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Centrifugal Pump Characteristics
Objectives
vTo obtain head, power and efficiency. Then the
characteristic graphs for a single centrifugal pump.
vInvestigate the result on discharge and total head of
operating two pumps in series.
vInvestigate the result on discharge and total head of
operating two pumps in parallel.
vInvestigate the effect of changing inlet head or flow
rate on pump performance. Investigate cavitation.
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Centrifugal Pump
Electric
Motor
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• Centrifugal pump is a mechanical device designed to move
a fluid by means of the transfer of rotational energy from
one or more driven rotors, called impellers.
• Fluid enters the rapidly rotating impeller along its axis and
is cast out by centrifugal force along its circumference
through the impeller’s vane tips.
• The action of the impeller increases the fluid’s velocity and
pressure and also directs it towards the outlet of the pump.
• The pump casing is specially designed to constrict the fluid
from the pump inlet, direct it into the impeller and then slow
and control the fluid before discharge.
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Centrifugal pumps are commonly used due to:
1.
2.
3.
4.
Simplicity
Compactness
Low cost
Ability to operate under wide conditions
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Characteristic
Ø Typically higher flow rates than positive displacement pumps
Ø Comparatively steady discharge
Ø Moderate to low pressure rise
Ø Large range of flow rate operation
Ø Sensitive to flow viscosity
Ø Inexpensive
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Applications of centrifugal pumps
• Centrifugal pumps are the most common type of fluid
mover in chemical industries.
• Centrifugal pumps are commonly used for pumping water,
solvents, organics, oils, acids, bases and any ‘thin’ liquids
in both industrial, agricultural and domestic applications.
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Introduction
• Centrifugal pump converts energy of a prime mover (an
electric motor or turbine) first into velocity or kinetic energy
and then into pressure energy of a fluid that is being pumped.
• The energy changes occur by virtue of two main parts of the
pump, the impeller and the volute or diffuser.
• The volute or diffuser is the stationary part that converts the
kinetic energy into pressure energy.
• The action of the impeller increases the fluid’s velocity and
pressure and also directs it towards the pump outlet.
• The pump casing is specially designed to constrict the fluid
from the pump inlet, direct it into the impeller and then slow
and control the fluid before discharge.
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• The impeller is the rotating part that converts driver energy
into the kinetic energy. The impeller consists of a series of
curved vanes.
• All of the forms of energy involved in a liquid flow system
are expressed in terms of meter of liquid i.e. head.
• Fluid enters the impeller at its axis (the ‘eye’) and exits along
the circumference between the vanes. The impeller, on the
opposite side to the eye, is connected through a drive shaft to
a motor and rotated at high speed (typically 500–5000 rpm).
• The rotational motion of the impeller accelerates the fluid out
through the impeller vanes into the pump casing.
• There are two basic designs of pump casing: volute and
diffuser. The purpose of both designs is to translate the fluid
flow into a controlled discharge at pressure.
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• In a volute casing, the impeller is offset, effectively creating
a curved funnel with an increasing cross-sectional area
towards the pump outlet.
Volute Case design
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• Same basic principle applies to diffuser. The fluid pressure
increases as fluid is expelled between a set of stationary
vanes surrounding the impeller.
Diffuser case design
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• Some centrifugal pumps contain diffusers.
• A diffuser is a set of stationary vanes that surround the
impeller.
• The purpose of the diffuser is to increase the efficiency of
the centrifugal pump by allowing a more gradual expansion
and less turbulent area for the liquid to reduce in velocity.
• The diffuser vanes are designed in a manner that the liquid
exiting the impeller will encounter an ever- increasing flow
area as it passes through the diffuser.
• This increase in flow area causes a reduction in flow
velocity, converting kinetic energy into flow pressure.
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Diffusers
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• When the impeller rotates, it spins the liquid sitting in the
cavities between the vanes outward and provides centrifugal
acceleration.
• As liquid leaves the eye of the impeller, a low-pressure area
is created causing more liquid to flow toward the inlet.
• Because the impeller blades are curved, the fluid is pushed
in a tangential and radial direction by the centrifugal force.
