Common Flow Regulation Methods For Centrifugal Pumps

Centrifugal pumps are widely used in industries such as water conservancy and chemical engineering, and the selection of their operating points and analysis of energy consumption are increasingly valued. Usually, the flow rate and pressure head of a centrifugal pump may not be consistent 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 essentially changes the operating point of the centrifugal pump. In addition to the correctness of centrifugal pump selection during the engineering design stage, the selection of operating points in the actual use of centrifugal pumps will also directly affect the energy consumption and cost of users. Therefore, it is particularly important to reasonably change the operating point of a centrifugal pump.
The working principle of a centrifugal pump is to convert the mechanical energy generated by the high-speed rotation of the electric 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 a centrifugal pump is established on the balance of energy supply and demand between the water pump and pipeline system. As long as one of the two conditions changes, its operating point will shift. The change of operating point is caused by two aspects: 1. The characteristic curve of the water pump itself changes, such as impeller cutting. 2. Changes in pipeline system characteristic curves, such as valve throttling.
Below is an analysis and comparison of these two methods:
1. Impeller cutting
When the speed is constant, the pressure head and flow rate of the pump are related to the diameter of the impeller. For pumps of the same model, the cutting method can be used to change the characteristic curve of the pump. Assuming that the original impeller diameter of the centrifugal pump is D, flow rate is Q, head is H, and power is P, and the cut impeller diameter is D', flow rate is Q', head is H', and power is P', the relationship between them is:
The above three equations are collectively referred to as the cutting law of pumps. The cutting law is based on a large amount of perceptual experimental data. It believes that if the cutting amount of the impeller is controlled within a certain limit (which is related to the specific speed of the water pump), the corresponding efficiency of the water pump before and after cutting can be considered unchanged. Impeller cutting is a simple and feasible method to change the performance of water pumps, known as variable diameter adjustment. It solves the contradiction between the limited type and specifications of water pumps and the diversity of water supply object requirements to a certain extent, expanding the scope of use of water pumps. Of course, impeller cutting is an irreversible process, and users must undergo precise calculations and measure economic rationality before implementation.
2. Valve throttling
The simplest way to change the flow rate of a centrifugal pump is to adjust the opening of the pump outlet valve, while keeping the pump speed constant (usually at the rated speed). Its essence is to change the position of the pipeline characteristic curve to change the operating point of the pump. When the valve is turned down to control flow, the water supply capacity of the pump itself remains unchanged, the head characteristic remains unchanged, and the pipe resistance characteristic will change with the change of valve opening. This method is easy to operate, with continuous flow, and can be adjusted freely between a certain maximum flow rate and zero, without the need for additional investment. It is widely applicable in various situations. But throttling regulation is to maintain a certain supply volume by consuming excess energy from the centrifugal pump, and the efficiency of the centrifugal pump will also decrease, which is not economically reasonable.
Nowadays, the preferred adjustment method for many enterprises is impeller cutting, as it can save energy and reduce consumption. However, accurate calculations must be made before impeller cutting to ensure that it can meet the operating conditions after cutting.