Pump cavitation

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Pump Cavitation
Cavitation should be avoided due to erosion damage and noise.
Cavitation occurs when P < Pv
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Vapor pressure: is the absolute pressure at which
a liquid will evaporate at certain temperature
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CAVITATION
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Concept of Cavitation
Definitions:
Some say when a pump makes a rattling or knocking sound along with
vibrations, it is cavitating.
Some call it slippage as the pump discharge pressure slips and flow
becomes erratic.
When cavitating, the pump not only fails to serve its basic purpose of
pumping the liquid but also may experience internal damage, leakage from
the seal and casing, bearing failure, etc.
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Meaning of the Term "Cavitation" in the Context of the
Centrifugal Pump
The term ‘‘cavitation’’ comes from the Latin word cavus, which means a
hollow space or a cavity.
Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and
collapse of cavities in a flowing liquid in regions of very low pressure.
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Types Of Bubbles Formed In
The Liquid
Vapor bubbles
Gas bubbles
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Vapor Bubbles
Vapor bubbles are formed due to the vaporization of a process liquid that is
being pumped. The cavitation condition induced by formation and collapse
of vapor bubbles is commonly p y referred to as Vaporous Cavitation.
Gas Bubbles
Gas bubbles are formed due to the presence of dissolved gases in the liquid
that is being pumped (generally air but may be any gas in the system). The
cavitation condition induced by the formation and collapse of gas bubbles is
commonly referred to as Gaseous Cavitation.
Both types of bubbles are formed at a point inside the pump where the local
static pressure is less than the vapor pressure of the liquid (vaporous
cavitation) or saturation pressure of the gas (gaseous cavitation).
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Vaporous Cavitation
It is the most common form of cavitation found in process plants.
Generally it occurs due to insufficiency of the available NPSH or internal
recirculation phenomenon. It generally manifests itself in the form of
reduced pump performance, excessive noise and vibrations and wear of
pump parts. The extent of the cavitation damage can range from a relatively
minor amount of pitting after years of service to catastrophic failure in a
relatively short period of time.
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Gaseous Cavitation
Occurs when any gas (most commonly air) enters a centrifugal pump along
with liquid. A centrifugal pump can handle air in the range of ½ % by
volume. If the amount of air is increased to 6%, the pump starts cavitating.
The cavitation condition is also referred to as Air binding. It seldom causes
damage to the impeller or casing. The main effect of gaseous cavitation is loss
of capacity.
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Mechanism of Cavitation
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Step One
Formation of bubbles inside the liquid
being pumped.
The bubbles form inside the liquid when it vaporises i.e. phase change
from liquid to vapor.
But how does vaporization of the liquid occur during a pumping operation?
Vaporization of any liquid inside a closed container can occur if either
pressure on the liquid surface decreases such that it becomes equal to or less
than the liquid vapor pressure at the operating temperature, or the
temperature of the liquid rises, raising the vapor pressure such that it
becomes equal to or greater than the operating pressure at the liquid surface.
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To understand vaporization, two important points
to remember are:
The static pressure and not the total pressure.
The terms pressure and head have different meanings and they
should not be confused
So, the key concept is ‐ vapor bubbles form due to vaporization of the liquid
being pumped when the local static pressure at any point inside the pump
becomes equal to or less than the vapor pressure of the liquid at the pumping
temperature.
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How does pressure reduction occur in a pump
system?
The reduction in local static pressure at any point inside the pump can occur
under two conditions:
The actual pressure drop in the external suction system is greater than that
considered during design. As a result, the pressure available at pump suction
is not sufficiently high enough to overcome the design pressure drop inside
the pump.
The actual pressure drop inside the pump is greater than that considered
during g the pump p p design.
g
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Pressure reduction in the external suction
system of the pump
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Pressure reduction in the external suction
system of the pump
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Dr.Ihab G.Adam
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Eng.Hisham Mohammed Khamys

