Pump cavitation

Cavitation in centrifugal pumps.

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Pump Cavitation
Cavitation should be avoided due to erosion damage and noise.
Cavitationoccurswhen 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
vibrationsitiscavitatingvibrations,itiscavitating.
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
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the seal and casing, bearing failure, etc.

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Meaning of the Term "Cavitation" in the Context of the
Centrifugal Pump
Th ‘ii’ f hLi d hi hThe term‘cavitation’comesfrom theLatinwordcavus,whichmeans 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
Vaporbubbles
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
o
fvaporbubbles is commonlyreferred to asVaporous Cavitation.p y p
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.
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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).
Vaporous Cavitation
It is the most common form of cavitation found in process plants.
GenerallyitoccursduetoinsufficiencyoftheavailableNPSHorinternalGenerallyitoccursduetoinsufficiencyoftheavailableNPSHorinternal
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
relativelyshortperiodoftime.
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relativelyshortperiodoftime.

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Gaseous Cavitation
Occurs when any gas (most commonly air) enters a centrifugal pump along
withliquid.Acentrifugalpumpcanhandleairintherangeof½%bywithliquid.Acentrifugalpumpcanhandleairintherangeof½%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.
Thebubblesforminsidetheliquidwhenitvaporisesi.e.phasechange
from liquid to vapor.
But how does vaporization of the liquid occur during a pumping operation?
Vaporizationofanyliquidinsideaclosedcontainercanoccurifeither
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Vaporizationofanyliquidinsideaclosedcontainercanoccurifeither
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.
To understand vaporization, two important points
to remember are:
ThestaticpressureandnotthetotalpressureThestaticpressureandnotthetotalpressure.
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
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being pumpedwhen the local static pressure atany pointinside 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
undertwoconditions:undertwoconditions:
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
durin
gthepumpdesign.
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gpp g
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
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).
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Dr.Ihab G.Adam
Eng.Hisham Mohammed Khamys
How pressure reduction occurs as the
fluid flows inside the pump?
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Step Two
Growth of bubbles
Unlessthereisnochangeintheoperatingconditions,newbubbles
continuetoformandoldbubblesgrowinsize.continuetoformandoldbubblesgrowinsize.
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.
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Thelifecycleofabubblehasbeenestimatedtobeintheorderof0.003
seconds.
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
theoutsideofthebubbleisgreaterthanthepressureinsidethebubbletheoutsideofthebubbleisgreaterthanthepressureinsidethebubble.
The bubble collapses.
The process is not an explosion but rather an implosion (inward bursting).
Hundredsofbubblescollapseatapproximatelythesamepointoneach
impeller vane.
Bubbles collapse non‐symmetricallysuch thatthe surroundingliquid
h fllh df ld
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Dr.Ihab G.Adam
Eng.Hisham Mohammed Khamys
p yy gq
rushes tofillthevoidforming aliquidmicro 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.

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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
loudnoisesvibrationsandanunsteadilyworkingpumploudnoises,vibrationsandanunsteadilyworkingpump.
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.
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Itrequires a lotofexperience and thorough investigation ofeffects 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
il i Ifhbbbl bi hh fhillapproximately1,700times.Ifthebubbles getbigenoughat theeyeoftheimpeller,
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 ex
pected head and flow.
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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
abnormalsoundsandvibrationsIthasbeenestimatedthatduringcollapseofabnormalsoundsandvibrations.Ithasbeenestimatedthatduringcollapseof
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
distin
guish between the noise due to a bad bearingor cavitation, operate the
h fl hd f llb d f
g g p
pumpwith no flow. The disappearance of noisewill 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
rightright.
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
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the temperature.
Net positive Suction Head
(NPSH)
Net positive Suction Head Available
(NPSHA)
Net positive Suction Head Required
(NPSHR)
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(NPSHA) (NPSHR)

