This document discusses various terms related to pumps, including types of pumps (positive displacement, centrifugal), pump components (impeller, casing), pump operation concepts (head, suction lift, cavitation, NPSH), and pump performance parameters (specific speed, affinity laws). It provides definitions and formulas for key terms like head, specific speed, NPSH, cavitation, and discusses how different types of pumps like centrifugal and rotary pumps operate.

1. There are various means and ways for conveying fluids (both compressible and non-
compressible) and two phase suspensions. Most widely used equipments can be classified as
follows.
Pumps
Fans
Blowers
Compressors
Out the four machines mentioned above, pumps are mostly used for transporting incompressible
fluids.
Pumps
Positive Displacement Centrifugal
Reciprocating
Rotary
Before starting detailed discussion on pumps, there are some terms that are commonly
encountered while dealing with pumps, especially centrifugal pumps. These terms will be briefly
discussed below.
HEAD:-
The mechanical energy equation is given below.
Pa/ρ + gZa/gc + αaVa2/2gc + ηWp = Pb/ρ + gZb/gc+ αbVb2/2gc + hf
Various terms encountered here are given below.
Pa/ρ & Pb/ρ Pressure Heads
gZa/gc & gZb/gc Height Heads
αaVa2/2gc & αbVb2/2gc Velocity Heads
ηWp Pump Work

2. hf Friction Losses During Flow From Section a
to Section b
The pressure at any point in a liquid can be thought of as being caused by a vertical
column of the liquid which, due to its weight, exerts a pressure equal to the pressure at the point
in question. The height of this column is called the static head and is expressed in terms of feet of
liquid.
The static head corresponding to any specific pressure is dependent upon the weight of
the liquid according to the following formula.
We can predict the approximate head of any centrifugal pump by calculating the
peripheral velocity of the impeller and substituting into the above formula. A handy formula for
peripheral velocity is:
The head developed is approximately equal to the velocity energy at the periphery of the
impeller.
Where
H = Total head developed in feet.
v = Velocity at periphery of impeller in feet per sec.
2
g = 32.2 Feet/Sec
D = Impeller diameter in inches
V = Velocity in ft./sec
SUCTION LIFT:-
It exists when the source of supply is below the
center line of the pump. Thus the STATIC SUCTION
LIFT is the vertical distance in feet from the centerline of the pump to the
free level of the liquid to be pumped.
Suction Lift – Showing Static Heads in a
Pumping System

3. Where the Pump is Located Above the Suction Tank.
(Static Suction Head)
SUCTION HEAD:-
It exists when the source of supply is above the centerline of the pump. Thus the STATIC
SUCTION HEAD is the vertical distance in feet from the centerline of the pump to the free level of the
liquid to be pumped.
Suction Head – Showing Static Heads
in a Pumping System Where the Pump
is Located Below the Suction Tank.
(Static Suction Head)
SPECIFIC SPEED:-
Specific speed (Ns) is a non-dimensional design index used to classify pump impellers as
to their type and proportions. It is defined as the speed in revolutions per minute at which a
geometrically similar impeller would operate if it were of such a size as to deliver one gallon per
minute against one foot head.
The understanding of this definition is of design engineering significance only, however,
and specific speed should be thought of only as an index used to predict certain pump
characteristics. The following formula is used to determine specific speed:

