Analyze the energy consumption of centrifugal pumps under different control methods

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Methods of adjusting centrifugal pumps: 1. Changing the pipeline characteristic curve. When centrifugal pumps are manufactured, they are designed to fully meet their rated performance parameters. When selecting such pumps, users take into account potential variations in pipeline resistance and flow rate, which leads to the use of pumps with high capacity; during operation, the flow rate is adjusted by reducing the size of the control valve. The simplest method is to use the opening degree of the pump outlet valve for control; essentially, this involves changing the position of the pipeline characteristic curve in order to alter the pump’s operating point. As shown in Figure 1, let the actual head of the centrifugal pump be ha, and the pipeline loss be h1. The centrifugal pump operates at the intersection point (qn, ha) of the pump characteristic curve h and the pipeline characteristic curve r. Now, in order to adjust the flow rate to q1, the outlet valve is closed; the valve loss is hv, as a result the pipeline curve becomes steeper and takes the form of r’, with the operating point shifting to b(q1, hb). It can be seen that using valves to regulate flow is a simple method, allowing the flow rate to vary continuously, but it results in significant energy loss. 2. Changing the characteristic curve of the centrifugal pump: According to the law of proportionality and the law of modification, two methods can be used to alter the characteristic curve of the pump – by changing its rotational speed or by 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. 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 net positive suction head requirement of the centrifugal pump, keeping it away from the cavitation zone and thereby decreasing the likelihood of cavitation occurring in the pump. 3. Series and parallel operation methods for centrifugal pumps: When a single centrifugal pump is not sufficient to meet the transportation requirements, these pumps can be operated in parallel or in series. When two centrifugal pumps of the same model are connected in parallel, the head change is minimal, but the total flow rate increases. The overall efficiency of the parallel-connected pumps 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 also the same as that of a single pump. Energy consumption analysis of pumps under different control methods: When analyzing the energy consumption under various control methods, only two commonly used approaches are considered, namely 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 identical approach. 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 the formula n = vqh/η. In this formula, n represents the shaft power in watts; h represents the effective head of the pump in meters; q represents the actual flow rate of the pump in m3/s; v represents the specific gravity of the fluid in kg/m3; and η represents the efficiency of the pump. When the flow rate is adjusted from qn to q1 using a valve, the shaft power consumed at operating point B is given by: nb = vq1h1/η. In this formula, vqnhv represents the actual useful power, in watts; vq1(h1-hv) represents the power lost due to the valve, in watts; and vq1h1(1/η-1) represents the power lost by the centrifugal pump, in watts. 2. Power consumption during variable-speed flow regulation: When conducting variable-speed analysis, the proportional law of centrifugal pumps is utilized; based on their operating conditions, the following analysis assumes that the variable-speed range of the centrifugal pumps is within ±20%, with little change in the efficiency of the pumps themselves. When the flow rate is adjusted to q1 using an electric motor for speed control, the shaft power consumed by the pump at operating point C is given by: nc = vq1hc/η. Through further transformation, it can also be expressed as: nc = vq1hc + vq1hc(1/η–1). Here, vq1hc represents the actual useful power, in watts; while vq1hc(1/η–1) represents the power lost by the centrifugal pump, also in watts. By comparing the variable-frequency speed control method with the outlet valve regulation method, the power saved is: δn=vq1(hb-hc)