Usually, although there are many external reasons for cavitation in liquid pumps, for example, in addition to the structure of the pump itself, it is also closely related to installation and operation, but according to the root cause of cavitation, part of the liquid is vaporized in the pump, so preventing Liquid vaporization is the fundamental measure to avoid cavitation in liquid pumps.
In order to prevent the liquid from vaporizing, on the one hand, the pressure of the liquid can be increased to increase its vaporization 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:
The first thing to understand is the NPSH (Net Positive Suction Head) concept. To successfully design an efficient cryogenic liquid pump system, it is necessary to ensure that the required NPSH is maintained at the pump inlet. This value is required at the suction flange of the pump and at each pump. It can be reliably achieved in all cases, and this value should be higher than the vapour pressure of the flowing medium. NPSH is the difference between the static pressure of the liquid at the pump inlet and the pressure at which the liquid starts to boil (saturate) at the pump inlet. The value of NPSH varies with different fluid media and different boiling temperatures. For compressing the specified liquid, the required value of NPSH is generally stated in the operation manual of the liquid pump manufacturer. The actual NPSH must be greater than or equal to the required NPSH in the piping, otherwise the maximum output rate of the pump will drop. To meet the required NPSH liquid pressure in the sump must be higher than the saturation pressure (PSL) to overcome pressure losses due to friction and cooling losses as the liquid flows in the piping to the pump.
H + Hp≥Hp + Hf + NPSH where H——the hydraulic head of the liquid, H=Ha + Hb
Ha——The height of the liquid level of the storage tank
Hb——The height difference between the storage tank and the liquid pump
Hp - the steam pressure of the storage tank
Hf——Pressure drop caused by fluid flow friction, that is, frictional pressure drop
Hq - pressure drop caused by heat leakage (ie cold loss)
NPSH - Net positive suction head required by the machine
Therefore, there are two ways to increase the suction head of the pump: (1) Raise the height of the liquid, that is, a sufficient hydraulic head—H, which can install the storage tank at a higher level (increase the value of Hb). According to practical experience, the foundation of the storage tank should be more than 1m higher than the foundation of the pump. (2) Increase the pressure of the storage tank by ─H through the supercharger.
In order to minimize the loss of Hq or Hf.
At the same time, pay special attention to the following points on the piping of the liquid pump:
(1) The inlet pipe should be kept as short as possible, and the "U"-shaped bend should be avoided in the pipeline.
(2) The inlet pipe and the outlet pipe must have a suitable diameter, the flow rate of the medium should be kept within the range specified by the national standard, and the flow resistance should be reduced as much as possible.
(3) In addition to the necessary number, the number of joints, elbows and valves should be minimized in the pipeline.
(4) The inlet pipe must be insulated as much as possible.
(5) In the fixed place of the valve, pipe fitting and pipeline, that is, in the place where the liquid may boil, the support feet should be fixed with a heat insulation board to prevent cold damage.
(6) A safety valve must be installed on the liquid pipeline between the two valves to avoid excess pressure caused by residual liquid when the pipeline valve is closed.
(7) It is necessary to add a section of bellows (the effective number of bellows turns 5 to 10 turns) or hose and filter on the inlet pipe of the pump to keep the flexibility of the pipeline system and the impurities in the liquid from entering the pump body. .
