DG type high pressure boiler feed water pump is used as high pressure boiler feed water or other high pressure clean water pumps. High-pressure boiler feed water pumps are widely used in industry, water conservancy, 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 boiler feedwater pump device at a certain instant. It indicates the working capacity of the feedwater pump. Generally, the flow and pressure head of the DG type high-pressure boiler feedwater pump may be inconsistent with the piping system, or due to changes in production tasks and process requirements, the flow of the feedwater pump needs to be adjusted. The essence is to change the working condition of the feedwater pump. . In addition to the correct selection of feedwater pumps in the engineering design stage, the selection of operating points in the actual use of boiler feedwater pumps will also directly affect the user's energy consumption and cost. Therefore, how to reasonably change the operating point of the boiler feed water pump is particularly important.
The working principle of the DG high-pressure boiler feed water 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 characteristic, the operating point of the boiler feed water pump is based on the balance of the energy supply and demand relationship between the feed water 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 feed water pump itself is changed, such as frequency conversion speed regulation, cutting impeller.
The following analysis and comparison of the three DG type high pressure boiler feed water pump flow adjustment methods:
1) Valve throttling
The easiest way to change the flow rate of the feedwater pump is to adjust the opening of the pump outlet valve, while the feedwater 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 operating point. When the valve is closed, the local resistance of the pipeline increases, the operating point of the feedwater pump 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, the water supply capacity of the feed water pump itself will not change, the head characteristic will not change, and the pipe resistance characteristic 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. But the throttling adjustment is to consume the excess energy of the boiler feed water pump to maintain a certain amount of supply, and the efficiency of the boiler feed water 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 feed pump to adjust the speed. When the speed of the feed 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 boiler feed water pump has a higher working efficiency. In addition, the use of variable frequency speed regulation not only helps reduce the possibility of cavitation in the boiler feed water pump, but also prolongs the start/stop process by presetting the speed up/down time, so that the dynamic torque is greatly reduced. It is small, which eliminates the destructive water hammer effect to a large extent, and greatly extends the life of the water supply pump and piping system.
In fact, variable frequency speed regulation also has limitations. In addition to higher investment and higher maintenance costs, when the feedwater pump speed is too large, it will cause a drop in efficiency. If it exceeds the range of the pump proportional law, it is impossible to adjust the speed without restriction.
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 feedwater pump), the corresponding efficiency of the feedwater pump before and after cutting can be regarded as constant. Cutting the impeller is a simple and easy way to change the performance of the feedwater pump, the so-called variable diameter adjustment, which to a certain extent solves the contradiction between the limited type and specification of the feedwater pump and the diversity of the requirements of the water supply object. The scope of application of multi-stage pumps has been improved. Of course, cutting the impeller is an irreversible process, and the user must perform accurate calculations and weigh the economic rationality before implementation.
