Working principle of variable-frequency water pumps

A Brief Discussion on Pump Selection and Speed Control 1 Introduction According to the GBJ13-86 Code for Design of Outdoor Water Supply Systems, when selecting pumps for water intake pumping stations, it is necessary to take into account the lowest raw water level that ensures a water supply reliability of 90–99%, as well as the maximum flow rate that the pumping station can handle. However, due to the laws of nature, winter in our country, from December to March, is the dry season for rivers – during this time the water levels in rivers are at their lowest. As a result, the static head required by the pumps is high, and the amount of water that can be supplied by the pumping stations is low, as shown at point a in Figures 1 and 2. During the peak water demand period in summer, from July to September, the water levels in rivers rise due to the abundant rainfall. Although the amount of water that can be supplied by the pumping stations increases significantly, the static head required by the pumps decreases, as shown at point b in Figures 1 and 2. There are significant seasonal variations in the two factors on which the design codes for outdoor water supply are based: the maximum water supply volume and the minimum water level. It is very unlikely that these two conditions occur at the same time. Following the codes mechanically in selecting the head and flow rate parameters for water pumping stations would lead to highly unreasonable outcomes; as a result, the actual operating conditions of such stations differ greatly from the designed parameters most of the time, resulting in high energy consumption and unnecessary expenses for infrastructure investment. However, if only the changes in water level and volume during normal years are considered and design is not carried out in accordance with regulatory requirements, then in the event of a drought during the peak water supply period in summer, when river water levels drop to their lowest levels while the demand for water supply is at its highest; or during the dry season in winter when the maximum amount of water supply is required due to some special circumstances, as shown at point c in Figures 1 and 2, the water pumping stations, which require substantial investment, will not be able to perform their intended function. 2 Changes in water level and volume, as well as existing problems: Taking the changes in the Yangtze River water level in the Nanjing area as an example, the average high water level during the wet summer season is 9.50 m (on the Wusong elevation scale, the same below). The average low water level during the winter dry season is 2.50 m, while the extreme low water level taken into account in the design, at 1.42 m, is almost never reached. The periods of higher water supply throughout the year are concentrated in July, August, and September, when river levels are high; whereas during the months of December, January, February, and March, when water levels are low, the demand for water is relatively low. In many cases, designers tend to focus on ensuring a secure water supply; they usually design the pumps in accordance with regulatory requirements, that is, by setting a minimum water level from which water can be drawn to ensure a water supply reliability of 90–99%, as well as determining the maximum flow rate that the pumping station can handle (as shown in points c in Figures 1 and 2). The elevation of the water plant’s reaction tank remains constant, but when the water level in rivers changes significantly throughout the seasons, the static head required by the pumps also changes considerably. The ideal design assumes that it is possible to adapt to changes in flow rate and head during actual operation simply by adjusting the number of water pumps operating in parallel, as shown at points a, b, and c in Figures 1 and 2. However, according to the author’s investigations, most water pumping stations need to adjust the opening degrees of the pipeline valves in order to coordinate with the number of pumps operating in parallel and thus adapt to changes in flow rate and head. As shown at points a1 and b1 in Figure 3, the energy associated with the remaining head between a1–a and b1–b is wasted on the valves, and the amount of energy lost over time is quite significant. Figure 1: Changes in river water levels during dry/peak periods, as well as variations in the water supply volume from the source pump stations in winter/summer. Figure 2: Changes in river water levels during dry/peak periods, as well as variations in the water supply volume from the source pump stations in winter/summer (2). Figure 3: Actual operating curves for most pump stations. Therefore, when selecting pumps based on extreme low water levels and maximum water supply volumes corresponding to a 100-year flood scenario (i.e., a water supply reliability of 90–99%), there will be a significant number of idle pumps at the pump stations; these pumps do not operate at their optimal operating points for most of the time, and adjustments have to be made by reducing the valve opening degrees. A large amount of idle fixed assets and high energy consumption in daily operations render the economic viability of the water pumping station irrelevant. 