This answer is compiled by Changsha Sanchang Pump Industry technicians and is for reference only!
High-temperature boiler feed water pumps have relatively high requirements on pump body structure, pressure resistance, and stability. Good flow channel design and impeller design can minimize the damage to boiler pumps caused by cavitation. This is also There is a new exam question for the water pump industry.
High-temperature boiler feed water pump cavitation or cavitation process is a process in which flowing liquid generates bubbles and then bursts. When the absolute speed of the fluid increases, due to the decrease in the static pressure of the fluid, for some specific particles of the fluid at a certain temperature, although no heat is input from the outside, they have reached the vaporization pressure, causing the particles to vaporize and generate bubbles. . Along the flow channel, if the static pressure of the fluid rises again and is greater than the vaporization pressure, the bubbles will burst rapidly, resulting in a huge condensation impact that is inwardly explosive. If the bubble collapse does not occur when the liquid is flowing, but occurs on the wall of the flow guide component, cavitation will cause the wall material to be eroded.
When the high-temperature boiler feed water pump operates under cavitation conditions, even if there is no corrosion of the wall material, it will be found that the noise of the high-temperature boiler feed water pump increases, the vibration intensifies, the efficiency decreases, and The head is reduced. Device cavitation margin: also known as effective cavitation margin. The cavitation margin of the device is provided by the suction device. At the inlet of the high-temperature boiler feed water pump, the liquid per unit weight has surplus energy exceeding the vaporization pressure and water head. Foreign countries call this the effective net positive suction head, that is, the liquid at the pump inlet (the position head is zero) has the value of the full head minus the vaporization pressure and the net head surplus, expressed as NPSHa. Its size is related to device parameters and liquid properties. Because the hydraulic loss of the suction device is proportional to the square of the flow rate, NPSHa decreases as the flow rate increases. NPSHa-Q is a declining curve.
The cavitation margin of the multi-stage high-temperature boiler feed water pump has nothing to do with the device parameters, but is only related to the motion parameters of the pump inlet part. The motion parameters are determined by the geometric parameters at a certain rotation speed and flow rate. This means that NPSHr is determined by the pump itself (geometric parameters of the suction chamber and impeller inlet part). For a given pump, no matter what kind of liquid (except for the very high viscosity, which affects the velocity distribution), flowing through the pump inlet at a certain rotation speed and flow rate, it will have the same pressure drop and the same NPSHr because the speed is the same. Therefore, NPSHr has nothing to do with the properties of the liquid (regardless of thermodynamic factors). The smaller the NPSHr, the smaller the pressure, and the smaller the NPSHa that the device must provide, so the better the anti-cavitation performance of the pump.
The NPSHr of the high-temperature boiler feed water pump is related to the flow conditions in the pump. It is the pressure drop at the inlet of the balanced pump determined by the high-temperature boiler feed water pump itself, which is to ensure that the pump does not generate steam. Erosion requires that the unit weight of liquid at the pump inlet has surplus energy exceeding the vaporization pressure head. Foreign countries call this the necessary net positive suction head. The physical meaning of pump NPSH indicates the degree to which the pressure of the liquid drops at the inlet of the pump. The so-called necessary net positive suction head refers to the requirement that the suction device must provide such a large net positive suction head to compensate for the pressure drop and ensure that cavitation does not occur in the pump.