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Centrifugal pump selection principle
Centrifugal pump is driven by impeller rotation of the centrifugal force generated by the fluid pressure, conveying fluid. In the selection of centrifugal pump, to determine the purpose and performance of the pump and select the pump. First of all, this choice from the choice of pump type and form of the beginning, then what principle to choose the pump? What is the basis? First, the principle of selection of centrifugal pumps 1, the type and performance of the selected pump in line with the device flow, head, pressure, temperature, cavitation flow, suction and other process parameters. 2, mechanical reliability, low noise, vibration 3, the economic should take into account the equipment costs, operating costs, maintenance costs and management costs of the lowest total cost. 4, centrifugal pump with high speed, small size, light weight, high efficiency, large flow, simple structure, infusion without pulsation, stable performance, easy operation and maintenance features such as convenience. Therefore, in addition to the following circumstances, the centrifugal pump should be used as far as possible: When there are measurement requirements, the choice of metering pump head is very demanding, the flow is small and not suitable for small flow High-lift centrifugal pump can be used when optional reciprocating pumps such as cavitation When the request is not high can also use a vortex pump. Low head, when the flow is large, the choice of axial flow pump and mixed flow pump. Medium viscosity (greater than 650 ~ 1000mm2 / s), consider the selection of rotor pump or reciprocating pump (gear pump, screw pump) medium 75% gas content, the flow is small and the viscosity is less than 37.4mm2 / s, the optional Vortex pump. On the occasion of frequent or inconvenient irrigation pump should be used with self-priming performance, such as self-priming centrifugal pumps, self-priming vortex pumps, pneumatic diaphragm pumps, electric diaphragm pumps. Second, centrifugal pump selection based on pump selection basis, should be based on process, water supply and drainage requirements, to be considered from five aspects, both the liquid delivery volume, the device lift, liquid properties, piping layout and operating conditions, etc. 1, Flow rate is one of the important performance data of the pump, it is directly related to the entire plant's production capacity and delivery capacity. Such as the design process can be calculated in the pump normal, minimum, maximum three kinds of flow. Select the pump, based on the maximum flow, taking into account the normal flow, in the absence of maximum flow, usually desirable 1.1 times the normal flow as the maximum flow. 2, the lift system required for installation of the pump is another important performance data, generally to enlarge the 5% -10% head lift to choose. 3, the liquid properties, including the name of the liquid medium, physical properties, chemical properties and other properties, the physical properties of the temperature c density d, viscosity u, medium particle diameter and gas content, etc., which involves the system lift, effective gas Erosion margin calculation and the type of suitable pump: Chemical nature, mainly refers to the liquid medium chemical corrosion and toxicity, is the selection of pump materials and choose an important basis for the type of shaft seal. 4, the installation of piping system conditions refers to the liquid from the height of the liquid into the liquid distance to the suction side such as the lowest level, the highest level of the discharge side of some data and pipe specifications and length, material, pipe specifications, the number , Etc., in order to carry out the Department of combing head calculation and NPSH check. 5, the operating conditions are many, such as the operation of the liquid T saturated steam P, suction side pressure PS (absolute), the discharge side of the tank pressure PZ, altitude, ambient temperature operation is gap or continuous, the pump position is fixed Or removable. Third, the central air conditioning circulating pump through the central air conditioning system project due to improper choice of circulating pump head, resulting in project failure case analysis, emphasizing the importance of a reasonable choice of circulating pump head and made a number of options for the design of central air-conditioning Reference value. 1, the problem raised In the central air conditioning system, the circulating pump in the summer to send chilled water, hot water in winter to the air conditioning terminal device. Engineering design should be in accordance with the air conditioning system water flow and system resistance to choose a good performance of the pump. HVAC design manual has a detailed design and calculation methods. The problem lies in the actual engineering design, some engineers did not follow the calculation method to calculate the design, but by the experience of course, the system and some air-conditioning equipment, accessories and other new products lack of careful study, resulting in the selected pump can not meet the requirements, Or cause the running cost to increase, even the water pump can not work normally, this has caused the air conditioner designer to attach great importance. 2, Theoretical analysis Air conditioning system, the size of the flow of water by the load and return water temperature to determine the system resistance obtained through hydraulic calculations. According to the flow and resistance of the pump selection, the operation should be in the high-efficiency area, the operating point for the pump performance curve and the intersection of pipeline characteristics curve. The project selected pump often appear two kinds of abnormal situation. 1) The design is more conservative, the actual value of the water system is lower, the estimated system resistance is larger, resulting in increased head lift when the pump is selected, so that the selected pump head is much larger than the system resistance under design flow. Flow QA is the system design flow at which pump head can be HB. The actual choice of pump head HS. In order to ensure the QA, we must change the pipeline characteristics, that is, by closing the small pump import and export of valves, so that the pipeline characteristics from â… to â…¡. Obviously, ΔP = HB-HA is completely throttled through the valve, which is very uneconomical, but also to avoid the situation in the project, if the same set of pump work in winter, due to smaller flow, throttling more serious, even more Not economical, and even cause pump instability [2]. 2) The design is too self-confident, the air-conditioning system resistance is estimated to be small, the pump head is less than the design flow under the system resistance. 3, Project Example 1: A project is a single high-rise building, the building height of 29m, the pump room is located in the basement of the main building. Design and selection of imported Carrier centrifugal freezers, a cooling capacity of 1163 kW, equipped with 2 circulating pumps, 1 with 1 prepared. At the beginning of the debugging exercise, we found that the pump motor current is too large, the pump outlet pipe vibration, and abnormal sound. Pump head only 0.28MPa, motor current I = 115A. Analysis of the reasons for the sub-catchment pressure difference of only 0.13MPa, the selected pump head is too large. At this point the pump operating point for low lift large flow, the motor is seriously overloaded [3]; serious pump cavitation, pipe jitter, abnormal sound; off the pump and freezer evaporator inlet and outlet valves to ensure that the evaporator import and export requirements The pressure difference Δp = (92 ± 5) kPa, the pump to resume normal work.