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Centrifugal Pump Selection
- Oct 05, 2018 -


Centrifugal impeller is agitated by rotation of the centrifugal force generated pressure fluid , transmission fluid . In the selection of centrifugal pumps , to determine the use and performance of pumps and pump selection . This option must first select the type and form of pump starts , then choose the pump to what principles it ? Based on what is ?
    A pump selection principle
1 , so that the selected pump type and performance in line with unit flow, head, pressure, temperature , cavitation flow , suction process parameters such requirements.
2, the mechanical aspects of high reliability, low noise, small vibration
3 , the economy and comprehensive consideration to the equipment costs , operating costs, maintenance costs and management fees the lowest total cost .
4 , centrifugal pump with a high speed , small size , light weight , high efficiency , large flow , simple structure, no pulse infusion , stable performance , easy operation and convenient maintenance .
     Therefore, the following exceptions should be used wherever possible centrifugal pumps :
     There measurement requirements , the selection of metering pumps
     Head demanding, flow is very small and not suitable for low flow high head pumps can be used , the choice of reciprocating pump , such as less demanding when cavitation vortex pump can also be used .
     Head low, traffic is heavy , the choice of axial and mixed flow pumps.
     Medium viscosity ( greater than 650 ~ 1000mm2 / s) , can be considered optional rotor pump or reciprocating pump ( gear pump , screw pump )
     Medium containing 75% of gas , flow is small and the viscosity of less than 37. 4mm2 / s , the choice of vortex pump .
     Start priming the pump for frequent or inconvenient occasions , should be used with self- priming pump , such as self- priming centrifugal pumps , self-priming vortex pump , pneumatic diaphragm pumps, electric diaphragm .
    Second, the pump selection basis
     Pump selection basis , should be based on process , drainage requirements, from five aspects to be considered , not only the amount of liquid delivery , installation head , the liquid nature of the piping and the operating conditions , etc.
     1 , the pump flow was selected one of the important performance data , which is directly related to the production capacity of the entire device and transmission capacity . Such as process design institute is able to calculate the pump is normal, minimum, maximum, three kinds of traffic. Select the pump at maximum flow rate as the basis, taking into account the normal flow , in the absence of maximum flow , it is usually preferable to the normal flow of 1.1 times as the maximum flow .
     2, the device system required pump head is selected another important performance data , the general use of amplification after 5% -10% margin to lift selection .
     3 , the liquid nature of the liquid medium including name, physical properties , chemical properties and other properties , physical properties with temperature c density d, viscosity u, medium diameter of solid particles and gas content, etc. , which relate to the lift system , the effective gas NPSH calculations and suitable pump types: chemical properties , mainly referring to the liquid medium corrosive and toxic chemicals , is the choice of materials and choice of what kind of pump shaft seal type of important basis.
     4 , the device piping system refers to a condition highly liquid feeding liquid from the liquid feed to send , such as suction side of the minimum level , the discharge side of the highest level and some data and pipe specifications and lengths , materials , fittings, specifications, quantity , in order to carry out the Department comb head NPSH calculations and checking .
     5, the operating conditions of many things, such as liquid saturated steam power operation T P, suction-side pressure PS ( absolute ) , the discharge side of the vessel pressure PZ, altitude , ambient temperature operation is a gap or continuous pump position is fixed thing can shift .
    Third, central air conditioning circulating pump
     Central air conditioning system works by circulating pump head due to improper selection , leading to project failure case analysis , emphasizing the rational choice importance circulating pump head , and made some selection method, the central air-conditioning design reference value.
     1 , the proposed
In the central air conditioning system, transporting chilled water circulating pump in summer and winter conveying water to the air terminal devices. Engineering design should follow the water flow and air-conditioning systems choose the good performance of the system resistance pumps. For HVAC design manual has detailed design calculations . The problem is that the actual engineering design , some engineers are not in accordance with the calculation method for the design calculations , but the rule of thumb for granted , for certain air conditioning systems and equipment, accessories and other new products, lack of serious study , resulting in the selected pump can not meet the requirements, or cause increased operating costs , and even the pump is not working properly , which had caused air conditioning designer 's attention.
     2 , theoretical analysis
     Air-conditioning system water flow by the size of the load and the supply and return water temperature to determine , through a hydraulic system resistance calculable . Press the pump flow and resistance selection , operation should be in the high zone, its operating point for the pump performance curve and the pipeline characteristic curve of intersection . The projects selected pump often there are two unusual situations .
     A ) the design of a more conservative , lower values of actual flow water system , greater resistance estimation system , resulting in increased lift when the pump selection , so that the chosen design flow circulating pump head than the system resistance is much greater.
Traffic QA system design flow , this flow rate pump head is HB can be. Actual choice of the pump head is HS. In order to ensure QA, will have to change the pipeline characteristic , namely through export off a small pump valves , so that by the pipeline characteristic curve becomes Ⅰ Ⅱ. Obviously , ΔP = HB-HA completely through the throttle valve , which is not economical , but also works to be avoided , and if using the same set of winter operation pump , flow rate becomes smaller as the throttle is more serious , the more not the economy , and even cause the pump operating point is unstable [ 2 ] .
     2 ) Design overconfident estimates for air conditioning system resistance is small, the selected pump head is less than the system design flow resistance.
     3 Projects
     Example 1: A project is a single high-rise buildings, building height 29m, pumping stations located in the basement of the main building . Design selection of imported Carrier centrifugal refrigeration machine, a cooling capacity of 1163 kW, equipped with two circulating water pumps , one with a device.
Just start debugging exercise, found the pump motor current is too large , powerful pump outlet pipe vibration and abnormal sounds. Pump head is only 0. 28MPa, the motor current I = 115A. Analyze the reasons for the sub-catchment dropout voltage of only 0. 13MPa, the selected pump head is too large . At this point the pump operating point for the low head high flow, serious motor overload [ 3 ] ; pump cavitation serious pipeline shake badly, abnormal sound ; off small pumps and freezer evaporator inlet and outlet valves, import and export requirements to ensure the evaporator differential pressure Δp = (92 ± 5) kPa, the pump resume normal operation.

