【正文】 態(tài)轉換到另一種流 動的狀態(tài)是漸進的，對于中間狀態(tài)，設備可稱為混流泵。葉輪中葉片越多，則液體的流動方向越好控制，那么液體循環(huán)流動時因波動引起的損失就越少。固定葉片前緣處的流體應該沒有受到沖擊?？紤]到液體在離泵中心 r到 r+dr 的距離內旋轉，如圖 d d. 所示。 （ 5）維修費用比其他類型的泵少。 （ 2）只有在有限條件下才能以最高效能運作：尤其是渦輪泵。汽 蝕的形成 討論 如下 。 靜態(tài)壓力： 在流 體中的靜態(tài)壓力是指單位面積上流體的的移動邊界與流體的正常作用力 之間的壓力差 。到 39。蒸汽壓力 取決于液體的溫度。然而，在實踐中 材料可以選擇，提高材料汽蝕作用下的承受能力，提供更長的使用壽 命和經濟 價值的結果，所以，注意泵的結構材料是重要的和富有成效的。 試想在一個密閉容器中，液體汽化，可發(fā)生在離心泵低于液體的蒸氣壓在泵的輸送溫度時，它的靜態(tài)壓力降低。大約每個葉輪葉片上都有數百相同點會 同時產生泡沫的破滅。氣泡內爆破， 簡而言之，汽蝕機制是所有有關泵輸送液體內部氣泡的形成，生長和崩潰的過程。 ，尤其是 它在重型車輛運行的時候。 ，用來取代閘閥。 ?如果可能的話，安裝誘導裝置。 4． 降低吸程要求 。 ： 。 Static pressure head, Step Three, Collapse of bubbles， As the vapor bubbles move along the impeller vanes, the pressure around the bubbles begins to increase until a point is reached where the pressure on the outside of the bubble is greater than the pressure inside the bubble. The bubble collapses. The process is not an explosion but rather an implosion (inward bursting). Hundreds of bubbles collapse at approximately the same point on each impeller vane. Bubbles collapse nonsymmetrically such that the surrounding liquid rushes to fill the void forming a liquid microjet. The micro jet subsequently ruptures the bubble with such force that a hammering action occurs. Bubble collapse pressures greater than 1 GPa (145x106 psi) have been reported. The highly localized hammering effect can pit the pump impeller. The pitting effect is Collapse of a Vapor Bubble 18 illustrated schematically in this the figure. After the bubble collapses, a shock wave emanates outward from the point of collapse. This shock wave is what we actually hear and what we call cavitation. The implosion of bubbles and emanation of shock waves (red color) . In nutshell, the mechanism of cavitation is all about formation, growth and collapse of bubbles inside the liquid being pumped. But how can the knowledge of mechanism of cavitation can really help in troubleshooting a cavitation problem. The concept of mechanism can help in identifying the type of bubbles and the cause of their formation and collapse. (5)Solution and Remedies： For vaporization problems (cavitation) ( cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the . Required. We shall look at each possibility: 1).Increase the suction head: ?Raise the liquid level in the tank ? Raise the tank ? Put the pump in a pit ? Reduce the piping losses. These losses occur for a variety of reasons that include : 1. The system was designed incorrectly. There are too many fittings and/or the piping is too small in diameter. 2. A pipe liner has collapsed. 3. Solids have built up on the inside of the pipe. 4. The suction pipe collapsed when it was run over by a heavy vehicle. 5. A suction strainer is clogged. 6. Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather 7. Something is stuck in the pipe, It either grew there or was left during the last time the system was opened . Maybe a check valve is broken and the seat is stuck in the pipe. 8. The inside of the pipe, or a fitting has corroded. 19 9. A bigger pump has been installed and the existing system has too much loss for the increased capacity. 10. A globe valve was used to replace a gate valve. 11. A heating jacket has frozen and collapsed the pipe. 12. A gasket is protruding into the piping. 13. The pump speed has increased. ? Install a booster pump ? Pressurize the tank 2) lower the fluid temperature ? Injecting a small amount of cooler fluid at the suction is often practical. ? Insulate the piping from the sun39。 1） Cavitation Enhanced Chemical Erosion Pumps operating under cavitation conditions bee more vulnerable to corrosion and chemical attack. Metals monly develop an oxide layer or passivated layer which protects the metal from further corrosion. Cavitation can remove this oxide or passive layer on a continuous basis and expose unprotected metal to further oxidation. The two processes (cavitation amp。 this applies especially to turbine pumps． (3)It is not usually selfpriming. (4)If a nonreturn valve is not incorporated in the delivery or suction line, the liquid will run back into the suction tank as soon as the pump stops． (5)Very viscous liquids cannot he handled efficiently． 3. Cavitation in centrifugal pump （ 1） The term ‘cavitation’ es from the Latin word cavus, which means a hollow space or a cavity. Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure. In any discussion on centrifugal pumps various terms like vapor pockets, gas pockets, holes, bubbles, etc. are used in place of the term cavities. These are one and the same thing and need not be confused. The term bubble shall be used hereafter in the discussion. In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump. Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles or Gas bubbles. bubbles are formed due to the vaporisation of a process liquid that is being 15 pumped. The cavitation condition induced by formation and collapse of vapor bubbles is monly referred to as Vaporous Cavitation. bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in the system). The cavitation condition induced by the formation and collapse of gas bubbles is monly referred to as Gaseous Cavitation. （ 2） Important Definitions: To enable a clear understanding of mechanism of cavitation, definitions of following impo