Fluid Flow System

In a marine fluid flow system, especially in piping system and centrifugal pumps, occurs a typical problem known as cavitation. That according to Matthew A. Carr (2012) “cavitation is the formation and subsequent collapse of bubbles (or cavities) of working fluid vapors.” These bubbles, which are compressible reducing the fluid density, are originated when the local pressure is too low (below the vapour pressure), or its temperature is too high making that some fluid became vapour, and when the pressure is increased the bubbles returns to its liquid phase collapsing.

When near solid surfaces such as pump impeller, which is location where it happens frequently in centrifugal pumps, or pipe wall the collapsed cavities result in shock waves that

CAVITATION IN MARINE CENTRIFUGAL PUMPS AND PIPES

In a centrifugal pump during its operation the fluid that enters is accelerated by the impeller. At impeller the pressure lowers as the velocity increases (Bernoulli equation).

The flow area at the eye of the impeller is usually smaller than either the flow area of the pump suction piping or the flow area through the impeller vanes. When the flowing liquid passes from the eye of the impeller the area that is smaller results in the increase of the velocity followed by the decrease of the pressure. If pressure is lower and/or the temperature is higher than the saturation pressure/temperature some fluid changes to vapour forming tiny bubbles. They are carried by the fluid and where the pressure risers; that is anywhere that the fluid velocity is reduced; and condense back resulting in its implosion that has enough energy, since due the impeller rotation action they attain very high velocity causing pitting when they reach the solid parts such as the outlet of the impeller or the pump casing around the impeller. In a centrifugal pump the local pressure drops experienced by the liquid flowing within its passages determine where cavitation occurs. Usually it is close to the inlet tips of the impeller

Provide that the effects that may be caused by it, the degraded performance and damage, affect the flow system of the ship.

2.1 NET POSITIVE SUCTION HEAD (NPSH)

The measure of how near a fluid is to saturation conditions is known as Net positive suction head (NPSH). NPSH is when the Net amount the pressure Head at Suction of the pump is superior to the saturation pressure of the fluid being pumped.

NPSH = psuction – psaturation

In order to ensure that cavitation does not appear in any place in the pump internals the pump manufacturer specifies a minimum pressure to be kept at standard local of the pump suction, considering that there are locations in the pump where, by cause of the design and flow rate, the pressure is lower than at the suction. This pressure is called the NPSHrequired. On the manufacturer-supplied pump curves the NPSH Required is commonly shown. The NPSH available is the NPSH that can be achieved at the pump suction by adjusting fluid system conditions. To prevent the pump from cavitating is necessary to preserve the NPSH available higher than the NPSH required. Then, in some circumstances the NPSH available will be increased and in order the NPSH required will be