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Net positive Suction Head Available
(NPSHA)
ThedifferencebetweenthetotalsuctionheadandtheThedifferencebetweenthetotalsuctionheadandthe
vapor pressure of the liquid at the suction flange.
Calculating NPSHA of a piping system
NPSHA = hsa–hvpa
where:
hsa Total suction head absolute
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hsa= Total suction head, absolute
hvpa = Vapor pressure of liquid at the pumping temperature, absolute. 
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 o
pen tank, or the absolutepressure above the liquid in a closed tank.p p q
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 Section Head and Cavitation (NPSH)
NPSH Available
Net Positive Suction Head    (NPSH)  
Suction supply open to atmosphere with 
section lift.
NPSH
A=P
B–(V
P+L
S+h
f)
Where
P
B= Barometric pressure in feet absolute. 
V V    f th li id t i
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V
P= Vapor pressure of the liquid at maximum
pumping temperature, in feet absolute.
L
s= Maximum static suction lift in feet.
h
f= Friction loss in feet in suction pipe at
required  capacity.
Net Positive Section Head and Cavitation (NPSH)
NPSH Available
Net Positive Suction Head    (NPSH)  
Suction supply open to atmosphere with 
section head.
NPSH
A=P
B+ L
H‐(V
P+h
f)
Where
P
B= Barometric pressure in feet absolute. 
V V    f th li id t i
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V
P= Vapor pressure of the liquid at maximum
pumping temperature, in feet absolute.
L
H=  Minimum static suction head in feet. 
h
f= Friction loss in feet in suction pipe at
required  capacity.

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Net Positive Section Head and Cavitation (NPSH)
NPSH Available
Net Positive Suction Head    (NPSH)  
Closed suction supply with suction head.
NPSH
A=P + L
H‐(V
P+h
f)
Where
P = Pressure on surface of liquid in closed
suction tank, in feet absolute. 
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V
P= Vapor pressure of the liquid at maximum
pumping temperature, in feet absolute.
L
H= Minimum static suction head in feet. 
h
f= Friction loss in feet in suction pipe at
required  capacity.
Net Positive Section Head and Cavitation (NPSH)
NPSH Available
Net Positive Suction Head    (NPSH)  
Closed suction supply with suction lift.
NPSH
A= P ‐(V
P‐L
S+h
f)
Where
P = Pressure on surface of liquid in closed
suction tank, in feet absolute. 
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V
P= Vapor pressure of the liquid at maximum
pumping temperature, in feet absolute.
h
f= Friction loss in feet in suction pipe at
required  capacity.
L
s= Maximum static suction lift in feet.

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Net Positive Suction Head Required
(NPSHR)
The reduction in total head as the liquid enters the 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
adischargecontrolvalveuntilapredeterminedamountofdeteriorationadischargecontrolvalveuntilapredeterminedamountofdeterioration
(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 by means 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 per minute
Q = capacity, at best efficiency, in gallons per minute
hsv = net positive suction head required by maximum diameter impeller at best
efficiency, in feet
Dependingonimpellerdesignsuctionspecificspeedswillvaryinnumericalvalue
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Dependingonimpellerdesign,suctionspecificspeedswillvaryinnumericalvalue
from below 4,000 to above 11,000 with the higher values indicating lower net positive
suction head requirements.
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  
Net Positive Suction Head    (NPSH)  
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:
NPSH
Margin Ratio(NPSHA/NPSHR)
Low  1.1 ‐1.3
High  1.2 ‐1.7 
Very High  1.7 ‐2.5 
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yg 75
Pump Cavitation and NPSH
Cavitation should be avoided due to
erosion damage and noise.
Cavitation occurs when P < Pv
Net positive suction head
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NPSH required curves are created
through systematic testing over a range
of flow rates V.

