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Relationship Between Water Pump Flow and Lift

Published on 2020/02/21

The pump flow and lift are important parameters for examining the performance of the pump. Correctly understanding the relationship between these parameters can help choose the most suitable pump model.
chemical centrifual pumps

What is flow, head, and power?

Flow: The flow of the pump is also called the water delivery volume, which refers to the amount of water that the pump delivers in a unit of time. It is represented by the symbol Q, and its units are liters/second, cubic meters/second, and cubic meters/hour.

Head: The head of a pump refers to the height at which the pump can lift water. It is usually represented by the symbol H, and its unit is meter. The lift of the centrifugal pump is based on the centerline of the impeller and is divided into two parts. The vertical height from the centerline of the pump impeller to the water surface of the water source, that is, the height at which the pump can suck up the water, is called the suction lift, or simply the suction lift.

The vertical height from the centerline of the pump impeller to the water surface of the water tank, that is, the height that the pump can press the water up, is called the pressure head, referred to as the pressure head. That is, pump head = suction head + pressurized water head It should be noted that the head marked on the general pump refers to the head generated by the pump itself, and it does not include the lost head caused by frictional resistance of the pipeline flow. When choosing a water pump, pay attention not to be ignored. Otherwise, water will not be pumped.

Power: The amount of work done by the machine in a unit of time is called power. It is usually represented by the symbol N. Commonly used units are: kg • m / s, kilowatts, horsepower. Generally, the power unit of an electric motor is expressed in kilowatts; the power unit of a diesel or gasoline engine is expressed in horsepower. The power transmitted by the power machine to the pump shaft is called shaft power, which can be understood as the input power of the pump. Generally speaking, the pump power refers to the shaft power.

The performance curve of centrifugal pump flow and head

The main design and operation parameters of centrifugal pumps are flow and head. The design technical parameters should be close to or close to the operating process parameters. When the pump will affect each other between these two parameters, all types of pumps and pumps of various specifications and models have their own characteristic curves, as shown below:
Performance curve of centrifugal pump flow and head

The actual operation of the pump should try to work in a state of high efficiency. When the design (model of the pump) is determined, if the actual head of the pump is too high, it will not only reduce the efficiency of the pump, but also seriously affect the actual flow of the pump. Conversely, if the pump head is selected too high and the actual operation head is too low, it will also affect the efficiency of the pump and cause excessive flow during actual operation.It is also likely to increase the power of the pump and exceed the rated current of the motor to generate heat.

Factors Affecting Pump Head

Head and flow, as well as power, are very important reference indicators for selecting a pump. Some people think that a high head is good. This view is not accurate. The height of the head is affected by many aspects.
The lift height of the pump depends on the structure of the pump, such as the diameter of the impeller, the bending of the blades, the speed, etc. The original height of the lift is determined by these. Then in practice, especially in the process of laying the pipeline, What is the effect of the head and flow? I believe everyone knows that if the pump is unchanged, if the height of the drainage using a large water pipe is definitely not as high as that of a smaller water pipe, the elbow of the pipe will also What also affects is the slope of the pipeline, so in general, when choosing, you must understand it in detail to avoid the situation where the water cannot be pumped.

Similarly, we must also understand that the flow and the head are inversely proportional. At different heights, the flow is not the same, especially when the highest height of the head is reached, there is no flow, and no water can be discharged at this time.
water pump
The lift of a centrifugal pump is used to overcome height and resistance. When a high lift pump works at a high lift point, its flow is the design point. The flow of the pump will increase, the motor will be overloaded, and it will burn the motor if it exceeds a certain level. For example, a feed pump has a head of 50 meters and a flow of 50 cubic meters per hour. When it feeds water to a height of 50 meters, its flow is 50 cubic meters. The height and resistance reduce its flow rate may reach more than 80-90 cubic meters hours, at which time the motor will heat up or burn out.

Many users believe that the lower the pumping head, the smaller the motor load. Under this misunderstanding and misunderstanding, when choosing a water pump, the pump head is often selected very high. In fact, for centrifugal pumps, 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 increase with the increase of the pump head, and the cause of the motor burning will be reduced due to the increase of the pump head. Therefore, the higher the pump head, the smaller the flow rate, and the smaller the power consumption. Conversely, the lower the head, the larger the flow, and the greater the power consumed.
Therefore, in order to prevent the motor from being overloaded, it is generally required that the actual pumping head of the pump should not be lower than 60% of the nominal head. Therefore, when the high head is used for pumping at too low head, the motor is easily overloaded and generates heat, and the motor can be burned under severe conditions. For emergency use, you must install a gate valve on the water outlet pipe to adjust the water output (or block the small water outlet with wood or other objects) to reduce the flow and prevent the motor from overloading.

Reminder: Pay attention to the temperature rise of the motor. If you find that the motor is overheated, you should turn off the flow of the water outlet or shut down in time. This point is also easy to cause misunderstanding. Some people think that blocking the water outlet and forcibly reducing the flow will increase the load on the motor. In fact, the opposite is true. The outlet pipes of regular high-power centrifugal pump drainage and irrigation units are equipped with a gate valve. In order to reduce the load of the motor when the unit starts, the gate valve should be closed first, and the gate valve should be gradually opened after the motor starts.

Pump head and flow calculation formula

Pump head H = (P2-P1) / ρg + (v2-v1) 2 / 2g + (z2-z1) + hf

H——Head, m;

P1, P2-the pressure of the liquid at the pump inlet and outlet, Pa;

v1, v2——the flow velocity of the fluid at the pump inlet and outlet, m / s;

z1, z2-height of inlet and outlet, m;

hf-pipeline and local pressure loss

ρ——Liquid density, kg / m3;

g——gravitational acceleration, m / s2.

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