CAVITATION IN CENTRIFUGAL PUMP

Prepared by : DEBJYOTI MONDALDEPARTMENT OF MECHANICAL ENGINEERINGASANSOL ENGINEERING COLLEGE4TH YR BATCH :- 2013-2017

Introduction Types of cavitation? Effects of cavitation Cavitation in Centrifugal Pumps What is the cause of cavitation & how to

avoid it? Effects on Performance Model Conclusions References

OUTLINE

What is cavitation? Cavitation is the formation of vapour bubbles of a flowing

liquid in a region where the pressure of the liquid falls below its vapour pressure.

In general a liquid is set to cavitate when vapour bubbles are observed to form and grow because of pressure reduction.

Introduction

Cavitation Inception It is the phenomenon of rupture of a volume

of liquid when it is subjected to sufficiently low pressure.

Liquid may forms a cavity close to the fixed surface.

It may occur behind the blade of a rapidly rotating propeller.on any surface vibrating under water.

Travelling cavitation: Composed of individual transit cavities and moves

with the liquid. May appear at the low pressure points along the

solid boundary or in the high turbulence region.

Fixed Cavitation: Develops after inceptions. This involves detachment of liquid flow from the

rigid boundary of an immersed body to form a cavity attached to the boundary.

Cavities may have highly turbulence boiling surface or it may also have smooth surface.

Types Of Cavitation

Vortex Cavitation: Cavities are found in the cores of vortices, which

form in zones of high shear. It often occurs on the blade tips of ships

propellers. also referred as Tip cavitation.

Vibratory Cavitation: the forces causing cavities to form and collapse

are due to continuous series of high amplitude, high frequency pressure pulsation in the liquid, generated by a submerged surface, which vibrates, normal to it’s face.

Harmful Effects:

Loss in efficiency of the performance of hydraulic machines like turbines, pumps & Propellers.Production of the noise, which is undesirable in the case of submarines. Increase in the drag force of cavitating parts Undesirable vibrations due to collapse of the bubbles are due to shock waves set up. Material damage due to erosion.

Effects of Cavitation

The beneficial effects of Cavitation:

Cavitation can be used for agitation and mixing.

A cavitation noise boomer can be used as sound source for an echo ranging survey of ocean bottom conditions

Jet cavitations can be used very effectively for tunneling through rock.

Cavitation poses problem in operation, of all three types of centrifugal pumps, viz. radial, mixed and axial flow pumps, whenever high discharge, high rotational speed, or low head is encountered .

All The three factors lead to the higher pressure differences between two sides of the impeller blade, pressure becoming lower on the suction side & higher on the pressure side. If the pressure at any point in the impeller reaches a critical temperature, cavitation will result.

Sign of Cavitation: knocking noises and vibrations can be heard when it is operating. erratic power consumption and fluctuations or reductions.

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. The pressure required to operate a pump without causing cavitation is called net positive suction head (NPSH). Therefore the pressure head available at the pump inlet should exceed the NPSH required. This is specified by the pump manufacturer, and is a function of the pump design.

Cause of Cavitation

As a general rule, centrifugal pumps located less than 4 meters above the water level should not experience cavitation.

The following guidelines should be applied to avoid the problem:

Minimize the number of valves and bends in the suction line Use eccentric reducers, not concentric Ensure the straight side of the eccentric reducer is installed along the

top of the suction line Suction length should be as short as possible Suction pipe should be at least the same diameter as the pump inlet

connection Use long radius bends Increase the size of valves and pipe work Do not allow air into the suction line Ensure adequate submergence over the foot valve. The submergence should be at least 5.3 times the suction diameter.

How to avoid cavitation?

The most important effect of cavitation as far as pumps are concerned is the drop in the head and efficiency.

In the case of turbines it causes damage to the turbine runner.

Cavitation cannot be allowed to develop in the pump to such a stage as to cause damage. This is because the cavities rapidly choke the narrow passages, restricting the discharge. This is one reason why damage observations in pumps are far less numerous than in turbines.

The performance of a pump at a given speed is described in terms of discharge, head, power and efficiency. If a graph of head, BHP or efficiency of the pump at constant discharge and speed is drawn against Hsv (Net Positive Suction Head) pumps of different specific speed will show different behavior.

EFFECT ON THE PERFORMANCE

BEHAVIOUR OF PUMPS WITH VARIATION IN Hsv AT CONSTANT DISCHARGE

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1. Cavitation is transient unsteady phenomenon characterized by a growth of holes or Cavities.

2. Along with harmful effects like loss in efficiency of performance of hydraulic machines like turbines, pumps, production of noise it has got beneficial effects also such as in control devices, for agitation and mixing in special processes.

3. Cavitation creates problem in operation of all three types of centrifugal pumps viz. radial, mixed and axial flow pumps, whenever high discharge, high rotational speed or low head is encountered.

CONCLUSIONS

4. The most important effect of cavitation on pumps is drop in head and efficiency. Cavitation can’t be allowed to develop in pump to such a stage as to cause damage as in turbine runners.

5. For the pumps with Ns<25 the curves BHP, head and efficiency Vs Hs drops abruptly as compared to gradual drop of pump having Ns>60.So pumps with low specific speed are more susceptible to cavitation as compared to high specific speed pumps.

6. The impeller outlet conditions have no influence on cavitations.

1. Dr R.K Bansal(1998) “A Texatbook of Fluid machanics and hydraulic machine”page no. 9th eddition.

2. https://en.wikipedia.org/wiki/Cavitation

3. http://www.rotechpumps.com/all-you-need-to-know-about-cavitation-in-centrifugal-pumps

REFERENCES

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