Basics Concepts of Operation, Maintenance, and Troubleshooting of a Centrifugal Pump.

Basic Concepts of Operation, Maintenance,
and Troubleshooting of Centrifugal Pumps
By Okafor Christopher Ogochukwu
Introduction
The operating manual of any centrifugal pump often starts with a general
statement, “Your centrifugal pump will give you completely trouble free and
satisfactory service only on the condition that it is installed and operated with due
care and is properly maintained.” Despite all the care in operation and
maintenance, engineers often face the statement “the pump has failed i.e. it can no
longer be kept in service”. Inability to deliver the desired flow and head is just one
of the most common conditions for taking a pump out of service. There are other
many conditions in which a pump, despite suffering no loss in flow or head is
considered to have failed and has to be pulled out of service as soon as possible.
These include seal related problems (leakages, loss of flushing, cooling, quenching
systems, etc), pump and motor bearings related problems (loss of lubrication,
cooling, contamination of oil, abnormal noise, etc), leakages from pump casing,
very high noise and vibration levels, or driver (motor or turbine) related problems.
The list of pump failure conditions mentioned above is neither exhaustive nor are
the conditions mutually exclusive. Often the root causes of failure are the same but
the symptoms are different. A little care when first symptoms of a problem appear
can save the pumps from permanent failures. Thus the most important task in such
situations is to find out whether the pump has failed mechanically or if there is
some process deficiency, or both. Many times when the pumps are sent to the
workshop, the maintenance people do not find anything wrong on disassembling it.
Thus the decision to pull a pump out of service for maintenance / repair should be
made after a detailed analysis of the symptoms and root causes of the pump failure.
Also, in case of any mechanical failure or physical damage of pump internals, the
operating engineer should be able to relate the failure to the process unit’s
operating problems.
Any operating engineer, who typically has a chemical engineering background and
who desires to protect his pumps from frequent failures must develop not only a
good understanding of the process but also thorough knowledge of the mechanics
of the pump. Effective troubleshooting requires an ability to observe changes in

performance over time, and in the event of a failure, the capacity to thoroughly
investigate the cause of the failure and take measures to prevent the problem from
re-occurring.
The fact of the matter is that there are three types of problems mostly encountered
with centrifugal pumps:
design errors
poor operation
poor maintenance practices
This article is being presented in three parts, covering all aspects of operation,
maintenance, and troubleshooting of centrifugal pumps. The article has been
written keeping in mind the level and interests of students and the beginners in
operation. Any comments or queries are most welcome.
Working Mechanism of a Centrifugal Pump
A centrifugal pump is one of the simplest pieces of equipment in any process plant.
Its purpose is to convert 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 impeller is the rotating part that
converts driver energy into the kinetic energy. The volute or diffuser is the
stationary part that converts the kinetic energy into pressure energy.
Note: All of the forms of energy involved in a liquid flow system are expressed in
terms of feet of liquid i.e. head.
Generation of Centrifugal Force
The process liquid enters the suction nozzle and then into the eye (center) of a
revolving device known as an impeller. 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 towards the inlet. Because the impeller blades are
curved, the fluid is pushed in a tangential and radial direction by the centrifugal
force. This force acting inside the pump is the same one that keeps water inside a
bucket that is rotating at the end of a string.
Figure A.01 below depicts a side cross-section of a centrifugal pump indicating the
movement of the liquid.

Figure A.01: Liquid flow path inside a centrifugal pump
Conversion of Kinetic Energy to Pressure Energy
The key idea is that 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.
This kinetic energy of a liquid coming out of an impeller is harnessed by creating a
resistance to the flow. The first resistance is created by the pump volute (casing)
that catches the liquid and slows it down. In the discharge nozzle, the liquid further

decelerates and its velocity is converted to pressure according to Bernoulli’s
principle.
Therefore, the head (pressure in terms of height of liquid) developed is
approximately equal to the velocity energy at the periphery of the impeller
expressed by the following well-known formula:
A handy formula for peripheral velocity is:
This head can also be calculated from the readings on the pressure gauges attached
to the suction and discharge lines. Pump curves relate flow rate and pressure (head)
developed by the pump at different impeller sizes and rotational speeds. The
centrifugal pump operation should conform to the pump curves supplied by the
manufacturer. In order to read and understand the pump curves, it is very important
to develop a clear understanding of the terms used in the curves. This topic will be
covered later.

