图3-3-4 (b)上示出压水室计算断面上液流速度up.c与过流断面尺寸系数Kn和比转速m,之间的关系，从图上可知，n, 降低将导致压水室计算断面上流速显著增大。因为它个断面的磨损量，通常是最大的，所以它决定了整个压水室的寿命。
Effect of Slurry Pump Speed on the Life of Fragile Parts
The repair cycle related to replacement of worn parts is not only related to the service life of worn parts, but also to the relationship between parts determined by relative wear resistance.
In fact, the most significant part is the relationship between the wear volume and the specific speed of the pump wearing parts (impeller, impeller and inlet side of the pressure chamber), so that is the relationship between parts life and specific speed.
Under other conditions, the wear of parts is related to the flow rate of solid-liquid mixture and the concentration of solid particles. However, if no obvious separation of solid particles is observed in the hole of the part, it can be considered that the wear rate is only related to the velocity of liquid flow, and the concentration of solid particles is taken as a fixed value, which is equal to the average value. This is for sealers.
When the gap size is constant, the velocity of the mixture at the seal is proportional to the 1/2 power of the internal static pressure drop. The pressure drop is equal to the pressure difference between the outlet pressure of the impeller and the reaction force produced by the fluid flow distortion in the cavity. It can be approximated that the pressure, the back pressure in the cavity and the pressure difference between the two are proportional to the quadratic of the velocity of liquid flow. At this time, the velocity of the mixture at the seal is proportional to the 1/2 power of the static pressure. Because the wear on the surface of the seal is proportional to the quadratic velocity of the mixture and the number of particles passing through it, it is used for qualitative analysis to be proportional to the pressure, that is, to the 3/2 power of the head (because of P-H). (The leakage of the seal which determines the number of solid particles passing through the seal is proportional to the speed when other conditions are the same, that is, to H.)
When the flow rate and rotational speed are fixed, the lift is inversely proportional to the fourth third power of the specific speed. That is to say, when the solid-liquid mixture containing medium and fine particles is transported in the flow passage of the pump chamber, no obvious separation of solid particles is observed. Therefore, when comparing the wear of the water chamber, as mentioned above, the relationship between them and the velocity of liquid flow can be used, assuming that the concentration of solid particles is constant. However, during the transition to conveying gravel solid-liquid mixtures, the solid particles are strongly redistributed along the section of the water chamber, and their concentration increases significantly on the outer wall.
Figure 3-3-4 (b) shows the relationship between up.c of liquid flow velocity on the calculated section of the water chamber and the size coefficient Kn and specific speed m of the cross section. From the graph, it can be seen that n, lower will lead to a significant increase of flow velocity on the calculated section of the water chamber. Because the wear of this section is usually the largest, it determines the life of the whole water chamber.
Quantitative analysis shows that when the wear rate is proportional to the square of the liquid flow velocity, the head increases (i.e. the specific speed n, decreases), and when other parameters are constant, the wear rate of the pressurized water chamber increases sharply, that is, the life of the pressurized water chamber decreases. The relationship between the wear rate or life of the pressurized water chamber and the specific speed can be determined by comparing the fluid velocity calculated from the data in the second section of the third chapter of this chapter.
II. THE EFFECT OF SOLID PARTICLE CONCENTRATION
In the flow passage between impeller blades, with the increase of particle concentration on the blade front, the solid particles will be strongly redistributed. Therefore, when determining blade wear as a function of specific speed n, the local velocity of liquid flow and solid particle concentration must be taken into account. However, when pumping fine sediment, it should be considered that the wear of the outlet side (which determines the diameter of the impeller outlet) is not the wear of all working faces, but the wear of the outlet side (which determines the diameter of the impeller outlet) has an effect on the change of pump characteristics.
The pump with specific speed ns = 80 is most widely used. Therefore, in evaluating the relative life of vulnerable parts, the life of n = 80 pump parts is used as the standard.
Fig. 3-7-14 shows the approximate life relationship T/T=80 for pump parts when pumping medium-fine solid-liquid mixtures. Assume n, = 80 pump chamber life as 1. The life curve of impeller and impeller entry seals is drawn in the following two situations, that is, N, =80 pump pressure chamber life exceeds impeller life expectancy 1 times (dotted line) and 1.5 times (dot line).
From Figure 3-7-14, it can be seen that when the specific speed changes, the life of the pressurized water chamber and seal changes the most and the impeller changes the least. When ns = 60, the life of impeller and water chamber is approximately the same. When the specific speed increases, the life of water chamber exceeds the life of impeller. Comparing with the impeller, the pressurized water chamber has larger size, quality and corresponding cost; disassembly is quite difficult, so when reducing the life of the pressurized water chamber, the cost of technical service of the pump is increased, that is to say, the quality of use is reduced. In practice at home and abroad, the slurry pump with specific speed NN and < 80 is seldom used.
The above analysis of the relative life of pump parts is correct only when the solid-liquid mixture of fine particles is pumped by the pump. When the solid liquid mixture of large particles is pumped, the life of impeller is not determined by the wear volume at the outlet edge of the blade, but by the wear amount at the inlet edge of the blade, and the wear volume is independent of the specific speed. In addition, compared with the wear rate of other parts, the wear volume of the impeller entry seal is obviously reduced, because large particles do not enter the gap. Slurry pump