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If the pump operates to the rightof 
point A then the required suction head 
Net Positive Suction Head    (NPSH)  
Cavitation 
point A, then the required suction head 
is greaterthan the availablesuction 
head. This means that vapour bubbles 
will occurin the suction pipe.
While Operation to the leftof point A 
means that vapour bubbles will not 
form, and so Cavitations will not be a 
bl
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problem.
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 highvelocityin fillingthevoid left bythe bubble is a loss,andq g y g y ,
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.
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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 cavitationi.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
oftenwillsoundasifitispumpingrocksandoftenwillsoundasifitispumpingrocks,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
awayastheflowisreducedthroughthepump
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awayastheflowisreducedthroughthepump.
How to stop vaporization Cavitation
By increasing the suction head 
Net Positive Suction Head    (NPSH)  
•Raise the liquid level in the tank 
•Elevate the supply tank. . 
•Reducethe piping losses.
•Install a booster pump . 
•Pressurizethe tank. 
•Be sure the tank vent is open and not obstructed.
•Some vents can  freeze in cold weather. 
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How to stop vaporization Cavitation
By lowering the fluid inlet temperature
Net Positive Suction Head    (NPSH)  
• 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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How to stop vaporization Cavitation
By decrease the fluid velocity
Net Positive Suction Head    (NPSH)  
•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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How to stop vaporization Cavitation
By reducing  the net positive suction head required (NPSHR)
Net Positive Suction Head    (NPSH)  
•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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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
pipingthepumpmustdevelopenoughheadtoequalthepiping,thepumpmustdevelopenoughheadtoequalthe
equivalent of one bar pressure which extends from the
earth’s atmosphere.
 Since Since thethe weightweight ofof waterwater isis approximatelyapproximately 80008000 timestimes
thatthat ofof air,air, thethe centrifugalcentrifugal pumppump cancan produceproduce onlyonly 11//80008000 ofof
itsits ratedrated liquidliquid pressurepressure..
anotheranother explanationexplanation: :‐ ‐
 A Acentrifugal centrifugalpump pumpis isa arotodynamic rotodynamicpump pump
69
 A Acentrifugal centrifugalpump pumpis isa arotodynamic rotodynamicpump pump. .
 As As headhead == pressurepressure //spsp..WeightWeight ofof thethe pumpedpumped liquidliquid..
 the the pressurepressure differencedifference createdcreated withwith airair willwill bebe onlyonly
aroundaround 11//800800 timestimes thatthat withwith waterwater duedue toto thethe densitydensity
differencedifference..
Priming Process
“It is a process of air
/vaporremoval from
pump suction and
casing by filling it
upwithliquid”.
70
upwithliquid.

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Positive displacement pumps primingPositive displacement pumps priming
 When Whenpumpingpumpinglow low‐ ‐viscosityviscosityliquidsliquids ,,aafootfootvalve valveppg ppg y yq q
may may bebe usedused toto helphelp keepkeep thethe pumppump primedprimed..
Alternately,Alternately, aa vacuumvacuum devicedevice maymay bebe usedused toto primeprime
thethe pumppump..
 When When handlin
ghandlingliquidsliquids ofof higherhigher viscosity,viscosity,footfoot
71
g gq q g g y, y,
valves valves areare usuallyusually notnot requiredrequired becausebecause liquidliquid isis
retainedretained inin thethe clearancesclearances andand actsacts asas aasealseal whenwhen
thethe pumppump isis restartedrestarted..
C.P Priming Methods
Install a foot valve in the suction piping.
EvacuatetheairinthesystemwithapositiveEvacuatetheairinthesystemwithapositive
displacement priming pump.
Fill the pump with liquid prior to starting it using a
hose or a small pot.

Makeabypasslinefromdischargeline.
72
Makeabypasslinefromdischargeline.
Convert the application to a self priming pump that maintains a reservoir of liquid at its suction.

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37
Priming Chambers   
1‐Single Chamber tank 
73
2‐Double Chamber Tank:
74

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38
Testing for Air in Centrifugal Pumps 
 The presence of only small quantities of air can result in The presence of only small quantities of air can result in 
id bl di  i  i i l id bl di  i  i i l % f i % f i considerable reduction in capacity, since only considerable reduction in capacity, since only 2 2% free air % free air 
will cause a will cause a 10 10% reduction in capacity, and % reduction in capacity, and 4 4% free air will % free air will 
reduce the capacity by reduce the capacity by 43 43..55%. %. 
 Entrained air is one of the most frequent causes of shaft Entrained air is one of the most frequent causes of shaft 
breakage. It also may cause the pump to lose its prime and breakage. It also may cause the pump to lose its prime and 
greatly accelerate corrosion.greatly accelerate 
corrosion.
75
 If air is present, the pump is likely to operate with a certain If air is present, the pump is likely to operate with a certain 
amount of internal noise. This noise can be described as a amount of internal noise. This noise can be described as a 
"gravel noise" "gravel
 noise" 
 We will assume that calculations We will assume that calculations 
have already been made to assure have already been made to assure 
that the NPSH available is greater that the NPSH available is greater 
than that required by the pump, than that required by the pump, 
(the noise is not a
 result of (the noise is not a result of 
cavitation). cavitation). 
 When the source of suction supply When the source of suction supply 
is above the centerline of the pump, is above the centerline of the pump, 
a check for air leaks can be made by a check for air leaks can be made by 
collecting a sample in a "bubble collecting
 a sample in a "bubble 
bottlebottle“ “. .
 Since the pressure at the suction Since the pressure at the suction 
chamber of the pump is above chamber of the pump is above 
atmos
pheric pressure, a valve can atmospheric pressure, a valve can 
76
p p,p p,
be installed in one of the tapped be installed in one of the tapped 
openings at the high point in the openings at the high point in the 
chamber and liquid can be fed into chamber and liquid can be fed into 
the "bubble bottle." The presence of the "bubble bottle." The presence of 
air or vapor will show itself in the air or vapor
 will show itself in the 
"bubble bottle.""bubble bottle."