General Components of Centrifugal Pumps
A centrifugal pump has two main components:
I. A rotating component comprised of an impeller and a shaft
II. A stationary component comprised of a casing, casing cover, and bearings.
The general components, both stationary and rotary, are depicted in Figure B.01.
The main components are discussed in brief below.
Figure B.02 shows these parts on a photograph of a pump in the field.
Figure B.01: General components of Centrifugal Pump

Figure B.02: General components of a Centrifugal Pump
Stationary Components
Casing:
Casings are generally of two types: volute and circular. The impellers are fitted
inside the casings.
1. Volute casings build a higher head; circular casings are used for low head and
high capacity.
o A volute is a curved funnel increasing in area to the discharge port as shown in
Figure B.03. As the area of the cross-section increases, the volute reduces the
speed of the liquid and increases the pressure of the liquid.

Figure B.03: Cut-away of a pump showing volute casing
o One of the main purposes of a volute casing is to help balance the hydraulic
pressure on the shaft of the pump. However, this occurs best at the manufacturer's
recommended capacity. Running volute-style pumps at a lower capacity than the
manufacturer recommends can put lateral stress on the shaft of the pump,
increasing wear-and-tear on the seals and bearings, and on the shaft itself. Double-
volute casings are used when the radial thrusts become significant at reduced
capacities.
2. Circular casing have stationary diffusion vanes surrounding the impeller
periphery that convert velocity energy to pressure energy. Conventionally, the
diffusers are applied to multi-stage pumps.
o The casings can be designed either as solid casings or split casings. Solid casing
implies a design in which the entire casing including the discharge nozzle is all
contained in one casting or fabricated piece. A Split casing implies two or more
parts are fastened together. When the casing parts are divided by horizontal plane,
the casing is described as horizontally split or axially split casing. When the split is
in a vertical plane perpendicular to the rotation axis, the casing is described as
vertically split or radially split casing. Casing Wear rings act as the seal between
the casing and the impeller.

Figure B.04: Solid Casing
Suction and Discharge Nozzle
The suction and discharge nozzles are part of the casings itself. They commonly
have the following configurations.
1. End suction/Top discharge (Figure B.05) - The suction nozzle is located at the
end and concentric to the shaft while the discharge nozzle is located at the top of
the case perpendicular to the shaft. This pump is always of an overhung type and
typically has lower NPSHr because the liquid feeds directly into the impeller’s eye.
2. Top suction Top discharge nozzle (Figure B.05) -The suction and discharge
nozzles are located at the top of the case perpendicular to the shaft. This pump can
either be an overhung type or between-bearing type but is always a radially split
case pump.

Basics Concepts of Operation, Maintenance, and Troubleshooting of a Centrifugal Pump.

Figure B.05: Suction and Discharge Nozzle Locations
3. Side suction / Side discharge nozzles - The suction and discharge nozzles are
located at the sides of the case perpendicular to the shaft. This pump can have
either an axially or radially split case type. Seal Chamber and Stuffing Box Seal
chamber and Stuffing box both refer to a chamber, either integral with or separate
from the pump case housing that forms the region between the shaft and casing
where sealing media are installed. When the sealing is achieved by means of a
mechanical seal, the chamber is commonly referred to as a Seal Chamber. When
the sealing is achieved by means of packing, the chamber is referred to as a
Stuffing Box. Both the seal chamber and the stuffing box have the primary
function of protecting the pump against leakage at the point where the shaft passes
out through the pump pressure casing. When the pressure at the bottom of the
chamber is below atmospheric, it prevents air leakage into the pump. When the
pressure is above atmospheric, the chambers prevent liquid leakage out of the
pump. The seal chambers and stuffing boxes are also provided with cooling or
heating arrangement for proper temperature control. Figure B.06 below depicts an
externally mounted seal chamber and its parts.

Figure B.06: Parts of a simple Seal Chamber
Gland: The gland is a very important part of the seal chamber or the stuffing box.
It gives the packing or the mechanical seal the desired fit on the shaft sleeve. It can
be easily adjusted in axial direction. The gland comprises of the seal flush, quench,
cooling, drain, and vent connection ports as per the standard codes like API 682.
o Throat Bushing: The bottom or inside end of the chamber is provided with a
stationary device called throat bushing that forms a restrictive close clearance
around the sleeve (or shaft) between the seal and the impeller.
o Throttle bushing refers to a device that forms a restrictive close clearance
around the sleeve (or shaft) at the outboard end of a mechanical seal gland.
o Internal circulating device refers to device located in the seal chamber to
circulate seal chamber fluid through a cooler or barrier/buffer fluid reservoir.
Usually it is referred to as a pumping ring.
o Mechanical Seal: The features of a mechanical seal will be discussed in Part-II
of this article.

