A brief discussion on the control methods and energy consumption analysis of centrifugal pumps

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A Brief Discussion on the Regulation Methods and Energy Consumption Analysis of Centrifugal Pumps 【Keywords】Centrifugal pump, regulation methods, energy consumption analysis 【Abstract】Several main methods for regulating the flow rate of centrifugal pumps are analyzed through the characteristic curve diagrams of centrifugal pumps and piping systems: regulation via the outlet valve, speed control of the pump, and series/parallel connection of pumps. The energy consumption losses associated with outlet valve regulation and pump speed variation were analyzed using characteristic curve diagrams, and a comparison was made. It was found that using pump speed variation to control flow in centrifugal pumps results in better energy savings compared to using outlet valves, and that the efficiency of energy saving depends on the magnitude of flow changes. In practical applications, attention should be paid to the range of speed adjustment in order to make better use of the speed control feature of centrifugal pumps. Several main methods of flow rate regulation for centrifugal pumps were analyzed through the characteristic curve diagrams of centrifugal pumps and piping systems: outlet valve regulation, pump speed variation regulation, and pump series/parallel connection regulation. The energy consumption losses associated with outlet valve regulation and pump speed variation were analyzed using characteristic curve diagrams, and a comparison was made. It was found that using pump speed variation to control flow in centrifugal pumps results in better energy savings compared to using outlet valves, and that the efficiency of energy saving depends on the magnitude of flow changes. In practical applications, attention should be paid to the range of speed adjustment in order to make better use of the speed control feature of centrifugal pumps. 　　Centrifugal pumps are a type of general-purpose fluid machinery widely used in chemical industry systems. It has many advantages, such as a wide range of performance capabilities (including adaptability to flow rate, head pressure, and the properties of the conveyed medium), small size, simple structure, easy operation, and low operating costs. Typically, the flow rate and head of the selected centrifugal pump may not match the requirements of the piping system, or changes in production tasks or process requirements may arise; in such cases, it is necessary to adjust the pump’s flow rate, which essentially means changing the operating point of the centrifugal pump. The operating point of a centrifugal pump is determined by both the pump’s characteristic curve and the characteristics curve of the piping system; therefore, altering either of these curves can achieve flow rate regulation. Currently, the main methods for regulating the flow rate of centrifugal pumps include control via control valves, variable speed control, and parallel or series connection of pumps. Due to the different principles behind various regulation methods, each of them has its own advantages and disadvantages, as well as different levels of energy loss. In order to find the best flow regulation method that results in minimal energy consumption, it is necessary to have a comprehensive understanding of the relationship between the flow regulation methods of centrifugal pumps and their energy consumption. 　　1 Main methods for adjusting pump flow rate 　　1.1 Changing the pipeline characteristic curve 　　The simplest way to adjust the flow rate of a centrifugal pump is by controlling the opening degree of the pump’s outlet valve; essentially, this involves changing the position of the pipeline characteristic curve in order to alter the pump’s operating point. 　　1.2 Changing the characteristic curve of centrifugal pumps According to the law of proportionality and the law of modification, two methods can be used to alter the characteristic curve of a pump: changing its rotational speed or modifying its structure (such as by reducing the outer diameter of the impeller). These methods enable the adjustment of flow rate (along with changes in head pressure). However, for pumps that are already in use, it is not convenient to change their structure, and such structural changes reduce the versatility of the pumps. Although it can be economical and convenient to adjust the flow rate in some cases, this method is rarely used in production. Here, only the method of adjusting flow rate by changing the speed of the centrifugal pump is analyzed. As analyzed from Figure 1, when the flow rate is reduced from Q1 to Q2 by adjusting the pump speed, the pump’s speed (or the motor speed) decreases from n1 to n2. At this speed of n2, the pump’s performance curve Q-H intersects the pipeline performance curve He=H0+G1Qe2 (which remains unchanged) at point A3(Q2, H3); point A3 represents the new operating point after adjusting the speed to change the flow rate. This adjustment method offers significant, rapid, and safe results; it can extend the pump’s service life and save energy. Additionally, operating at a lower speed helps to reduce the NPSHr of the centrifugal pump, keeping it away from the cavitation zone and thereby decreasing the likelihood of cavitation occurring in the pump. The disadvantage is that to change the pump’s speed, it is necessary to use frequency conversion technology to adjust the speed of the prime mover (usually a motor); the principle behind this is complex, the investment required is high, and the range of flow rate adjustment is limited. 　　1.3 Parallel and series operation modes of pumps When a single centrifugal pump is not sufficient to meet the transportation requirements, parallel or series operation of centrifugal pumps can be employed. By connecting two centrifugal pumps of the same model in parallel, the head change is minimal, but the total flow rate increases. The overall efficiency of the pumps in parallel is the same as that of a single pump ； When centrifugal pumps are connected in series, the total head increases while the flow rate remains roughly the same; the overall efficiency of the series-connected pumps is equal to that of a single pump. 　　2 Energy consumption analysis of the pump under different control methods In the analysis of energy consumption under various control methods, this article focuses solely on two commonly used approaches: valve control and variable speed control of the pump. Since the purpose of parallel and series operation of centrifugal pumps is to increase head or flow rate, they are not widely used in the chemical industry. Their energy consumption can be analyzed using Figure 2, with a basically similar approach. 　　2.1 Power consumption when the valve is used to regulate flow rate When a centrifugal pump is in operation, the power input by the motor to the pump shaft is given by: N = vQH/η Where N represents the shaft power, in watts ； 　　Q —— effective head of the pump, m ； 　　H — actual flow rate of the pump, m3/s ； 　　v——fluid specific gravity, N/m3 ； 　　η —— the efficiency of the pump. 　　When the flow rate is adjusted from Q1 to Q2 using a valve, the shaft power consumed at operating point A2 is: NA2 = vQ2H2/η. vQ2H3 represents the actual useful power, in watts ； 　　vQ2(H2－H3) —— Power lost at the valve, W ； 　　vQ2H2(1/η–1) —— Power loss of the centrifugal pump, W. 　　2.2 Power consumption during variable-speed flow regulation 　When conducting variable-speed analysis, the proportional law of centrifugal pumps is utilized; based on the conditions under which it is applied, the following analysis assumes that the speed range of the centrifugal pump is within ±20%, with little change in the pump’s efficiency. When the flow rate is adjusted to Q2 using an electric motor for speed control, the shaft power consumed by the pump at operating point A3 is given by: NA3 = vQ2H3/η. Similarly, after some transformations, it can be expressed as: NA3 = vQ2H3 + vQ2H3(1/η – 1) (2). Here, vQ2H3 represents the actual useful power, in watts ； 　　vQ2H3(1/η–1) —— Power loss of the centrifugal pump, W. 　　3 Conclusions 　　Regarding the two main methods of flow rate control currently used in centrifugal pumps, namely adjustment of the outlet valve and adjustment of the pump’s speed, the energy savings achieved through adjusting the pump’s speed are much greater than those obtained by adjusting the outlet valve, as can be seen from the analyses of their power consumption. The relationship graph between flow rate and head of the centrifugal pump allows for a more intuitive understanding of the energy consumption patterns under the two control methods. Reducing the flow rate by adjusting the pump speed also helps to decrease the likelihood of cavitation in centrifugal pumps. The greater the reduction in flow rate, the higher the energy-saving efficiency of variable-speed regulation; that is, the greater the power loss due to valve regulation. However, if the pump’s speed is changed too much, its efficiency will decrease, exceeding the range dictated by the pump’s proportional law. Therefore, in practical applications, it is necessary to consider various factors in order to determine the optimal method for flow rate regulation.