• The energy created by the centrifugal force is kinetic energy.
• The amount of energy given to the liquid is proportional to
the velocity at the edge or vane tip of the impeller.
• The faster the impeller revolves or the bigger the impeller
is, then the higher will be the velocity of the liquid at the
vane tip and the greater the energy imparted to the liquid.
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• The power is applied to the fluid by the impeller.
• The impeller is direct connected through a drive shaft to an
electric motor.
• The use of characteristic curves to present the operating
characteristics of a pump is commonly done in industry.
• Characteristic curves can be used to aid engineers in the
selection of pumps needed for their process and determine
the maximum efficiency of a pump over a range of
operating conditions
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The fluid flows into the rotor at
a given radius and flows out at
greater radius. In the pump
kinetic, potential and pressure
energy is changed.
Fluid flows axially through the
inflow part located in the center
of the rotor, then the direction of
flow is changed in the radial
direction by the action of the
rotor blade. The kinetic energy
of the fluid is increased within
the rotor and converted to
pressure energy at the outlet.
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— Converts
kinetic
energy to
pressure
energy
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Typical centrifugal pump that shows the relative locations
of the pump suction, impeller, volute, and discharge
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Working Principle of a centrifugal pump
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• Impellers can be either single- suction or double-suction.
• A single-suction impeller allows liquid to enter the center of
the blades from only one direction.
• A double-suction impeller allows liquid to enter the center
of the impeller blades from both sides simultaneously.
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Main Parts of Centrifugal Pumps
1. Impeller
• The impeller is the essential part of a centrifugal pump.
• The performance of the pump depends on the impeller
diameters and design.
• The pump’s TDH (Total Dynamic Head) is basically defined
by the impeller’s inner and outer diameter and the pump’s
capacity is defined by the width of the impeller vanes.
• There are three possible types of impellers, open, enclosed
and semi open impellers, each impeller for suitable
application.
• Standard impellers are made of cast iron or carbon steel,
while impeller for aggressive fluids and slurries require high
end materials to ensure a long pump life.
• The impeller is driven by a shaft which is connected to the
shaft of an electric motor
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Open
impeller
Enclosed
impeller
Semi Open Impeller
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2. Casing
—
Which is an air-tight passage surrounding the
impeller designed to direct the liquid to the
impeller and lead it away
—
Volute casing or diffuser case in which the area of
the flow increases gradually.
3. Suction Pipe
4. Delivery Pipe
5. The Shaft: which is the bar by which the power is
transmitted from the motor drive to the impeller.
6. The driving motor: which is responsible for
rotating the shaft. It can be mounted directly on the
pump, above it, or adjacent to it.
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• The actual head rise (H) produced by a centrifugal pump is a
function of the flow rate (Q).
• It is possible to determine the head-flow relationship by
appropriate selection of the geometry of the impeller blades
• Pumps are designed so that the head decreases with
increasing flow since such a design results in a stable flow
rate when the pump is connected to a piping system.
• Typical head flow curve for a pump is:
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Typical performance curves for centrifugal pump.
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— A centrifugal pump uses the conservation of energy principle.
It changes velocity energy into pressure energy .
— As the differential head (H) increases ,the flow rate (Q)
decreases .
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Static and Dynamic Head
w The static Head is the difference between Suction Head and
Delivered Head.
w As the Suction Head changes the Static Head changes.
w When the pump is operating the liquid will be moving within
the pipe wok and so a loss due to friction will occur.
w Dynamic Head
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Losses due to Friction
— A centrifugal pump incurs head losses due to friction.
— The friction is caused by the fluid changing direction when
travelling through the pump and by clearances within the
pump. These losses vary with both head and flow.
Actual performance curve
Subtracting the losses from the ideal gives the actual
performance curve for the pump.
Ideal Head – Losses = Actual Head
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Equipment Setup
• Two motor-driven centrifugal pumps mounted on a stainless
steel plinth with a water reservoir and pipework for
continuous circulation.
• The pumps can be configured for single pump operation, two
pumps in parallel or two pumps in series by using manually
operated ball valves.
