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Centrifugal pumps are a kind of universal fluid machinery widely used in chemical industrial systems. It has a wide range of performance adaptation (including flow, pressure head and adaptability to the nature of the transmission medium), small size, simple structure, easy operation, low operating cost and many other advantages. In general, the flow rate and pressure head of the selected centrifugal pump may be inconsistent with the requirements in the pipeline, or due to changes in production tasks and process requirements. At this time, the flow adjustment of the pump is required, essentially changing the working point of the centrifugal pump. The working point of the centrifugal pump is determined by the characteristic curve of the pump and the characteristic curve of the pipeline system. Therefore, changing any of the characteristic curves can achieve the purpose of flow regulation. At present, the flow adjustment methods of centrifugal pumps mainly include control valves, variable speed controls, and pump and series adjustments. Due to the different principles of various adjustment methods, in addition to their own advantages and disadvantages, the resulting energy loss is not the same. In order to seek the best, the smallest energy consumption, the most energy-efficient flow adjustment methods, we must fully understand the flow regulation of centrifugal pumps. The relationship between methods and energy consumption.
1, the main way of pump flow regulation
1.1 Change the characteristic curve of the pipeline The simplest method to change the flow of the centrifugal pump is to use the opening of the outlet valve of the pump to control. The essence is to change the position of the characteristic curve of the pipeline to change the working point of the pump.
1.2 Change the characteristic curve of the centrifugal pump According to the law of proportionality and cutting law, changing the speed of the pump and changing the pump structure (such as the outer diameter method of the cutting impeller) can change the characteristic curve of the centrifugal pump so as to adjust the flow (simultaneously change the pressure The purpose of the head). However, for a pump that is already in operation, the method of changing the pump structure is inconvenient, and because the structure of the pump is changed, the versatility of the pump is reduced, although it is economical and convenient to adjust the flow rate at certain times [1], and it is also very important in production. Use less. Only the method of changing the rotational speed of the centrifugal pump to adjust the flow is analyzed here. From the analysis in Figure 1, when changing the pump speed adjustment flow rate from Q1 down to Q2, the pump speed (or motor speed) from n1 down to n2, the speed curve is n2 pump characteristic curve QH and the line characteristics He = H0 +G1Qe2 (with no change in pipe special curve) is delivered at point A3 (Q2, H3). Point A3 is a new operating point after adjusting the flow rate by speed control. This adjustment method has obvious, fast, safe and reliable adjustment effect, can prolong the service life of the pump and save electric energy. In addition, reducing the speed operation can effectively reduce the NPSHr of the centrifugal pump, keeping the pump away from the cavitation area and reducing the centrifugal pump. The possibility of cavitation occurs [2]. The disadvantage is that changing the rotational speed of the pump requires changing the rotational speed of the prime mover (usually the motor) through frequency conversion technology, the principle is complex, the investment is large, and the flow regulation range is small.
1.3 Pump series and parallel adjustment method When a single centrifugal pump can not meet the delivery task, it can use the parallel or serial operation of the centrifugal pump. Using two parallel centrifugal pumps of the same type in parallel, although the pressure head changes little, but the total delivery flow is increased. The total efficiency of the parallel pumps is the same as the efficiency of a single pump; the total pressure head increases when the centrifugal pumps are connected in series. The flow rate does not change much, and the total efficiency of the tandem pump is the same as that of a single pump.
2. Analysis of Energy Consumption of Pumps under Different Regulation Modes In the analysis of energy consumption under different adjustment methods, the article only analyzes the two adjustment methods that are widely used: valve adjustment and pump variable speed adjustment. Because the purpose of the parallel and series operation of the centrifugal pump is to increase the pressure head or flow, it is not used in the chemical industry. The energy consumption of the centrifugal pump can be analyzed in combination with Figure 2, and the method is basically the same.
2.1 Power consumption when the valve regulates flow When the centrifugal pump is running, the power N of the motor input pump shaft is:
N=vQH/η
Where N-axis power, w;
Q——effective head of the pump, m;
H - the actual flow of the pump, m3/s;
v - specific gravity of fluid, N/m3;
η - the efficiency of the pump.
When using valves to regulate flow from Q1 to Q2, the shaft power consumed at operating point A2 is:
NA2=vQ2H2/η
vQ2H3 - actual useful power, W;
vQ2 (H2-H3) - Loss of power on the valve, W;
vQ2H2 (1/η-1)—The power lost by the centrifugal pump, W.
2.2 The power consumption when changing the flow at variable speeds The proportionality law of the centrifugal pump is used in the variable speed analysis. According to the application conditions, the following analysis refers to that the transmission range of the centrifugal pump is within ±20%, and the efficiency of the centrifugal pump itself changes. Not much [3]. When using a motor to adjust the flow rate to flow rate Q2, the shaft power consumed by the pump at the operating point A3 is:
NA3=vQ2H3/η
The same transformation can be obtained:
NA3=vQ2H3+vQ2H3(1/η-1) (2)
Where vQ2H3 - actual useful power, W;
vQ2H3 (1/η-1)—The power lost by the centrifugal pump, W.
3. Conclusions For the two main flow regulating methods, centrifugal valve's universal outlet valve adjustment and pump variable speed adjustment, the energy consumption of pump variable speed adjustment is much greater than that of outlet valve adjustment. This can be analyzed from the power consumption of both. Contrast analysis with power consumption shows. Through the relationship between the flow rate and lift of the centrifugal pump, the energy consumption relationship under the two adjustment modes can be more intuitively reflected. Reducing the flow rate by adjusting the speed of the pump also helps to reduce the possibility of cavitation in the centrifugal pump. When the flow rate decreases, the energy-saving efficiency of variable-speed adjustment is also greater, that is, the greater the power consumption of the valve adjustment, but the pump speed is too large and will cause the pump efficiency to decrease, which exceeds the range of pump proportional law. Therefore, in practical applications At the same time, it should be considered in many aspects, and the best flow adjustment method should be integrated between the two.
Analysis of Regulating Mode and Energy Consumption of Centrifugal Pump
Through the characteristic curve diagram of centrifugal pump and piping system, several main methods of centrifugal pump flow regulation were analyzed: outlet valve adjustment, pump speed adjustment and series and parallel adjustment of pump. The energy loss of the two methods of outlet valve regulation and pump speed regulation was analyzed by using the characteristic graph, and a comparison was made. It was pointed out that the variable speed of the centrifugal pump can be used to adjust the flow rate compared with the outlet valve to save energy and energy efficiency. Related to the size of the flow changes. In practical applications, attention should be paid to the range of variable speed adjustment in order to better apply the variable speed adjustment of the centrifugal pump.