The working principle of centrifugal pumps are high-speed rotation of the motor mechanical energy into kinetic energy of the liquid had been promoted and the potential energy is an energy transfer and conversion process. According to this characteristic, we can see that the working conditions of centrifugal pump is set up pumps and piping system at the energy balance on the supply-demand relationship, as long as one of the two changes in the status of their workers will transfer point. Changes in working conditions at both caused by: 1. Pipeline system to change the characteristic curves, such as cutting valve; two. Pump characteristic curve itself changes, such as Frequency, cutting impeller, pumps in series or parallel.

Following a number of ways to carry out this analysis and comparison:

1, valve cutting

     To change the centrifugal flow of the simplest way is to adjust the pump outlet valve opening, and the pump speed remains unchanged (normally rated speed), and its essence is to change the location of curve lines in order to change working conditions at the pump. As shown in Figure 1, pump characteristic curves of Q-H curve with the line of Q-Σh intersection A full-time for the valve pumps working condition at the limit. Clearance small valves, the pipeline to increase the local resistance, water pumps working condition at point B to move to the left, the corresponding decrease in the flow. When the entire valve clearance, which is equivalent to an infinite resistance, flow rate to zero, when the pipeline characteristic curves coincide with the ordinate. As can be seen from Figure 1 to clearance small valves to control flow of the water supply capacity of water pump itself remains unchanged, the same lift characteristics, the management will be with the resistance characteristics of the valve opening change. This method is simple, continuous flow can be at a maximum flow with adjustable between zero and no additional investment, the application of a wide range of occasions. However, regulation is based on consumption expenditure of the surplus centrifugal energy (shaded area in Figure) to maintain a certain degree of supply, the efficiency of centrifugal pump will also drop, the economy not on reasonable.

 2,Change frequency and Adjust speed

     Deviate from the efficient working condition at are a pumps are the basic conditions necessary speed. When the pump speed changes, the valve opening remains unchanged (usually the largest opening), the same characteristics of pipeline systems, and water supply capacity and lift characteristics change. As shown in Figure 2, A condition for the pump balance point (also called operating point), corresponding to the efficiency of ηa. To reduce the flow speed can be reduced, working conditions at this time point B, corresponding to the efficiency of ηb, efficient pumps are still in the region. If the valves to regulate the ways to cut down expenditure, the case points to C, the corresponding efficiency ηc, pump efficiency. It can be seen, rated at less than the required flow of traffic cases, VVVF lift when cutting smaller than the valve, so VVVF power supply needed to reduce expenditure than the smaller valves, Figure 2 the shadow is part of the savings VVVF power supply. It is clear that expenditure compared with the valve, the energy saving effect VVVF very prominent, and the efficiency of the higher centrifugal pump. In addition, the use of Frequency Control, not only to reduce the centrifugal pump has happened the possibility of cavitation, but also can speed through or / deceleration time to extend the pre-boot / shutdown process, so that greatly reduce dynamic torque and thus to a large extent eliminate the devastating effects of water hammer, water pumps and greatly extended the life span of the piping system.

     In fact, there is also the limitations of Frequency Control, in addition to investment in larger, higher maintenance costs, when the pump speed is too large will result in reduced efficiency, the ratio of the law beyond the scope of pumps can not be unlimited speed.

3, cutting impeller

     When a certain speed, the pump pressure head, flow rate and impeller diameter are related. Of the same type of pump, cutting method can change the pump characteristic curve. For centrifugal pump impeller diameter of the original D, the flow for the Q, head for the H, power of P, after the cutting of impeller diameter D ', flow rate Q', head to H ', power P', while their mutual relations :

     These three are collectively referred to as pump-type law of the cutting. Cutting the law are set up at a large number of perceptual tests on the basic information, it believed that if the cutting impeller control within certain limits (this cutting and pumps more than a limited number of the switch), then the corresponding cutting before and after the pump efficiency can be regarded as unchanged . Is to change the water pump impeller cutting performance of a simple approach, the so-called adjustable-conditioning, which to some extent at the pump type, size objects with limited water supply requirements and the contradictions between diversity and expand the the use of pumps. Of course, cutting is an irreversible process impeller, the user must accurately calculate and measure the economic rationality before the implementation.

4, pumps in series and parallel

     Pump Units Series 1 refers to the export pumps to the entrance of another transmission fluid pump. The simplest models of the two the same, the same performance of centrifugal pumps in series as an example: as shown in Figure 3, equivalent to single-series performance curve of the lift pump performance curve at the same flow superposition circumstances, the Series A job at the flow and pump head job than the single point of B, but less than one pump at each of the 2-fold serial This is because after the pump head on the one hand, the increase in resistance is greater than the increase in pipeline, causing the head to the flow of surplus increase in volume of traffic on the other hand, makes the resistance increase in the total suppression of the increased lift. Pumps running in series, it should be noted after a booster pump is able to bear. Before each export pump valve should be closed, then the order to open the field of water supply pumps and valves.

     Refers to the two pumps in parallel, or two or more pumps to the same fluid pressure transmission pipeline, which aims at the same pressure head to increase flow. Still the same two simple models, the same parallel performance of centrifugal pump as an example: as shown in Figure 4, parallel performance curve is equivalent to a single flow pump performance curve at the head equal superposition, and parallel operating point A the volume and lift than a single pump of the job point B, but stop taking possession of factors, not the same when the single-pump 2 times.

     If it is purely for the purpose to increase the traffic, then what should be used in series or parallel lines characteristic curve depends on the extent of the flat, the more flat line curve, parallel flow after a single pump will be close to 2 times the run-time, thus more than flow series at the time of even greater operational benefits.

5. conclusions

 Although cutting the valve will cause energy losses and waste, but in some occasions still easy a quick easy way flow regulator; VVVF their good energy-saving effect of a high degree of automation and be more and more customers ; cutting impeller pump generally used for clearance, because of changes in the structure of the pump, less common; pumps in series and parallel connection is only applicable to a single pump can not meet the delivery of the mission, but in series or in parallel instead of the number of excessive uneconomical. In practical applications should be considered from various aspects, methods of flow regulation in a variety of the best programs are integrated to ensure efficient operation of the centrifugal pump.