Axial flow pump is a kind of high specific speed (500 ~ 1200) vane pump, the flow of large head low flow rate of about 0.1 ~ 50 m ³ / s range, head is generally less than 25 meters; most in 4 to 15 meters . Flow in the role of rotating wing blades, resulting in axial movement along the axle. Because of its blades like a propeller, it is also called a propeller pump. In axial flow pumps, the flow of water acts like a movement on a helical surface, that is, on the one hand, along the shaft and on the other hand, the impeller rotates. The rotating water flow with tangential velocity that flows out of the impeller, if it goes directly into the pipe, is completely lost in the kinetic energy of this part of the rotation. Therefore, it is necessary to eliminate the rotational movement of the liquid and convert its kinetic energy into pressure energy so as to achieve the purpose of improving the efficiency of the water pump. Therefore, the guide vane is provided. The number of vanes is generally one or less than the number of impeller blades. While the number of impeller blades is related to the number of revolutions, and the number of blades is Z = 5 ~ 6 for the low specific speed axial-flow pump (ns = 500 ~ 600) Z = 4; high specific speed axial flow pump (ns> 1000), Z can take three or two. For adjustable axial-flow pump impellers (ie blades can rotate) Z> 4 causes difficulties in turning the mechanism. The distance between the guide vane inlet side and the exit side of the impeller also has certain requirements, generally 0.1D, D is the guide vane diameter; if this distance is too small, the axial pump operation is unstable, and if the distance is too large, it increases Hydraulic loss. The theoretical head produced by an axial-flow pump is very similar to a centrifugal pump, but considering the same axial velocity at the outlet of the axial-flow pump as the inlet, there are: H∞ = u2 (v2u-v1u) / g If the inlet , Then the above formula is: H∞ = u2v2u / g The theoretical flow is: QT = vF Where v-- liquid in the impeller axial speed, m / s; F-- liquid cross-sectional area at the exit, . Axial flow pump work, cavitation will occur. That is, when the pressure on the back of the blade drops to a saturation pressure lower than the working water temperature, the liquid begins to evaporate to generate bubble. When the bubble enters the region with higher pressure along the streamline, the pressure rapidly shrinks to generate a hydraulic impact and the pressure on the blade surface Cause serious erosion damage. When the area of ​​the bubble expands further, the back of the blade will be completely covered by the bubble. At this time, the elimination of the bubble is not on the blade but behind the blade so that there is no erosion on the blade. However, at this moment, the bubble blocks the blade Between the channel, so the pump flow, pressure, efficiency, etc. are declining, and produce noise and vibration, damage to the normal work of the pump. Axial flow pump can be installed upright, horizontal or inclined. Compared with the centrifugal pump, the axial flow pump has the advantages of simple and compact structure, light weight, high efficiency, and can transport dirty liquids. Large flow axial pump, the blade can also be designed to adjust the type, the external load changes, changing the blade mounting angle, thus changing the performance of the pump, so that the pump has a better economy. Axial flow pump head small and large flow in the power plant is mainly used for circulating pumps, and the use of more widespread. However, axial-flow pump installation and maintenance work is more complex than horizontal centrifugal pumps. Axial flow pump characteristics of the curve are as follows: 1, axial flow pump Q-H characteristic curve at the blade mounting angle steepness, turning point (K point). This characteristic curve, for the condenser tube running in the accumulation of resistance and increased resistance, can still meet the traffic flow requirements substantially unchanged. However, due to the steepness is too large, the axial pump power or with the decrease of water sharp increase. 2, axial flow pump work most favorable range is not small, because in the efficiency curve, once left the highest efficiency point, no matter left or right, the efficiency drops rapidly. 3, axial pump power decreases with the delivery volume increases, which is completely different from the centrifugal pump. Pump idle power at power even more than 2 times the normal case of power, so you must use a larger capacity motor, and also easy to overload when starting. Therefore, the axial flow pump should be activated when the outlet valve is opened, and it is not allowed to adjust the water supply with the small outlet valve in operation.
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