Centrifugal sand pumps are widely used in water conservancy, chemical industry and other industries, and the selection of their operating points and the analysis of energy consumption are also increasingly valued. The so-called working point refers to the actual water output, head, shaft power, efficiency and suction vacuum height of the sand pump water 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, the flow rate of the pump needs to be adjusted, which is essentially 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 sand pumps 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: 1. The characteristic curve of the pipeline system changes, such as valve throttling; 2. The characteristic curve of the pump itself changes, such as frequency conversion speed regulation, cutting impeller, and water pumps connected in series or in parallel.
The following methods are analyzed and compared:
- Valve throttling
The easiest way to change the flow rate of the centrifugal sand pumps 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 flow is controlled by closing the small valve, 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 (the shaded part in the figure) 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 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. Compared with valve throttling, the energy-saving effect of frequency conversion speed regulation is very prominent, and the work efficiency of 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 extend the start/stop process by presetting the speed up and 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.
- 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. Assuming that the diameter of the original impeller of the centrifugal pump is D, the flow rate is Q, the lift is H, and the power is P, and the diameter of the impeller after cutting is D′, the flow rate is Q’, the head is H’, and the power is P’, then the relationship between them is : The cutting law is as follows:
- For low and medium specific speed pumps:
Q1/Q2=(D1/D2)2 H1/H2=(D1/D2)2 N1/N2=(D1/D2)4
- For high specific speed pump:
Q1/Q2=D1/D2 H1/H2=(D1/D2)2 N1/N2=(D1/D2)3
- Centrifugal sand pumpsin series and in parallel
Pumps in series means that the outlet of one pump delivers fluid to the inlet of another pump. Take the simplest two centrifugal pumps of the same model and same performance in series as an example: the performance curve of the series is equivalent to the head of the performance curve of a single pump. When the flow rate is the same, the flow and head of the series working point A are higher than that of the single pump. The working point B of the pump is large, but it is less than twice that of a single pump. This is because after the pumps are connected in series, the increase in head is greater than the increase in pipeline resistance, resulting in an increase in flow due to the excess head. The increase increases the resistance and restrains the increase of the total lift. 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, whether to use parallel 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 parallel connection is to twice that of a single pump, so that When connected in series, the flow rate is greater, which is more conducive to operation.