• Manual valves are used to control the flow and facilitate the
study of suction effects, including demonstration of air
release.
• In parallel operation the two pumps draw from a shared inlet
pipe of a wider diameter than the pump inlet, reflecting a
typical industrial configuration of parallel pumping.
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Cut-away view of ball valve components:
1 body, 2 seat, 3 floating ball, 4 lever handle, 5 stem
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• The Electronic sensors measure the pump outlet pressure of
each pump, the shared pump inlet pressure, the flow rate and
the water temperature.
• The pump speed of the first pump is accurately controlled by
an advanced electronic inverter within the IFD7 and can be
varied over the full range.
• The inverter calculates the torque produced at the motor
drive shaft, allowing the power used by the pump to be
derived.
• The second pump runs at inherent motor speed.
• This combination of control facilities allows a wide range of
different configurations to be investigated.
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• The pump operating parameters are controlled using the
supplied software via an interface device.
• The software also allows all sensor outputs to be logged.
• The speed of one of the pumps may be varied to allow the
collection of performance data over a range of parameters.
• Outlet pressures may be varied to control the flow rate.
• Flow through the system may be set to allow single pump
operation, series pump operation or parallel pump operation.
• Each pump has impellers that can be easily accessed and
replaced without tools.
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Front View of FM51 Series and Parallel Pumps Demonstration
Unit
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Top View of Series and Parallel Pumps Demonstration Unit reservoir (1);
manual gate valve (2); paddle-wheel flow sensor (3); pump 1 (4); electronic
pressure sensors (5); manual ball valve (6); temperature sensor (7); baseplate
(8); system drain valve (9); drain hole (10); storage position (11); Pump 2 (12);
electronic pressure sensors (13); ball valve (14) at the outlet of Pump 2 (12);
ball valve (15); Drain valves in the pump casings (16); Drain valves in the
pump casings (17); 3-way valve (18); electronic pressure sensors (19)
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Outlet Manual Gate valve
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TECHNICAL SPECIFICATIONS
Max flow rate: 2.2 l/s (parallel pumping, both pumps 50Hz)
Max head: 6.0 m (single pump) – 12.0 m (series)
Pump speed: 1,800 rpm (pump 1) – 1,500 rpm (pump 2)
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§ The ArmSOFT software enables to control the pump speed 0
to 100%. Feedback from the sensors is then displayed in real
time for the end user with simultaneous data logging.
§ The data trend is also displayed graphically in real time and
can be exported to another platform such as Excel for further
analysis.
IFD7-G: 220 -240V / 1Ph / 60Hz
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Baseplate
• The baseplate supports the other components of the system.
Réservoir
• The reservoir is formed of clear acrylic and is mounted on the
baseplate.
• The reservoir may be filled through the open top.
• Flow exits the reservoir from the lower front pipe connection,
is drawn through the pump, and re-enters the reservoir from
the upper rear pipe connection.
• A central baffle prevents air from being drawn into the
pipework.
• A drain hole in the base allows the reservoir to be drained
after use.
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Pumps
• The two pumps are motor-driven centrifugal pumps.
• On pump 1 the speed of the motor is adjustable to give a
range of 0 to 100%, allowing operation as a single pump for
pump performance analysis.
• Pump 2 is an identical model but is run at its design speed,
which is equivalent to a setting of 80% on the variable-speed
pump for a 50 Hz electrical supply, or 100% for a 60 Hz
supply.
• The pump bodies and cover plates are made from clear
acrylic, allowing the impellers to be observed.
• Each plate is secured with six thumbscrews and may be
removed to allow the impellers to be changed.
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Impellers
• When supplied, both pumps are fitted with impellers with
backward-curved blades.
Inlet Valve
• A manual ball valve controls the inlet (suction) head supplied
to the pumps.
• This valve should be fully open except when investigating
the effect of inlet pressure on pump performance and
cavitation formation.
• The valve is operated by turning the handle on the top.
• The valve is fully open when the handle is in line with the
pipework, and is fully closed when the handle is at right
angles to the pipework.
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Gate Valve
• Manual gate valve controls the overall flow rate through the
system.
• The valve is operated by turning the wheel on the top.
