How to choose the self-priming centrifugal pump correctly

How to choose the self-priming centrifugal pump correctly

Self-priming centrifugal pumps rely on the centrifugal force of the impeller to stir the fluid rotation to generate pressure and transport the fluid. When choosing a self-priming centrifugal pump, determine the purpose and performance of the pump and select the pump type. This choice must first start with the choice of pump type and form, so what principles are used to select the pump? What is the basis?
   1. Principle of pump selection
1. Make the type and performance of the selected pump meet the requirements of process parameters such as device flow, head, pressure, temperature, cavitation flow, suction stroke and so on.
2. High mechanical reliability, low noise and low vibration
3. Economically, the total cost of equipment, operation, maintenance, and management costs should be considered superior and low.
4. Self-priming centrifugal pump has the characteristics of high speed, small volume, 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, self-priming centrifugal pumps should be used as much as possible:
    When there are metering requirements, choose a metering pump
    High head requirements, small flow rate and no suitable small flow high-lift self-priming centrifugal pumps can be used when reciprocating pumps are used, and vortex pumps can also be used when cavitation requirements are not high.
    When the head is very low and the flow rate is large, an axial flow pump and a mixed flow pump can be used.
    When the viscosity of the medium is large (greater than 650 ~ 1000mm2 / s), rotor pump or reciprocating pump (gear pump, screw pump) can be considered
    When the gas 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 used.
    For occasions where the starting is frequent or the pump is inconvenient, the pump with self-priming performance should be selected, such as self-priming self-priming centrifugal pump, self-priming vortex pump, pneumatic (electric) diaphragm pump.
   2. Basis for pump selection
    The basis for pump selection should be considered from five aspects according to the process flow, water supply and drainage requirements, including liquid delivery volume, device head, liquid properties, pipeline layout, and operating conditions, etc.
    1. The flow rate is one of the important performance data of the pump selection, which is directly related to the production capacity and delivery capacity of the entire device. For example, in the process design of the design institute, three flow rates of normal, superior small and superior pump can be calculated. When choosing a pump, based on superior large flow rate, taking into account the normal flow rate, when there is no superior large flow rate, it can usually take 1.1 times the normal flow rate as the superior large flow rate.
    2. The head required by the device system is another important performance data for pump selection. Generally, the head is selected after amplifying the 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 and gas content in the medium, etc. This involves the system head, effective gas Calculation of corrosion allowance and suitable pump type: chemical properties, mainly referring to the chemical corrosiveness and toxicity of the liquid medium, is an important basis for the selection of pump materials and the type of shaft seal.
    4. The piping layout conditions of the device system refer to the liquid delivery height, the liquid delivery distance, the liquid delivery direction, the suction side is superior to the low liquid level, and the discharge side is superior to the high liquid level. Some data and pipeline specifications and their lengths, materials, and pipe specifications , Quantity, etc., in order to carry out the calculation of the system comb head and check the cavitation surplus.
    5. There are many operating conditions, such as liquid operation T, saturated steam power P, suction side pressure PS (absolute), discharge side container pressure PZ, altitude, ambient temperature, whether the operation is gapped or continuous, and the position of the pump is fixed Is still removable.
   3. Central air conditioning circulating water pump
    Through the analysis of cases of failure of the project due to improper selection of circulating pump head in the central air conditioning system project, the importance of reasonable selection of circulating pump head is emphasized, and some selection methods are proposed, which have reference value for the design of central air conditioning.
    1. Questions raised
In the central air conditioning system, circulating water pumps deliver chilled water in summer and hot water in winter to the terminal device of the air conditioner. The engineering design should select a pump with good performance according to the water flow and resistance of the air conditioning system. Relevant HVAC design manuals have detailed design calculation methods. The problem is that in the actual engineering design, some engineers did not carry out the design calculation according to the calculation method, but took it for granted based on experience and lacked serious research on the system and some new products such as air conditioning equipment and accessories. Or it may cause an increase in operating costs, and even the water pump may not work properly, which has to be highly valued by air-conditioning designers.
    2. Theoretical analysis
    The water flow of the air conditioning system is determined by the load and the temperature difference between the supply and return water, and the system resistance is obtained by hydraulic calculation. The pump selected according to the flow rate and resistance should be in the high efficiency zone during operation, and its working point is the intersection of the pump performance curve and the pipeline characteristic curve. The pumps selected in the project often have two abnormal conditions.
