Cavitation in Centrifugal Pumps
Cavitation is derived from the word 'cavity', an empty space or a hollow space. It is caused by the 'boiling off' of a liquid at normal temperature and low pressure. It is worsened by the presence of dissolved air which is released when pressures are reduced.
If a liquid contains much dissolved air, then as the pressure is reduced (which happens when the pump takes suction), the air comes out of solution and forms cavities or bubbles. Each cavitation bubble grows from a nucleus to a finite size and is carried by the flowing liquid into a region of higher pressure, which is the vanes of an impeller. Here, these bubbles condense and burst or collapse. Condensation takes place violently and is accompanied by a tremendous increase of pressure which has a character of water hammering and also a rise in temperature.
Bubbles follow each other very rapidly and the bursting results in shockwaves which attack the metal, causing cavitation pitting in the least resistant places. Erosion due to cavitation is a major problem and is increased in high velocity situation.
In pumps carrying liquids a phenomenon known as cavitation occurs. Low pressure regions occur in the flow at points where high local velocities exist. If vapourisation occurs due to these low pressure areas then bubbles occur, these expand as they move with the flow and collapse when they reach a high pressure region. Such formation and collapse of bubbles is very rapid and collapse near a surface can generate very high pressure hammer blows which results in pitting, noise, vibration, and fall off in the pump efficiency. This phenomenon is not usually very pronounce in reciprocating pumps. Incipient cavitation i.e. cavitation which is just beginning can occur when suction lift capability cannot meet supply requirements and the output reduces until the two coincide. Under these conditions of operation the pump runs noisely and cavitation damage can occur. By throttling the discharge, or reducing pump speed, rough running of the pump and possible damage can be avoided.
Circumstances that Develop Cavitation
- If the has to take suction from a great distance, i.e. the suction head is too great.
- If the normal discharge rate has been exceeded, i.e. the pump speed has been increased above rated speed.
- Improper shape of pump entry passages, or due to some obstruction in the inlet side.
- When the pump is handling liquids with low vapour pressures, such as LPG and LNG.
Effects of Cavitaions
- The metal gets eroded and is seen as small pittings. The rate of erosion may be increased if the liquid itself has corrosive properties.
- Audible rattling or cracking sounds may be heard, which can reach a pitch of dangerous vibrations.
- Due to the flow disturbances set up, the entire performance of the machine may deteriorate, and the head and efficiency of the pump may fall below acceptable value.
Regions Attacked by Cavitation
- At the starting stage, the collapse of bubbles terminates within the impeller passages.
- In the fully developed stage, the main part of vapour bubble collapse is in the impeller itself and the rest is carried out of the impeller into the diffuser ring and the volute casing, and sometimes even to the next stage of the pump.