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How to avoid cavitation phenomenon of centrifugal liquid pump?
Cavitation is a pump damage process. Centrifugal pump in operation, the pressure inside the impeller is different, the inlet pressure is low, the outlet pressure is higher. The liquid gasification temperature is pressure related; the lower the pressure (or higher), the corresponding gasification temperature is lower (or higher). If the temperature of the liquid entering the pump is higher than the gasification temperature corresponding to the inlet pressure, some of the liquid will vaporize to form bubbles. When the bubbles are brought to the area with higher pressure by the liquid, the vapor will re-condense into a liquid due to the corresponding increase of the gasification temperature, and the bubbles rapidly rupture. At this moment, since the density of gas and liquid is different by a few hundred times, the bubbles suddenly condense and suddenly shrink in volume, and the surrounding liquid will rush to the space occupied by the bubbles at a very high speed and violently impact inside the liquid . If this phenomenon occurs on the surface of the blade, the metal material is eroded due to repeated high impact stress, so it is called cavitation. The external cause of the cavitation of the liquid pump, though a lot, is closely related to the installation and operation, for example, in addition to the structure of the pump itself. However, according to the root cause of cavitation, part of the liquid is vaporized in the pump, Gasification is the fundamental measure to avoid cavitation of liquid pumps. In order to prevent the gasification of the liquid, on one hand, the pressure of the liquid can be increased so as to increase its gasification temperature; on the other hand, the introduction of external energy should be reduced to prevent the liquid temperature from increasing. The following should be noted for this purpose: First of all, to understand the concept of NPSH (net positive suction head), to successfully design an efficient cryogenic liquid pump system, you must ensure that NPSH required at the pump inlet is maintained, Inhalation of the hair roots can be reliably achieved under all conditions and should be higher than the steam pressure of the flowing medium. NPSH is the difference between the hydrostatic pressure at the inlet of the pump and the pressure at which the liquid begins to boil (saturate) at the inlet of the pump. The value of NPSH varies with the fluid medium and the boiling temperature. The value required for compressing a given liquid, NPSH, is generally stated in the pump manufacturer's operating manual. The actual NPSH in the piping must be equal to or greater than the required NPSH, otherwise the maximum pump output rate will decrease. The liquid pressure in the NPSH sump must be above the saturation pressure (PSL) to satisfy the required pressure drop in order to overcome the pressure loss caused by friction and the loss of cooling capacity as the liquid flows to the pump in the pipe. These can be clearly seen from the pump suction condition of FIG. 1. H + Hp ≧ Hp + Hf + NPSH where H - liquid hydraulic pressure head, H = Ha + Hb Ha - storage tank level Hb - storage tank and the liquid pump Height difference Hp - Vapor pressure in tank Hf - Pressure drop due to liquid flow friction, ie friction pressure drop Hq - Pressure drop due to thermal leakage (ie, cold loss) NPSH - Net positive suction pressure required by the machine Therefore, there are two ways to increase the suction head of the pump: (1) Increase the height of the liquid by a sufficient hydraulic head -H, which will allow the tank to be installed at a higher level (to increase the value of Hb) . According to practical experience, the basis of the tank should be higher than the base of the pump more than 1m better. (2) Increase tank pressure by supercharger ─H. In order to minimize the loss of Hq or Hf, special attention should be paid to the following points on the liquid pump piping: (1) The inlet pipe should be kept as short distance as possible, and "U" elbows should be avoided in the piping. (2) Imported pipe, outlet pipe must have a suitable diameter, the flow rate of the medium should be maintained within the scope of the national standard, and as far as possible to reduce the flow resistance. (3) Except for the necessary number, the number of joints, elbows and valves in piping shall be minimized. (4) The inlet pipe must be insulated as much as possible. (5) In the fixed position of valves, fittings and pipes, that is, where the liquid may boil, the support legs should be insulated with a thermal insulation plate to prevent the cold damage. (6) A safety valve must be fitted to the liquid line between the two valves to prevent overpressure caused by residual liquid when the line valve is closed. (7) In the pump inlet pipe to be added to a period of bellows (bellows effective number of laps 5 to 10 laps) or hose and filter to keep the piping system flexibility and liquid impurities do not enter the pump body . Also note the following points: 1) reduce the pump installation height, in order to improve the pump inlet pressure. Increase the liquid undercooling, so not easy to gasification. 2) to strengthen the cooling of the liquid pipeline to prevent liquid due to absorption of heat caused by the temperature rise and gasification; 3) Do not allow the liquid pump running idle time is too long. Because when the liquid pump outlet valve is closed, the effective power is zero. The power consumed by the motor is only used to agitate the liquid in the pump, which will cause the temperature of the liquid to rise, resulting in gasification. General provisions, before the start of the outlet valve to open 1/3 is appropriate; 4) liquid oxygen adsorber to pre-cooling thoroughly. Because if the pre-cooling is not complete, liquid oxygen into the adsorber will be part of the gasification, the adsorber pressure increases, liquid oxygen flow decreased, while reducing power consumption is not much. Part of the power consumption will be in the form of heat to the liquid oxygen, so that liquid oxygen temperature. Therefore, precooling the adsorber should be until the liquid can be released. Bypass valve closing process should not be too hasty; 5) In the event of cavitation, exhaust should be immediately carried out until the pump stopped to ensure the safety of the liquid pump.