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Flow path of fluid inside the pump
The internal suction system is comprised of the pump’s suction nozzle and
impeller. Figures 5 and 6 depict the internal parts in detail.
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In Figure 7, it can be seen that the passage from the suction flange (point 2)
to the impeller suction zone (point 3) and to the impeller eye (point 4) acts
like a venturi i.e. there is gradual reduction in the cross‐section area.
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In the impeller, the point of minimum radius (eye) with reference to pump
centerline is referred to as the “eye” of the impeller (Figure 8).
Dr.Ihab G.Adam
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How pressure reduction occurs as the
fluid flows inside the pump?
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Step Two
Growth of bubbles
Unless there is no change in the operating conditions, new bubbles
continue to formand old bubbles grow in size.
The bubbles then get carried in the liquid as it flows from the impeller
eye to the impeller exit tip along the vane trailing edge.
Due to impeller rotating action, the bubbles attain very high velocity and
eventually reach the regions of high pressure within the impeller where
they start collapsing.
The life cycle of a bubble has been estimated to be in the order of 0.003
seconds.
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Step Three
Collapse of bubbles
As the vapor bubbles move along the impeller vanes, the pressure around
the bubbles begins to increase until a point is reached where the pressure on
the outside of the bubble is greater than the pressure inside the bubble.
The bubble collapses.
The process is not an explosion but rather an implosion (inward bursting).
Hundreds of bubbles collapse at approximately the same point on each
impeller vane.
Bubbles collapse non‐symmetrically such that the surrounding liquid
p y y g q
h f ll h d f l d
rushes to fill the void forming a liquid micro jet.
The micro jet subsequently ruptures the bubble with such force that a
hammering action occurs.
Bubble collapse pressures greater than 1 GPa (145x106 psi) have been
reported. The highly localized hammering effect can pit the pump impeller.
Dr.Ihab G.Adam
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The pitting effect
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Formation and collapse of a bubble
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Impeller Cavitation Regions
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General Symptoms of Cavitation and its
Effects on Pump Performance and Pump
Parts
loud noises, vibrations and an unsteadily working pump.
 Fluctuations in flow and discharge pressure take place with a sudden and
drastic reduction in head rise and pump capacity.
Depending upon the size and quantum of the bubbles formed and the
severity of their collapse, the pump faces problems ranging from a partial
loss in capacity and head to total failure in pumping along with irreparable
damages to the internal parts.
It requires a lot of experience and thorough investigation of effects of
cavitation on pump parts to clearly identify the type and root causes of
cavitation.
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1-Reduction in capacity of the pump
The formation of bubbles causes a volume increase decreasing the space available
for the liquid and thus diminish pumping capacity.
For example, when water changes state from liquid to gas its volume increases by
approximately 1,700 times. If the bubbles get big enough at the eye of the impeller,
the pump “chokes” i.e. loses all suction resulting in a total reduction in flow.
The unequal and uneven formation and collapse of bubbles causes fluctuations in
the flow and the pumping of liquid occurs in spurts.
This symptom is common to all types of cavitations.
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2-Decrease in the head developed
Bubbles unlike liquid are compressible. The head developed diminishes drastically
because energy has to be expended to increase the velocity of the liquid used to fill
up the cavities, as the bubbles collapse.
As mentioned earlier, The Hydraulic Standards Institute defines cavitation as
condition of 3 % drop in head developed across the pump. Like reduction in
capacity, this symptom is also common to all types of cavitations.
Thus, the hydraulic effect of a cavitating pump is that the pump performance
drops off of its expected performance curve, referred to as break away, producing a
lower than expected head and flow.
p
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3-Abnormal sound and vibrations
 It is movement of bubbles with very high velocities from low‐pressure area to a
high‐pressure area and subsequent collapse that creates shockwaves producing
abnormal sounds and vibrations. It has been estimated that during collapse of
bubbles the pressures of the order of 10000 atm develops.
 The sound of cavitation can be described as similar to small hard particles or
gravel rapidly striking or bouncing off the interior parts of a pump or valve.
Various terms like rattling, knocking, crackling are used to describe the
abnormal sounds.
 People can easily mistake cavitation for a bad bearing in a pump motor. To
distinguish between the noise due to a bad bearing or cavitation, operate the
g g p
h fl h d f ll b d f
pump with no flow. The disappearance of noise will be an indication of
cavitation.
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Damage to pump parts
Cavitation erosion or pitting
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Cavitation Damage on Impellers
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The two pictures are of the same area of a centrifugal pump impeller.
The one on the left shows a typical cavitation 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.
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The picture shows damage at the low
pressure side of the leading edge of an
impeller vane due to suction cavitation.
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Net Positive Suction Head
(NPSH)
What is Net Positive Suction Head??
Net Positive Suction Head. NPSH is what the pump needs, the minimum
requirement to perform its duties.
Therefore, NPSH is what happens in the suction side of the pump, including what
goes on in the eye of the impeller.
NPSH takes into consideration the suction piping and connections, the elevation
and absolute pressure of the fluid in the suction piping, the velocity of the fluid and
the temperature.
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Net positive Suction Head
(NPSH)
Net positive Suction Head Available
(NPSHA)
Net positive Suction Head Required
(NPSHR)
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Net positive Suction Head Available
(NPSHA)
The difference between the total suction head and the
vapor pressure of the liquid at the suction flange.
Calculating NPSHA of a piping system
NPSHA = hsa – hvpa
where:
hsa = Total suction head head, absolute
hvpa = Vapor pressure of liquid at the pumping temperature, absolute.
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hsa = hpsa + hss – hfs
where:
hpsa = suction surface pressure, absolute, on the surface of the liquid from which
the pump takes its suction. This will be the atmospheric pressure, in the case
of an open tank, or the absolute pressure above the liquid in a closed tank.
hss = static suction head. In other words, the height of the liquid surface in the