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39
Self Priming Pumps
Self‐priming pumps overcomes the air binding
problem by mixing air with water to create a
pumpablefluidwithpumpingpropertiesmuchpumpablefluidwithpumpingpropertiesmuch
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
centrifugalpumpinthatithasaaterreser oir
77
centrifugalpumpinthatithasawaterreservoir
built into the unit may be above the impeller or in front of the impeller.
a achargecharge ofof liquidliquid sufficientsufficient toto
primeprime thethe pumppump mustmust bebe
retainedretained inin thethe casingcasing (Fig(Fig..A)A)
 When When thethe pumppump starts,starts, thethe
rotating rotatingimpeller impellercreates createsa arotating rotatingimpeller impellercreates createsa a
partial partial vacuumvacuum ;;airair fromfrom thethe
suctionsuction pipingpiping isis drawndrawn intointo
thisthis vacuumvacuum andand isis entrainedentrained inin
thethe liquidliquid drawndrawn fromfrom thethe
primingpriming chamberchamber (Fig(Fig..B),B), thenthen
thethe primingpriming cyclecycle startsstarts..
 This Thiscycle cycleis isrepeated repeateduntil untilall all
Fig. A
78
 This Thiscycle cycleis isrepeated repeateduntil untilall all
of of thethe airair fromfrom thethe suctionsuction
pipingpiping hashas beenbeen expelledexpelled andand
replacedreplaced byby pumpagepumpage andand thethe
primeprime hashas beenbeen establishedestablished
(Fig(Fig..C)C)..
Fig. B Fig. C

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40
Self Priming pumps 
Piping System Considerations
 Insure Insure thatthat adequateadequate liquidliquid isis retainedretained InIn thethe primingprimingchamber chamberchamber chamber. .
 The The staticstatic liftlift andand suctionsuction pipingpiping shouldshould bebe minimizedminimized
toto keepkeep primingpriming timetime toto aa minimumminimum..
 Keep Keep AllAll connectionsconnections inin thethe suctionsuction pipingpiping shouldshould bebe
leakleak‐ ‐free free asas airair couldcould bebe suckedsucked inin..
79
Aprimingbypass
line should be
installed.
The suctionpiping
80
pp g
should be designed such that no high points are created

41. 29/10/1435
41
Priming Time Calculation
 1 1. Select the correct size and speed pump . Select the correct size and speed pump 
 2 2. Calculate the . Calculate the NPSH AvailableNPSH Availablefor the   for the   
s ystem. system. yy
NPSHNPSH
A A
= P= P‐ ‐(L (L
s s
+ V+ V
p p
+ h+ h
f f
) ) 
 P = Pressure on surface of liquid in feet absolute P = Pressure on surface of liquid in feet absolute 
LL
s s
= Maximum static lift in feet = Maximum static lift in feet 
VV
p p
= Vapor pressure of the liquid at maximum   = Vapor pressure of the liquid at maximum   
pumping temperature in feet absolute pumping temperature in feet absolute 
81
pumping temperature in feet absolute. pumping temperature in feet absolute. 
h h
f f
= Suction pipe friction loss in feet at the required   = Suction pipe friction loss in feet at the required   
capacity. capacity. 
 3 3.Determine the effective static lift. .Determine the effective static lift. 
LL
es es
= Ls x Sp. Gr.= Ls x Sp. Gr.
4. Enter the priming time curve at
the effective static lift then
downwarddownward toto thethe bottombottom
coordinatecoordinate toto determinedetermine thethe
primingpriming timetime (PT(PT
Les Les
) )toto achieveachieve thethe
givengiven liftlift..
5inserttheprimingtimeintothe5.inserttheprimingtimeintothe
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 lth           i ft 
82
length            in feet. 
Dp = Nominal pipe diameter. 
Ds = Nominal pump suction  
diameter.