4. Where
N = Pump speed in RPM
Q = Capacity in gpm at the best efficiency point
H = Total head per stage at the best efficiency point
CAVITATION:-
Consider that a pump transfers fluid from a section a to section b. The power calculated
from the mechanical energy balance equations dependent upon the inlet and outlet pressures.
Consider a case where the suction or inlet pressure becomes almost equal to or just above the
vapor pressure of the fluid. The fluid sucked in will have bubbles of air entrained in it and this
will be a serious cause of pump capacity and thus efficiency. In other words, when the pressure
of the liquid is reduced to a value equal to or below its vapor pressure the liquid begins to boil
and small vapor bubbles or pockets begin to form. As these vapor bubbles move along the
impeller vanes to a higher pressure area above the vapor pressure, they rapidly collapse. This
phenomenon is termed as cavitation. The collapse or "implosion" is so rapid that it may be heard
as a rumbling noise, as if you were pumping gravel. The accompanying noise is the easiest way
to recognize cavitation. Vibration and mechanical damage such as bearing failure can also occur
as a result of operating in excessive cavitation, with high and very high suction energy pumps.
On the other hand what will happen if the suction pressure falls below the vapor pressure of the
fluid? The answer is that the pump will be unable to suck any fluid and thus no fluid will be
transferred through the pump. In such a case the pump will be vapor locked. This generally
happens when the pump is started to operate. The sure of such a problem is Priming which
means that the operating fluid be manually or mechanically circulated through the pump so that
the pump becomes void of air or vapors and the pump is able to operate on the designed
pressure.
NET POSITIVE SUCTION HEAD (NPSH):-
The Hydraulic Institute defines NPSH as the total suction head in feet absolute,
determined at the suction nozzle and corrected to datum, less the vapor pressure of the liquid in
feet absolute. Simply stated, it is an analysis of energy conditions on the suction side of a pump
to determine if the liquid will vaporize at the lowest pressure point in the pump.
A liquid increases greatly in volume when it vaporizes. One cubic foot of water at room
temperature becomes 1700 cu. ft. of vapor at the same temperature. It is obvious from this
example that if we are to pump a fluid effectively, we must keep it in liquid form. NPSH is
simply a measure of the amount of suction head present to prevent this vaporization at the lowest
pressure point in the pump.
As the liquid passes from the pump suction to the eye of the impeller, the velocity
increases and the pressure decreases. There are also pressure losses due to shock and turbulence
as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the
velocity and decreases the pressure of the liquid. The NPSH Required is the positive head in feet
absolute required at the pump suction to overcome these pressure drops in the pump and
maintain the majority of the liquid above its vapor pressure. The NPSH Required varies with

5. speed and capacity within any particular pump. Pump manufacturer's curves normally provide
this information.
Net Positive Suction Head (NPSH) NPSH Available is a function of the system in which
the pump operates. It is the excess pressure of the liquid in feet absolute over its vapor pressure
as it arrives at the pump suction. Fig shows four typical suction systems with the NPSH
Available formulas applicable to each. It is important to correct for the specific gravity of the
liquid and to convert all terms to units of "feet absolute" in using the formulas. Various terms
used are
PB = Barometric pressure in feet absolute.
VP = Vapor pressure of the liquid at maximum pumping temperature, in feet absolute.
P = Pressure on surface of liquid in closed suction tank, in feet absolute.
Ls = Maximum static suction lift in feet.
LH = Minimum static suction head in feet.
hf = Friction loss in feet in suction pipe at required capacity

6. In an existing system, the NPSH Available can be determined by a gauge on the pump
suction. The following formula applies:
Where
Gr = Gauge reading at the pump suction expressed in feet (plus if above atmospheric,
minus if below atmospheric) corrected to the pump centerline.
hv = Velocity head in the suction pipe at the gauge connection, expressed in feet.
SUCTION SPECIFIC SPEED:-
In designing a pumping system, it is essential to provide adequate NPSH available for
proper pump operation. Insufficient NPSH available may seriously restrict pump selection, or
even force an expensive system redesign. On the other hand, providing excessive NPSH
available may needlessly increase system cost. Suction specific speed may provide help in this
situation.
Suction specific speed (S) is defined as:
Where
N = Pump speed RPM
GPM = Pump flow at best efficiency point at impeller inlet (for double suction
impellers divide total pump flow by two).
NPSHR = Pump NPSH required at best efficiency point.
An example:
Flow 2,000 GPM; head 600 ft. What NPSHA will be required?
Assume: at 600 ft., 3500 RPM operation will be required.

7. SUCTION ENERGY:-
The amount of energy in a pumped fluid, that flashes into vapor and then collapses back
to a liquid in the higher pressure area of the impeller inlet, determines the extent of the noise
and/or damage from cavitation. Suction Energy is defined as:
Suction Energy = De x N x S x Sg
Where
D e = Impeller eye diameter (inches)
Sg = Specific gravity of liquid (Sg - 1.0 for cold water)
AFFINITY LAWS:-
The affinity laws express the mathematical relationship between the several variables
involved in pump performance. They apply to all types of centrifugal and axial flow pumps.
They are as follows:
1. With impeller diameter D held constant:
2. With speed N held constant
Where:

8. Q = Capacity, GPM
H = Total Head, Feet
BHP = Brake Horsepower
N = Pump Speed, RPM
FUNDAMENTAL FORMULAE FOR CENTRIFUGAL PUMPS:-
Where
GPM = gallons per minute
CFS = cubic feet per second
Lb. = pounds
Hr. = hours
BBL = barrel (42 gallons)

