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渣漿泵的主要零部件
一、葉輪
葉輪是離心泵中傳遞能量的主要部件。對葉輪的主要要求是:單級葉輪能給予液體較大的理論揚(yáng)程，以便在達(dá)到高揚(yáng)程時(shí)采用較少的級數(shù)，使機(jī)器結(jié)構(gòu)緊湊，葉輪的效率較高，抗汽血性能好以及性能曲線形狀滿足工藝生產(chǎn)要求等。下面分析與這些要求有關(guān)的葉輪結(jié)構(gòu)形式和幾何參數(shù)等。
離心泵的葉輪大多數(shù)為后彎葉片型葉輪，只有高速部分流泵和旋渦泵等采用徑向葉片葉輪。常用的后彎葉輪的葉片數(shù)一般在6~12 片之間。比轉(zhuǎn)速在60~250之間的泵，葉片常為6片,低比轉(zhuǎn)速泵可取9片,高比轉(zhuǎn)速泵可為4~5片。通常,增加葉片數(shù)可以改善液體流動情況,適當(dāng)提高泵的揚(yáng)程;但葉片數(shù)增加后又會使葉片摩擦損失變大，使流道通流面積變小，從而會降低效率，容易導(dǎo)致汽蝕。反之,若葉片數(shù)過少，每個(gè)葉片的負(fù)荷增加,對液體的導(dǎo)流作用減小，又會使泵的揚(yáng)程下降。
離心泵葉輪的葉片形狀有兩類:圓柱面狀葉片和扭曲葉片。石油儲運(yùn)工程用離心泵的比轉(zhuǎn)速小于90的，般都采用圓柱面狀葉片它垂直于葉輪前后蓋板制造較容易。對比轉(zhuǎn)速大于90的泵，常采用扭曲葉片。
后彎葉片型葉輪葉片的出口葉片角P2A和進(jìn)口葉片角B人對泵的性能有重要的影響。葉片出口葉片角B:u一般是在15-~40°之間石消儲運(yùn)工程用為泵為20-30。比轉(zhuǎn)述較低的泵，選擇較大的B2.可以增加揚(yáng)程.減小直徑D2,，從而減少圓盤摩擦損失,提高泵的效率;但增大Bax時(shí)，在相同流量下增加葉輪出口速度C:壓液室的水力損失增加尤其是非設(shè)計(jì)流量下沖擊規(guī)時(shí)規(guī)，在以使H-Q性能曲線出現(xiàn)駝峰。此外必校時(shí)，葉道中液流相對速度w2更小，使流動擴(kuò)散損失越嚴(yán)重，因此，為獲得平坦下降的性能曲線，不宜選過大的總間，
葉輪進(jìn)口角 就是在葉片入口處，葉片工作面的切線(嚴(yán)路地說應(yīng)該是在流面上葉片骨線的切線)與圓周切線間的夾角，通常是接設(shè)計(jì)流量下波流進(jìn)葉道時(shí)絕對速度c1的方向角來定的。當(dāng)流量偏離設(shè)計(jì)流量時(shí)，進(jìn)口液流角p與葉片進(jìn)口角隊(duì)間的差記。為了提高泵的抗汽蝕性能，一般采用正沖角Ap(3°~10)。因?yàn)檎龥_角能增大葉片進(jìn)口角，減小葉片的彎曲，從而增加葉片進(jìn)口過流面積，降低葉片進(jìn)口處的C1和w1;另一方面,采用正沖角時(shí)，在非設(shè)計(jì)流量下,液體在葉片非工作面形成旋渦，由于這里是低壓,旋渦不易向高壓區(qū)擴(kuò)散，因而旋渦是穩(wěn)定的,對汽蝕影響較小。綜上所述,葉片進(jìn)口角應(yīng)在18°~-25°范圍內(nèi)。
就離心泵葉輪的結(jié)構(gòu)形式來看,可分為閉式.半開式和開式三種,如圖1- 58所示。
閉式葉輪具有蓋板和輪盤，流道是封閑的，如圖1-58(a)所示。這種時(shí)輪水力效率較高，適用于高揚(yáng)程，輸送潔凈的液體，半開式葉輪只有輪盤。流道是半開啟的:如圖，運(yùn)用于輸送含固體顆粒和雜質(zhì)的液體，它的葉片和輪盤可由整塊鍛件銑制成一個(gè)整體。強(qiáng)度較高，且制造較容易;開式葉輪既無蓋板，又無輪盤，流道完全敞開，如圖1 58(c)所示，常用來輸送漿狀或糊狀液體。離心泵葉輪還分為單吸式和雙吸式兩種,單吸式構(gòu)造簡單，液體從葉輪一側(cè)被吸入;雙吸式葉輪,如圖1- 58(d)所示,構(gòu)造比較復(fù)雜，液體從葉輪兩側(cè)吸入。顯然，雙吸式具有較大的吸液能力,抗汽蝕性能較好，而且基本上可以消除軸向力,適用于流量較大的情況。
另外，閉式葉輪和半開式葉輪后蓋板與泵殼之間的縫隙內(nèi)，液體的壓力較入口側(cè)高，這便產(chǎn)生了指向葉輪吸入口方向的軸向推力，使葉輪向吸入口竄動，引起葉輪與泵殼接勝處磨損。嚴(yán)重時(shí)造成渣漿泵振動。為此，可在后蓋板上鉆幾個(gè)小孔，稱為平衡孔，讓一部分高壓液體漏到低壓區(qū)，降低葉輪兩側(cè)的壓力差。這種方法雖然他便，但由于液體通過平衡孔短路回流，增加了內(nèi)泄漏量，因而降低了泵的效率。
Main parts of slurry pump
I. impeller
Impeller is the main part of centrifugal pump to transfer energy. The main requirements for impellers are: the single-stage impellers can give a larger theoretical lift of liquid, so as to adopt less stages when reaching the high lift, so as to make the machine structure compact, the efficiency of impellers is high, the resistance to steam blood is good, and the performance curve shape can meet the production requirements. The impeller structure and geometric parameters related to these requirements are analyzed below.
