Through the characteristic curve of centrifugal pump and pipeline system, several main ways of centrifugal pump flow regulation are analyzed: outlet valve regulation, pump shift regulation and pump series and parallel regulation. The energy consumption loss of the two methods of outlet valve regulation and pump speed regulation is analyzed by the characteristic curve, and the comparison is made. It is pointed out that the variable speed regulation flow of the centrifugal pump can save energy and save energy by adjusting the flow rate with the outlet valve. It is related to the size of the flow change. In practical applications, attention should be paid to the range of shift regulation to better apply the centrifugal pump shift regulation.
Centrifugal pumps are a general-purpose fluid machine widely used in chemical industrial systems. It has a wide range of performance (including flow, pressure head and adaptability to the nature of the transport medium), small size, simple structure, easy operation, low operating costs and many other advantages. Generally, the flow rate and pressure head of the selected centrifugal pump may be inconsistent with the requirements in the pipeline, or the production task and process requirements may change. At this time, the flow adjustment of the pump is required, which essentially changes 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 piping system. Therefore, changing the characteristic curve of any one can achieve the purpose of flow regulation. At present, the flow adjustment mode of the centrifugal pump mainly includes the regulating valve control, the shift control, and the parallel and series adjustment of the pump. Due to the different principles of various adjustment methods, in addition to their own advantages and disadvantages, the energy loss is not the same. In order to find the best, the lowest energy consumption, the most energy-saving flow regulation, it is necessary to fully understand the flow regulation of the centrifugal pump. The relationship between mode and energy consumption.
1, the main way of pump flow adjustment
1.1 Change the pipeline characteristic curve
The easiest way to change the flow rate of a centrifugal pump is to use the opening of the pump outlet valve to control it. The essence is to change the position of the pipeline characteristic curve to change the working point of the pump.
1.2 Change the centrifugal pump characteristic curve
According to the law of proportionality and the law of cutting, changing the speed of the pump and changing the structure of the pump (such as the outer diameter of the cutting impeller) can change the characteristic curve of the centrifugal pump to achieve the purpose of adjusting the flow rate (while changing the pressure head). However, for the pump that has been operated, the method of changing the structure of the pump is not convenient, and the versatility of the pump is reduced due to the change of the structure of the pump, although it is economical to adjust the flow rate at some time [1], and is also very good in production. Less adopted. Here, only the method of changing the rotational speed of the centrifugal pump to adjust the flow rate is analyzed. From the analysis in Figure 1, when changing the pump speed regulation flow from Q1 to Q2, the pump speed (or motor speed) drops from n1 to n2, the pump characteristic curve QH and the pipeline characteristic curve He=H0 at the speed of n2 G1Qe2 (the pipe curve does not change) is assigned to point A3 (Q2, H3), and point A3 is the new working point after the flow rate is adjusted by the speed regulation. The adjustment method is obvious, fast, safe and reliable, which can prolong the service life of the pump and save energy. In addition, the speed operation can also effectively reduce the NPSHr of the centrifugal pump, keep the pump away from the cavitation zone, and reduce the centrifugal pump. The possibility of cavitation [2]. The disadvantage is that changing the speed of the pump requires changing the speed of the prime mover (usually the motor) through frequency conversion technology, the principle is complicated, the investment is large, and the flow adjustment range is small.
1.3 pump string, parallel adjustment mode
When a single centrifugal pump cannot meet the delivery task, the parallel or series operation of the centrifugal pump can be used. Two parallel pumps of the same type are used in parallel. Although the head change is not large, the total delivery flow is increased. The total efficiency of the parallel pump is the same as that of a single pump; when the centrifugal pump is connected in series, the total head is increased. The flow rate does not change much, and the total efficiency of the series pump is the same as that of a single pump.
2. Analysis of energy consumption of pumps under different adjustment modes
In the analysis of energy consumption under different adjustment modes, the article only analyzes the two adjustment methods of valve regulation and pump variable speed regulation which are widely used at present. Since the parallel and serial operation of the centrifugal pump is aimed at increasing the pressure head or the flow rate, it is not widely used in the chemical industry, and its energy consumption can be analyzed in combination with Fig. 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 is the shaft power, w;
Q——the effective head of the pump, m;
H——the actual flow of the pump, m3/s;
V——fluid specific gravity, N/m3;
η - the efficiency of the pump.
When the flow is adjusted from Q1 to Q2 with a valve, the shaft power consumed at operating point A2 is:
NA2=vQ2H2/η
vQ2H3 - actual useful power, W;
vQ2 (H2-H3) - the power lost on the valve, W;
vQ2H2 (1/η-1) - the power lost by the centrifugal pump, W.
2.2 Power consumption when shifting the flow rate
In the shift analysis, the proportional law of the centrifugal pump is used. According to the application conditions, the following analysis refers to the shift range of the centrifugal pump within ±20%, and the efficiency of the centrifugal pump itself does not change much [3]. When the motor is used to adjust the flow rate to the 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 - the actual useful power, W;
vQ2H3 (1/η-1) - the power lost by the centrifugal pump, W.
2.3 Comparison of energy consumption
3 conclusions
For the current two kinds of main flow regulation methods, such as the outlet valve regulation and the pump variable speed adjustment, the pump variable speed adjustment saves much more energy than the outlet valve adjustment. This can be analyzed from both power consumption and power consumption. See the comparative analysis. Through the relationship between the flow rate and the lift of the centrifugal pump, the energy consumption relationship between the two adjustment modes can be more intuitively reflected. Reducing the flow rate through pump shift adjustment also helps to reduce the possibility of cavitation in the centrifugal pump. When the flow rate decreases, the energy-saving efficiency of the shift regulation is also greater, that is, the valve adjustment loss power is larger, but when the pump shift is too large, the pump efficiency is lowered, which exceeds the pump proportional law range. Therefore, in practical applications It should be considered from many aspects, and the best flow adjustment method is integrated between the two.