Bearing housing
The bearing housing encloses the bearings mounted on the shaft. The bearings
keep the shaft or rotor in correct alignment with the stationary parts under the
action of radial and transverse loads. The bearing house also includes an oil
reservoir for lubrication, constant level oiler, jacket for cooling by circulating
cooling water.
Rotating Components
1. Impeller
The impeller is the main rotating part that provides the centrifugal acceleration to
the fluid. They are often classified in many ways.
o Based on major direction of flow in reference to the axis of rotation
Radial flow
Axial flow
Mixed flow
o Based on suction type
Single-suction: Liquid inlet on one side.
Double-suction: Liquid inlet to the impeller symmetrically from both sides.
o Based on mechanical construction (Figure B.07)
Closed: Shrouds or sidewall enclosing the vanes.
Open: No shrouds or wall to enclose the vanes.
Semi-open or vortex type.

Figure B.07: Impeller types
Closed impellers require wear rings and these wear rings present another
maintenance problem. Open and semi-open impellers are less likely to clog, but
need manual adjustment to the volute or back-plate to get the proper impeller
setting and prevent internal re-circulation. Vortex pump impellers are great for
solids and "stringy" materials but they are up to 50% less efficient than
conventional designs. The number of impellers determines the number of stages of
the pump. A single stage pump has one impeller only and is best for low head
service. A two-stage pump has two impellers in series for medium head service.
A multi-stage pump has three or more impellers in series for high head service.
o Wear rings: Wear ring provides an easily and economically renewable
leakage joint between the impeller and the casing. Clearance becomes too large
that the pump’s efficiency will be lowered causing heat and vibration problems.
Most manufacturers require that you disassemble the pump to check the wear ring
clearance and replace the rings when this clearance doubles.
2. Shaft
The basic purpose of a centrifugal pump shaft is to transmit the torques
encountered when starting and during operation while supporting the impeller and
other rotating parts. It must do this job with a deflection less than the minimum
clearance between the rotating and stationary parts.

Shaft Sleeve (Figure B.08): Pump shafts are usually protected from erosion,
corrosion, and wear at the seal chambers, leakage joints, internal bearings, and in
the waterways by renewable sleeves. Unless otherwise specified, a shaft sleeve of
wear, corrosion, and erosion resistant material shall be provided to protect the
shaft. The sleeve shall be sealed at one end. The shaft sleeve assembly shall extend
beyond the outer face of the seal gland plate. (Leakage between the shaft and the
sleeve should not be confused with leakage through the mechanical seal).
Figure B.08: A view of a shaft sleeve
Coupling: Couplings can compensate for axial growth of the shaft and transmit
torque to the impeller. Shaft couplings can be broadly classified into two groups:
rigid and flexible. Rigid couplings are used in applications where there is
absolutely no possibility or room for any misalignment. Flexible shaft couplings
are more prone to selection, installation and maintenance errors. Flexible shaft
couplings can be divided into two basic groups: elastomeric and non-elastomeric.
Elastomeric couplings use either rubber or polymer elements to achieve
flexibility. These elements can either be in shear or in compression. Tire and
rubber sleeve designs are elastomer in shear couplings; jaw and pin and bushing
designs are elastomer in compression couplings.
Non-elastomeric couplings use metallic elements to obtain flexibility. These
can be one of two types: lubricated or non-lubricated. Lubricated designs
accommodate misalignment by the sliding action of their components, hence the

need for lubrication. The non-lubricated designs accommodate misalignment
through flexing. Gear, grid and chain couplings are examples of non-elastomeric,
lubricated couplings. Disc and diaphragm couplings are non-elastomeric and non-
lubricated.
Auxiliary Components
Auxiliary components generally include the following piping systems for the
following services:
o Seal flushing , cooling , quenching systems
o Seal drains and vents
o Bearing lubrication , cooling systems
o Seal chamber or stuffing box cooling, heating systems
o Pump pedestal cooling systems
Auxiliary piping systems include tubing, piping, isolating valves, control valves,
relief valves, temperature gauges and thermocouples, pressure gauges, sight flow
indicators, orifices, seal flush coolers, dual seal barrier/buffer fluid reservoirs, and
all related vents and drains.
All auxiliary components shall comply with the requirements as per standard codes
like API 610 (refinery services), API 682 (shaft sealing systems) etc.
Definition of Important Terms
The key performance parameters of centrifugal pumps are capacity, head, BHP
(Brake horse power), BEP (Best efficiency point) and specific speed. The pump
curves provide the operating window within which these parameters can be varied
for satisfactory pump operation. The following parameters or terms are discussed
in detail in this section.
Capacity
Head
o Significance of using Head instead of Pressure
o Pressure to Head Conversion formula
o Static Suction Head, hS
o Static Discharge Head, hd
o Friction Head, hf
o Vapor pressure Head, hvp
o Pressure Head, hp
o Velocity Head, hv
o Total Suction Head HS