suction tank above or below the pump centerline. (Positive if the liquid level
is above the pump, negative if the liquid level is below the pump).
hfs = friction head loss, between the liquid surface in the suction tank and the
suction flange of the pump.
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Net Positive Suction Head (NPSH)
Net Positive Section Head and Cavitation (NPSH)
NPSH Available
Suction supply open to atmosphere with
section lift.
NPSHA =PB – (VP +LS +hf)
Where
PB= Barometric pressure in feet absolute.
V V f th li id t i
VP= Vapor pressure of the liquid at maximum
pumping temperature, in feet absolute.
Ls = Maximum static suction lift in feet.
hf = Friction loss in feet in suction pipe at
required capacity.
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NPSH Available
Suction supply open to atmosphere with
section head.
NPSHA =PB + LH ‐ (VP +hf)
Where
PB= Barometric pressure in feet absolute.
V V f th li id t i
LH = Minimum static suction head in feet.
required capacity.
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NPSH Available
Closed suction supply with suction head.
NPSHA =P + LH ‐ (VP +hf)
Where
P = Pressure on surface of liquid in closed
suction tank, in feet absolute.
LH = Minimum static suction head in feet.
required capacity.
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NPSH Available
Closed suction supply with suction lift.
NPSHA = P ‐ (VP ‐ LS +hf)
Where
P = Pressure on surface of liquid in closed
suction tank, in feet absolute.
Ls = Maximum static suction lift in feet.
required capacity.
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Net Positive Suction Head Required
(NPSHR)
The reduction in total head as the liquid enters the
pump.
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NPSH testing
Two procedures are used for NPSHR testing as follows:‐
 Establishing a constant NPSHA and then varying the pump flow by means of
a discharge control valve until a predetermined amount of deterioration
(usually 3 percent) in the pump discharge head performance is observed
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An alternate procedure is to hold capacity constant while the NPSH available is
reduced by either throttling or changing the vacuum on the pump suction
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Methods of reducing NPSHA
1‐Valve suppression
In the valve suppression method of testing, a valve, located in the
suction line leading to the pump, is used to reduce the suction pressure
by throttling, thus creating varying NPSHA conditions.
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2‐Vacuum method
An alternate to valve suppression testing is to create a vacuum on the suction
side of the pump . This is done by using a tank or reservoir in which the pressure
above the liquid is reduced bymeans of an ejector or a vacuum pump.
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The objective of all these testing methods is to establish the NPSH requirements,
at various flow rates, for a given impeller‐casing combination and to construct an
NPSHr versus flow capacity curve.
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Suction specific speed
A dimensionless rating number which indicates the relative ability of centrifugal
pumps to operate under conditions of low available net positive suction head.
where:
S = suction specific speed
N = rotative speed, in revolutions perminute
Q = capacity, at best efficiency, in gallons per minute
hsv = net positive suction head required by maximum diameter impeller at best
efficiency, in feet
Depending on impeller design design, suction specific speeds will vary in numerical value
from below 4,000 to above 11,000 with the higher values indicating lower net positive
suction head requirements.
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General rule to avoid cavitation
NPSHa > NPSHr + 3 ft or more safety margin
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NPSH margin
According to the Hydraulic Institute, NPSH margin is required above the NPSHR
f h h f f
of the pump to suppress incipient cavitation. The amount of margin is a function
of Suction Energy and the critical nature of the application as follows:
NPSHMargin Ratio (NPSHA/NPSHR)
Low 1.1 ‐ 1.3
High 1.2 ‐ 1.7
Very y High g 1.7 7 ‐ 2.5
5
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Pump Cavitation and NPSH
Cavitation should be avoided due to
erosion damage and noise.
Cavitation occurs when P < Pv
Net positive suction head
NPSH required curves are created
through systematic testing over a range
of flow rates V.
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Cavitation
 If the pump operates to the right of
point A, A then the required suction head
is greater than the available suction
head. This means that vapour bubbles
will occur in the suction pipe.
 While Operation to the left of point A
means that vapour bubbles will not
form, and so Cavitations will not be a
problem.
bl
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If the available NPSH is not greater than that required by the pump, the following
serious problems can result :‐
1. A marked reduction in head and capacity (the energy expended in accelerating
the liquid to high velocity in filling q g y g the void left by the bubble is a loss, and
causes the drop in head, while the loss in capacity is the result of pumping a
mixture of vapor and liquid instead of liquid).
2. Excessive vibration can occur when sections of the impeller are handling vapor
and the other sections handling liquid.
3. Probably the most serious problem is pitting and erosion of the pump parts,
resulting in reduced life.
4. Erratic flow rate with spurts of liquid being thrown from the discharge pipe.
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The solid line curves represent a condition of
adequate NPSHa whereas the dotted lines
depict the condition of inadequate NPSHa
i.e. the condition of cavitation.
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Impeller damaged by cavitation
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Detecting a low NPSHA problem
A centrifugal pump in the field that is cavitating
often will sound as if it is pumping rocks, and
frequently the discharge pressure will pulsate. A
simple way to determine if the problem is a flow‐induced
NPSH problem is to slowly shut down on
the discharge block valve. If the problem is flow‐induced,
the noise and the pulsations should go
away as the flow is reduced through the pump.
pump
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How to stop vaporization Cavitation
By increasing the suction head
• Raise the liquid level in the tank
• Elevate the supply tank. .
• Reduce the piping losses.
•Install a booster pump .
• Pressurize the tank.
• Be sure the tank vent is open and not obstructed.
• Some vents can freeze in cold weather.
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By lowering 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.
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By decrease the fluid velocity
•Remove obstructions in the suction piping.
• Reduce the speed of the pump.
• Reduce the capacity of the pump.
• Do not install an elbow too close to the pump suction.
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By reducing 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%,
• Use a lower speed pump.
• Use a pump with a larger impeller eye opening.
• 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.
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8/25/2014 68 Pump fitted with inducer