9. Sp.Gr. = specific gravity
H = head in feet
psi = pounds per square inch
In. Hg. = inches of mercury
hv = velocity head in feet
V = velocity in feet per second
g = 32.16 ft/sec2 (acceleration of gravity)
A = area in square inches
I.D. = inside diameter in inches
BHP = brake horsepower
Eff. = pump efficiency expressed as a decimal
Ns = specific speed
N = speed in revolutions per minute
v = peripheral velocity of an impeller in feet per second
D = Impeller in inches
Nc = critical speed
f = shaft deflection in inches
P = total force in pounds
L = bearing span in inches
m = constant usually between 48 and 75 for pump shafts
E = modules of elasticity, psi - 27 to 30 million for steel
CONSTRUCTION OF CENTRIFUGAL PUMPS:-
As a rule, the casing for the liquid end of a pump with a single-suction impeller is made
with an end plate that can be removed for inspection and repair of the pump. A pump with a
double-suction impeller is generally made so one-half of the casing may be lifted without
disturbing the pump.
Since an impeller rotates at high speed, it must be carefully
machined to minimize friction. An impeller must be balanced to
avoid vibration. A close radial clearance must be maintained
between the outer hub of the impeller and that part of the pump
casing in which the hub rotates. The purpose of this is to minimize
leakage from the discharge side of the pump casing to the suction
side.
Centrifugal pump impellers.
A. Single-suction.
B. Double-suction

10. In most centrifugal pumps, the shaft is fitted with a replaceable sleeve. The advantage of
using a sleeve is that it can be replaced more economically than the entire shaft.
Seal piping (liquid seal) is installed to cool the mechanical seal. Most pumps in saltwater
service with total head of 30 psi or more are also fitted with cyclone separators. These separators
use centrifugal force to prevent abrasive material (such as sand in the seawater) from passing
between the sealing surfaces of the mechanical seal. There is an opening at each end of the
separator. The opening at the top is for "clean" water, which is directed though tubing to the
mechanical seals in the pump. The high-velocity "dirty" water is directed through the bottom of
the separator, back to the inlet piping for the pump.
OPERATION OF CENTRIFUGAL PUMP:-
Liquid enters the rotating impeller on the suction side of the casing and enters the eye of
the impeller. Liquid is thrown out through the opening around the edge of the impeller and
against the side of the casing by centrifugal force. This is where the pump got its name. When
liquid is thrown out to the edge of the casing, a region of low pressure (below atmospheric) is
created around the center of the impeller; more liquid moves into the eye to replace the liquid
that was thrown out. Liquid moves into the center of the impeller with a high velocity (speed).
Therefore, liquid in the center of the impeller has a low pressure, but it is moving at a high
velocity.
Liquid moving between the blades of the impeller spreads out, which causes the liquid to
slow down. (Its velocity decreases.) At the same time, as the liquid moves closer to the edge of
the casing, the pressure of the liquid increases. This change (from low pressure and high velocity
at the center to high pressure and low velocity at the edge) is caused by the shape of the opening
between the impeller blades. This space has the shape of a diffuser, a device that causes the
velocity-pressure relationship of any fluid that moves through it to change.
A centrifugal pump is considered to be a nonpositive-displacement pump because the
volume of liquid discharged from the pump changes whenever the pressure head changes. The
pressure head is the combined effect of liquid weight, fluid friction, and obstruction to flow. In a
centrifugal pump, the force of the discharge pressure of the pump must be able to overcome the
force of the pressure head; otherwise, the pump could not deliver any liquid to a piping system.
The pressure head and the discharge pressure of a centrifugal pump oppose each other. When the
pressure head increases, the discharge pressure of the pump must also increase. Since no energy

11. can be lost, when the discharge pressure of the pump increases, the velocity of flow must
decrease. On the other hand, when the pressure head decreases, the volume of liquid discharged
from the pump increases. As a general rule, a centrifugal pump is usually located below the
liquid being pumped. (NOTE: This discussion assumes a constant impeller speed.).
When a centrifugal pump is started, the vent line must be opened to release entrained air.
The open passage through the impeller of a centrifugal pump also causes another problem. It's
possible for liquid to flow backwards (reverse flow) through the pump. A reverse flow, from the
discharge back to the suction, can happen when the pressure head overcomes the discharge
pressure of the pump. A reverse flow can also occur when the pump isn't running and another
pump is delivering liquid to the same piping system. To prevent a reverse flow of liquid through
a centrifugal pump, a check valve is usually installed in the discharge line.
With a check valve in the discharge line, whenever the pressure above the disk rises
above the pressure below it, the check valve shuts. This prevents liquid from flowing backwards
through the pump.
ROTARY PUMPS:-
A number of types are included in this classification, among which are the gear pump, the
screw pump, and the moving vane pump. Unlike the centrifugal pump, which we have discussed,
the rotary pump is a positive displacement pump. This means that for each revolution of the
pump, a fixed volume of fluid is moved regardless of the resistance against which the pump is
pushing. As you can see, any blockage in the system could quickly cause damage to the pump or
a rupture of the system. You, as a pump operator, must always be sure that the system is properly
aligned so a complete flow path exists for fluid flow. Also, because of their positive
displacement feature, rotary pumps require a relief valve to protect the pump and piping system.
The relief valve lifts at a preset pressure and returns the system liquid either to the suction side of
the pump or back to the supply tank or sump.
Rotary pumps are also different from centrifugal pumps in that they are essentially self-
priming. As we saw in our discussion of centrifugal pumps, the pump is located below the liquid
being pumped; gravity creates a static pressure head which keeps the pump primed. A rotary
pump operates within limits with the pump located above the source of supply.
A good example of the principle that makes rotary pumps self-priming is the simple
drinking straw. As you suck on the straw, you lower the air pressure inside the straw.
Atmospheric pressure on the surface of the liquid surrounding the straw is therefore greater and
forces the liquid up the straw. The same conditions basically exist for the gear and screw pump
to prime itself.