o Total Discharge Head Hd
o Total Differential Head HT
NPSH
o Net Positive Suction Head Required NPSHr
o Net Positive Suction Head Available NPSHa
Power (Brake Horse Power, B.H.P) and Efficiency (Best Efficiency Point, B.E.P)
Specific Speed (Ns)
Affinity Laws
Capacity
Capacity means the flow rate with which liquid is moved or pushed by the pump to
the desired point in the process. It is commonly measured in either gallons per
minute (gpm) or cubic meters per hour (m3/hr). The capacity usually changes with
the changes in operation of the process. For example, a boiler feed pump is an
application that needs a constant pressure with varying capacities to meet a
changing steam demand. The capacity depends on a number of factors like:
Process liquid characteristics i.e. density, viscosity
Size of the pump and its inlet and outlet sections
Impeller size
Impeller rotational speed RPM
Size and shape of cavities between the vanes
Pump suction and discharge temperature and pressure conditions
For a pump with a particular impeller running at a certain speed in a liquid, the
only items on the list above that can change the amount flowing through the pump
are the pressures at the pump inlet and outlet. The effect on the flow through a
pump by changing the outlet pressures is graphed on a pump curve.
As liquids are essentially incompressible, the capacity is directly related with the
velocity of flow in the suction pipe. This relationship is as follows:

Head
Significance of using the “head” term instead of the “pressure” term
The pressure at any point in a liquid can be thought of as being caused by a vertical
column of the liquid due to its weight. The height of this column is called the static
head and is expressed in terms of feet of liquid. The same head term is used to
measure the kinetic energy created by the pump.
In other words, head is a measurement of the height of a liquid column that the
pump could create from the kinetic energy imparted to the liquid. Imagine a pipe
shooting a jet of water straight up into the air, the height the water goes up would
be the head. The head is not equivalent to pressure. Head is a term that has units of
a length or feet and pressure has units of force per unit area or pound per square
inch.
The main reason for using head instead of pressure to measure a centrifugal
pump's energy is that the pressure from a pump will change if the specific gravity
(weight) of the liquid changes, but the head will not change. Since any given
centrifugal pump can move a lot of different fluids, with different specific
gravities, it is simpler to discuss the pump's head and forget about the pressure. So
a centrifugal pump’s performance on any Newtonian fluid, whether it's heavy
(sulfuric acid) or light (gasoline) is described by using the term ‘head’. The pump
performance curves are mostly described in terms of head.
Note:A given pump with a given impeller diameter and speed will raise a
liquid to a certain height regardless of the weight of the liquid.
Pressure to Head Conversion formula
The static head corresponding to any specific pressure is dependent upon the
weight of the liquid according to the following formula:

Newtonian liquids have specific gravities typically ranging from 0.5 (light, like
light hydrocarbons) to 1.8 (heavy, like concentrated sulfuric acid). Water is a
benchmark, having a specific gravity of 1.0.
This formula helps in converting pump gauge pressures to head for reading the
pump curves. The various head terms are discussed below.
Note: A given pump with a given impeller diameter and speed will raise a liquid to a
certain height regardless of the weight of the liquid.
Note: The Subscript’s’ refers to suction conditions and ‘d’ refers to discharge
conditions.
o Static Suction Head, hS
o Static Discharge Head, hd
o Friction Head, hf
o Vapor pressure Head, hvp
o Pressure Head, hp
o Velocity Head, hv
o Total Suction Head HS
o Total Discharge Head Hd
o Total Differential Head HT
o Net Positive Suction Head Required NPSHr
o Net Positive Suction Head Available NPSHa

Static Suction Head (hS) : Head resulting from elevation of the liquid relative to
the pump center line. If the liquid level is above pump centerline, hS is positive. If
the liquid level is below pump centerline, hS is negative. Negative hS condition is
commonly denoted as a “suction lift” condition

Static Discharge Head (hd): It is the vertical distance in feet between the pump
centerline and the point of free discharge or the surface of the liquid in the
discharge tank.