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Pump Priming
* Why you must prime a centrifugal pump ?
 To remove air from the pump cavities and the suction
piping, the pump must develop enough head to equal the
equivalent of one bar pressure which extends from the
earth’s atmosphere.
 Since the weight of water is approximately 8000 times
that of air, the centrifugal pump can produce only 1/8000 of
its rated liquid pressure.
another explanation:‐
 A centrifugal pump is a rotodynamic pump
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pump.
 As head = pressure / sp.Weight of the pumped liquid.
 the pressure difference created with air will be only
around 1/800 times that with water due to the density
difference.
Priming Process
“ It is a process of air
/vapor removal from
pump suction and
casing by filling it
up with liquid ”.
70
.

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Positive displacement pumps priming
When pumping low‐viscosity liquids , a foot valve
may be used to help keep the pump primed.
Alternately, a vacuum device may be used to prime
the pump.
When handling liquids of higher viscosity, foot
71
g q g y,
valves are usually not required because liquid is
retained in the clearances and acts as a seal when
the pump is restarted.
C.P Priming Methods
 Install a foot valve in the suction piping.
 Evacuate the air in the system with a positive
displacement priming pump.
 Fill the pump with liquid prior to starting it using a
hose or a small pot.
 Make a bypass line from discharge line.
 Convert the application to a self priming pump that
maintains a reservoir of liquid at its suction.
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Priming Chambers
1‐ Single Chamber tank
73
2‐ Double Chamber Tank:
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Testing for Air in Centrifugal Pumps
 The presence of only small quantities of air can result in
considerable reduction in capacity, since only 2% free air
will cause a 10% reduction in capacity, and 4% free air will
reduce the capacity by 43.5%.
 Entrained air is one of the most frequent causes of shaft
breakage. It also may cause the pump to lose its prime and
greatly accelerate corrosion.
 If air is present, the pump is likely to operate with a certain
amount of internal noise. This noise can be described as a
"gravel noise"
75
 We will assume that calculations
have already been made to assure
that the NPSH available is greater
than that required by the pump,
(the noise is not a result of
cavitation).
 When the source of suction supply
is above the centerline of the pump,
a check for air leaks can be made by
collecting a sample in a "bubble
bottle“.
 Since the pressure at the suction
chamber of the pump is above
atmospheric pressure, a valve can
76
p p ,
be installed in one of the tapped
openings at the high point in the
chamber and liquid can be fed into
the "bubble bottle." The presence of
air or vapor will show itself in the
"bubble bottle."

