When the speed n is constant, list the functional relationships of the head (H), shaft power (N), efficiency (η), and the allowable suction vacuum height (HS) with the flow rate (Q), namely: H=f( Q); N=F(Q); Hs= Ψ(Q); η = φ(Q), we use curves to express these equation relationships, and call these curves the characteristic curves of centrifugal pumps. The effective head H, shaft power N and efficiency η of the centrifugal pump are all related to the infusion flow Q, which are the main performance parameters of the centrifugal pump.
The characteristic curve of the centrifugal pump is determined by the design intention and actual test at a certain speed. The three characteristic curves of H-Q, η-Q and N-Q are measured by the pump manufacturer before leaving the factory, which is a certain type of centrifugal. The characteristic curve of the pump at a certain speed is listed in the product catalog for user reference. The characteristic curve of a centrifugal pump is usually drawn with Cartesian first coordinate system. The abscissa represents the flow rate of the pump, and the ordinate represents the head of the pump. The flow and head curve of a specific centrifugal pump is a downward curved line, indicating that the pump head decreases and its flow increases.
The characteristic curve of the centrifugal pump reflects the change law of the basic performance of the pump, and can be used as the basis for pump selection and use. The characteristic curve of different pumps is different, and the characteristic curve of the pump is provided by the equipment manufacturer. Strictly speaking, every pump has a specific characteristic curve. On the pump characteristic curve, a set of head, shaft power, and efficiency values corresponding to any flow point can be found. This set of corresponding parameters is usually called working conditions, which correspond to a set of working conditions at the highest efficiency point. The condition is called the best condition. Although the characteristic curves of various types of centrifugal pumps are different, they all have a common changing trend.
(1) H-Q curve (flow-head curve) shows the relationship between pump head and flow. The pressure head of a centrifugal pump generally decreases as the flow rate increases. The centrifugal pump with a specific speed of less than 80 has the characteristics of rising and falling (that is, the middle is convex, the two sides are bent down), which is called the hump performance curve. The centrifugal pump with a specific speed between 80 and 150 has a flat performance curve. Centrifugal pumps with specific speeds above 150 have a steep drop performance curve. Generally speaking, when the flow rate is small, the head is high, and the head gradually decreases as the flow rate increases.
(2) N-Q curve (flow-power curve) shows the relationship between pump shaft power and flow. The shaft power of the centrifugal pump increases with the increase of the flow, and the shaft power is the smallest when the flow is zero. Therefore, when the centrifugal pump is started, the outlet valve of the pump should be closed to reduce the starting current and protect the motor. The shaft power increases with the flow. When the flow Q=0, the corresponding shaft power is not equal to zero, but a certain value (about 60% of normal operation). This power is mainly consumed in mechanical losses. At this time, because the water pump is full of water, if the centrifugal pump has been running for a long time, the temperature in the pump will continue to rise, and the pump casing and bearings will also heat up. In severe cases, the pump body may be thermally deformed. At this time, the lift When the maximum value is reached, the water outlet valve is gradually opened at this time, the flow rate will gradually increase, and the shaft power will slowly increase.
(3) η-Q curve (flow-efficiency curve) shows the relationship between pump efficiency and flow. The flow-efficiency curve is shaped like a hill. When the flow is zero, the efficiency is also equal to zero. As the flow increases, the efficiency gradually increases, but after increasing to a certain value, the efficiency decreases. At this time, the efficiency has a maximum value. Near the most efficient efficiency point, the efficiency is relatively high, and this area is called the high efficiency area.
It can be seen from the characteristic curve that when Q=0, η=0; as the flow rate increases, the efficiency of the pump rises and reaches a maximum value. After the flow rate increases again, the efficiency decreases. It shows that the centrifugal pump has the highest efficiency point at a certain speed, which is called the design point. The pump works most economically at the flow rate and pressure head corresponding to the highest efficiency, so the Q, H, and N values corresponding to the highest efficiency point are called the best working condition parameters. The performance parameters marked on the nameplate of the centrifugal pump refer to the condition parameters of the highest efficiency point of the pump during operation. According to the requirements of conveying conditions, centrifugal pumps are often impossible to operate at the optimal operating point, so generally only a working range can be specified, which is called the high efficiency zone of the pump, which is usually about 92% of the highest efficiency, as shown in the figure. In the range indicated by the number, when choosing a centrifugal pump, the pump should work within this range as much as possible.
