Analysis Of Common Adjustment Methods For Centrifugal Pumps

Dec 11, 2024

The operating point of a centrifugal pump is when there is a change in the energy supply and demand of the pump and pipeline system, and the operating point will correspondingly change. The change in operating conditions is caused by two factors:
1, Changes in pipeline system characteristic curves, such as valve throttling;
2, The characteristic curve of the water pump itself changes, such as variable frequency speed regulation, cutting impeller, water pump series or parallel connection.
1, Valve throttling
The direct method to change the flow rate of a centrifugal pump is to adjust the opening of the pump outlet valve, while keeping the rated speed unchanged. Essentially, changing the position of the pipeline characteristic curve changes the operating point of the pump. For example, the intersection point A between the pump characteristic curve Q-H and the pipeline characteristic curve Q - ∑ h is the limit operating point of the pump when the valve is fully opened. When the valve is closed, the local resistance in the pipeline increases, and the operating point of the water pump shifts to point B to the left, resulting in a corresponding decrease in flow rate
2, Variable frequency speed regulation
The deviation zone of the operating point is the basic condition for the speed regulation of the water pump. When the speed of the water pump changes, the valve opening remains unchanged, the pipeline system characteristics remain unchanged, and the water supply capacity and head characteristics change accordingly. As shown in Figure 2, A is the equilibrium operating point (also known as the working point) of the water pump, corresponding to the efficiency η a. To reduce the flow rate, the speed can be lowered. At this point, the operating point is B, corresponding to the efficiency η b, and the water pump is still in the zone. Variable frequency speed regulation also has limitations. In addition to high investment and maintenance costs, when the water pump speed changes too much, it will cause a decrease in efficiency, exceeding the range of the pump proportional law, and it is impossible to achieve unlimited speed regulation.
3, Cutting impeller
The cutting law holds that if the cutting amount of the impeller is controlled within a certain limit, the corresponding efficiency of the water pump before and after cutting is considered constant. Cutting impellers is a simple and feasible way to change the performance of water pumps, and variable diameter adjustment has to some extent solved the limitations of water pump types and specifications, expanding the scope of water pump use.
4, Series and parallel connection of water pumps
When running water pumps in series, it is necessary to pay attention to whether the latter pump can withstand the pressure increase. Before starting, the outlet valve of each pump should be closed, and then the pump and valve should be opened in sequence to supply water to the outside.
Parallel connection of water pumps refers to two or more pumps delivering fluid to the same pressure pipeline, with the aim of increasing flow at the same pressure head.
If the purpose is only to increase the flow rate, the flatter the pipeline characteristic curve, the greater the flow rate in parallel than in series, which is more conducive to operation.
5, Conclusion
Although valve throttling can cause energy loss and waste, it is still a quick and easy way to regulate flow in some simple situations; Variable frequency speed regulation is increasingly favored by users due to its good energy-saving effect and high degree of automation; Cutting impellers are generally used in clean water pumps, but due to changes in the pump structure, their versatility is poor; Series and parallel connection of water pumps are only suitable for situations where a single pump cannot meet the conveying task, and having too many series or parallel pumps is not economical. In practical applications, suitable water pumps should be considered from multiple aspects in order to be economically reasonable and long-lasting.