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提高離心系本身抗汽蝕性能的措施
1)泵設(shè)計(jì)時(shí)選擇合適的幾何參數(shù)，減小泵的必需汽蝕余量
在泵設(shè)計(jì)時(shí)，適當(dāng)加大葉輪進(jìn)口面積(通過(guò)增大入口直徑和進(jìn)口寬度來(lái)實(shí)現(xiàn))，合理選擇葉片進(jìn)口沖角和葉片數(shù)，適當(dāng)加大葉輪前蓋板進(jìn)口段的曲率半徑和葉輪表面的光滑度，均可以減小必需汽蝕余量NPSH,，提高抗汽蝕性能，如圖1-22與圖1-23所示。
2)首級(jí)葉輪采用雙吸式葉輪
雙吸式葉輪單側(cè)通過(guò)的流量為總流量的一半，減小了葉輪進(jìn)口流速，從而減小NPSH.提高抗汽蝕性能。
3)采用前置誘導(dǎo)輪
誘導(dǎo)輪裝在泵第一級(jí)葉輪前面，如圖1- 24所示。當(dāng)液體流過(guò)誘導(dǎo)輪時(shí)，誘導(dǎo)輪對(duì)液體作功，使液體在進(jìn)入離心泵前已增壓，從而提高泵的抗汽蝕性能。同時(shí)，由于誘導(dǎo)輪是軸流式的,其外緣速度高，在離心力的作用下，一且產(chǎn)生氣泡，只能在誘導(dǎo)輪的外緣處沿軸向流動(dòng),不會(huì)堵塞流道。所以誘導(dǎo)輪作為軸流式葉輪，可以在一定程度上預(yù)防產(chǎn)生汽蝕。
4)采用抗汽蝕材料
應(yīng)選用抗汽蝕性能強(qiáng)的材料制造葉輪，以延長(zhǎng)葉輪使用壽命。實(shí)踐證明,材料的強(qiáng)度、硬度、韌性和化學(xué)穩(wěn)定性越高，葉道表面越光滑，抗汽蝕性能越好。常用的材料有稀土合金鑄鐵、含鎳鉻不銹鋼、鋁青銅和磷青銅等。
五、特殊液體汽蝕性能參數(shù)的計(jì)算
泵的必需汽蝕余量是在采用溫度為20C的清水條件下試驗(yàn)得到的。泵在輸送烴類(lèi)介質(zhì)和高溫水時(shí)的必需汽蝕余量NPSH'比輸送常溫水時(shí)的NPSH.,要小。因此，用水來(lái)確定特
殊液體的汽蝕余量一般是偏向安全的。美國(guó)API 610標(biāo)準(zhǔn)中規(guī)定，輸送水以外液體(如烴類(lèi))的必需汽蝕余量不能采用降低系數(shù)或修正系數(shù)，應(yīng)采用清水試驗(yàn)所得到的汽蝕余量值。
輸送特殊液體時(shí)泵必需汽蝕余量NPSH'計(jì)算公式為:渣漿泵
圖1- 25是美國(guó)水力協(xié)會(huì)對(duì)泵送碳復(fù)化合物時(shí)汽蝕余量的修正值△h.的曲線(xiàn)。在圖的橫坐標(biāo)上找到被輸送液體的溫度，由此向上作線(xiàn)與表示該液體汽化壓力的科級(jí)交于一點(diǎn)，過(guò)該點(diǎn)作與修正曲線(xiàn)平行的斜線(xiàn)，與右邊的坐標(biāo)軸相交，交點(diǎn)的坐標(biāo)值即為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.
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