    1) The design is relatively conservative, the actual flow rate of the water system is low, and the estimated system resistance is large, which leads to an increase in the head when the pump is selected, so that the selected circulating pump head is much larger than the system resistance at the design flow rate.
Flow rate QA is the system design flow rate, at which the pump head can be HB. The actual pump head selected is HS. In order to ensure QA, it is necessary to change the pipeline characteristics, that is, by closing the inlet and outlet valves of the small water pump, the pipeline characteristic curve is changed from I to II. Obviously, ΔP = HB-HA is completely throttled by the valve, which is very uneconomical, and it is also a situation that needs to be avoided in the project. If the same set of pumps is used for operation in winter, the throttle will be more serious due to the smaller flow rate. It is uneconomical, and even makes the working point of the pump unstable [2].
    2) The design is too confident, the resistance of the air conditioning system is too small, and the selected pump head is less than the system resistance at the design flow rate.
    3. Project examples
    Example 1: Project A is a single high-rise building with a building height of 29m. The pump room is located in the basement of the main building. The design uses one imported Carrier centrifugal freezer with a cooling capacity of 1163 kW, equipped with two circulating water pumps, one for each and one for standby.
When starting the debugging exercise, it was found that the current of the pump motor was too large, the pump outlet pipe vibrated severely, and there was an abnormal sound. The pump head is only 0.28MPa, and the motor current is I = 115A. The reason for the analysis is that the pressure difference of the sub-collector is only 0.13MPa, and the pump head selected is too large. At this time, the working point of the pump is low head and large flow, and the motor is seriously overloaded [3]; the pump has serious cavitation, severe piping jitter, and abnormal sound; close the small pump and the inlet and outlet valves of the evaporator of the refrigerator to ensure the import and export requirements of the evaporator The differential pressure Δp = (92 ± 5) kPa makes the pump return to normal operation.
    The pump head is 0.48 MPa, the pressure difference of the sub-collector is 0.10 MPa, and the pressure difference of the evaporator is 0.1 MPa, the system resistance is not large, and most of the pressure head of the pump is completely consumed on the closed valve.
    Solution: replace a low-lift pump, the test data is shown in Table 2 (new pump). Comparing the data in Table 2, the motor current is reduced from 82A to 40A, and its economic operation is self-evident.
Example 2: Project B is an area air conditioner, and the cooling and heating station provides cooling and heating to multiple buildings. Design 2907kW refrigerators, 3 circulating water pumps, 2 for 1 backup. When the system is commissioned, a freezer and a circulating water pump are turned on. After a few minutes, the pump outlet pipe vibrated violently and accompanied by abnormal sound, the freezer could not be started, and the fault showed that the circulating water volume of the chilled water was insufficient. Check that the system valves are all open, the water filters are all cleaned, and the system exhaust is complete. Nameplate parameters: Q is 500m3. h-1, H is 0.475MPa, N is 90kW; test data: evaporator inlet and outlet pressure difference is 0. 02MPa, water pump inlet and outlet pressure difference is 0. 14MPa, water collector pressure is 0.27MPa, water separator pressure is 0 .40 MPa. According to the pump performance curve, when the head H = 0.14MPa, the flow rate Q should be greater than 500m3 when the system valve is fully open. h-1 is correct, and the water flow indicator is self-locking at this time, indicating insufficient flow. This shows that the pump works abnormally. In fact, for such a large system, the pump head H = 0.14MPa is also impossible. There are two reasons for the superior final investigation: ① The inlet brush water filter is installed at the inlet of the pump. The mesh of the filter is too small, which causes the inlet resistance of the pump to be too large, resulting in serious cavitation and deterioration of the performance of the pump; Both are very small and the system resistance is large. Remove the brush filter screen and run, the pressure difference between the sub-collector and the collector reaches 0.45MPa and the pump head is 0.52MPa If the two pumps are running at the same time, the circulation flow increases, and the system resistance also increases. Obviously the water pump cannot guarantee the normal operation of the system.
Solution: replace the water filter screen; recalculate the hydraulic system of the system and replace the water pump. This not only caused a large economic loss, but also affected the normal operation of the air conditioning system, the lesson is profound.
    4 Conclusion
1) It is not difficult to select the circulating water pump of the air conditioning system, but it must be paid enough attention by the designer, and the experience should be combined with the theoretical calculation.
2) The selection of the pump head cannot be considered that the larger the more the insurance, but the economics of operation should be paid attention to.
3) At present, air-conditioning products are updated very quickly, so they should be very cautious when selecting them, and their performance should be carefully studied

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