3 Principles for the selection and speed control design of economical water pumps: The rated parameters of a water pump correspond to the point at which its efficiency is highest; within a certain range around this point, as long as the efficiency remains at least 10% above its maximum value, it is considered to be in a region of high efficiency. The ideal design would be one in which the flow rate and head range of the pumping station fall within the efficient operating range of the selected pump, but in practice it is not always possible to find a pump that meets these ideal conditions. Moreover, in practical engineering applications, it is common for the efficient operating range of a single water pump to not cover the entire range of flow rates and head requirements of the pumping station; in such cases, it is necessary to use multiple pumps operating in parallel to meet these requirements. When water pumps are connected in parallel, they operate on the principle of constant head with flow rates adding together (as shown in Figure 4). The Q-H curve of the water pump becomes flatter, making it more suitable for pumping stations where flow rates vary significantly while head changes only slightly. Figure 4: Diagram of the parallel operation mode of water pumps. Figure 5: Changes in the speed regulation characteristics of water pumps and changes in the pipeline characteristic curve due to variations in river water levels. An increase in river water levels results in a decrease in the static head of the water pumps, with the pipeline characteristic curve shifting downward parallelly. At this point, the operating condition often moves outside the efficient range of the water pump. If the speed of the water pump can be changed simultaneously, the pump’s q-h characteristic curve will shift downward in a parallel manner. As a result, the pump’s q-h curve and the pipeline’s characteristic curve can intersect within the region labeled ABCD (as shown in Figure 5). At all the operating points within this region, the pump is able to adapt to changes in both flow rate and head pressure. Thus, the potential energy of the water level is fully utilized, saving electricity consumption. According to the law of similar operating conditions for water pumps, we have: qn/ q0 = nn/n0 (1), hn/ h0 = (nn/n0)2 (2), pn/ p0 = (nn/n0)3 (3). Here, n0, q0, h0, and p0 represent respectively the rotational speed, flow rate, head, and power of the pump when it is operating at full speed. nn, qn, hn, pn represent the speed, flow rate, head, and power of the variable-speed pump, respectively. Therefore, speed regulation can precisely compensate for the drawback that, when water pumps operate in parallel, the q-h curve becomes flatter, making it difficult to adapt to large changes in raw water level with only minor changes in flow rate. As can be seen from the two scenarios in Figures 1 and 2, the normal operation of the water pumping station takes place at point b during summer, when water levels are high, head is low, and the flow rate is high, as well as at point a during winter, when water levels are low, head is high, and the flow rate is low, along with the areas between these points. Thus, the starting points for selecting pumps based on economic considerations and for determining speed control principles can be divided into two types: (1) Using point b in Figure 1 as the reference point for pump selection, with the water pump operating at point b within its corresponding high-efficiency range. If the water flow rate at point b can be met by a single pump, then economic operation in the area between point a and a-b can be achieved by reducing the operating speed of the pump unit. If the water flow rate at point b requires several pumps to operate in parallel, then economic operation in the area between point a and a-b can be accomplished by reducing the number of pumps operating in parallel along with lowering their operating speeds. (2) Using point a in Figure 2 as the reference point for pump selection, the pump should operate within its corresponding high-efficiency range on the left side of point a. If the water flow at point a can be met by a single pump, then economic operation in the area from point b to a-b can be achieved by reducing the operating speed of the pump unit. If the water flow at point a requires several pumps to be operated in parallel, then economic operation in that area can be accomplished by reducing the number of pumps in parallel along with lowering their operating speeds. 