Most of the impellers of centrifugal pumps are backward curved impeller, only high-speed partial flow pump and vortex pump adopt radial impeller. The commonly used number of blades of backward curved impeller is generally between 6 and 12. For pumps with specific speed between 60-250, the blades are usually 6 pieces, 9 pieces for low specific speed pumps and 4-5 pieces for high specific speed pumps. Generally, increasing the number of blades can improve the flow of liquid and increase the head of pump properly; however, increasing the number of blades will increase the friction loss of blades and reduce the flow passage area, which will reduce the efficiency and easily lead to cavitation. On the contrary, if the number of blades is too small, the load of each blade will increase, the diversion effect on the liquid will decrease, and the head of the pump will decrease.
There are two types of blade shapes of centrifugal pump impeller: cylindrical surface blade and twisted blade. When the specific speed of centrifugal pump used in petroleum storage and transportation engineering is less than 90, it is easy to use cylindrical blade which is perpendicular to the front and back cover plate of impeller. Compared with the pump whose speed is more than 90, the twisted blade is often used.
The outlet blade angle P2a and the inlet blade angle B of the backward curved blade type impeller blade have an important influence on the performance of the pump. The blade angle B: u at the blade outlet is generally 20-30 for the stone elimination storage and transportation project between 15-40 °. Compared with the pump with lower rotation, the larger B2 can increase the lift and reduce the diameter D2, so as to reduce the disc friction loss and improve the efficiency of the pump; but when Bax is increased, the impeller outlet speed C is increased under the same flow rate; the hydraulic loss of the pressure chamber is increased, especially the impact gauge under the non design flow rate, so as to make the H-Q performance curve appear hump. In addition, when it has to be calibrated, the relative velocity W2 of the liquid flow in the blade channel is smaller, which makes the flow diffusion loss more serious. Therefore, in order to obtain a flat decline performance curve, it is not suitable to select a large total chamber,
The impeller inlet angle is the angle between the tangent line of the blade working surface (strictly speaking, it should be the tangent line of the blade bone line on the flow surface) and the tangent line of the circumference, which is usually determined by the direction angle of the absolute velocity C1 when the wave flows into the blade channel under the design flow. When the flow deviates from the design flow, the difference between the inlet flow angle P and the blade inlet angle is recorded. In order to improve the anti cavitation performance of the pump, the positive impact angle AP (3 ° ~ 10) is generally used. Because the positive impact angle can increase the inlet angle of the blade and reduce the bending of the blade, thus increasing the flow area at the inlet of the blade and reducing the C1 and W1 at the inlet of the blade; on the other hand, when the positive impact angle is adopted, under the non design flow rate, the liquid forms a vortex on the non working surface of the blade. Because it is low pressure, the vortex is not easy to spread to the high pressure area, so the vortex is stable and has little impact on cavitation. To sum up, the blade inlet angle should be in the range of 18 ° - 25 °.
According to the structure of centrifugal pump impeller, it can be divided into closed, semi open and open, as shown in figure 1-58.
The closed impeller has a cover plate and a disc, and the flow channel is sealed, as shown in Fig. 1-58 (a). This kind of impeller has high hydraulic efficiency, which is suitable for high lift and conveying clean liquid. The semi open impeller has only a disc. The runner is half open: as shown in the figure, it is used to transport liquid containing solid particles and impurities. Its blades and wheels can be milled into a whole by a whole forging. It has high strength and is easy to manufacture; the open impeller has neither cover plate nor disc, and the flow channel is completely open, as shown in Figure 158 (c), which is commonly used to transport slurry or paste liquid. The impeller of centrifugal pump is also divided into single suction and double suction. The single suction structure is simple, and the liquid is drawn in from one side of the impeller; the double suction impeller, as shown in figure 1-58 (d), has complex structure, and the liquid is drawn in from both sides of the impeller. Obviously, the double suction type has a larger suction capacity, better anti cavitation performance, and can basically eliminate the axial force, which is suitable for the case of large flow.
In addition, in the gap between the rear cover plate of closed impeller and semi open impeller and the pump housing, the liquid pressure is higher than that of the inlet side, which produces the axial thrust pointing to the suction port of the impeller, makes the impeller move towards the suction port, and causes the abrasion at the junction of the impeller and the pump housing. In serious cases, it will cause vibration of slurry pump. Therefore, several small holes can be drilled on the back cover plate, called balance holes, to let part of the high-pressure liquid leak into the low-pressure area, reducing the pressure difference on both sides of the impeller. Although this method is convenient, because the liquid is short circuited through the balance hole, the internal leakage is increased, so the efficiency of the pump is reduced.