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Self Priming Pumps
 Self‐priming pumps overcomes the air binding
problem by mixing air with water to create a
pumpable fluid with pumping properties much
like those of regular water .
 recirculating water within the pump on priming
cycle is the main operation concept.
 This type of pump differs from a standard
centrifugal pump in that it has a ater reser oir
water reservoir
built into the unit may be above the impeller or in
front of the impeller.
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 a charge of liquid sufficient to
prime the pump must be
retained in the casing (Fig. A)
 When the pump starts, the
rotating impeller creates a
partial vacuum ; air from the
suction piping is drawn into
this vacuum and is entrained in
the liquid drawn from the
priming chamber (Fig. B), then
the priming cycle starts.
 This cycle is repeated until all
Fig. A
78
of the air from the suction
piping has been expelled and
replaced by pumpage and the
prime has been established
(Fig. C).
Fig. B Fig. C

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Self Priming pumps
Piping System Considerations
 Insure that adequate liquid is retained In the priming
chamber.
 The static lift and suction piping should be minimized
to keep priming time to a minimum.
 Keep All connections in the suction piping should be
leak‐free as air could be sucked in.
79
A priming bypass
line should be
installed.
 The suction piping
80
p p g
should be designed
such that no high
points are created

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Priming Time Calculation
 1. Select . the correct size and speed pump
 2. Calculate the NPSH Available for the
system.
NPSHA = P‐(Ls + Vp + hf)
 P = Pressure on surface of liquid in feet absolute
Ls= Maximum static lift in feet
Vp= Vapor pressure of the liquid at maximum
pumping temperature in feet absolute
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absolute.
hf = Suction pipe friction loss in feet at the required
capacity.
 3.Determine the effective static lift.
Les = Ls x Sp. Gr.
 4. Enter the priming time curve at
the effective static lift then
downward to the bottom
coordinate to determine the
priming time (PTLes ) to achieve the
given lift.
 5. insert the priming time into the
following formula to calculate the
total system priming time:
 PTT = Total system priming time.
PTLES = Priming time in seconds
for the effective static lift
SPL = Total suction pipe
l th i f t
82
length in feet.
Dp = Nominal pipe diameter.
Ds = Nominal pump suction
diameter.

Pump cavitation

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