When designing water pumps, in order to expand the series or use the developed hydraulic model for optimization and reference, similar designs are often used. The law of similarity can express the relationship between the two, but it is very inconvenient to use the law of similarity to express a series of pumps. , The law cannot be seen intuitively, so on the basis of the law of similarity, pump scholars came up with the discriminant number of "specific speed".
The law of similarity only describes the relationship between the performance parameters of the pump at similar operating conditions. Due to the difference in pump impeller structure and hydraulic performance, the size of the pump is also different. In order to classify the pumps and group the same type of pumps into a series, it is necessary to have a comprehensive parameter that can reflect the common characteristics of the pump, as the standardization of the pump Foundation. This comprehensive parameter is the specific speed of the pump. The specific speed is a comprehensive characteristic number derived on the basis of the law of similarity, including flow, head, and revolutions. It is the basis for calculating pump structural parameters. For a particular pump, there are many specific speeds, such as the specific speed at the maximum flow rate and the specific speed at the minimum flow rate. But usually the specific speed of the pump we talk about is the specific speed of the pump at the best efficiency.
The specific speed is also called the specific speed or specific speed. The English name is Specific Speed, which means ns in the Chinese symbol. The specific speed is derived from a dimensionless analysis and is used to compare pumps with similar geometry but different parameters. The specific revolution of the pump is numerically equal to the revolution of a geometrically similar pump at a flow rate of 0.075 m/s and a head of 1 m.
1. Definition of specific speed of water pump
At the highest efficiency, the size of the pump is reduced (or enlarged) according to a certain proportion, so that the effective power Pu=0.7355kw, the head Hm=1m, and the flow Qm=0.075m3/s. At this time, the speed of the model pump is called It is the specific speed of the actual water pump similar to it.
2. Calculation formula of pump specific speed
According to the law of similarity:
The unit of calculation in our country is:
Q——Flow m3/s (1/2 for double-suction split pump);
H-head m (single-stage head for multi-stage pumps);
n——Rotation speed r/min.
In daily work, you will encounter the specific speed expressions of different countries. Different countries have different expressions of specific speed, and the final unit is also different. The conversion relationship between different specific speed formulas is as follows:
3. Problems that should be paid attention to when applying water pump specific speed formula
①Q and H refer to the flow and head when the pump is at the highest efficiency, and n is the design speed of the pump. For the same pump, the specific speed is a certain value.
②Q and H in the formula refer to the design flow and design head of the single-stage single-suction centrifugal pump. For single-stage double-suction centrifugal pumps, substitute Q/2 into the calculation; for horizontal multi-stage pumps or vertical multi-stage pumps, the head of the first stage impeller should be substituted into the calculation.
③The specific speed is based on pumping clean water at about 20°C.
④The unit of each parameter should be consistent with that of the model pump.
4. Discussion of the comparison speed
① For any pump, the specific speed is not a factorless number. Since it is not the actual speed, it is just a common standard used to compare the performance of various pumps. Therefore, the meaning of the unit itself is of little use and is generally omitted. The same pump has different ns values under different working conditions. The similarity criterion refers to the value under the working condition corresponding to the highest efficiency point.
② The specific speed is derived based on the similarity theory, which can be used as a similarity criterion, that is, the ns value of pumps with similar geometrical similarity is equal under similar working conditions. On the contrary, generally speaking, pumps with the same ns value are geometrically similar and similar in motion. But it cannot be said that pumps with equal ns must be geometrically similar. This is because there are many parameters that constitute the geometric shape of the pump. For example, a pump with the same ns=500 can be made into an axial flow type or a mixed flow type. The same low specific rotary pump impeller can use 6 blades or 7 Pieces of leaves. For these geometrically dissimilar pumps, ns may be equal. But for the same type of pump, when ns is equal, if the pump performance is to be good, that is, the geometric shape conforms to the flow law of guest inkstone, the geometric shape will not differ greatly, so generally speaking, they are geometrically similar.
③Although the specific speed is a factorial frequency, it does not affect its substantive significance as a similarity criterion. For geometrically similar pumps, under similar working conditions, the ns value calculated in a unified unit is equal.
④ Although the specific speed of the water pump is obtained according to a similar relationship, it contains the main parameters Q, H, and n values of the actual water pump. Therefore, it reflects the main performance of the actual water pump; from the formula of the specific speed of the pump, it can be seen that when the speed of the pump is constant, the larger ns is, the greater the flow of the pump and the lower the head; on the contrary, the smaller the specific speed. , Indicating that the smaller the flow of the pump, the higher the head.
⑤The shape, size, performance and efficiency of the impeller of the water pump vary with the specific speed. The specific speed can be used to classify water pumps, as listed in the following table:
⑥The performance of the water pump changes with the specific speed. Therefore, the specific speed is different, the shape of the performance curve of the water pump is also different, that is, the specific speed of the water pump can analyze the performance of the water pump.