4 Strategies for the selection of reliable water pumps and speed control design: In accordance with the design specifications of GBJ13-86, when selecting pumps for water intake stations, it is necessary to take into account the lowest water level at which a water supply reliability rate of 90–99% can be achieved, as well as the flow rate when the station’s water supply capacity is at its maximum – that is, the requirements specified at point C in Figures 1 and 2. However, as analyzed earlier in this article, the water intake pump station operates frequently within the range a-b due to the laws of nature; the condition at point c only occurs under special circumstances such as high temperatures and drought in summer, or when there is an extremely high demand for water supply in winter. This is precisely what ensures the reliability of the pump selection design for the water intake pump station. The water pump unit utilizes variable frequency speed control technology; it operates at a frequency below 50 Hz within the normal operating range between a and b. When necessary, based on actual conditions, the operating frequency can be increased to 55 Hz or even higher, entering an over-frequency operating mode. According to the principles outlined in equations (1), (2), and (3), this setup enables meeting the operating requirements at point c. 5 Points to Note (1) Matching of motor power: Due to the relationship in Equation (3), when operating at a speed higher than the rated value by increasing the frequency, it is necessary to verify the power of both the water pump and the motor. Since the shaft power of a water pump varies with flow rate and head, the power of the motor used in the water pump is selected based on the maximum shaft power when the pump operates alone. As can be seen from Figure 4, at the operating point f when the two water pumps operate in parallel, the flow rate is q1+2, and the head is h2. When converted to a single water pump, the head remains h2, and the flow rate is q1,2. This flow rate is less than the flow rate q1 when a single water pump is in operation; its shaft power p1,2 is also less than the shaft power p1 when a single pump is in use. The power of multiple pumps operating in parallel is even lower than that of a single pump operating alone. Therefore, when selecting a motor, its power rating is sufficient for conventional use. However, it should be verified that when the water pumps are operated in parallel with speed control, the output power of their motors is generally not less than 75% of the rated value. To ensure that the motor is also within the high-efficiency range under speed-regulated operation. When multiple water pumps are operated in parallel, it is still not possible to meet the requirements of maximum flow rate and highest head (i.e., point c); in such cases, the frequency needs to be adjusted to 55 Hz. Using equation (3), pp = (55/50)³ × p1 = 1.13 × p1 = 1.331 × p1 (4). Conversely, p1 = 0.751 × pp (5). Thus, when water pumps operate in parallel, it is possible to achieve safe operation at higher speeds without exceeding the motor’s power capacity. (2) Verification of the pump’s net positive suction head is necessary because the NPSH (required net positive suction head) of the pump increases as the speed rises under conditions of overspeed operation. However, the installation height of the pump remains constant, and the operating conditions at point C correspond to the maximum flow rate at the lowest water level. Therefore, when measures are taken to meet the requirements at point C, verifying the NPSH is an essential condition to ensure the safe operation of the pumping station. (3) Motor power factor: When water pumps operate in parallel, the motors are under light load conditions, and their power factor cosφ decreases to a certain extent; this issue can be resolved through capacitor compensation. When operating at an overspeed to meet the requirements for point c, the motor power gradually approaches full load or even a slight overload as the load increases; simultaneously, the power factor also rises, which may lead to a situation of excessive power factor compensation and resulting inefficiency. However, since point C represents an abnormal extreme condition that occurs very rarely, even if the power factor is not optimal, it can be disregarded as a low-probability event. (4) Considerations regarding mechanical strength: At present, the mechanical strength of domestic water pumps and motors is sufficient to meet the requirements for operation at slightly elevated speeds as mentioned above. This is because when manufacturing water pumps and motors for 50 Hz operating conditions, manufacturers only need to modify the process parameters while keeping the original mechanical structure unchanged. 6 Conclusion: When the water level of rivers changes significantly, the static head of water pumps also changes considerably. It is a law of nature that the water supply is low during low water levels in winter and high during high water levels in summer. The pump selection for water intake pumping stations should be based on the actual conditions, taking into account the water levels and flow rates during winter in normal years as well as those during summer. Appropriate pump types should be chosen, along with variable frequency speed control, to ensure the efficient operation of the pumping station – this meets the economic requirements of pump selection and speed control design. It is also necessary to verify whether the pumping station can meet the maximum water supply requirements at the lowest water levels, as this is essential for ensuring the reliability of the pump selection and speed control design. References: gbj13-86. Code for Design of Outdoor Water Supply. Qian Jian, Wu Zhicheng. Discussion on the Rationality of the Design Flow Rate for Water Intake in Water Treatment Plants. China Water & Wastewater, 2001(8). Jiang Naichang. Pumps and Pumping Stations (2nd edition). Beijing: China Architecture & Building Press, 1987.