Specific speed pump, the pump specific speed calculation formula

In the design of the pump, to extend the series or using the developed hydraulic model was optimized, reference, tend to adopt similar design, similarity law can express the relationship between the two, but if the similarity law is used to express a series of pumps, very inconvenient, can't intuitively know the law, on the basis of the similarity law, therefore, pump scholars up the discriminant of specific speed.

similarity law only shows that the pump in the similar point of the relationship between the performance parameters. Because of the different structure and hydraulic performance of pump impeller, the size of the size are different, in order to categorize the pump, the pump composed of a series of the same type, which requires a could reflect the comprehensive parameter of water pumps in common, as the basis of water pump normalization. The comprehensive parameter is the pump than revolution. Specific speed is derived on the basis of the similarity law of a including flow, lift and revolution, comprehensive features, it is the basis of the calculation of pump structure parameters. Than for a particular pump rotation speed has a lot of, such as specific speed under the maximum flow and minimum flow under specific speed. But we usually told by specific speed pump is a pump in the best efficiency of specific speed.

Specific Speed than revolutions or Specific Speed, also known as English name Specific Speed, in the Chinese symbol of ns. Specific speed from the dimensionless analysis, is used to compare the geometry similar but different parameters of the pump. Than the number of revolutions of the pump on the numerical value is equal to the geometric similar pump in the flow rate is 0. 075 m/s and head up to 1 m when revolutions.

1, the specific speed pumps to define

under the highest efficiency, reduced the size according to certain proportion of water pump, Or expand) , making the effective power Pu = 0. 7355 kw, head Hm = 1 m, flow Qm = 0. 075 m3 / s, then, the model of the pump speed is called a similar to its actual than the speed of water pump.

2. The pump specific speed

according to the law of similarity calculation formula can be obtained:

regulation calculation unit in our country is:

Q - — Flow m3 / s ( The double suction pump take 1/2) ;

       H— — Lift m ( The multistage pump for single-stage head) ;

       n— — Speed r/min.

in dealing with daily work, will meet specific speed expression in different countries. Specific speed expression way is different in different countries, and finally the unit also differ. The conversion relationship between the different specific speed formula is as follows:

3, application problems that should be noticed in the pump specific speed formula

1 Q and H refers to the capacity and head of the high efficiency water pump, n for the design of the pump speed. To the same pumps than speed for a certain value.

(2) the type of Q, H is refers to the single-stage single-suction centrifugal pump design flow and design head. For single stage double suction centrifugal pump with Q / 2 generation into the calculation; For horizontal or vertical multistage pump, multi-stage pump should be the primary impeller head in computing.

(3) than the revolution is about 20 ° C according to the pumping water.

4. Various parameters of the unit should be consistent with the model of the pump unit.

4, compare the discussion of the rotational speed

1) for any pumps, specific speed is not a dimensionless number. Because it is not the actual speed, it is only used to compare a common standard of various kinds of pump performance. As a result, its meaning not have much use, usually omit don't write. The same pump under different conditions have different ns value, as the similarity criterion refers to the value corresponding to the highest efficiency point conditions.

(2) the specific speed is, according to the similarity theory can be used as a similarity criterion, namely the mean geometric similar pump under similar conditions ns values are equal. Conversely, in general ns value equal to the pump, is similar to that of geometric similarity and movement. But can't say ns equal pump must be geometric similarity. This is because, constitute the parameters of the pump geometry is for instance with ns = 500 can be made of axial flow pump, can also be made of the mixed flow, the same low than turn water pump impeller can use six, seven pieces of blade is also available. These geometric similar pump, ns may be equal. But for the same kind pump, equals the ns, if you want to make the pump performance is good, that is geometry in line with the passenger flow law of inkstone, its geometry is not very big, so, in general are geometrically similar.

3. Although specific speed is due to the number, but does not affect it as the essence of the similarity criterion of significance. For pump geometric similarity, under similar conditions, with unified unit calculation of ns values are equal.

(4) the pump specific speed is according to the similarity relation, but which contains the actual pump main parameter Q, H, n values. Therefore, it reflects the actual pump main performance; Can be seen from the water pump than revolution formula: when the speed of the pump must be, the greater the ns, show that the flow of water pump is larger, the lower the head; On the contrary, the smaller than the number of revolutions, show that the flow of water pump is smaller, the higher the lift.

(5) the shape, size, performance and efficiency of the pump impeller is changed according to the specific speed. Water pump with specific speed can be classified, listed in the following table:

6. The performance of the pump with specific speed. Therefore, the specific speed is different, the shape of the water pump performance curve is different also, with the water pump specific speed pump performance can be analyzed.

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