• The valve is fully open (maximum flow) when the wheel is
turned fully anticlockwise (looking downwards on the valve),
and is fully closed (no flow) when the handle is turned fully
clockwise. The valve is marked on the wheel indicating the
correct turn directions.
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Pump Outlet Valves
• Each pump has a ball valve fitted on the outlet side.
• These valves are used to control the pattern of flow through
the system.
• The ball valve at the outlet of Pump 2 should be closed (set at
90° to the pipework) to produce flow through Pump 1.
• The outlet valves should be set in conjunction with the 3-way
valve to ensure the correct flow path.
3-Way Valve
• A 3-way valve is situated near the inlet to Pump 2.
• This valve is used to set the flow pattern for single, series or
parallel flow through the system.
• The valve is marked with the flow route through the valve.
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3-Way Valve
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Flow Sensor
• A paddle-wheel flow sensor is situated in the outlet
pipework to measure the flow rate through the pump.
• The output from the sensor is displayed on the equipment
software.
Pressure Sensors
• Three electronic pressure sensors are fitted to the equipment,
one in the inlet pipe and one at each pump outlet.
• Outputs from the pressure sensors are displayed on the
equipment software
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System Drain Valve
• A drain valve beneath the base plate controls flow from the
reservoir drain.
• The valve should be closed while the equipment is in use.
Pump drain valves
• Drain valves in the pump casings and allow the pump volutes
to be fully drained.
Temperature Sensor
• A temperature sensor is situated at the outlet from the
reservoir, to measure the temperature of the fluid within the
system.
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Single Pump
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Series Pumps
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Parallel Pumps
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Controlling flow rate using the gate valve
• The flow rate through the pump can be manually controlled
using the gate valve.
• This alters the back pressure on the pump, and hence the head
against which the pump must do work.
• The valve is marked with the direction in which the handle
should be turned for Open (maximum flow rate) and Closed
(minimum/no flow rate).
• When adjusting the flow rate, turn the valve handle smoothly
in small increments and observe the result of the change on
the software screen.
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Old version
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Theory
• The operating characteristics of a centrifugal pump is
illustrated by using graphs of pump performance.
• The three most commonly used graphical representations
of pump performance are:
Ø Change in total head produced by the pump, Ht
Ø Power input to the pump, Pm
Ø Pump efficiency, E
Total Head:
The change in total head produced as a result of the work
done by pump can be calculated as:
Ht = Change in static head + change in velocity head +
change in elevation
= Hs + Hv + He
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P in = fluid pressure at inlet in Pa
P out = fluid pressure at outlet in Pa
Vin = fluid velocity at inlet in m/s
Vout = fluid velocity at outlet in m/s
H e = Change in elevation = Vertical distance between inlet
and outlet = 0.075 m for this system
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Power Input
The mechanical power input to the pump can be calculated as:
Pm = rotational force x angular distance = 2. π. n. t
n = rotational speed of pump in revolutions per second
t
= shaft torque in Nm
Pump efficiency
The efficiency of the pump may be calculated as:
P h = hydraulic power imparted to fluid = Ht. Q. ρ. g
Q = volume flow rate in m 3/s
P m = mechanical power absorbed by pump =
2. π. n. t
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Each of these parameters is measured at constant pump speed,
and is plotted against the volume flow rate, Q, through the pump.
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• The Ht - Q curve shows the relationship between
head and flow rate.
• The change in head decreases as flow rate increases
(head increases as flow rate decreases). This type of
curve is referred to as a rising characteristic curve.
• A stable head - capacity characteristic curve is one in
which there is only one possible flow rate for a given
head.
• The Pm - Q curve shows the relationship between
the power input to the pump and the change in flow
rate through the pump.
• Outside the optimum operating range of the pump
this curve flattens, so that a large change in pump
power produces only a small change in flow velocity.
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• The E - Q curve shows the pump capacity at which
the pump operates most efficiently.
• The optimum operating capacity is 0.7 dm3/s, which
would give a head of 1.2 m.
