提高离心系本身抗汽蚀性能的措施
1)泵设计时选择合适的几何参数，减小泵的必需汽蚀余量
    在泵设计时，适当加大叶轮进口面积(通过增大入口直径和进口宽度来实现)，合理选择叶片进口冲角和叶片数，适当加大叶轮前盖板进口段的曲率半径和叶轮表面的光滑度，均可以减小必需汽蚀余量NPSH,，提高抗汽蚀性能，如图1-22与图1-23所示。
2)首级叶轮采用双吸式叶轮
    双吸式叶轮单侧通过的流量为总流量的一半，减小了叶轮进口流速，从而减小NPSH.提高抗汽蚀性能。
3)采用前置诱导轮
    诱导轮装在泵第一级叶轮前面，如图1- 24所示。当液体流过诱导轮时，诱导轮对液体作功，使液体在进入离心泵前已增压，从而提高泵的抗汽蚀性能。同时，由于诱导轮是轴流式的,其外缘速度高，在离心力的作用下，一且产生气泡，只能在诱导轮的外缘处沿轴向流动,不会堵塞流道。所以诱导轮作为轴流式叶轮，可以在一定程度上预防产生汽蚀。
4)采用抗汽蚀材料
    应选用抗汽蚀性能强的材料制造叶轮，以延长叶轮使用寿命。实践证明,材料的强度、硬度、韧性和化学稳定性越高，叶道表面越光滑，抗汽蚀性能越好。常用的材料有稀土合金铸铁、含镍铬不锈钢、铝青铜和磷青铜等。
五、特殊液体汽蚀性能参数的计算
    泵的必需汽蚀余量是在采用温度为20C的清水条件下试验得到的。泵在输送烃类介质和高温水时的必需汽蚀余量NPSH'比输送常温水时的NPSH.,要小。因此，用水来确定特

殊液体的汽蚀余量一般是偏向安全的。美国API 610标准中规定，输送水以外液体(如烃类)的必需汽蚀余量不能采用降低系数或修正系数，应采用清水试验所得到的汽蚀余量值。
输送特殊液体时泵必需汽蚀余量NPSH'计算公式为:渣浆泵
    图1- 25是美国水力协会对泵送碳复化合物时汽蚀余量的修正值△h.的曲线。在图的横坐标上找到被输送液体的温度，由此向上作线与表示该液体汽化压力的科级交于一点，过该点作与修正曲线平行的斜线，与右边的坐标轴相交，交点的坐标值即为0。

Measures to improve the cavitation resistance of centrifugal system

1) Select appropriate geometric parameters to reduce the necessary NPSH during pump design

In the design of the pump, increasing the impeller inlet area (by increasing the inlet diameter and width), selecting the blade inlet angle and blade number, increasing the curvature radius of the front cover plate inlet section of the impeller and the smoothness of the impeller surface can reduce the NPSH and improve the anti cavitation performance, as shown in Fig. 1-22 and Fig. 1-23.

2) The first stage impeller adopts double suction impeller

The flow through one side of the double suction impeller is half of the total flow, which reduces the inlet flow rate of the impeller, thereby reducing NPSH and improving the cavitation resistance.

3) Adopt front inducer

The inducer is installed in front of the first stage impeller of the pump, as shown in Figure 1-24. When the liquid flows through the inducer, the inducer works on the liquid, which makes the liquid pressurized before entering the centrifugal pump, so as to improve the anti cavitation performance of the pump. At the same time, because the inducer is axial-flow type, its outer edge speed is high, under the effect of centrifugal force, once bubbles are generated, they can only flow along the axial direction at the outer edge of the inducer, and will not block the flow channel. Therefore, as an axial-flow impeller, inducer can prevent cavitation to a certain extent.

4) Adopt anti cavitation material

The impeller shall be made of materials with strong cavitation resistance to prolong its service life. It has been proved that the higher the strength, hardness, toughness and chemical stability of the material, the smoother the blade surface and the better the cavitation resistance. The commonly used materials are rare earth alloy cast iron, nickel chromium stainless steel, aluminum bronze and phosphor bronze.

5、 Calculation of cavitation performance parameters of special liquid

The necessary NPSH of the pump is obtained under the condition of clean water at 20c. The NPSH 'required by the pump in the transportation of hydrocarbon medium and high temperature water is smaller than that in the transportation of normal temperature water. Therefore, water is used to determine the specific

NPSH of different liquids is generally safe. According to API 610, the reduction coefficient or correction coefficient shall not be used for the necessary NPSH of liquid (such as hydrocarbon) other than water, and the NPSH value obtained from clean water test shall be used.

The calculation formula of NPSH 'is: slurry pump

Figure 1-25 shows the curve of NPSH correction value △ h. for pumping carbon compound by American Hydraulic Association. Find the temperature of the liquid to be transported on the abscissa of the figure, and then make a line upward to intersect with the section level representing the vaporization pressure of the liquid at a point. Make a diagonal parallel to the correction curve through the point, intersect with the coordinate axis on the right, and the coordinate value of the intersection point is 0.