How to Choose a Centrifugal Pump?
A centrifugal pump is a mechanical device designed to move fluid by transferring rotational energy from one or more driven rotors, called impellers. Fluid enters the rapidly rotating impeller along its axis and exits through the impeller's blade tips along its circumference by centrifugal force. The action of the impeller increases the velocity and pressure of the fluid and directs it towards the pump outlet. The pump casing is specially designed to compress the fluid from the pump inlet, direct it to the impeller, and then slow and control the fluid before discharging.
Basis for selection
Each type of centrifugal pump has a certain scope of application and conditions of use. First of all, the type of pump should be determined according to the nature of the liquid to be transported and the requirements of production conditions, and then the specific model of the pump should be further determined. The centrifugal pump has the characteristics of high speed, small size, light weight, high efficiency, large flow, simple structure, no pulsation of infusion, stable performance, easy operation and convenient maintenance.
Therefore, except for the following situations, centrifugal pumps should be used as far as possible:
- When there is a metering requirement, choose a metering pump;
- When the lift requirement is very high, the flow rate is small and there is no suitable small flow and high lift centrifugal pump, the reciprocating pump can be selected, and the vortex pump can also be selected when the cavitation requirement is not high;
- When the lift is very low and the flow rate is large, axial flow pump and mixed flow pump can be selected;
- When the viscosity of the medium is relatively large (greater than 650~1000mm2/s), a rotor pump or a reciprocating pump (gear pump, screw pump) can be considered;
- When the air content of the medium is 75%, the flow rate is small and the viscosity is less than 37.4mm2/s, the vortex pump can be selected;
- For occasions with frequent startup or inconvenient pump filling, pumps with self-priming properties, such as self-priming centrifugal pumps, self-priming vortex pumps, and pneumatic (electric) diaphragm pumps, should be selected.
Steps of selecting centrifugal pump
- Determine the physical and chemical properties of the liquid being transported:
The physical and chemical properties of the liquid include temperature, viscosity, density, saturated vapor pressure, corrosiveness and toxicity, etc., whether it contains solid particles or bubbles. Only then can the type and model of the pump be determined, the material of the pump parts, the type of seals, and the measures to prevent pump corrosion and cavitation.
- Determine the flow rate of the pump:
According to the requirements of production conditions, calculate the amount of liquid that needs to be transported per unit time, and add a certain margin (generally 5%-10%) as the flow rate of the centrifugal pump.
- Calculate the lift of the pump:
According to the flow rate of the pump and the condition of the pipeline and device, calculate the liquid resistance loss of the pipeline, find out the lift required by the pump, and add a certain margin (5%-10%) as the basis for selecting the pump.
The rated flow generally adopts the maximum flow directly. If it is missing, it adopts 1.1-1.15 times of the normal flow. The rated lift is 1.05-1.1 times the lift required by the device. For media with viscosity greater than 20mm or containing solid particles, it needs to be converted into the rated flow and head when conveying clean water. According to the rated flow and head, find out the initially selected pump model, there may be several. Check whether the rated work of the pump falls within the high-efficiency working area of the pump according to the performance curve; check whether the NPSHA-must NPSH of the pump meets the requirements.
- Correction of viscous liquid:
If the kinematic viscosity of the liquid to be transported is less than 20cSt (1cSt=1mm2/s), it is not necessary to correct it because the flow rate and lift of the pump do not change much. According to the flow rate and head of the pump and the physical and chemical properties of the liquid, a type of centrifugal pump is initially selected on the series diagram (spectrogram) of a certain type of pump.
- Find the working point of the pump:
On the series diagram of centrifugal pump, select the characteristic curve of a certain type of centrifugal pump and draw the characteristic curve of the connecting pipeline of the pump to obtain the working point. See if the pump is operating in the higher efficiency range. If the flow and head of the selected centrifugal pump are slightly higher, it can be used as a margin for the pump, and the flow and head can be adjusted by adjusting the opening of the outlet valve. If the selected pump flow and head are too large, the cutting law method can be used to reduce the flow and head of the pump. If the selected pump operating point is not within the allowable range of efficiency, another type of centrifugal pump should be selected.
- Determine the installation height of the pump:
After the model of the centrifugal pump is determined, according to the local atmospheric pressure or the liquid surface pressure of the liquid to be conveyed, the temperature of the liquid and the saturated vapor pressure of the liquid at this temperature, the allowable vacuum height or allowable cavitation given on the centrifugal pump sample margin, determine the installation height of the pump.
- Determine the spare rate of the pump:
When one pump can meet the production requirements, one pump should be used as far as possible, and the parallel operation of two or more pumps is generally less efficient, which is uneconomical from the point of view of "energy saving". However, when the flow rate is large, one pump cannot meet the production needs or a spare pump is required, two or more pumps can be used in parallel. When the starting current of the large pump is too large and has an impact on the power system, two smaller pumps are often used in parallel and started one by one.
Standby rate refers to the ratio of the number of standby pumps to the number of operating pumps. For example, if two operating pumps are connected in parallel, one is operating and the other is standby, and the standby rate is 100%. If three pumps are connected in parallel, and two operate and one is standby, the standby rate is 50%.
The standby rate of the pump should comprehensively consider the influence of various factors, including the nature of the liquid, the technical requirements, the reliability of the pump itself, the difficulty of maintenance, the price of the pump, and so on. For example, the centrifugal oil pump of the heating furnace of the kerosene plant or the boiler home water pump of the thermal power plant, if the pump is not allowed to be stopped at any time during operation, one pump must operate one pump for standby, that is, the standby rate is 100%. For conveying liquids containing solid particles, because the parts of the pump are easily damaged, the pump is often shut down for maintenance, so a 100% backup rate must also be used. For intermittent operation equipment or when the pump can be stopped at any time, the standby pump can be omitted to reduce investment.