Multi-stage centrifugal explosion-proof pumps are widely used in mining, water conservancy, industry and other industries. The selection of operating conditions and the analysis of energy consumption have also been paid more and more attention. The so-called operating point refers to the actual water output, head, shaft power, efficiency, and suction vacuum height of the multi-stage pump device at a certain instant. It represents the working capacity of the pump. Generally, the flow and pressure head of the multi-stage centrifugal explosion-proof pump may be inconsistent with the piping system, or due to changes in production tasks and process requirements, the flow of the multi-stage pump needs to be adjusted. The essence is to change the working condition of the centrifugal pump. . In addition to the correct selection of the centrifugal pump in the engineering design stage, the selection of working conditions in the actual use of the multi-stage centrifugal pump will also directly affect the user's energy consumption and cost. Therefore, how to reasonably change the working conditions of the multi-stage centrifugal pump is particularly important.
The working principle of the multi-stage centrifugal explosion-proof pump is to convert the mechanical energy of the high-speed rotation of the motor into the kinetic energy and potential energy of the liquid to be lifted, which is a process of energy transfer and transformation. According to this feature, the operating point of the multistage centrifugal pump is based on the balance of the energy supply and demand relationship between the pump and the piping system. As long as one of the two conditions changes, the operating point will shift. The change of operating point is caused by two aspects: one. The characteristic curve of the piping system changes, such as valve throttling; two. The characteristic curve of the pump itself is changed, such as frequency conversion speed regulation, cutting impeller.
The main methods of flow adjustment of multi-stage centrifugal explosion-proof pumps are analyzed and compared below:
1) 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 the same (usually the rated speed). The essence is to change the position of the pipeline characteristic curve to change the pump's operating point. When the valve is closed, the local resistance of the pipeline increases, the pump operating point moves to the left, and the corresponding flow decreases. When the valve is fully closed, it is equivalent to infinite resistance and zero flow. At this time, the pipeline characteristic curve coincides with the ordinate. When the valve is closed to control the flow rate, the water supply capacity of the water pump itself remains unchanged, the head characteristics remain unchanged, and the pipe resistance characteristics will change with the change of the valve opening. This method is easy to operate, continuous in flow, and can be adjusted at will between a certain maximum flow and zero, without additional investment, and has a wide range of applications. However, the throttling adjustment is to consume the excess energy of the multi-stage centrifugal pump to maintain a certain supply, and the efficiency of the multi-stage centrifugal pump will also decrease accordingly, which is not economically reasonable.
2) Frequency conversion speed regulation
The deviation of the operating point from the high-efficiency zone is the basic condition for the pump to adjust the speed. When the speed of the water pump changes, the valve opening remains the same (usually the maximum opening), the piping system characteristics 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 for 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 working efficiency of multi-stage centrifugal pumps is higher. In addition, the use of variable frequency speed regulation not only helps to reduce the possibility of cavitation in multi-stage centrifugal pumps, but also prolongs the startup/shutdown process by presetting the speed-up/speed-down time, so that the dynamic torque is greatly improved. Reduced, thereby largely eliminating the extremely destructive water hammer effect, greatly extending the life of the pump and piping system.
In fact, variable frequency speed regulation also has its limitations. In addition to higher investment and higher maintenance costs, when the pump speed change is too large, it will cause a drop in efficiency. If the pump ratio law is exceeded, unlimited speed control is impossible.
3) Cutting impeller
When the speed is constant, the pressure head and flow rate of the pump are all related to the diameter of the impeller. 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 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 impeller is a simple and easy way to change the performance of the water pump, that is, the so-called variable diameter adjustment. It solves the contradiction between the limited type and specification of the water pump and the diversity of the water supply object requirements to a certain extent. The scope of use of stage pumps. Of course, cutting the impeller is an irreversible process, and the user must perform accurate calculations and weigh the economic rationality before implementation.
