Cavitation is designed as the formation process of vapor cavities and bubbles in a fluid. In a centrifugal pump, cavitation has a significant effect on the efficiency of a pump. It causes reduction on the pump’s efficiency. This results in a fluctuating rate of flow and of discharge pressure. Cavitation also cause damage of pump bearings, seals and even wearing rings of the pump. These are as a result of excessive pump vibration. Cavitation is an undesirable occurrence, it majorly occurs in pumps and propellers. It results in excessive noise and vibrations. It also damages some components and leads to low performance of the pumps and even the propellers. Cavitation has become an issue in the renewable energy department because it occurs on blade surface of the tidal steam turbines.
Why cavitation occurs
- Poor pump inlet condition. The common causes of disruption of flow which result in cavitation are:
- a) When there is a clogged inlet
- b) Excess long inlet piping
- c) Restricted and collapsed inlets
- d) when there is higher fluid viscosity
- Discharge cavitation
When there is higher discharge pressure, instead of discharging, some of the liquid move freely in the pump. Then pressure reduction is caused by the liquid which has been trapped at a higher speed in between the housing and the impeller. This creates the conditions similar to those of suction cavitation.
How to recognize pump cavitation
Cavitation can be detected through sight or sound. Factors that indicate presence of cavitation are: noise, bearing failure, vibrations, unusually high-power consumption, and impeller erosion.
How to prevent cavitation
There are some engineering systems that have been developed to prevent cavitation. In this project I used a system of cavitation prevention known as a Control System.
Control System
Cavitation may persist for long periods of time causing damage to mechanical conditions of a pump. The prolonged cavitation leads to reduction in the flow rate ,Q, and the overall head generated. It also causes a reduction in the performance and power of work turbines. It is therefore important to detect and prevent cavitation at an early stage to avoid its development in the centrifugal machines.
Design procedure of the control system
The system is designed to measure and collect data that indicate cavitation. The system is connected to a compete circuit and located in a rig for testing. The complete or closed circuit enables the changing of the temperature of the liquid and even the barometric surface level on the free end. This tank is designed in such a way that it avoids the inclusion of gas in the aspiration pump.
The following are present inside the tank: 2 electric heaters with specifications 17kW and 9kW, a vacuum inlet which controls the atmospheric pressure on the free end tank surface level, a spray nozzle device, and 2 calm screens. Two valves which are operating manually one at the rate part of the tank and another one at the upper side of the tank. The electrical, mechanical and hydraulic parameters are detected inside the tank by the use of sensors. The pressure downwards and the upward pressure, the temperature of the liquid and the rate of flow of pressure in the tank are some of the hydraulic parameters measured.
The measuring range for the pressure transducers is between -1 and 1 while for the delivery side is between -1 and 3. The instruments provided need a supply of 24Vdc power, the signal output produced is of 4-20mA.
The distance of separation of the flow meter and the curves of the pipe is 10 meters. The separation of the nozzle from the outward part of the flow meter is 5 meters. The flow meters requires a 230Vac power and its output signal varies from 4mA to 20mA. Its fluid flow rate varies from 0 to 0.0158 cubic meters per second. 12 meters distance is ensured from the suction pump to ensure pressure distribution and a uniform velocity is reached. A transducer with the specifications ranging from 0 to 1 bar is used to measure the tank pressure. Its output varies from 1 to 5Vdc. The liquid temperature in the tank is detected using a thermal resistance installed in the tank. The temperature is necessary in calculating the pressure of the vapor.
The mechanical quantities that were measured are: vibration of the pump and the rotational speed of the pump. In order to acquire the vibrations, an accelerometer is installed in the pump. The accelerometers are in the range of -500 to 500g while their sensitivity is 100mV/g. The pump rotational speed is regulated by an inverter installed in the pump. The specifications of the inverter are, Maximum power of 4kW, the highest value of current needed is 15 A and the frequency is varied from 0 to 400Hz. The inverter output is varied from 0 to 10V, and is converted to frequency first and then to revolutions per minute in the centrifugal pump.
The electrical quantities are measured by the use of a meter for power measurement. This enables the current flow and voltage values to be obtained and also the absorbed power by the pump.
