The cavitation of a water pump is caused by the vaporization of water, which is the process of water transforming from liquid to vapor. The vaporization of water is related to temperature and pressure to a certain extent. At a certain pressure, water only begins to vaporize when the temperature reaches a certain value; If the pressure drops to a certain value at a certain temperature, water will also vaporize, and this pressure is called the vaporization pressure of water at that temperature. If during the flow process, the pressure in a certain local area is equal to or lower than the vaporization pressure corresponding to the water temperature, water vaporizes at that location. After vaporization occurs, many small bubbles of steam mixed with gas will form. When bubbles flow from the low-pressure area to the high-pressure area along with water flow, they rupture under the action of high pressure, and high-pressure water flows towards the space occupied by these original bubbles at an extremely high speed, forming an impact force. The metal surface is severely damaged due to fatigue under water hammer pressure. Therefore, we refer to the entire process of bubble formation, development, and rupture leading to material damage as cavitation phenomenon.
There are many reasons for the vibration of the unit and pump room building caused by water pump vibration analysis. Some factors are both related and interact with each other, and can be summarized into the following four main reasons.
1. Electrical aspect
The motor is the main equipment of the unit, and the imbalance of magnetic force inside the motor and the imbalance of other electrical systems often cause vibration and noise. If the radial alternating magnetic pulling force between the stator and rotor is generated by the interaction of harmonic magnetic flux between the stator and rotor teeth during the operation of asynchronous motors, or if the magnetic center of the stator and rotor is inconsistent or the air gap difference in all directions exceeds the allowable deviation value during the operation of large synchronous motors, it may cause periodic vibration and noise of the motor.
2. Mechanical aspect
The unbalanced quality, rough manufacturing, poor installation quality, asymmetric axis of the unit, excessive swing of the rotating components of the motor and water pump, poor mechanical strength and stiffness of the components, wear and damage of bearings and sealing components, as well as resonance caused by the critical speed of the water pump and the natural frequency of the unit, can all generate strong vibration and noise.
3. Hydraulic aspect
Uneven flow velocity and pressure distribution at the inlet of the water pump, pressure pulsation, liquid flow, deviation and detachment at the inlet and outlet of the pump, non rated operating conditions, and pump cavitation caused by various reasons are common causes of pump unit vibration. The rapid pressure changes and water hammer effects in the water supply pipeline caused by dynamic transition processes such as pump startup and shutdown, valve opening and closing, working condition changes, and emergency shutdown accidents often lead to vibration in the pump room and unit.
4. Hydraulic and other aspects
Unreasonable or mismatched design of the inlet channel of the unit, improper submergence depth of the water pump, and unreasonable starting and stopping sequence of the unit can worsen the inlet conditions, generate vortices, induce cavitation or aggravate the vibration of the unit and pump room. When using a unit that breaks the siphon vacuum to cut off the flow, if the air in the hump section is difficult to carry during startup, the siphoning time will be too long; The design of the unit's flap door with flow interruption is unreasonable, opening and closing at times, constantly hitting the flap door seat; Uneven settlement or poor rigidity of the foundation supporting the water pump and motor can also cause vibration of the unit.
