闭式叶轮具有盖板和轮盘,流道是封闲的,如图1-58(a)所示。这种时轮水力效率较高,适用于高扬程，输送洁净的液体,半开式叶轮只有轮盘。流道是半开启的:如图，运用于输送含固体颗粒和杂质的液体，它的叶片和轮盘可由整块锻件铣制成一个整体。强度较高，且制造较容易;开式叶轮既无盖板,又无轮盘,流道完全敞开,如图1 58(c)所示，常用来输送浆状或糊状液体。离心泵叶轮还分为单吸式和双吸式两种,单吸式构造简单，液体从叶轮一侧被吸入;双吸式叶轮,如图1- 58(d)所示,构造比较复杂，液体从叶轮两侧吸入。显然，双吸式具有较大的吸液能力,抗汽蚀性能较好,而且基本上可以消除轴向力,适用于流量较大的情况。
Main parts of slurry pump
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。