The main methods of flow regulation in centrifugal pumps

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Centrifugal pumps are widely used in industries such as water management and chemical engineering, and increasing attention is being paid to the selection of their operating points as well as the analysis of their energy consumption. The so-called operating point refers to the actual flow rate, head, shaft power, efficiency, and suction vacuum level of the water pump at a given moment; it represents the pumping capacity of the water pump. Typically, the flow rate and head of a centrifugal pump may not match those of the piping system; or due to changes in production tasks or process requirements, it is necessary to adjust the pump’s flow rate, which essentially means changing the operating point of the centrifugal pump. In addition to the correctness of the centrifugal pump selection during the engineering design phase, the choice of operating conditions during the actual use of the centrifugal pump also directly affects the user’s energy consumption and costs. Therefore, it is particularly important to know how to reasonably change the operating point of the centrifugal pump. The working principle of a centrifugal pump is to convert the mechanical energy generated by the high-speed rotation of the electric motor into the kinetic and potential energy of the liquid being pumped, which is a process of energy transfer and conversion. Based on this characteristic, it can be seen that the operating point of a centrifugal pump is determined by the balance between the energy supply and demand in the pump and pipeline system; whenever there is a change in either of these elements, the operating point will shift. Changes in operating conditions are caused by two factors: first, changes in the characteristic curve of the piping system, such as valve throttling ； II. Changes in the characteristic curve of the water pump itself, such as variable frequency speed control, cutting of the impeller, or connecting pumps in series or parallel. The following methods will be used for analysis and comparison: Valve throttling. The simplest way to change the flow rate of a centrifugal pump is to adjust the opening degree of the pump’s outlet valve, while keeping the pump’s speed constant (usually at the rated speed). Essentially, this involves changing the position of the pipeline characteristic curve in order to alter the pump’s operating point. When the valve is closed, the local resistance in the pipeline increases, causing the operating point of the water pump to shift to the left, with a corresponding decrease in flow rate. When the valve is fully closed, the resistance is infinitely large and the flow rate is zero; at this point, the pipeline characteristic curve coincides with the vertical axis. When the valve is closed to control the flow rate, the water supply capacity of the pump itself remains unchanged, as does its head performance; however, the pipe resistance characteristic changes as the valve opening degree changes. This method is easy to operate and provides a continuous flow rate; it can be adjusted freely between a certain maximum flow rate and zero, without any additional investment. It is suitable for a wide range of applications. However, throttling regulation relies on consuming the excess energy of the centrifugal pump to maintain a certain supply volume, which in turn reduces the efficiency of the pump; hence it is not economically reasonable.   Variable frequency speed control: The deviation of the operating point from the high-efficiency zone is a basic requirement for speed control of water pumps. When the speed of the water pump changes, the valve opening remains constant (usually at its maximum value); the characteristics of the piping system stay unchanged, while the water supply capacity and head performance change accordingly. When the required flow rate is less than the rated flow rate, the head generated by variable frequency speed control is lower than that achieved through valve throttling; therefore, the power required for water supply under variable frequency speed control is also lower than that required for valve throttling. It is evident that, compared to valve throttling, variable frequency speed control offers a significant energy-saving effect, and centrifugal pumps achieve higher efficiency. Furthermore, the use of variable frequency speed control not only helps to reduce the likelihood of cavitation in centrifugal pumps, but it also allows the start-up and shutdown processes to be extended by presetting the acceleration/deceleration times. This results in a significant reduction in dynamic torque, thereby substantially eliminating the highly destructive water hammer effect and extending the lifespan of the pump and pipeline systems. In fact, variable-frequency speed control also has its limitations. In addition to high investment costs and maintenance expenses, excessive changes in the pump’s speed lead to a decrease in efficiency; once beyond the range dictated by the pump’s proportional law, it becomes impossible to adjust the speed indefinitely. Cutting the impeller: At a constant rotational speed, both the head and flow rate of the pump are related to the diameter of the impeller. For pumps of the same model, the cutting method can be used to alter their characteristic curves. The cutting law is based on a large amount of empirical experimental data; it states that if the amount of cutting on the impeller is kept within certain limits (with this limit being related to the specific speed of the water pump), then the efficiency of the water pump before and after cutting can be considered constant. Cutting the impeller is a simple and effective method for altering the performance of water pumps, namely what is known as diameter adjustment. To a certain extent, it resolves the contradiction between the limited range of pump types and specifications and the diverse requirements of water supply applications, thereby expanding the scope of use for water pumps. Of course, cutting the impeller is an irreversible process, and users must conduct precise calculations and assess the economic viability before proceeding.   Series and parallel connection of water pumps: Series connection of water pumps means that the outlet of one pump feeds fluid into the inlet of another pump. Taking the simplest case of two centrifugal pumps of the same model and with identical performance connected in series as an example: as shown in Figure 3, the performance curve for the series connection is equivalent to the sum of the head values from the individual pump’s performance curves at the same flow rate. The flow rate and head at point A in the series configuration are both higher than those at point B for a single pump (WeChat official account: Pump Manager), but they do not reach twice the values of a single pump. This is because, when pumps are connected in series, the increase in head is greater than the increase in pipeline resistance; this excess head leads to an increase in flow rate. On the other hand, the increased flow rate further raises the resistance, thereby limiting the increase in total head. When water pumps are operated in series, it is necessary to ensure that the subsequent pump can withstand the increased pressure. Before starting, the outlet valves of each pump must be closed, after which the pumps and valves are turned on in sequence to supply water outward. Parallel operation of water pumps refers to the use of two or more pumps to deliver fluid into the same pressure pipeline, with the aim of increasing flow rate while maintaining the same head pressure. Still taking the simplest case of two identical centrifugal pumps of the same model and performance connected in parallel as an example, the performance curve for parallel operation is equivalent to the sum of the flow rates corresponding to the performance curve of a single pump, with equal head values. Both the flow rate and head at the parallel operation point A are greater than those at the single-pump operation point B, but due to pipe resistance, they still do not reach twice the values of the single pump. If the sole purpose is to increase flow rate, then whether to use parallel or series configuration should depend on how flat the pipe characteristic curve is. The flatter the curve, the closer the flow rate in a parallel setup will be to twice that achieved when a single pump operates alone; thus, this configuration yields a higher flow rate than the series setup and is more advantageous for operation.  Conclusion Although valve throttling results in energy loss and waste, it remains a fast and straightforward method for flow regulation in some simple applications ； Variable frequency speed control is gaining increasing favor among users due to its excellent energy-saving effects and high degree of automation ； Cutting impellers are generally used in clean water pumps; due to the change in the pump’s structure, their versatility is limited ； Pump series and parallel connections are only applicable when a single pump is not sufficient to meet the transportation requirements, and using too many pumps in series or parallel is actually uneconomical. In practical applications, multiple factors should be considered to determine the optimal solution among various flow regulation methods, thereby ensuring the efficient operation of centrifugal pumps.