• When selecting a pump for an application where the
typical operating capacity is known, a pump should
be selected so that its optimum efficiency is at or
very near that capacity
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• Single pump may be insufficient to produce the performance
required.
• Combining two pumps increases the pumping capacity of the
system.
• Two pumps may be connected in series, so that water passes
first through one pump and then through the second.
• When two pumps operate in series, the flow rate is the same
as for a single pump but the total head is increased.
• The combined pump head-capacity curve is found by adding
the heads of the single pump curves at the same capacity.
• Combining two pumps increases the pumping capacity of the
system.
• Two pumps may be connected in parallel, so that half the
flow passes through one of the pumps and the other half
through the second pump.
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• When two pumps operate in parallel the total head increase
remains unchanged but the flow rate is increased. The headcapacity curve is found by adding the capacities of the single
pump curves at the same head.
SERIES
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PARALLEL
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• Pumps in serial mode:
H = H1+ H2
• Pumps in parallel mode:
H = (H1+H2)/2
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Centrifugal Pump Characteristics
Cavitation
• Cavitation is unwanted phenomenon that has a serious
negative impact on operating functions of centrifugal
machines with a liquid working fluid and the lifespan.
• It can affect many aspects of a pump, but it is often the
pump impeller that is most severely impacted.
• A relatively new impeller that has suffered from cavitation
typically looks like it has been in use for many years; the
impeller material may be eroded and it can be damaged
beyond repair.
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• The cavitation phenomenon takes place due to local
evaporation of the fluid caused by a lowering of the pressure.
• If the pressure falls below the vapor pressure of the fluid,
steam bubbles are created.
• When the bubbles reach regions of higher pressure, they
implode in contact with the surfaces of the machine,
generating pressure waves that cause erosion.
• If the cavitation persists for a long time, it may cause severe
mechanical consequences.
• The onset of cavitation has a negative influence on the
performance of the pump. It causes a decrease in the flow
rate and the total head generated.
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• It occurs because there is not enough pressure at the
suction end of the pump, or insufficient Net Positive Suction
Head available (NPSHa).
• As the liquid passes from the suction side of the impeller to
the delivery side, the bubbles implode. This creates a shock
wave that hits the impeller and creates pump vibration and
mechanical damage, possibly leading to complete failure of
the pump at some stage.
• In abnormal operating conditions, especially in cavitation
conditions, vibrations and noise are present; many studies
use acoustic noise and vibrations to detect cavitation
formation.
• If the cavitation persists for a long time, it may cause severe
mechanical consequences.
• It is necessary to detect and avoid the development of
cavitation in centrifugal pump.
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Ø If the pressure at any point is less than the vapor pressure of
the liquid at the temperature at that point, vaporization will
occur.
Ø This is likely to arise in the suction side where the lowest
pressures are experienced.
Ø The vaporized liquid appears as bubbles within the liquid, and
these subsequently collapse with such force that mechanical
damage may be sustained.
Ø This condition, known as cavitation, is accompanied by a
marked increase in noise and vibration in addition to the loss
of head.
Ø In addition to the potential for physical damage to the pump
from cavitation, both from the resulting vibration and from
the explosive force of the collapsing bubbles of vapor, pumps
cannot pump vapor effectively. Hence if cavitation occurs
then the pump may not be capable of developing the suction
head necessary to reach the required operating point.
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Ø Net Positive Suction Head (NPSH), the NPSH will involve
running the pump with water at different capacities, while
throttling (reducing the flow in) the inlet (suction) side.
Ø The suction pressures at which the first sign of vaporization
appear are noted for each capacity. These are converted into
head values and are published on the pump characteristic
curve as the Net Positive Suction Head Required (NPSHr)
or just NPSH.
Ø NPSH is the amount by which the pressure at this point
must exceed the vapor pressure of the liquid.
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Ø The Net Positive Suction Head Available (NPSHa) depends
on the system in which the pump is used, and is calculated
according to system conditions.
Ø The basic calculation for an existing system using water as
the working fluid may be approximated as:
NPSHa = H atm. – Vapor + H in + H v
H atm = Barometric (ambient) pressure, expressed as a
head of water in mm.
