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Causes of propeller cavitation and ship propeller repair

Cavitation is a symptom of the evaporation of a flowing liquid so that it forms steam bubbles due to the reduced pressure of the liquid to below its vapor saturation point. For example, water at a pressure of 1 atm will boil and become steam at a temperature of 100 degrees Celsius. But if the pressure is lowered then the water will be able to boil at a lower temperature even if the pressure is low enough then the water can boil at room temperature.

When a liquid boils, bubbles form in the liquid vapor. This can happen to the liquid that is flowing in the pump or in the pipe. Places of low pressure and/or high velocity in the flow, it will be very prone to cavitation. For example, in a pump, the part that will easily cavitate is on the suction side. Cavitation in this section is caused by the suction pressure being too low.

From bubble formation to bubble burst it only takes about 0.003 seconds. These bubbles will be carried away by the fluid flow until they are finally in an area that has a pressure greater than the saturated vapor pressure of the liquid. In that area the bubble will burst and will cause a shock to the nearby wall. The liquid will enter suddenly into the space formed by the bursting of the vapor bubbles, resulting in a collision. This event will cause mechanical damage to the pump so that it can cause the wall to be perforated or pocked. This event is called cavitation erosion as a result of the continuous collision of steam bubbles that burst on the wall.

In addition, cavitation also causes noise, vibration, corrosion caused by the chemical reaction of gases and metals, and can also cause pump performance to suddenly decrease so that the pump cannot work properly.

Ways that can be used to avoid cavitation include:

  1. The suction side pressure should not be too low, the pump should not be placed far above the level of the liquid being pumped because it causes a large static head.
  2. The flow velocity in the suction pipe should not be too large. The part that has a high speed then the pressure will be low. Therefore, the amount of flow velocity must be limited, by limiting the diameter of the suction pipe it should not be too small.
  3. Avoiding installations in the form of sharp turns. At sharp turns, the fluid flow velocity will increase while the fluid pressure will decrease so that it becomes vulnerable to cavitation.
  4. The suction pipe is made as short as possible, or the suction pipe is selected one number higher to reduce friction losses.
  5. Does not impede the flow of liquid on the suction side.
  6. The total head of the pump must match that required under actual operating conditions.

Cavitation is common in ship propellers and pump impellers. The way it works is that when the water fluid passes through the foil, the top of the foil will accelerate itself due to the influence of the shape of the foil (the concept of Cf and Cd Foil). When the speed increases, the pressure decreases.

The fluid flow that occurs is in the form of a very fast swing. If you remember the T-s diagram in thermodynamics, there is a liquid section on the right and a vavor on the left of the vapor dome, because the pressure drops, the fluid will load past the saturation limit and bubbles will form containing vavor.

The enlargement of the vavor is influenced by the concept of heat transfer, such as the process of boiling. Cavitation process that can damage the foil edge due to continuous bursting of bubbles. there are 3 parameters, namely the surface tension of the bubble and the pressure of the vapor and the pressure of the liquid in this case water, if the liquid pressure is greater than the surface tension with the cross-sectional area of ​​the bubble, the bubble will burst and the liquid pressure will press the surface of the foil (the concept of displacement due to pressure differences).

Propeller Repair

Damage to the propulsion, especially the propulsion leaf affects the performance of the ship where the power transferred from the engine cannot be absorbed optimally (in other words, power losses occur in the propeller). For example, if the propeller leaf is bent or bent, there may be a change in the propeller pitch for a certain r/R ratio, this causes an increase in the propeller load (propeller load) so that to achieve service speed a larger driving motor power is required (propeller load curve). will go up and out of the curve of the engine power range / engine envelope) and if forced then the main motor will work hard (MCR continuously) this will endanger the motor, if used continuously then the possibility of the motor will be damaged (piston rod, piston and parts -section berg other crust).

The ship propeller Marine engine spares regeneration process is carried out when the ship is in the dock (docking process), generally damage to the propeller occurs in the blade where the propeller leaf is the mediator between the ship and the water so that the ship can run, for example damage to the propeller leaf such as : experiencing fouling, erosion due to cavitation, cracking and bending due to collision and so on. As previously explained, with the non-optimal propeller conditions, the propeller performance will decrease.

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