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Selection of centrifugal pump

Update:23-04-2020
Summary:

Centrifugal pump is to produce pressure by the centrifu […]

Centrifugal pump is to produce pressure by the centrifugal force of the impeller agitating the fluid rotation and to deliver the fluid. In the selection of centrifugal pump, to determine the purpose and performance of the pump and select the pump type. This choice must start with the selection of the type and form of the pump, then what principle should be adopted to select the pump? What is the basis?
1、 Pump selection principle
1. Make the type and performance of the selected pump meet the requirements of flow, head, pressure, temperature, cavitation flow, suction head and other process parameters of the device.
2. High mechanical reliability, low noise and small vibration
3. In economy, the total cost of equipment cost, operation cost, maintenance cost and management cost should be considered comprehensively.
4. Centrifugal pump is characterized by high speed, small volume, light weight, high efficiency, large flow, simple structure, no pulsation of infusion, stable performance, easy operation and maintenance.

Therefore, in addition to the following situations, centrifugal pumps should be selected as far as possible: when there are metering requirements, the metering pump with high head requirements, small flow and no suitable high head centrifugal pump with small flow can be selected, the reciprocating pump can be selected, and the vortex pump can be selected if the cavitation requirements are not high. When the head is very low and the flow is very large, axial flow pump and mixed flow pump can be selected. When the viscosity of medium is large (more than 650-1000mm2 / s), the gas content of medium of rotor pump or reciprocating pump (gear pump and screw pump) can be considered to be 75%, the flow is small and the viscosity is less than 37. When 4mm2 / s, vortex pump can be selected. In case of frequent start-up or inconvenient pump filling, pumps with self-priming performance shall be selected, such as self-priming centrifugal pump, self-priming vortex pump and pneumatic (electric) diaphragm pump.
2、 Pump selection shall be based on the basis of pump selection, process flow, water supply and drainage requirements, and shall be considered from five aspects, namely, liquid delivery volume, device head, liquid property, pipeline layout, operation and operation conditions, etc
1. Flow rate is one of the important performance data of the pump, which is directly related to the production capacity and delivery capacity of the whole plant. For example, the design institute can calculate the normal, minimum and maximum flow of the pump in the process design. When selecting the pump, the maximum flow shall be taken as the basis and the normal flow shall be considered. When there is no maximum flow, 1 of the normal flow shall be taken generally. 1 time as the maximum flow.
2. The head required by the device system is another important performance data of the pump selection, which is generally selected by enlarging the head by 5% - 10% margin.
3. Liquid properties, including liquid medium name, physical properties, chemical properties and other properties, physical properties include temperature C, density D, viscosity u, solid particle diameter in the medium and gas content, etc., which involves system head, effective cavitation allowance calculation and appropriate pump type: chemical properties, mainly referring to chemical corrosion and toxicity of liquid medium, are pump materials and An important basis for selecting the type of shaft seal.
4. The pipeline layout conditions of the device system refer to some data such as delivery height, delivery distance, delivery direction, suction side minimum liquid level, discharge side maximum liquid level, pipe specification and length, material, pipe fitting specification, quantity, etc., so as to facilitate the calculation of comb head and the check of NPSH.
5、 There are many operating conditions, such as liquid operation T saturated steam force P, suction side pressure PS (absolute), discharge side vessel pressure PZ, altitude, ambient temperature operation is intermittent or continuous, pump position is fixed or movable.
3、 Through the analysis of the failure cases caused by the improper selection of the head of the circulating water pump in the central air conditioning system engineering, the importance of the reasonable selection of the head of the circulating water pump is emphasized, and some selection methods are put forward, which is of reference value to the design of the central air conditioning system.

1. In the central air-conditioning system, the circulating water pump delivers chilled water in summer and hot water to the air-conditioning terminal device in winter. The engineering design shall select the water pump with good performance according to the water flow and system resistance of the air conditioning system. There are detailed design calculation methods in the HVAC design manual. The problem is that in the actual engineering design, some engineers did not design and calculate according to the calculation method, but took it for granted based on experience, lack of careful research on the system, some air conditioning equipment, accessories and other new products, resulting in the selected water pump can not meet the requirements, or
As a result, the operation cost increases, even the water pump can not work normally, which has to be paid great attention by the air conditioning designers.
2. Theoretical analysis the water flow of air conditioning system is determined by load and water temperature difference between supply and return, and the system resistance is obtained by hydraulic calculation. The pump selected according to the flow and resistance should be in the high efficiency area during operation, and its working point is the intersection of the pump performance curve and the pipeline performance curve. However, there are often two kinds of abnormal pump selection steps, methods and selection requirements in engineering. The so-called reasonable pump selection is to comprehensively consider the comprehensive technical and economic indicators such as the investment and operation cost of pump unit and pump station, so as to make them conform to the principles of economy, safety and applicability. Specifically, there are several aspects as follows: 1. It must meet the requirements of using flow and head, that is, it is required that the operation work points of the pump (the intersection of the device characteristic curve and the pump performance curve) are often kept in the high-efficiency section, so as to save power and not easily damage the parts. The selected pump should not only be small in volume, light in weight and cheap in cost, but also have good characteristics and high efficiency. It has good anti cavitation performance, which can not only reduce the excavation depth of the pump room, but also prevent the pump from cavitation, so that the operation is stable and the service life is long. According to the selected pump station, the project investment is small and the operation cost is low. 2. Selection step A. list the basic data: medium characteristics: medium name, specific gravity, viscosity, corrosiveness, toxicity, etc. The particle diameter and content of the factor contained in the medium. Medium temperature: (℃) the required flow of industrial pumps can ignore the leakage in the pipeline system in the process flow, but the influence of process change on the flow must be considered. If agricultural pump is to use open channel for water delivery, leakage and evaporation must also be considered. Pressure: suction tank pressure, drainage tank pressure, pressure drop in the piping system (head loss). Pipeline system data (pipe diameter, length, type and number of pipeline accessories, geometric standard from suction tank to pressure tank, etc.).
If necessary, the device characteristic curve shall also be made. When designing and arranging pipelines, attention shall be paid to the following matters:
A. Reasonable selection of pipe diameter, large pipe diameter, small liquid flow speed and small resistance loss under the same flow rate, but high price and small pipe diameter will lead to a sharp increase in resistance loss, increase the head of the selected pump, increase the belt power, increase the cost and operation cost. Therefore, it should be considered comprehensively from the perspective of technology and economy. The maximum pressure that the discharge pipe and its pipe joint can bear shall be considered.
B. The pipeline shall be arranged as straight as possible, the accessories in the pipeline and the length of the pipeline shall be minimized. When it is necessary to turn, the bending radius of the elbow shall be 3-5 times the diameter of the pipeline, and the angle shall be greater than 90 ℃.
C. The discharge side of the pump must be equipped with valves (ball valves or stop valves, etc.) and check valves. The valve is used to regulate the working point of the pump. The check valve can prevent the pump from reversing when the liquid flows back and prevent the pump from being hit by water hammer. (when the liquid flows back, it will produce huge reverse pressure, which will damage the pump.) B. determine the flow head flow. If the production process has given the minimum, normal and maximum flow, it should be considered as the maximum flow.
If only the normal flow is given in the production process, a certain margin should be considered. For large flow pumps with NS > 100 and low unintended head pumps, the flow margin is taken as 5; for small flow pumps with NS < 50 and high head pumps, the flow margin is taken as 10; for pumps with 50 ≤ ns ≤ 100, the flow margin is also taken as 5; for pumps with poor quality and poor operation conditions, the flow margin is taken as 10. If the basic data only gives weight flow, it should be converted into volume flow.

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