     Pump head is 0. 48MPa, manifold pressure is 0. 10MPa, evaporator pressure is 0. 1MPa, the system resistance is not large, but most of the pressure head pump off completely consumed in small valve.
     The solution: the replacement of a low- lift pumps , test data, as shown in Table 2 ( new pump ) . Comparative data in Table 2 , the motor current is reduced to 82A 40A, its operating economy is self-evident .
Example 2: B works as a regional air conditioning, refrigeration heating station to a number of buildings centralized cooling and heating . Design 2907kW freezing machine 2, circulating pump , 3 sets of 2 with a device. Commissioning the system to open a freezer , a circulating pump . A few minutes later , the pump outlet pipe vibration severe and accompanied by abnormal sounds , freezer not start, fault display chilled water circulation water is not enough . Check the system valve is fully open , water filters all cleaned exhaust system more thoroughly . Nameplate parameters : Q is 500m3. h-1, H is 0. 475MPa, N is 90kW; test data: evaporator pressure drop 0. 02MPa, pump differential pressure 0. 14 MPa, manifold pressure 0. 27MPa, manifold pressure 0 . 40 MPa. According to the pump performance curve , when the head H = 0. 14MPa when the valve is fully open in the system case, the flow rate Q should be greater than 500m3. h-1 fishes at a time when self-locking flow indicator , indicating insufficient flow . Thus, the pump working properly . In fact , such a large system , the pump head H = 0. 14MPa is impossible. Eventually identified two reasons : ① pump inlet installed imported brush water filters, filter mesh is too small , causing the pump inlet resistance is too large , resulting in severe cavitation and pump performance deterioration ; ② pump flow and head are is very small, the system greater resistance . The brush filter run the filter removed , minutes, manifold pressure up to 0. 45MPa, pump head is 0. 52MPa. If two pumps running simultaneously , circulating flow rate increases , the system resistance also increases. Obviously the pump can not be guaranteed system uptime .
Solution: Replace the water filter strainer ; re- hydraulic calculation system , replace the water pump . This not only caused great economic losses , but also affect the normal operation of the air conditioning system , the lesson is profound.
     

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