Four Main Methods Of Centrifugal Pump Flow Regulation

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, the flow rate of the pump needs to be adjusted, which is essentially to change the operating point of the centrifugal pump. 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. 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 transfer 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 pipeline system. As long as one of the two conditions changes, its operating point will shift. The change of the working point is caused by two aspects: one is the change of the characteristic curve of the pipeline system, such as valve throttling; the other is the change of the characteristic curve of the pump itself, such as frequency conversion speed regulation, cutting impeller, and water pumps in series or parallel.

These methods are analyzed and compared below.

01 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 (generally 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 decreases. 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 change of 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, and is widely applicable. But the throttling adjustment is to maintain a certain amount of supply by consuming the excess energy of the centrifugal pump, and the efficiency of the centrifugal pump will also decrease accordingly.

02 Frequency conversion speed regulation

The working condition point deviates from the high-efficiency zone is the basic condition for the speed regulation of the water pump. When the speed of the water pump changes, the opening of the valve 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 frequency conversion speed regulation is smaller than valve throttling, so the water supply power required by frequency conversion speed regulation is also smaller than valve throttling. Obviously, compared with valve throttling, the energy-saving effect of frequency conversion speed regulation is very prominent, and the work efficiency of centrifugal pump is also higher. In addition, after adopting frequency conversion speed regulation, it not only helps to reduce the possibility of cavitation in centrifugal pumps, but also prolongs the start/stop process by presetting the speed up/down time, so that the dynamic torque is greatly reduced , thereby largely eliminating the destructive water hammer effect and greatly extending the life of pumps and piping systems.

However, 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 exceeds the range of the pump proportional law, and it is impossible to adjust the speed without limit.

03 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 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. range of use. Of course, cutting the impeller is an irreversible process, and the user must perform accurate calculations and measure economic rationality before implementation.

04 Pumps in series and parallel

Pumps in series means that the outlet of one pump sends fluid to the inlet of another pump. After the pumps are connected in series, on the one hand, the increase in the head is greater than the increase in the resistance of the pipeline, resulting in an increase in the flow due to the excess head; on the other hand, the increase in the flow increases the resistance, which inhibits the increase in 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.

Parallel connection of water pumps means that two or more pumps deliver fluid to the same pressure pipeline, the purpose of which is to increase the flow rate when the pressure head is the same.

If it is purely for the purpose of increasing the flow rate, should it be connected in parallel or in series? It should depend on the flatness of the pipeline characteristic curve. The flatter the pipeline characteristic curve, the closer the flow rate after parallel connection is to twice that of single pump operation, which is larger than the flow rate in series connection, which is more conducive to operation.

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