Determination of Basic Parameters of Centrifugal Pump Impeller
In the design of centrifugal pumps, the following design parameters are generally required: flow rate Q, head H, speed N, cavitation allowance NPSHr, and efficiency.
To preliminarily determine the structural scheme of the centrifugal tip based on the above parameters and calculate the specific speed Ns of the impeller, then use some empirical data (formulas) to determine the main geometric dimensions of the impeller.
In the hydraulic design of centrifugal and mixed flow pump impellers, it is assumed that the liquid flow inside the impeller is axisymmetric, and the velocity at any point in the flow field can be decomposed into axial velocity components and circumferential velocity components within the axial plane (including the plane of the impeller axis).
The distribution of axial velocity components is determined by the axial rotation projection of the impeller channel and the boundary conditions at the impeller inlet, while the distribution of circumferential velocity is determined by the distribution of axial velocity components and blade placement angles. Therefore, the first step in impeller design is to determine the axial velocity and circumferential velocity.
There are three methods to determine the main geometric dimensions of the impeller:
(1) Model conversion method. This method is simple and reliable. The specific method is to first use a centrifugal pump similar to the one designed, and then enlarge or reduce all geometric dimensions of its flow section.
(2) Speed coefficient method. The velocity coefficient method is essentially a similar design method. Compared with the model conversion method, its difference is that it is based on a series of similar centrifugal pumps, rather than on a single similar centrifugal pump. That is to say, the velocity coefficient method is based on the principle of similarity, using statistical coefficients of a series of excellent centrifugal pumps to calculate the main dimensions of the impeller.
(3) Theoretical calculation method. This method is based on the basic equation of the vane pump, and provides a relatively accurate calculation of the impeller outer diameter and blade outlet angle, usually requiring iterative calculations.
The above three methods are usually used in combination and complement each other.
