.

EFFECTS OF CAVITATION:
The effects of cavitation are hydraulic (low efficiency due to flow instability) and mechanical (surface
damage, noise and vibration). In addition, it may also lead to surface erosion. It is difficult to avoid
cavitation in hydro turbines which cannot be avoided completely but can be reduced. Computing
two-phase cavitating flows is a big challenge since the cavitating bubbles or clouds have very
complicated dynamics. Cavitation has also become a concern in the renewable energy sector as it
may occur on the blade surface of tidal stream turbine. Although the collapse of a small cavity is a
relatively low-energy event, highly localized collapses can erode metals, such as steel, over time. The
pitting on turbine blade is as shown in the Figure 1. The pitting caused by the collapse of cavities
produces great wear on components and can dramatically shorten a propeller or pump's lifetime.
After a surface is initially affected by cavitation, it tends to erode at an faster rate. The cavitation pits
increase the turbulence of the flow of the fluid and create crevices that act as nucleation sites for
additional cavitation bubbles. The pits also increase the components' surface area and leave behind
residual stresses. This makes the surface more prone to stress corrosion.

CAVITATION EROSION

Cavitation erosion is the process of surface deterioration and surface material loss due to the
generation of vapor or gas pockets inside the flow of liquid. These pockets are formed due to low
pressure well below the saturation vapor pressure of the liquid and erosion caused by the
bombardment of vapor bubbles on the surface. Cavitation erosion can occur on the surfaces of
metals and non metals. It may produce undesirable noise levels and reduce the useful life of very
valuable property. In the case of pumps, cavitation erosion risks are increased by a smaller inlet pipe
diameter and inlet restrictions, combined with higher liquid viscosity. Cavitation erosion can damage
and destroy critical and valuable equipment, such as industrial/military/power station equipment
and parts, such as pump impellers, delicately balanced high-speed propellers and turbine blades,
causing failures leading to potential risk of life and injury for workers and others like loss of revenue,
due to equipment downtime and the extra costs of failure analysis, repair and replacement.

THOMA CAVITATION NUMBER

Prof. D. Thoma suggested a dimensionless number, called as Thoma’s cavitation factor ‘σ’[6]. Typical
sigma curve is shown in Figure 2. It can be seen that as σ decreases, initially there is no effect on the
efficiency. With further decrease in σ, efficiency first increases then decreases sharply. Accordingly,
critical value of sigma (σcr) is defined and it is recommended to run the machines (pump/turbines)
above σcr for cavitation free operation. σ = (Hb-Hs)/H = (Hatm-Hv-Hs)/H Where, Hb is barometric
pressure head in meter of water, Hs is suction pressure at the outlet of reaction turbine, H is a net
head on the turbine.
To prevent cavitation

 avoid low pressure - pressurize supply tanks if necessary

 reduce fluid temperature
 use larger suction pipe diameters - reduce minor losses
 use cavitation resistant materials (Stellite 6, CA15 S.Steel 410BHN etc.)  or
coatings (Polyurethane Coating, Reinforced Epoxy Coatings etc.)
  small amounts of air supplied to the suction system may reduce the amount of
cavitation damage
 keep available NPSH(Net Positive Suction Head) well above required NPSH.

Cavitation, How to reduce It

To cure vaporization problems you must either increase the suction head, lower the fluid

temperature, decrease the fluid velocity, or decrease the net positive suction head required

(NPSHR). We shall look at each possibility:

How to increase the suction head

 Raise the liquid level in the tank

 Elevate the supply tank.
 Put the pump in a pit.
 Reduce the piping losses.
 Retrofit the pump with a higher specific speed impeller.
 Install a booster pump or inducer.
 Pressurize the tank.
 Be sure the tank vent is open and not obstructed. Some vents can freeze in cold
weather.

Lower the fluid inlet temperature

 Injecting a small amount of cooler fluid at the suction is often practical.

 Insulate the suction piping from the sun’s rays.
 Be careful of discharge re-circulation and vent lines re-circulated to the pump suction;
they can heat up the suction fluid.

Decrease the fluid velocity

 Remove obstructions in the suction piping

 Do not run the impeller too close to the pump cutwater.
 Reduce the speed of the pump.
 Reduce the capacity of the pump.
 Do not install an elbow too close to the pump suction.

Reduce the net positive suction head required (NPSHR)

 Use a double suction pump. Double suction designs can reduce the net positive
suction head required (NPSHR) by as much as 27%, or in some cases it will allow you to
raise the pump speed by 41%
 Use a lower speed pump.
 Use a pump with a larger impeller eye opening.
 If possible install an inducer. These inducers can cut net positive suction head
required (NPSHR) by almost 50%.
 Use several smaller pumps. Three half-capacity pumps can be cheaper than one
large pump plus a spare. This will also conserve energy at lighter loads.

It is a general rule of thumb that hot water and gas free hydrocarbons can use up to
50% of normal cold water net positive suction head required (NPSHR) requirements
or 10 feet (3 meters), whichever is smaller.

How to increase the suction head

Assignment No.
Assignment #/Presentation Report?
(Heading/Topic)
(Session 2007-2011)Submitted To:Sir Engr.
Muhammad UbaidullahSubmitted
By:XXXXXXXXXXXXXXXXX(Roll # XXX)(Class
XXX)Department of Telecom
EngineeringGovernment College University
Faisalabad
Assignment On: Topic