Impeller cutting law and cutting parabola of AF foam pump

A pump with constant speed has only one H-Q characteristic curve. In order to expand the working range of the pump, the method of cutting the outer diameter of the impeller is often used to change the working range from "one line" to "one surface". The relationship between the performance parameters before and after impeller cutting is approximately determined by the cutting law.

1. Cutting law

When the impeller cutting amount is small, it can be considered that the outlet angle and flow area of the blades before and after cutting are approximately the same, and the pump efficiency is approximately the same, that is, when the rotating speed is constant, n '= n, (the parameter with skimming represents the parameter after cutting). In this way, before and after cutting, the impeller outlet clearance triangle is close to the phase system. The relationship between the flow before and after cutting is as follows:

The relationship between lift is:

The relationship between power is:

Equation (1-64), equation (1-65) and equation (1-66) are expressions of cutting law. They show that the characteristic curve of the pump can be changed under the condition of constant pump speed by reducing D2.

2. Cutting parabola

For the same pump, at the same speed, after the impeller is cut one by one, the line of each corresponding operating point is called cutting parabola. From formula (1-64) and formula (1-65), we can get:

Combine the above two formulas to obtain:

Where k is a constant varying with the operating conditions.

Equation (1-67) shows that the proportional relationship between head and flow before and after impeller cutting is a quadratic parabola equation passing through the origin, which is called cutting parabola. The points on the cutting parabola are the corresponding working points of the impeller before and after cutting, but they are not similar working conditions, because the impeller before and after cutting does not maintain geometric similarity. When the amount of impeller cutting is small, the efficiency of corresponding points is considered to be equal, so the parabola cutting is also called equal efficiency line.

The cutting amount of impeller should not be too large, otherwise the flow area and blade outlet angle will change greatly, the efficiency will decrease obviously, which will make the cutting law invalid, so the maximum cutting amount of impeller is specified. It is related to the impeller shape (specific speed NS), as shown in table 1-8. AF foam pump

According to table 1-8, each pump can get a Q-H characteristic curve corresponding to the minimum allowable cutting diameter (d2m), as shown in Figure 1-38.