Friction Head (hf): The head required to overcome the resistance to flow in the
pipe and fittings. It is dependent upon the size, condition and type of pipe, number
and type of pipefittings, flow rate, and nature of the liquid.

Vapor Pressure Head (hvp): Vapor pressure is the pressure at which a liquid and
its vapor co-exist in equilibrium at a given temperature. The vapor pressure of
liquid can be obtained from vapor pressure tables. When the vapor pressure is
converted to head, it is referred to as vapor pressure head, hvp. The value of hvp
of a liquid increases with the rising temperature and in effect, opposes the pressure
on the liquid surface, the positive force that tends to cause liquid flow into the
pump suction i.e. it reduces the suction pressure head.

Pressure Head (hp): Pressure Head must be considered when a pumping system
either begins or terminates in a tank which is under some pressure other than
atmospheric. The pressure in such a tank must first be converted to feet of liquid.
Denoted as hp, pressure head refers to absolute pressure on the surface of the
liquid reservoir supplying the pump suction, converted to feet of head. If the
system is open, hp equals atmospheric pressure head.

Velocity Head (hv): Refers to the energy of a liquid as a result of its motion at
some velocity ‘v’. It is the equivalent head in feet through which the water would
have to fall to acquire the same velocity, or in other words, the head necessary to
accelerate the water. The velocity head is usually insignificant and can be ignored
in most high head systems. However, it can be a large factor and must be
considered in low head systems.

Total Suction Head (HS): The suction reservoir pressure head (hpS) plus the
static suction head (hS) plus the velocity head at the pump suction flange (hVS)
minus the friction head in the suction line (hfS).
HS = hpS + hS + hvS – hfS
The total suction head is the reading of the gauge on the suction flange, converted
to feet of liquid.

Total Discharge Head (Hd): The discharge reservoir pressure head (hpd) plus
static discharge head (hd) plus the velocity head at the pump discharge flange
(hvd) plus the total friction head in the discharge line (hfd).
Hd= hpd + hd + hvd + hfd
The total discharge head is the reading of a gauge at the discharge flange,
converted to feet of liquid.


Total Differential Head (HT): It is the total discharge head minus the total
suction head or
HT = Hd + HS (with a suction lift)
HT = Hd - HS (with a suction head)
NPSH
When discussing centrifugal pumps, the two most important head terms are NPSHr
and NPSHa.
Net Positive Suction Head Required, NPSHr. NPSH is one of the most widely used
and least understood terms associated with pumps. Understanding the significance
of NPSH is very much essential during installation as well as operation of the
pumps.
Pumps can pump only liquids, not vapors
The satisfactory operation of a pump requires that vaporization of the liquid being
pumped does not occur at any condition of operation. This is so desired because
when a liquid vaporizes its volume increases very much. For example, 1 ft3 of
water at room temperature becomes 1700 ft3 of vapor at the same temperature.
This makes it clear that if we are to pump a fluid effectively, it must be kept
always in the liquid form.
Rise in temperature and fall in pressure induces vaporization
The vaporization begins when the vapor pressure of the liquid at the operating
temperature equals the external system pressure, which, in an open system is
always equal to atmospheric pressure. Any decrease in external pressure or rise in
operating temperature can induce vaporization and the pump stops pumping. Thus,
the pump always needs to have a sufficient amount of suction head present to
prevent this vaporization at the lowest pressure point in the pump.
NPSH as a measure to prevent liquid vaporization
The manufacturer usually tests the pump with water at different capacities, created
by throttling the suction side. When the first signs of vaporization induced
cavitation occur, the suction pressure is noted (the term cavitation is discussed in
detail later). This pressure is converted into the head. This head number is
published on the pump curve and is referred as the "net positive suction head
required (NPSHr) or sometimes in short as the NPSH. Thus the Net Positive
Suction Head (NPSH) is the total head at the suction flange of the pump less
the vapor pressure converted to fluid column height of the liquid.

