with high pressure. But the cavitation erosion will accompany with the usage of the globe valve. As fluid flow through the globe valve via S type pass, great pressure drop between valve inlet and outlet will cause cavitation erosion. Not only this phenomenon will damage the trim and wall easily, but also will increase the operation cost, In a power plant, feed water is supplied to pre-heater, and then into boiler drum. The globe valve is employed feed-water bypass valve, which can be used in low flow rate operating condition. When the globe valve are adjusted under low flow rate with pilot controls, the plug of the valve between inlet and outlet will cause great pressure drop, which will cause cavitation erosion to damage the plug, valve seat and so on. This problem, two approaches will be adopted as described in the following:
1. Orifices different ratio will be installed either alone or in series connection upstream of valves. These orifices aim to the flow dissipate energy in order to reduce the pressure drop occurred at control valve, and subsequently change the flow field distribution of valve. Based on the result, orifices with ratio 0.5 and 0.42 in series can reduce the vibration caused by cavitation and increase the value of inception cavitation.
2. Use FloWorks software to simulate the flow field distribution of the inside valve. From preliminary study, it shows that the position of cavitation occurrence always appears in the area between plug head and seat. Therefore, five types of plug head designs are developed to analyze this influence. The results indicate that if the appearance of the plug head is revised to ladder-like type, the cavitation index will be elevated, and the phenomenon of cavitation will be improved. |
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