Vapor = Vapor pressure of water at maximum expected
temperature, expressed as an equivalent head of
water in mm.
H in = Gauge (sensor) pressure at inlet (note that value is
relative to atmosphere, and thus in some
circumstances may be negative), expressed as a
head of water in mm.
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NPSH is calculated in mm of water.
Ø In some pump datasheets it may be expressed in inches of
water. It may also be calculated as a pressure by summing
the component pressures.
Ø To convert velocity head to equivalent pressure, use
REQUIREMENT:
On a single graph, plot the pump capacity against the total
head for each set of data.
On a single graph, plot the Net Positive Suction Head Available
against pump capacity.
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—
For proper pump operation (no cavitation) :
(NPSH)A > (NPSH)R
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Manufacturers
determine conservatively
how much NPSH is
needed to avoid
cavitation in the pump
–Systematic
experimental testing
•NSPH required
(NPSHR) is plotted on
pump chart
– Caution: different axis
scale is common – read
carefully
•Plot NPSH vs NSPH
required to give safe
operating range of pump
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• The suction of static pressure is progressively reduced in
order to study the inception of the initial bubbles and their
development.
• This is done by throttling the suction valve in order to
increase the suction losses and reduce NPSH value, while
properly regulating the discharge valve to retain the flow
rate.
• The NPSH value is calculated for every operating point from
the equation:
HV is the water vapor pressure as function of temperature
P is the absolute static pressure
v is the mean flow velocity in the suction pipe.
• The NPSH value is expressed in terms of the dimensionless
Thoma’s cavitation number:
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σ=NPSH / Htot
where Htot is the total head calculated from:
where z is the vertical distance between the discharge and
suction measuring points.
The σ and NPSH values are further specified with the use of a
subscript that determines the intensity of cavitation.
Reference: Georgios Mousmoulis Nilla Karlsen Davies,
Dimitrios Papantonis, Experimental analysis of
cavitation in a centrifugal pump using acoustic
emission, vibration measurements and flow
visualization, European Journal of Mechanics /B Fluids
75(2019) 300–311,
85
If the value of σ is less than the critical value, σc then cavitaion
will occur in the pump.
The Cavitaion factor σ is a function of specific speed, efficiency
of the pump and number of vanes. The value of σc depends
upon the specific speed of the pump Ns.
The following empirical relation is used to obtain the value of
σc
Q = discharge of the pump, m 3/s
N = Speed of Pump,
Hm = Manomatric height , m
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What is vapor pressure?
• At a specific combination of pressure and temperature, which
is different for different liquids, the liquid molecules turn to
vapor. An everyday example is a pot of water on the kitchen
stove. When boiled to 100 o Celsius, atmospheric pressure
bubbles form on the bottom of the pan and steam rises.
• This indicates vapor pressure and temperature have been
reached and the water will begin boiling.
• Vapor pressure is defined as the pressure at which liquid
molecules will turn into vapor. It should be noted that the
vapor pressure for all liquids varies with temperature.
• It is also important to understand that vapor pressure and
temperature are linked.
• A half full bottle of water subjected to a partial vacuum will
begin to boil without the addition of any heat whatsoever.
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Constant pressure acting on liquid (atmospheric pressure)
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Properties of water at different temperatures
By regulating the pressure at which water is subjected its vapor
pressure can be changed and it will eventually boil at room
temperature.
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The impact of cavitation on a pump
• Cavitation causes pump performance deterioration,
mechanical damage, noise and vibration which can ultimately
lead to pump failure.
• Vibration is a common symptom of cavitation, and many
times the first sign of an issue.
• Vibration causes problems for many pump components,
including the shaft, bearings and seals.
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Pitting action in wall of impeller due to cavitation
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Impeller damage by cavitation
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Impeller damage.
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How to avoid cavitation
• Cavitation can be avoided most easily during the design stage.
The key is to understand Net Positive Suction Head (NPSH)
and take it into account throughout the design process. In
order to understand this term more easily it is helpful to
break it down:
• Net refers to that which is remaining after all deductions
have been made
• Positive is obvious
• Suction Head refers to the pressure at the pump inlet flange.
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Thank you
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