NPSHr is a function of pump design
NPSH required is a function of the pump design and is determined based on actual
pump test by the vendor. 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 is always positive since it is expressed in terms of absolute fluid
column height. The term "Net" refers to the actual pressure head at the pump
suction flange and not the static suction head.
NPSHr increases as capacity increases
The NPSH required varies with speed and capacity within any particular pump.
The NPSH required increase as the capacity is increasing because the velocity of
the liquid is increasing, and as anytime the velocity of a liquid goes up, the
pressure or head comes down. Pump manufacturer's curves normally provide this
information. The NPSH is independent of the fluid density as are all head terms.
Note: It is to be noted that the net positive suction head required (NPSHr) number
shown on the pump curves is for fresh water at 20°C and not for the fluid or
combinations of fluids being pumped.
Net Positive Suction Head available, NPSHa.
NPSHa is a function of system design Net Positive Suction Head 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, to
be sure that the pump selected does not cavitate. It is calculated based on system or
process conditions.
NPSHa calculation
The formula for calculating the NPSHa is stated below:

Note:
1. 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 formula.
2. Any discussion of NPSH or cavitation is only concerned about the suction side
of the pump. There is almost always plenty of pressure on the discharge side of the
pump to prevent the fluid from vaporizing.
NPSHa in a nutshell
In a nutshell, NPSH available is defined as:
NPSHa = Pressure head + Static head - Vapor pressure head of your product –
Friction head loss in the piping, valves and fittings. “All terms in feet absolute”
In an existing system, the NPSHa can also be approximated by a gauge on the
pump suction using the formula:
NPSHa = hpS - hvpS ± hgS + hvS
hpS = Barometric pressure in feet absolute.
hvpS = Vapor pressure of the liquid at maximum pumping temperature, in feet
absolute.
hgS = Gauge reading at the pump suction expressed in feet (plus if above
atmospheric, minus if below atmospheric) corrected to the pump centerline.

hvS = Velocity head in the suction pipe at the gauge connection, expressed in
feet.
Significance of NPSHr and NPSHa
The NPSH available must always be greater than the NPSH required for the pump
to operate properly. It is normal practice to have at least 2 to 3 feet of extra NPSH
available at the suction flange to avoid any problems at the duty point.
Power and Efficiency
Brake Horse Power (BHP)
The work performed by a pump is a function of the total head and the weight of the
liquid pumped in a given time period.
Pump input or brake horsepower (BHP) is the actual horsepower delivered to the
pump shaft.
Pump output or hydraulic or water horsepower (WHP) is the liquid horsepower
delivered
by the pump. These two terms are defined by the following formulas.

The constant 3960 is obtained by dividing the number or foot-pounds for one
horsepower (33,000) by the weight of one gallon of water (8.33 pounds).
BHP can also be read from the pump curves at any flow rate. Pump curves are
based on a specific gravity of 1.0. Other liquids’ specific gravity must be
considered.
The brake horsepower or input to a pump is greater than the hydraulic horsepower
or output due to the mechanical and hydraulic losses incurred in the pump.
Therefore the pump efficiency is the ratio of these two values.
Best Efficiency Point (BEP)
The H, NPSHr, efficiency, and BHP all vary with flow rate, Q. Best Efficiency
Point (BEP) is the capacity at maximum impeller diameter at which the efficiency
is highest. All points to the right or left of BEP have a lower efficiency.
Significance of BEP
BEP as a measure of optimum energy conversion
When sizing and selecting centrifugal pumps for a given application the pump
efficiency at design should be taken into consideration. The efficiency of
centrifugal pumps is stated as a percentage and represents a unit of measure
describing the change of centrifugal force (expressed as the velocity of the fluid)
into pressure energy. The B.E.P. (best efficiency point) is the area on the curve
where the change of velocity energy into pressure energy at a given gallon per
minute is optimum; in essence, the point where the pump is most efficient.
BEP as a measure of mechanically stable operation
The impeller is subject to non-symmetrical forces when operating to the right or
left of the BEP. These forces manifest themselves in many mechanically unstable
conditions like vibration, excessive hydraulic thrust, temperature rise, and erosion
and separation cavitation. Thus the operation of a centrifugal pump should not be

