Solve the pump cavitation phenomenon

      The pressure drop is the condition of cavitation, the rapid rupture of a large number of bubbles in water is the internal cause of cavitation. Therefore, to improve and reduce the cavitation damage of the main drainage pump impeller of the coal mine, it is necessary to increase the inlet pressure of the pump .

In the coal mine pump is equivalent to the "heart" of the coal mine, the performance of the pump has a direct impact on the production safety of coal mines, coal mine drainage equipment to ensure the integrity and stability of the main drainage pump is of great significance. The pump impeller is the pump of the mechanical energy into water pressure and kinetic energy. In terms of the pump, as long as the impeller performance can guarantee the normal operation of the pump. However, for a long time, one of the major problems plaguing the normal operation of the pump is the damage of the impeller of the pump. The main reason for this failure is often the cavitation phenomenon of the impeller. Therefore, the reason why the pump produces cavitation and how to prevent it Cavitation is a crucial issue. This is also the designer in the design of the coal mine drainage equipment design, the installation process to consider the key issues that the actual use of the main pump impeller to destroy the main reason is due to the water in the solid particles Wear, increase the severity of mine water, thereby increasing the possibility of cavitation occurs. To this end, this article specializes in the specific circumstances of the main drainage pump coal mine, on the existence of solid particles impeller damage research.

1, cavitation phenomenon causes

When the absolute pressure at the pump inlet is reduced to the cavitation pressure at the current water temperature, the water evaporates. Water forms gas at the water inlet, creating many small bubbles in the water inlet. When these small bubbles flow into the high pressure area with water, the bubbles rapidly rupture and the surrounding liquid immediately fills the original bubbles and cavities. Due to the short bubble rupture time, a hydraulic impact up to several hundred MPa is formed. Bubbles continue to form and burst, a huge hydraulic impact to the frequency of tens of thousands of times per second repeatedly on the impeller, a long time, it will gradually impeller blades due to fatigue and flaking; the same time, the bubble is also mixed There are some active gases (such as oxygen), the smooth layer of metal due to electrolysis and gradually become rough. Metal surface roughness is destroyed, the more rapid mechanical erosion. In addition, the formation and rupture of bubbles in the process, the overcurrent will make the temperature difference between the two ends, the cold end and the hot end of the formation of galvanic couple potential difference, so that the metal surface electrolysis, smooth metal layer due to electrolysis gradually Become rough. In the mechanical erosion, chemical corrosion and electrochemical joint role, the metal surface quickly appeared honeycomb pitting, and gradually formed voids and damage, this phenomenon is called cavitation.

2, cavitation phenomenon after the serious impact on the pump

When cavitation develops to a certain extent, it will affect the performance of the pump and prevent its normal operation. Mainly for the following aspects:

(1) pump performance changes Cavitation at birth, the pump outside the characteristics of no significant effect. Cavitation development to a certain extent, the pump power, efficiency, flow and head and other parameters will suddenly decline. When cavitation is fully developed, the effective flow area of ​​the flow will be reduced a lot, resulting in interruption of flow, can not work.

(2) Caused by vibration and noise Bubble burst, the liquid particles impact each other, resulting in noise and vibration of the unit, both excited each other so that the pump produces a strong vibration, known as cavitation resonance phenomenon.

(3) The destruction of the surface of the overcurrent component Cavitation damage will greatly shorten the life of the pump, erosion and corrosion serious, it will produce leaf rupture or perforation and other major accidents.

3, impeller wear experimental analysis

Qualitative analysis of the location and extent of the impeller damage using the method of painting on the impeller blades Experimental conditions: (1) Replace the mineral water with a certain amount of sand to make the density and mineral water the same (ρ = 1025kg / m3) , Respectively, in the case of cavitation and cavitation does not occur under the wear test: (2) with water in the case of cavitation wear test. Pump running in the above two conditions 10h, the impeller damage observed and analyzed. In practice, the most serious damage to the impeller of the main drainage pump in the coal mine is the inlet pressure surface and the water absorption surface of the blade. The material of the blade is peeled off to form pits. Even severe blade spalling occurs: the pressure surface at the blade outlet is relatively worn. Back cover from the blade inlet to the central blade uneven, subject to varying degrees of damage. Practice has proved that only the blade coated with paint imports more obvious signs of solid particles friction, the entire impeller vane flow channel without cavitation point. Cavitation loss, the blade inlet, the blade inlet pressure surface, the blade outlet pressure surface of the paint has more serious friction of solid particles traces, especially the blade suction surface has a large area of ​​cavitation point exists in the injury site and The extent is basically the same as the actual damage condition of the main drainage pump impeller, which is better than that without cavitation. Under the condition 2, there is no abrasion mark in the experiment of the clear water, only the cavitation point and the degree of cavitation are much smaller than that of the sandy water, which shows that the water containing the sand grains enlarges the cavitation range. It can be seen from the above that under the same fluid medium conditions, the cavitation damage is more serious than that without cavitation; under the same cavitation condition, the medium with solid particles is more serious than without solid particles. Therefore, cavitation is the main cause of damage to the impeller of coal mine drainage pump. Elimination of cavitation can reduce the damage of water pump, and the elimination of solid particles in water can reduce the damage to a greater extent. The experimental results show that although it is unclear whether cavitation has any relationship with the size of solid particles, the objective existence of damage can be verified.

4, measures and conclusions

It can be seen from the above experiment that the pressure drop is the condition of cavitation, the rapid rupture of a large number of bubbles in water is the internal cause of cavitation. Therefore, in order to improve and alleviate the cavitation destruction of the main drainage pump impeller of coal mine, the following two aspects should be improved:

(1) Design, manufacture. 1) Change the design of the impeller shape and optimize the structural parameters of the impeller to improve the external conditions generated by the cavitation; 2) The material with good anti-cavitation performance should be selected for the components flowing through the blades and other water; 3) Reduce the pressure loss of the suction pipe Σ h, the suction pipe system, including the end of the valve, water filters, pipes, elbows, etc., so that these parts of the installation design is reasonable and reduce losses, but also reduce the pump cavitation phenomenon an important way; 4) to reduce the pump itself must Of the NPSH, to this end, may be appropriate to increase hand-wheel impeller diameter inhalation, or bottomless valve drainage.

(2) the use of. 1) In the installation of the allowed conditions, minimize the suction height of the pump. This allows the pump operation to allow larger allowance NPS. Under normal circumstances, the installation height of 2 ~ 3.5m, reduce pump cavitation phenomenon. 2) To reduce the density of well water, mine water containing pulverized coal and silt, in order to reduce the density of mineral water to reduce pump cavitation, sedimentation should be done before the mine drainage. 3) Reduce the average flow of water into the pump suction.

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