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The Main Methods of Flow Adjustment of Centrifugal Pump

Categories:Technology Service Author: Origin:Origin Time of issue:2019-04-27
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Centrifugal pump is widely used in water conservancy, chemical industry and other industries, the choice of its operating point and energy consumption analysis is increasingly valued. The so-called working point, refers to the pump device in a certain instantaneous actual water output, head, shaft power, efficiency and suction vacuum height, etc., it represents the working capacity of the pump. Usually, the centrifugal pump flow, pressure head may not be consistent with the pipeline system, or due to the production task, process requirements change, the need to regulate the flow of the pump, its essence is to change the centrifugal pump working point. In addition to the engineering design stage of the centrifugal pump selection is correct, the actual use of the centrifugal pump operating point will also directly affect the user's energy consumption and cost. Therefore, how to reasonably change the centrifugal pump operating point is particularly important. The working point of the centrifugal pump is based on the balance between the supply and demand of energy of the pump and the pipeline system. As long as one of the two situations changes, the working point will shift. The change of operating point is caused by two aspects: first, the change of piping system characteristic curve, such as valve throttling; Second, the characteristics of the water pump itself curve change, such as frequency conversion speed, cutting impeller, water pump series or parallel.

The following methods are analyzed and compared:
Valve closure: the simplest way to change the centrifugal pump flow is to adjust the pump outlet valve opening, and the pump speed remains unchanged (generally rated speed), its essence is to change the position of the pipeline characteristics curve to change the pump working point. When the valve is turned off, the local resistance of the pipe increases and the working point of the pump moves to the left, thus reducing the corresponding flow. When the valve is completely closed, it is equivalent to infinite resistance and zero flow. At this time, the pipeline characteristic curve coincides with the vertical coordinate. When the valve is closed to control the flow, the water supply capacity of the pump itself remains unchanged, the lift characteristics remain unchanged, and the pipe resistance characteristics will change with the change of the valve opening. This method is simple to operate, continuous flow, can be adjusted at will between a certain maximum flow and zero, and no additional investment, applicable to a wide range of occasions. But throttling regulation is to consume the excess energy of the centrifugal pump to maintain a certain amount of supply, and the efficiency of the centrifugal pump will also decline, which is not reasonable economically.

Variable frequency speed regulation and deviation of working point from high efficiency zone are the basic conditions for pump speed regulation. When the pump speed changes, the valve opening remains the same (usually the maximum opening), the piping system characteristics remain the same, and the water supply capacity and lift characteristics change accordingly.
In the case of the required flow less than the rated flow, the head of variable frequency speed regulation is smaller than the valve throttling, so the need for variable frequency speed regulation of water supply power is smaller than the valve throttling. Obviously, compared with valve throttling, frequency conversion speed saving effect is very prominent, centrifugal pump work efficiency is higher. In addition, using variable frequency speed regulation, is not only beneficial to reduce the risk of developing cavitation in centrifugal pump, and can be controlled by the acc/dec time to extend the preset starting/stopping process, thus greatly reducing the dynamic torque, thus eliminated vary greatly and destructive water hammer effect, greatly extend the life span of the pump and piping system.

In fact, frequency conversion speed regulation also has limitations, in addition to large investment, higher maintenance costs, when the pump speed will be too large will cause efficiency decline, beyond the scope of the pump proportional law, it is impossible to unlimited speed.

Cutting impeller: when the speed is certain, the pump pressure head, flow and impeller diameter. For the same type of pump, cutting method can be used to change the characteristics of the pump curve.

The cutting law is based on a large number of perceptual test data, it thinks that if the cutting amount of the impeller is controlled within a certain limit (the cutting limit is related to the specific revolution of the pump), then the corresponding efficiency of the pump before and after the cutting can be regarded as unchanged. Cutting impeller is a simple and easy way to change the performance of water pump, that is, the so-called reducing diameter adjustment, which to a certain extent solves the contradiction between the limited type and specification of water pump and the diversity of water supply object requirements, and expands the scope of use of water pump. Of course, the cutting impeller is an irreversible process; the user must be accurately calculated and measured before the economic rationality can be implemented.

Series parallel: water pump series refers to the outlet of a pump to the inlet of another pump to transfer fluid. In the most simple two same model and same performance of a centrifugal pump series, for example: series performance curve is equivalent to a single pump performance curve of the head under the same flow superposition, and obtain a series of flow and head are bigger than single pump working point B, but are short of single pump 2 times the size of, this is because the pump series after on the one hand, the increase in lift is greater than the pipeline resistance increases, the surplus of lift force flow increases, the increase of flow rate and increase the resistance on the other hand, inhibit the increase of total head. , water pump series operation, must pay attention to the latter a pump can withstand the boost. Before the start of each pump outlet valve should be closed, and then the sequence of open the pump and valve to supply water.

Water pump parallel refers to two or more than two pumps to the same pressure pipeline delivery of fluid; its purpose is to increase the flow in the same head. Still in the most simple of two same type, same centrifugal pump in parallel as an example, the performance of parallel performance curve is equivalent to a single pump performance curve of the flow under the condition of the head is equal to superposition, the capacity and head of the parallel working point A were bigger than single pump working point B, but consider the pipe resistance factor, also short of single pump 2 times.

If the purpose is purely to increase the flow rate, then whether to use parallel or series should depend on the flatness of the pipeline characteristic curve. The flatter the pipeline characteristic curve is, the more the flow rate after parallel is close to twice that of the single pump operation, so that the flow rate is greater than that in series, which is more conducive to operation.

Conclusion: Although valve throttling may cause energy loss and waste, it is still a fast and easy flow regulation 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 impeller is generally used for cleaning water pump, because of the change of the structure of the pump, the generality is poor; Pump series and parallel is only suitable for a single pump cannot meet the task of conveying the situation, and series or parallel too many but not economic. In practical application, we should consider from many aspects and synthesize the best scheme in various flow regulation methods to ensure the efficient operation of the centrifugal pump.