outside the furthest left or right effic iency curves published by the manufacturer.
Performance in these areas induces premature bearing and mechanical seal failures
due to shaft deflection, and an increase in temperature of the process fluid in the
pump casing causing seizure of close tolerance parts and cavitation.
BEP as an important parameter in calculations
BEP is an important parameter in that many parametric calculations such as
specific speed, suction specific speed, hydrodynamic size, viscosity correction,
head rise to shutoff, etc. are based on capacity at BEP. Many users prefer that
pumps operate within 80% to 110% of BEP for optimum performance.
Specific Speed
Specific speed as a measure of the geometric similarity of pumps Specific speed
(Ns) is a non-dimensional design index that identifies the geometric similarity of
pumps. It is used to classify pump impellers as to their type and proportions.
Pumps of the same Ns but of different size are considered to be geometrically
similar, one pump being a size-factor of the other.
Specific speed Calculation
The following formula is used to determine specific speed:
As per the above formula, 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 flow 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.
Specific speed as a measure of the shape or class of the impellers
The specific speed determines the general shape or class of the impellers. As the
specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or
eye diameter, D1, decreases. This ratio becomes 1.0 for a true axial flow impeller.
Radial flow impellers develop head principally through centrifugal force. Radial
impellers are generally low flow high head designs. Pumps of higher specific
speeds develop head partly by centrifugal force and partly by axial force. A higher
specific speed indicates a pump design with head generation more by axial forces
and less by centrifugal forces. An axial flow or propeller pump with a specific
speed of 10,000 or greater generates its head exclusively through axial forces.
Axial flow impellers are high flow low head designs.

Specific speed identifies the approximate acceptable ratio of the impeller eye
diameter (D1) to the impeller maximum diameter (D2) in designing a good
impeller.
Ns: 500 to 5000; D1/D2 > 1.5 - radial flow pump
Ns: 5000 to 10000; D1/D2 < 1.5 - mixed flow pump
Ns: 10000 to 15000; D1/D2 = 1 - axial flow pump
Specific speed is also used in designing a new pump by size-factoring a smaller
pump of the same specific speed. The performance and construction of the smaller
pump are used to predict the performance and model the construction of the new
pump.
Suction specific speed (Nss)
Suction specific speed (Nss) is a dimensionless number or index that defines the
suction characteristics of a pump. It is calculated from the same formula as Ns by
substituting H by NPSHr.
In multi-stage pump the NPSHr is based on the first stage impeller NPSHR.
Specific speed as a measure of the safe operating range Nss is commonly used as a
basis for estimating the safe operating range of capacity for a pump. The higher the
Nss is, the narrower is its safe operating
range from its BEP. The numbers range between 3,000 and 20,000. Most users
prefer that their pumps have Nss in the range of 8000 to 11000 for optimum and
trouble-free operation.
The Affinity Laws

The Affinity Laws are mathematical expressions that define changes in pump
capacity, head, and BHP when a change is made to pump speed, impeller diameter,
or both.
According to Affinity Laws:

Capacity, Q changes in direct proportion to impeller diameter D ratio, or to
speed N ratio:
Q2 = Q1 x [D2/D1]
Q2 = Q1 x [N2/N1]

Head, H changes in direct proportion to the square of impeller diameter D ratio,
or the square of speed N ratio:
H2 = H1 x [D2/D1]2
H2 = H1 x [N2/N1]2

BHP changes in direct proportion to the cube of impeller diameter ratio, or the
cube of speed ratio:
BHP2 = BHP1 x [D2/D1]3
BHP2 = BHP1 x [N2/N1]3
Where the subscript: 1 refers to initial condition, 2 refer to new condition If
changes are made to both impeller diameter and pump speed the equations can be
combined to:
Q2 = Q1 x [(D2xN2)/(D1xN1)]
H2 = H1 x [(D2xN2)/(D1xN1)]2
BHP2 = BHP1 x [(D2xN2)/(D1xN1)]3
This equation is used to hand-calculate the impeller trim diameter from a given
pump performance curve at a bigger diameter.
The Affinity Laws are valid only under conditions of constant efficiency.
Understanding Centrifugal Pump Performance Curves
The capacity and pressure needs of any system can be defined with the help of a
graph called a system curve. Similarly the capacity vs. pressure variation graph for
a particular pump defines its characteristic pump performance curve. The pump
suppliers try to match the system curve supplied by the user with a pump curve that
satisfies these needs as closely as possible. A pumping system operates where the

pump curve and the system resistance curve intersect. The intersection of the two
curves defines the operating point of both pump and process. However, it is
impossible for one operating point to meet all desired operating conditions. For
example, when the discharge valve is throttled, the system resistance curve shift
left and so does the operating point.
Figure D.01: Typical system and pump performance curves
Developing a system curve
The system resistance or system head curve is the change in flow with respect to
head of the system. It must be developed by the user based upon the conditions
of service. These include physical layout, process conditions, and fluid
characteristics. It represents the relationship between flow and hydraulic losses in a
system in a graphic form and, since friction losses vary as a square of the flow rate,
the system curve is parabolic in shape. Hydraulic losses in piping systems are
composed of pipe friction losses, valves, elbows and other fittings, entrance and
exit losses, and losses from changes in pipe size by enlargement or reduction in
diameter.

