Prepared By:

Sami El-haj 201000679Ibrahim Barakeh 201000681

Presented To Prof.Dr. Ali Hammoud

Cavitations In Pumps

Cavitations - a common problem in pumps and control valves - causing serious wear and tear and damage. Under the wrong condition, cavitations will reduce the components life time dramatically.

What is cavitations:-• Cavitations is defined as the rapid formation and collapse of

vapor bubbles or pockets in a liquid, due to dynamic action, and resulting in the formation of cavities on the surfaces of solid boundaries. These solid boundaries can exist in any number of structures including hydrofoils, pipes, and fittings but, in our industry, the primary victims are impellers and propellers.

The two pictures are of the same area of a centrifugal pump impeller. The one on the left shows a typical cavitations pattern during flow. Bubbles are forming to the left and imploding at the impeller’s surface in the upper right. The picture on the right shows the actual damage caused by continuous implosion of bubbles in the same area.

The upper pictures is a series of actual photographs of bubble collapse below a solid boundary. It also shows the formation of the re entrant micro jet. In stage 7 you ‐can see a small dot at the bottom of the bubble. It grows in size in the following stages and appears to penetrate the upper surface in stage 14. Stage 18 is just prior to total collapse and generation of the shock wave.

What is the cause? When water enters a pump, its velocity increases causing a reduction in pressure within the pumping unit. If this pressure falls too low, some of the water will vaporize, forming bubbles entrained in the liquid. These bubbles collapse violently as they move to areas of higher pressure creating the noise and vibration from the pump.

When the NPSH available falls below the vapor pressure of the liquid in a pump’s flow passages. Most published definitions of the Net Positive Suction Head required by a pump (NPSHr) state that it is the head required to prevent cavitations.

The NPSH available to a centrifugal pump combines the effects of atmospheric pressure, water temperature, supply elevation, and the dynamics of the suction piping. The following equation illustrates this relationship. All values are in feet of water.

NPSHa = Ha +/ Hz – Hf + Hv Hvp‐ ‐Where:

Ha is the atmospheric pressureHz is the vertical distance from the surface of the water to the centerline of the pump’s suction (positive or negative)Hf is the friction formed in the suction pipingHv is the velocity head at the pump’s suctionHvp is the vapor pressure of the water at its ambient temperature

The Effects & Signs:-Cavitations is a common occurrence but is the least understood of all pumping problems.Your pump is cavitating if knocking noises and vibrations can be heard when it is operating. Other signs may be erratic power consumption and fluctuations or reductions in pump output.If you continue to operate your pump when it is cavitating, it will be damaged. Impeller surfaces and pump bowls will pit and wear, eventually leading to mechanical destruction.

How to avoid cavitations:-

Cavitations can in general be avoided by

• increasing the distance between the actual local static pressure in the fluid – and the vapor pressure of the fluid at the actual temperature

This can be done by:• reengineering components initiating high speed velocities and low static pressures• increasing the total or local static pressure in the system• reducing the temperature of the fluid

To begin with first one

Reengineering of Components Initiating High Speed Velocity and Low

Static Pressure

Cavitations and damage can be avoided by using special components designed for the actual rough conditions.

• Conditions as huge pressure drops can - with limitations - be handled by Multi Stage Control Valves

• Difficult pumping conditions - with fluid temperatures close to the vaporization temperature - can be handled with a special pump - working after an other principle than the centrifugal pump.

Second:-

Increasing the Total or Local Pressure in the System

By increasing the total or local pressure in the system, the distance between the static pressure and the vaporization pressure is increased and vaporization and cavitations may be avoided.The ratio between static pressure and the vaporization pressure, an indication of the possibility of vaporization, is often expressed by the Cavitations Number. Unfortunately it may not always be possible to increase the total static pressure due to system classifications or other limitations. Local static pressure in the component maythen be increased by lowering the component in the system. Control valves and pumps should in general be positioned in the lowest part of the systems to maximize the static head.This is common for boiler feeding pumps receiving hot condensate (water close to 100oC) from a condensate receiver.

Temperature(oC)

Vapor Pressure(kN/m2)

0 0.65 0.910 1.215 1.720 2.325 3.230 4.335 5.640 7.745 9.650 12.555 15.760 2065 2570 32.175 38.680 47.585 57.890 7095 84.5100 101.33

Third:-Reducing the Temperature of the Fluid

The vaporization pressure is highly dependable of the fluid temperature. Water, our most common fluid, is an example:

As we can see - the possibility of evaporation and cavitations increases dramatically with the water temperature.Cavitations can be avoided by locating the components in the coldest part of the system. By example it is common to locate the pumps in heating systems at the "cold" return lines.The same is the situation for control valves. Where it is possible they should be located on the cold side of heat exchangers.

Finally, Thank for your listening