Centrifugal pumps are widely used in water conservancy, the chemical industry, HVAC and other industries, and more and more attention has been paid to the selection of working conditions and the analysis of energy consumption. The so-called working point refers to the actual water output, head, shaft power, efficiency and suction vacuum height of the pump device at a certain moment, which indicates the working capacity of the pump. Usually, the flow rate and pressure head of the centrifugal pump may be inconsistent with the pipeline system, or due to changes in production tasks and process requirements, it is necessary to adjust the flow rate of the pump, which is essential to change the operating point of the centrifugal pump. In addition to the correct selection of the centrifugal pump in the engineering design stage, the selection of the working point of the centrifugal pump in actual use will also directly affect the energy consumption and cost of the user. Therefore, how to reasonably change the operating point of the centrifugal pump is particularly important.
The working principle of the centrifugal pump is to convert the mechanical energy of the high-speed rotation of the motor into the kinetic energy and potential energy of the lifted liquid, which is a process of energy transmission and conversion. According to this characteristic, the operating point of the centrifugal pump is based on the balance between the energy supply and demand of the pump and the piping system. As long as one of the two conditions changes, its operating point will shift. The change in operating point is caused by two aspects:
- Changes in the characteristic curve of the piping system, such as valve throttling;
- The characteristic curve of the water pump itself changes, such as frequency conversion speed regulation, cutting impeller, and water pumps in series or parallel.
4 Methods of centrifugal pump flow adjustment
Valve throttling
The easiest way to change the flow rate of a centrifugal pump is to adjust the opening of the pump outlet valve, while the pump speed remains unchanged (usually the rated speed). The essence is to change the position of the pipeline characteristic curve to change the operating point of the pump. When the valve is closed, the local resistance of the pipeline increases, the operating point of the pump moves to the left, and the corresponding flow decrease. When the valve is fully closed, the resistance is infinite and the flow rate is zero. At this time, the pipeline characteristic curve coincides with the ordinate. When the valve is closed to control the flow, the water supply capacity of the pump itself remains unchanged, the lift characteristic remains unchanged, and the pipe resistance characteristic will change with the valve opening. This method is easy to operate, the flow rate is continuous, and it can be adjusted freely between a certain maximum flow rate and zero without additional investment. It is applicable to a wide range of occasions. But the throttling adjustment is to consume the excess energy of the centrifugal pump to maintain a certain supply, and the efficiency of the centrifugal pump will also decrease accordingly, which is not economically reasonable.
Frequency conversion speed regulation
The working point deviating from the high-efficiency zone is the basic condition for the speed regulation of the water pump. When the speed of the pump changes, the valve opening remains unchanged (usually the maximum opening), the characteristics of the piping system remain unchanged, and the water supply capacity and head characteristics change accordingly.
When the required flow rate is less than the rated flow rate, the head of the frequency conversion speed regulation is smaller than the valve throttling, so the water supply power required by the frequency conversion speed regulation is also smaller than the valve throttling. Obviously, compared with valve throttling, the energy-saving effect of frequency conversion speed regulation is very prominent, and the work efficiency of the centrifugal pump is higher. In addition, after adopting frequency conversion speed regulation, it is not only beneficial to reduce the possibility of cavitation in the centrifugal pump but also can prolong the start/stop process by presetting the speed up/down time, so that the dynamic torque is greatly reduced, thus eliminating the extremely destructive water hammer effect to a large extent, and greatly extending the life of the pump and piping system.
In fact, frequency conversion speed regulation also has limitations. In addition to large investment and high maintenance costs, when the speed of the water pump is too large, it will cause a decrease in efficiency, which is beyond the scope of the pump proportional law, and it is impossible to adjust the speed without limit.
Cutting impeller
When the speed is constant, the pressure head and flow rate of the pump are related to the diameter of the impeller. For the same type of pump, the characteristic curve of the pump can be changed by the cutting method.
The cutting law is based on a large amount of perceptual test data. It believes that if the cutting amount of the impeller is controlled within a certain limit (this cutting limit is related to the specific revolution of the water pump), the corresponding efficiency of the water pump before and after cutting can be regarded as unchanged. . Cutting the impeller is a simple and easy way to change the performance of the water pump, the so-called variable diameter adjustment, which to a certain extent solves the contradiction between the limited type and specification of the water pump and the diversity of water supply objects, and expands the scope of the water pump. have to use the range. Of course, cutting the impeller is an irreversible process, and the user must perform accurate calculations and measure economic rationality before implementation.
Pumps in series and parallel
Pumps in series mean that the outlet of one pump sends fluid to the inlet of another pump. Take the simplest two centrifugal pumps of the same model and performance in series as an example: as shown in Figure 3, the series performance curve is equivalent to the head of the single pump performance curve. Both the flow rate and the head are larger than the working point B of the single pump (public account: pump steward), but they are not twice as high as that of the single pump. As a result, the excess head promotes the increase of the flow rate, and on the other hand, the increase of the flow rate increases the resistance, which inhibits the increase of the total head. When pumps are operated in series, attention must be paid to whether the latter pump can withstand the boost. Before starting, the outlet valve of each pump must be closed, and then the pumps and valves are opened in sequence to supply water to the outside.
The parallel connection of water pumps means that two or more pumps deliver fluid to the same pressure pipeline, and its purpose is to increase the flow rate when the pressure head is the same. Still take the simplest parallel connection of two centrifugal pumps of the same model and the same performance as an example, the parallel performance curve is equivalent to the single pump performance curve. The operating point B of the single pump must be larger, but considering the pipe resistance factor, it is also less than twice that of the single pump.
If it is purely for the purpose of increasing the flow rate, whether to use a parallel connection or series connection should depend on the flatness of the pipeline characteristic curve. The flatter the pipeline characteristic curve, the closer the flow rate after the parallel connection is to twice that of single pump operation, so The flow rate is larger than that in series, which is more conducive to operation.
Although valve throttling will cause energy loss and waste, it is still a fast and easy flow adjustment method in some simple occasions; frequency conversion speed regulation is more and more favored by users because of its good energy-saving effect and high degree of automation; Cutting impellers are generally used in clean water pumps. Due to the change of the pump structure, the versatility is poor; the series and parallel connections of water pumps are only suitable for situations where a single pump cannot meet the delivery task, and too many units connected in series or parallel are uneconomical. In practical application, many aspects should be considered, and some reliable schemes should be synthesized among various flow adjustment methods to ensure the efficient operation of centrifugal pumps.
