Centrifugal pump working principle and performance characteristics 3-1-1 Centrifugal pump working principle The main working parts are the impeller and the pump housing.The impeller is usually composed of 5 to 7 arc blades and the front and rear circular cover. Use the key and the nut to fix the pump shaft at one end.The nut for the fixed impeller usually adopts the left-hand thread to prevent the repeated starting from loosening due to the inertia.The other end of the shaft extends through the stuffing box and extends out of the pump housing, driven by the prime mover. Helical, also known as the spiral shell or volute Figure 3-1 cantilever single-stage centrifugal pump 3-1-1 centrifugal pump works Full of liquid in the pump with the impeller rotation, resulting in centrifugal force, thrown to the surrounding The formation of low pressure in the center of the impeller, the liquid is sucked into the impeller under the action of the liquid surface pressure. The liquid flowing out of the impeller is increased in pressure and speed, and the volute - convergence and diversion. The diffuser A increases and the flow velocity decreases, Kinetic energy into pressure energy, and then discharged. Impeller non-stop rotation, suction row on the continuous increase of liquid through the pump energy is the result of the prime mover through the impeller on the liquid work .3-1-2 liquid in the impeller Flowing To simplify the study, we assume that: (1) the liquid consists of an infinite number of exactly the same The flow trajectories of all the liquid particles formed by the unit flow are the same and all agree with the section of the blade, and the flow states of the liquid particles are the same on the same radius, only when the blade has infinitely many blades, the thickness is infinitely thin and the cross-sectional shape is exactly the same The ideal impeller can be achieved. (2) the liquid flow without friction, impact and eddy current loss Set the liquid is non-sticky ideal liquid, the flow in a collision-free whirlpool ideal conditions 3-1-2 centrifugal pump lift The impeller that causes the impeller to drive the liquid at high speed and transfer the mechanical energy to the delivery lift or delivery head is closely related to the size and speed of the impeller, and the flow rate obviously changes with the working lift. It is necessary to study and decide the lift of the centrifugal pump Various factors and the relationship between head and flow That lift the equation of the centrifugal pump 3-1-2 liquid flow in the impeller Impeller to the rotation, the liquid particles have two kinds of movement: peripheral speed - with the speed of impeller movement with u said; relative speed - with respect to the impeller speed of movement, expressed in w, which is tangent to the blade profile absolute speed - relative to the pump housing movement speed; is u and w and the vector path of liquid mass entering and leaving the impeller can be represented by AC in the figure.3-1-2 Flow of liquid in the impeller The three velocity vectors u, w and c of any particle in the impeller are Constitute a speed triangle, as shown in Figure C and u between the angle between w and u that is used to represent the opposite direction; C peripheral velocity C with Cu that the radial velocity of Cr is expressed as 3-1- 2 liquid flow in the impeller under the symbol subscript 1, refers to the parameters of the impeller inlet plus subscript 2, refers to the parameters at the exit of the impeller.In the impeller, the speed of the triangle in the direction of u, w have been identified , And U = nD / 603-1-2 flow of liquid in the impeller D - the diameter of the impeller where the particle is located, mm; B - the width of the impeller where the particle is located, in m; - the coefficient of displacement Is 0.75 ~ 0.95), in order to consider the blade thickness to reduce the cross-sectional area of ​​the flow channel; v - volumetric efficiency of the pump.It can be seen that when the flow rate, speed and size of the impeller are established, the velocity triangles To determine .3-1-2 lift equation According to the knowledge of liquid mechanics, we can introduce lift equation: from the figure (blade exit angle on the theory We can conclude that the head depends mainly on the impeller diameter and the theoretical value of the closed head of the pump (Q = 0) as: Ht = u2 / g, to increase H, it must be increased Large D2 or increased nD2 related to the pump profile and weight n limited by the pump cavitation performance Centrifugal pump n generally not exceed 8000 ~ 10000r / min single-stage pump H usually does not exceed 150m Centrifugal pump head changes with the flow rate When using radial blades, that is, 2 = 90, that is, H and Q have nothing to do with backward bending blades, that is, 2 0, Q increases, then Ht decreases. When using forward bending blades, ie 2> 90, Q increases Then Ht increase 3-1-2 lift equation more than three cases the size and n the same centrifugal pump, Q the same time, 2 (forward bend) the greater the higher the H, the surface to the front curved blade is appropriate , Taking into account the various losses, multipurpose back bending blade Ht has nothing to do with the nature of the fluid transported (character) If the pump is air, the air density is only 1/800 of the water, the pressure difference For example, a water pump with H of 100m, which delivers the same H gas when it is being vented, only produces a pressure differential of 1.268kPa between the suction and discharge ports, which at atmospheric pressure only sucks up about 12.9cm Centrifugal pump has no self-absorption capacity Figure 3-5 Centrifugal pump constant speed characteristic curve theory analysis 3-1-3 Flow rate-head curve Ht and Qt are downhill straight line Ht and Qt is also downhill straight line (the slope is small) there is friction, Vortex, impact and other hydraulic losses along the path along the friction loss and flow rate (flow) is proportional to the non-design conditions into and out of the impeller impact loss, (design conditions = zero) Qt-H curve in order to reduce the head loss of these two parts 3-1-3 Flow-Head Curve ηv Seal Leakage Internal Leakage and Shaft Seal External Leakage due to Multistage Leakage Multistage Leakage There are also interstage leaks when the pump is equipped with a balance hole (pipe) or balance disc, with additional Of the total volume loss of the total amount of leakage is generally 4% to 10% of the theoretical flow Q-H curve to consider the loss of leakage flow g 3-1-3 Flow - power curve Based on Qt and Ht, find the pump hydraulic power Ph = ÏgQtHt can make a Qt-Ph curve.If the Ph plus mechanical friction power loss, you can get the theoretical flow and shaft power curve Qt-P and then Qt-P curve of the Qt value minus the corresponding Of the leakage flow g, you can get the actual flow and shaft power curve Q-P3-1-3 Flow - Power Curve Mechanical damage Loss include: shaft seal and bearing mechanical friction loss accounted for 1% to 5% of shaft power, with mechanical shaft seal less loss; impeller disk friction loss is the cover to make both sides of the fluid due to centrifugal force and the formation of reflux The resulting energy loss is about 2% to 10% of the shaft power, which is proportional to the square of the fifth power of the impeller D2 and n. Increasing n and correspondingly decreasing the impeller outer diameter (constant H) reduces the disk Friction loss .3-1-3 Flow-efficiency curve The efficiency at each flow rate is calculated from Q-H curve and Q-P curve. Η = ÏgQH / P Then, the relation curve Q-η is obtained. Figure 3-6 Centrifugal pump Constant-speed characteristic curve 3-1-3 Actual-measured constant-speed characteristic curve The actual constant-speed characteristic curve is determined by the manufacturer through experiments. (1) Centrifugal pumps use backward bending blades, and their Q-H curve trend declines. Due to the different exit angles of the blades, the curve shapes can be divided into three types: 3-1-3 Measured constant speed characteristic curve Steep descending (high specific speed) Blade exit angle is small, and Q changes little when H changes H changes do not want Q change occasions (bilge pump ballast pump, etc.) flat (medium low pump) blade exit angle is slightly larger, H change Q change larger for those who often need to To adjust the Q but do not want to throttle loss is too large (condensate pump, boiler feed pump) 3-1-3 measured constant speed characteristic curve Hump peak blade exit angle larger Q-H curve is relatively flat, and In the small Q when the impact loss is large, so the Q-H curve will appear hump hump Q-H curve of the pump may occur during the work surge should be avoided as far as possible to limit the use of appropriate export leaves and leaves the blade angle, you can avoid Hump ​​3-1-3 Measured constant speed characteristic curve (2) QP curve is tilted upwards, ie the shaft power increases with increasing Q. Minimum shaft power (35% ~ 50%) at Q = 0 The pump's H (Also known as closed head) is not very high Pump off the discharge valve Lower starting current can reduce the voltage fluctuations But closed operation, the efficiency is zero, the pump will heat 3-1-4 pipeline characteristics and pump Point liquid flow through the pipeline required head pressure and flow between the functional relationship between the two parts including the first position, pressure head, and the flow has nothing to do Consumption to overcome the pipeline resistance curve A is to show the relationship between the above characteristics of the pipeline The general shape of the curve 3-1-4 pipe characteristics curve and the pump operating point Static pressure head Hu is a level Pipe resistance h = Q2, a parabola is dependent on the inclination of the hydrostatic head resistance depends on the ordinate line starting position when the pipeline resistance changes, such as increasing the value of K, as the curve becomes steeper
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