12. In the tank must be vented to allow air into the tank to provide atmospheric pressure on
the surface of the liquid. To lower the pressure on the suction side of the pump, the clearances
between the pump parts must be close enough to pump air. When the pump starts, the air is
pumped through the discharge side of the pump and creates the low-pressure area on the suction
side, which allows the atmospheric pressure to force the liquid up the pipe to the pump. To
operate properly, the piping leading to the pump must have no leaks or it will draw in air and can
lose its prime.
Rotary pumps are useful for pumping oil and other heavy viscous liquids. In the engine
room, rotary pumps are used for handling lube oil and fuel oil and are suitable for handling
liquids over a wide range of viscosities.
Rotary pumps are designed with very small clearances between rotating parts and
stationary parts to minimize leakage (slippage) from the discharge side back to the suction side.
Rotary pumps are designed to operate at relatively slow speeds to maintain these clearances;
operation at higher speeds causes erosion and excessive wear, which result in increased
clearances with a subsequent decrease in pumping capacity.
Classification of rotary pumps is generally based on the types of rotating element. Some
types have been briefly mentioned below.
1- GEAR PUMPS:-
The simple gear pump has two spur gears that mesh together and revolve in opposite
directions. One is the driving gear, and the other is the driven gear. Clearances between the gear
teeth (outside diameter of the gear) and the casing and between the end face and the casing are
only a few thousandths of an inch. As the gears turn, the gears unmesh and liquid flows into the
pockets that are vacated by the meshing gear teeth. This creates the suction that draws the liquid
into the pump. The liquid is then carried along in the pockets formed by the gear teeth and the
casing. On the discharge side, the liquid is displaced by the meshing of the gears and forced out
through the discharge side of the pump.

13. Simple gear pump
2- SCREW PUMPS:-
Several different types of screw pumps exist. The differences between the various types
are the number of intermeshing screws and the pitch of the screws. Figure shows a double-screw,
low-pitch pump; and figure 9-11 shows a triple-screw, high-pitch pump. Screw pumps are used
aboard ship to pump fuel and lube oil and to supply pressure to the hydraulic system. In the
double-screw pump, one rotor is driven by the drive shaft and the other by a set of timing gears.
In the triple-screw pump, a central rotor meshes with two idler rotors.
In the screw pump, liquid is trapped and forced through the pump by the action of
rotating screws. As the rotor turns, the liquid flows in between the threads at the outer end of
each pair of screws. The threads carry the liquid along within the housing to the center of the
pump where it is discharged.
Most screw pumps are now equipped with mechanical seals. If the mechanical seal fails,
the stuffing box has the capability of accepting two rings of conventional packing for emergency
use.

14. 3- SLIDING VANE PUMPS:-
The sliding-vane pump figure below has a cylindrically bored housing with a suction
inlet on one side and a discharge outlet on the other side. A rotor (smaller in diameter than the
cylinder) is driven about an axis that is so placed above the center line of the cylinder as to
provide minimum clearance between the rotor and cylinder at the top and maximum clearance at
the bottom.
The rotor carries vanes (which move in and out as the rotor rotates) to maintain sealed
spaces between the rotor and the cylinder wall. The vanes trap liquid on the suction side and
carry it to the discharge side, where contraction of the space expels liquid through the discharge
line. The vanes slide on slots in the rotor. Vane pumps are used for lube oil service and transfer,
tank stripping, bilge, aircraft fueling and defueling and, in general, for handling lighter viscous
liquids.