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Process Operation Technology

Oil, gas and petrochemical process industries


    Talib Al_Munawri

    Posts : 337
    Join date : 2009-07-03
    Age : 30
    Location : Sultanate of Oman


    Post by Talib Al_Munawri on Fri Sep 04, 2009 10:32 am


    Cavitation means different things to people.

    It has been described as:

    A reduction in pump efficiency
    A reduction in the head of the pump.
    The formation of bubbles in a low-pressure area of the pump volute.
    A noise that can be heard when the pump is running.
    Damaged that can be seen on the pump impeller and volute.

    All of the above is right.

    Cavitation implies cavities or holes in the fluid we are pumping.
    These holes can also be described as bubbles, so cavitation is really
    about the formation of bubbles and their collapse.
    Bubbles form when ever liquid boils.

    It is important not to associate boiling with hot to touch. Liquid oxygen will boil and no one would ever call that hot.

    Fluids boil when the temperature of the fluid gets too hot or the
    pressure on the fluid gets too low.
    At an ambient sea level, a pressure of 14.7 psia, water will boil at 212 F. When the pressure is lowered, the water will boil at a much lower temperature and conversely if the pressure is increased,
    the water will not boil until it gets to a higher temperature.
    It is now possible to explain cavitation in more details

    The capacity of the pump is reduced because:

    Bubbles take up space and it is not possible to have bubbles and liquid in the same place at the same time.
    If the bubble gets big enough at the eye of the impeller, the pump will lose its suction and will require priming.
    The head is often reduced because:
    Bubbles unlike liquid are compressible. It is compression that can change the head.
    The bubbles form in a lower pressure area because they cannot form in a high-pressure area.

    A noise is heard because:

    Any time a fluid moves faster than the speed of sound in the pumping medium, a sonic boom will be heard. It is also explained by the imploding of the bubbles when it moves to a higher pressure zones in the pump.
    Pump parts show damage because:
    The bubbles try to collapse on its self.
    This is called imploding, the opposite of exploding.
    The bubble is trying to collapse from all sides, but if the bubble
    is lying against a piece of metal such as the impeller or volute,
    it can not collapse from that side. Then, the fluid comes in from the
    opposite side at this high velocity proceeded by a shock wave that can
    cause all kinds of damage.

    There is a very characteristic round shape to the liquid as it bangs against the metal creating the impression that the metal was hit with a “ball peen hammer”.

    Some solutions that can be recommended to avoid cavitation includes:

    •Increasing the suction head

    1.Raising the liquid level in the tank
    2.Raising the tank
    3.Reduce the piping losses (due to strainer clogged, a gasket leaking, etc)
    4.Pressurizing the tank
    5.Installing a booster pump

    •Lowering the fluid temperature

    1.Injecting a small amount of cooler fluid at the suction.

    •Reducing NPSHR

    1.Using a double suction pump. This can reduce NPSHR by as much as 27%
    2.using a lower speed pump
    3.using a pump with a larger impeller eye opening
    4.Installing an inducer. It can cut NPSHR to 50%

      Current date/time is Sun Mar 18, 2018 9:00 am