Developing a Pump performance Curve
A pump's performance is shown in its characteristics performance curve where its
capacity i.e. flow rate is plotted against its developed head. The pump performance
curve also shows its efficiency (BEP), required input power (in BHP), NPSHr,
speed (in RPM), and other information such as pump size and type, impeller size,
etc. This curve is plotted for a constant speed (rpm) and a given impeller diameter
(or series of diameters).
It is generatedby tests performed by the pump manufacturer. Pump curves are
based on a specific gravity of 1.0. Other specific gravities must be considered by
the user.
Normal Operating Range
A typical performance curve (Figure D.01) is a plot of Total Head vs. Flow rate for
a specific impeller diameter. The plot starts at zero flow. The head at this point
corresponds to the shut-off head point of the pump. The curve then decreases to a
point where the flow is maximum and the head minimum. This point is sometimes
called the run-out point. The pump curve is relatively flat and the head decreases
gradually as the flow increases. This pattern is common for radial flow pumps.
Beyond the run-out point, the pump cannot operate. The pump's range of operation
is from the shut-off head point to the run-out point. Trying to run a pump off the
right end of the curve will result in pump cavitation and eventually destroy the
pump.
In a nutshell, by plotting the system head curve and pump curve together, you can
determine:
1. Where the pump will operate on its curve?
2. What changes will occur if the systems head curve or the pump performance
curve changes?
Two Basic Requirements for Trouble-Free Operation of Centrifugal
Pumps
Centrifugal pumps are the ultimate in simplicity. In general there are two basic
requirements that have to be met at all the times for a trouble free operation and
longer service life of centrifugal pumps. The first requirement is that no cavitation
of the pump occurs throughout the broad operating range and the second
requirement is that a certain minimum continuous flow is always maintained
during operation. A clear understanding of the concept of cavitation, its symptoms,

its causes, and its consequences is very much essential in effective analyses and
troubleshooting of the cavitation problem.
Just like there are many forms of cavitation, each demanding a unique solution,
there are a number of unfavorable conditions which may occur separately or
simultaneously when the pump is operated at reduced flows. Some include:
o Cases of heavy leakages from the casing, seal, and stuffing box
o Deflection and shearing of shafts
o Seizure of pump internals
o Close tolerances erosion
o Separation cavitation
o Product quality degradation
o Excessive hydraulic thrust
o Premature bearing failures
Each condition may dictate a different minimum flow low requirement. The final
decision on recommended minimum flow is taken after careful “techno-
economical” analysis by both the pump user and the manufacturer. The
consequences of prolonged conditions of cavitation and low flow operation can be
disastrous for both the pump and the process.Such failures in hydrocarbon services
have often caused damaging fires resulting in loss of machine, production, and
worst of all, human life.
Thus, such situations must be avoided at all cost whether involving modifications
in the pump and its piping or altering the operating conditions. Proper selection
and sizing of pump and its associated piping can not only eliminate the chances of
cavitation and low flow operation but also significantly decrease their harmful
effects.
References
1.“ Trouble shooting Process Operations”, 3rd Edition 1991, Norman P.Lieberman,
PennWell Books
2. “Centrifugal pumps operation at off-design conditions”, Chemical Processing
April, May, June 1987, Igor J. Karassik
3. “Understanding NPSH for Pumps”, Technical Publishing Co. 1975, Travis F.
Glover
4. “Centrifugal Pumps for General Refinery Services”, Refining Department, API
Standard 610, 6th Edition, January 1981
5. “Controlling Centrifugal Pumps”, Hydrocarbon Processing, July 1995, Walter
Driedger

6. “Don’t Run Centrifugal Pumps Off The Right Side of the Curve”, Mike
Sondalini
7. “Pump Handbook” , Third Edition , Igor j. Karassik , Joseph P.Messina , Paul
cooper
Charles C.Heald

Basics Concepts of Operation, Maintenance, and Troubleshooting of a Centrifugal Pump.

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Basics Concepts of Operation, Maintenance, and Troubleshooting of a Centrifugal Pump.