Cavitation

What is CAVITATION

Fig-1 (pic of impeller having cavitation effect). If the pressure at any point inside the pump drops below VP of the liquid, then under such condition the liquid can 'Flash'(boiling). i.e. changing from liquid to gaseous phase. The vapor bubbles or cavities are carried along with the stream through the impeller eye to regions where the pressure within the impeller shroud rises due to centrifugal force. In the higher pressure zone the bubbles collapse or implode with tremendous shock and noise. This phenomenon is called CAVITATION. Ref Fig-2

Why it happens

The absolute pressure at the pump suction nozzle and cavitation problems are closely linked. If the lift is high and pump operates beyond BEP, then suction velocity increases to such an extent that it causes further drop in suction pressure. when the pumps runs under such conditions then the risk of liquid getting vaporised cannot be ruled out. However, for cavitation to occur, the pump need not necessarily work beyond BEP towards the right hand side of curve. It will also happen if the pump is working far beyond left of BEP. Also, cavitation can happen at any point on the curve if NPSHA is less then NPSHR.

There is limit to which abs pressure in the suction pipe can be reduced. Centrifugal pump with ordinary design does not function properly when the liquid contains vapors.

How to prevent Cavitation

Pump must be selected to operate at BEP. This is most important factor. If there is variation at site, necessary arrangement, if possible is to be done to make pump operate at BEP.

To provide enough head on the pump suction side (say by reducing the suction lift by taking pump near the suction source) so that the suction pressure at the impeller eye will remain above the liquid VP.

We can also reduct the discharge to bare minimum and thus reducing the NPSHR. This can be done by installing pulley drive system.

Where and why it forms

Impeller eye forms the zone of lowest suction pressure because:-

Ref Fig-3

1) Rotating impeller causes forced vortex formation just in front of impeller eye causing pressure to drop.

2) Further pressure drop takes place because of abrupt change in flow direction and sudden increase in velocity due to centrifugal force.

3) Rotating vanes causes shock to flow and increases friction resulting in further drop.

This is why impeller eye zone after the inlet vane tip portion is prone to severe action of cavitation. Proper filing of vane inlet tip and finishing of the eye area may reduce gravitation to some degree.

Cavitation and liquid properties

1) Specific Gravity

If the sp.gr. of liquid increases, the danger of cavitation also increases as to that extent the available NPSH is reduced.

2) Liquid Temperature

Liquids have the ability to dissolve gases or air in varying quantities. The amount of gas/air which goes in to the solution depends on liquid temperature and pressure on its free surface. The volume of gas/air absorbed by the liquid decreases as temperature rises. However, dissolved gases are released from the liquid when the pressure decreases.

What is difference between Static Head And Static Lift

What is difference between Static Head And Static Lift. When in a pumping system, the source of liquid is below pump datum, head is defined as Static Suction Lift. It is the distance between pump datum and the liquid surface. This attracts -ve sign. Example- Pump operating to lift water from well and pump it to overhead tank. In this example, water is drawn from below the pump datum. Fig-1.

When in a pumping system, the source of liquid is above pump datum, head is defined as Static Suction Head. It is the distance between pump datum and the liquid surface. It is also called Flooded Suction. Example- pump operating to transfer water from ground level tank to overhead tank. Ref Fig-2

Term Pressure

Atmospheric Pressure

Fig-1 Atmospheric pressure balances a column of mercury(Hg) to the height of 760 mm at sea level or water column of 10.33 m. The atmospheric pressure at sea level is 1.033 kg/cm2 at normal temperature (15.550C or 600F).

Figure shows a vessel filled with mercury exposed to atmospheric pressure. A capillary is placed in the center of the vessel. Due to the atm pr, the liquid, i.e. Mercury, rises up to 760mm.

The atm. pr. drops at the rate of about 1 meter for every 1000 meters increase in altitude, above mean sea level.

The Physical Atmosphere (Atm.)

It measures 1.033kg/cm2 = 760 mm of Hg (at sea level). This should not be confused with the Technical Atmosphere of 1kg/cm2. The standard atmospheric is 1.013 bar or 760 mm of Hg column.

The Technical Atmosphere (Atm.)

The unit of 1 atm = 1kg/cm2 = 735.5 mm of Hg is called the technical atmosphere.

