Inline centrifugal pump performance curve has three main curves: flow-lift curve, flow-power curve, flow-efficiency curve.
A. Flow-lift characteristic curve It is the basic performance curve of the centrifugal pump. Centrifugal pumps with a specific speed less than 80 have the characteristics of ascending and descending (both middle and convex) and the hump performance curve. A centrifugal pump with a specific speed between 80 and 150 has a flat performance curve. Centrifugal pumps with a specific number of revolutions above 150 have a steep drop performance curve. Generally speaking, when the flow rate is small, the lift is high, and as the flow rate increases, the lift gradually decreases.
B. Flow-power curve The shaft power increases with the flow. When the flow rate Q=0, the corresponding shaft power is not equal to zero, but is a certain value (about 60% of normal operation). This power is mainly consumed by mechanical losses. At this time, the water pump is full of water. If it is operated for a long time, the temperature inside the pump will rise continuously. The pump casing and the bearing will heat up. In severe cases, the pump body may be thermally deformed. We call it "squeaky head". The head is at the maximum value. When the outlet valve is gradually opened, the flow rate will gradually increase and the shaft power will increase slowly.
C. Flow-efficiency curve Its curve is like a hill shape. When the flow rate is zero, the efficiency is also equal to zero. As the flow rate increases, the efficiency gradually increases, but after increasing to a certain value, the efficiency decreases. The highest value, near the highest efficiency point, the efficiency is relatively high, this area is called high efficiency area
Eight major misunderstandings of pipeline centrifugal pumps:
1. Inline Centrifugal Pump Pipeline Matching Some pipe centrifugal pump users believe that this can improve the actual lift, in fact, the actual lift of the pump = total lift ~ loss head. When the pump model is determined, the total lift is certain; the loss head is mainly from the pipeline resistance. The smaller the diameter, the greater the resistance, so the larger the lift head is. Therefore, after the pipe diameter is reduced, the actual lift of the pump can not be increased. On the contrary, it will decrease, resulting in a drop in pump efficiency. In the same way, when the small-diameter water pump is pumped with a large water pipe, the actual lift of the water pump will not be lowered, but the loss head will be reduced due to the decrease of the resistance of the pipeline, so that the actual lift is improved. It is also believed that the small-diameter water pump will greatly increase the motor load when pumping water with a large water pipe. They think that the water in the water outlet pipe will have a large pressure on the pump impeller after the pipe diameter is increased, thus greatly increasing the motor load. As everyone knows, the magnitude of the liquid pressure is only related to the height of the lift, and has nothing to do with the cross-sectional area of the water pipe. As long as the head is fixed, the impeller size of the pump is constant, and the pressure acting on the impeller is constant regardless of the diameter of the pipe. Only when the pipe diameter is increased, the water flow resistance will decrease, and the flow rate will increase, and the power consumption will also increase appropriately. However, as long as it is within the rated head range, the pump can work normally regardless of the pipe diameter, and it can also reduce the pipeline loss and improve the pump efficiency.
2. High lift pump for low lift pumping Some pipeline centrifugal pump users believe that the lower the pump lift, the smaller the motor load. Under the misunderstanding of this misunderstanding, when the pump is purchased, the lift of the pump is often selected very high. In fact, for a centrifugal pump, when the pump model is determined, the power consumption is proportional to the actual flow of the pump. The flow rate of the pump will decrease with the increase of the lift, so the higher the lift, the smaller the flow and the smaller the power consumption. Conversely, the lower the lift, the greater the flow and the greater the power consumed. Therefore, in order to prevent motor overload, it is generally required that the actual pumping stroke of the pump should not be less than 60% of the nominal head. Therefore, when the high lift is used for low lift pumping, the motor is easily overloaded and heated, and in severe cases, the motor can be burnt. If it is used in an emergency, a gate valve for regulating the amount of water (or a small outlet with wood or the like) must be installed on the outlet pipe to reduce the flow and prevent the motor from being overloaded. Pay attention to the temperature rise of the motor. If the motor is overheated, turn off the flow of the small outlet or shut down. This is also easy to misunderstand. Some operators think that blocking the water outlet and forcibly reducing the flow will increase the motor load. In fact, on the contrary, the gate valve of the regular high-power centrifugal pump irrigation and drainage unit is equipped with a gate valve. In order to reduce the motor load when the unit is started, the gate valve should be closed first, and then the gate valve is gradually opened after the motor is started.
3. There are many elbows on the inlet pipe of the inline centrifugal pump. If there are many elbows on the inlet pipe, it will increase the local flow resistance. And the elbow should turn in the vertical direction, and it is not allowed to turn in the horizontal direction to avoid collecting air.
4. When installing the water inlet pipe, the horizontal section is horizontally or upwardly tilted. This will cause air to collect in the water inlet pipe, reduce the vacuum degree of the water pipe and the water pump, and reduce the water absorption head of the water pump and reduce the water discharge. The correct way is: the horizontal section should be slightly inclined to the water source, should not be horizontal, and should not be lifted upwards.
5. The inlet of the centrifugal pump of the pipeline is directly connected to the elbow. This will cause uneven distribution of water flow through the elbow into the impeller. When the inlet pipe diameter is larger than the water pump inlet, an eccentric reducer should be installed. The flat part of the eccentric reducer is to be mounted on the upper part, and the inclined part is placed below. Otherwise, the air is collected, the amount of water is reduced or the water is not pumped, and there is a crash. If the inlet pipe and the water inlet of the pump are equal in diameter, a straight pipe shall be added between the water inlet and the elbow of the pump. The length of the straight pipe shall not be less than 2 to 3 times the diameter of the water pipe.
6. The outlet pipe of the pipeline centrifugal pump is above the normal water level of the outlet. If the outlet is above the normal water level of the outlet, the pump head is increased, but the flow is reduced. If the water outlet must be higher than the water level of the pool due to terrain conditions, the elbow and short pipe should be installed at the nozzle to make the water pipe siphon type and reduce the height of the water outlet.
7. The inlet pipe is not in the correct position.
A. The inlet of the inlet pipe is farther from the bottom of the pool and the wall of the pool than the diameter of the inlet. If there is dirt such as sediment at the bottom of the pool, the distance from the bottom of the pool to the bottom of the pool is less than 1.5 times the diameter, which may result in poor water intake during pumping or inhalation of debris and blockage of the water inlet.
B. When the water inlet of the inlet pipe is not deep enough, this will cause a vortex on the water surface around the inlet pipe, affecting the inflow and reducing the amount of water. The correct installation method is: the water depth of the small and medium-sized water pump should not be less than 300 ~ 600mm, and the large water pump should not be less than 600 ~ 1000mm.
8. When the bottom valve is set, the bottom section of the inlet pipe is not vertical. If installed, the valve cannot be closed by itself, causing water leakage. The correct installation method is: the inlet pipe with the bottom valve, the bottom section is preferably vertical. If it cannot be installed vertically due to terrain conditions, the angle between the water pipe axis and the horizontal plane should be 60° or more. Pipeline centrifugal pump is the most widely used and widely used pump type in various fields. The majority of users have learned the eight misunderstandings of the pipeline centrifugal pump and try to avoid the above. Changsha Zhonglian Pump Industry Co., Ltd. is a professional manufacturer of pipeline centrifugal pumps with rich production experience and perfect testing equipment to ensure the stability, reliability and long service life of product quality.
