Do you know how many ways to adjust the capacity?Let us talk about it today.
Centrifugal pump is widely used in water conservancy, chemical industry and other industries. The selection of its operating point and the analysis of energy consumption are also paid more and more attention. The so-called operating point refers to the actual water output, head, shaft power, efficiency and vacuum height of the pump device at a certain instant, which represents the working capacity of the pump. Generally, the flow and pressure head of centrifugal pump may be inconsistent with the pipeline system, or due to the change of production task and process requirements, the flow of pump needs to be adjusted, which is essentially to change the operating point of centrifugal pump. In addition to the correct selection of centrifugal pump in the engineering design stage, the selection of operating point in the actual use of centrifugal pump will also directly affect the energy consumption and cost of users. Therefore, how to reasonably change the operating point of centrifugal pump is particularly important.
The working principle of centrifugal pump is to convert the mechanical energy of motor rotating at high speed into the kinetic energy and potential energy of the lifted liquid, which is a process of energy transfer and conversion. According to this feature, the operating point of centrifugal pump is based on the balance of energy supply and demand of pump and pipeline system. As long as one of the two conditions changes, the operating point will be transferred. The change of operating point is caused by two aspects: one is the change of characteristic curve of pipeline system, such as valve throttling; the other is the change of characteristic curve of water pump itself, such as frequency control, impeller cutting, series or parallel connection of water pump.
The following are the analysis and comparison of these methods:
1. Valve throttling
The simplest way to change the flow of centrifugal pump is to adjust the opening of the valve at the pump outlet, while the pump speed remains unchanged (generally rated speed), which is essentially 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 working point of the water pump moves to the left, and the corresponding flow decreases. When the valve is fully closed, the resistance is infinite and the flow is zero. At this time, the pipeline characteristic curve coincides with the ordinate. When the small valve is closed to control the flow, the water supply capacity of the pump itself and the head characteristics are unchanged, and the pipe resistance characteristics will change with the change of the valve opening. This method is easy to operate and has continuous flow. It can be adjusted between a certain maximum flow and zero at will without additional investment. It is widely used. But throttling regulation is to consume the extra energy of centrifugal pump to maintain a certain supply, and the efficiency of centrifugal pump will also decline, which is not reasonable economically.
2. Cutting impeller
When the speed is fixed, the pressure head and flow of the pump are related to the impeller diameter. For the same type of pump, the cutting method can be used to change the characteristic curve of the pump.
The cutting law is based on a large number of perceptual test data. It holds that if the cutting amount of impeller is controlled within a certain limit (the cutting limit is related to the specific speed of water pump), the corresponding efficiency of water pump before and after 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 variable diameter regulation, which to a certain extent solves the contradiction between the limitation of the 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 impeller cutting is an irreversible process, users must go through accurate calculation and measure the economic rationality before implementation.
3. Pumps in series and parallel
The pump in series refers to the delivery of fluid from the outlet of one pump to the inlet of another pump. Take the simplest two identical centrifugal pumps in the same type and the same performance as an example: as shown in Fig. 3, the series performance curve is equivalent to the head of the single pump performance curve which is superimposed on the same flow rate. The flow and lift of the A at the series operation point are larger than that of the single pump working point B (official account number: Pump housekeeper), but they can not reach 2 times of the single pump, because the pump head is connected in series and the lift head is on the one hand. On the other hand, the increase of flow makes the resistance increase, which restrains the increase of total head. When the water pump operates in series, it must be noted whether the latter pump can bear the pressure rise. Before starting, the outlet valve of each pump shall be closed, and then the pump and valve shall be opened in sequence to supply water.
Parallel connection of water pump means that two or more pumps deliver fluid to the same pressure pipeline, the purpose of which is to increase the flow at the same time when the pressure head is the same. Taking the most simple two centrifugal pumps of the same model and performance as an example, the parallel performance curve is equivalent to the flow of single pump performance curve when the head is equal. The flow and head of parallel operation point a are larger than that of single pump operation point B, but considering the pipe resistance factor, the flow and head of parallel operation point a are less than two times of that of single pump.
If the purpose is purely to increase the flow, then whether to use parallel or series connection should depend on the flatness of the pipeline characteristic curve. The flatter the pipeline characteristic curve is, the flow after parallel connection will be closer to 2 times of that of single pump operation, so that the flow is larger than that of series connection, which is more conducive to operation.
Although valve throttling will cause energy loss and waste, it is still a fast and easy way of flow regulation in some simple occasions; frequency conversion speed regulation is more and more popular with users because of its good energy saving effect and high automation degree; cutting impeller is generally used for clean water pump, because of the change of pump structure, the versatility is poor; series and parallel pumps are only suitable for a single set Pump can not meet the delivery task, and the number of series or parallel is too much, but not economic. In practical application, we should consider from many aspects and synthesize the best scheme among various flow regulation methods to ensure the efficient operation of centrifugal pump.