The Concept of Vacuum

Refer Fig-2 (click here) Theoretically zero atm. pr. would be attained in a perfect vacuum. This is known as Absolute ZERO pr. All practical pr. are then made either relative to this datum or to the atm. pr. Pl refer the figure above

Vapor Pressure (VP)

Ref Fig-3 (click here) Substance/ Matter exists in three phases: 1) Solid, 2) Liquid and 3) Gas.

The term vapor pressure is frequently used in connection with centrifugal pumps, Particularly in Thermal plants and Process Industries.

All liquids have a tendency to evaporate when exposed to atm. The rate at which this evaporation occurs depends on the molecular energy of the liquid.(i.e. type of liquid and its temp.) and the condition of pr. adjoining the liquid surface.

Ref. fig above Consider a liquid contained in a sealed in beaker A. Assuming the constant temp is maintained, some of liquid molecules have sufficient energy to break away from the liquid and enter the air in the vapor form as in A. As the time passes, the air will contain vapor molecules to the extent that some of them will be forced to re-enter the liquid by the partial air pressure as shown in B. Eventually equilibrium will be attained when the rate at which molecules break away from liquid, same will re-enter back in to the liquid. As shown in C. Under such condition, the air above the liquid is saturated with liquid molecule and the pr. on the liquid surface is called its vapor pressure at prevailing temp.

Under vacuum, the pressure of vaporisation is accelerated. The boiling point of liquid has direct relation with the pressure above its surface. At higher pr. the boiling point increases and vice versa.

When a liquid has a very high vapor pr. is evaporates readily even at room temp. It is called a "Volatile Liquid" (like Petrol).

V.P. has great effects on working of centrifugal pumps. The energy available at the eye of the impeller gets reduced as liquid vapor pr. increases. In section of NPSH, will study further.

What is NPSH ?

Net Positive Suction Head(NPSH)

A liquid can not be sucked inside the pump. Liquid has to be pushed inside the suction pipe Fig-1. For a liquid to be pushed into a suction pipe some external energy, force or head is required. Atm. pr. provides this head on the free surface of liquid from which the pump draws the liquid.

Theoretically, at sea level the maximum suction lift is 10 m (for water). Actually because of effects of several factors, it is less than this value. Pipeline friction, liquid VP are some of the constraints that determine maximum allowable suction lift.

So what is NPSH?

In Net Positive Suction Head (NPSH)
Net stands for remaining Positive stands for greater then ZERO (pressure) Suction head stands for abs. energy at the pump suction flange in meters of liquid above VP at pumping temperature (in relation to Zero abs. Pr.)
Ref Fig-2 to understand- 

CASE - 1 With zero suc. lift, NPSH is 10m (10-0).

CASE - 2 With 3m suction lift, NPSH is 7m (10-3).

CASE - 3 With 5 m suction head, NPSH is 15m (10+5).

We have ignored other factors like hf losses and VP of liquid involves for simplicity

NPSH Available

It can be defined as the absolute energy in mlc available at pump inlet nozzle and corrected to pump datum minus VP of liquid.

Factors to be considered for NPSH.

1) Atmospheric pr at pumping site converted in mlc.
2) Abs VP of liquid at pumping temp. in mlc.
3) Friction losses in suction pipe in mlc.
4) Suction lift or head in mlc.
5) Pressure head in closed suction vessel in mlc.
This is the characteristic of the system in which the pump operates (site condition). The NPSHA value is to be given to the manufacturer by us when we want to buy the pump.

NPSH Required

This is required or minimum NPSH of the pump (NPSHR)
It can be defined as the positive head in meters abs. necessary at the impeller eye to overcome the internal pressure drop within the impeller and maintain the liquid pressure above its VP and thus ensure stable flow in to the impeller. The pump designer decides the NPSHR value of the pump model.

NPSHR is the function of the impeller eye peripheral velocity in m/s and suction velocity of liquid at pump inlet in m/s. It logically follows that for the same pump when run at 1500 rpm requires less NPSHR then the pump with same discharge and head running at 2900 rpm.

NPSHA must be always more then NPSHR by at least 0.5m.

CAVITATION can occur at any point if NPSHA is less then NPSHR.