Centrifugal Compressor_Centrifugal Compressor.pdf

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Part 7
Definition :
A compressor is a device that transfers
energy to a gaseous fluid for the purpose
of raising the pressure of the fluid.
Applications of compressed gas vary from
consumer products, such as the home
refrigerator, to large complex
petrochemical plant installations.
Compressors - Introduction

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Compressors - Introduction

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Compressors - Classifications

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Centrifugal Compressor
Theory of Operation

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Centrifugal Compressor
 The most common type of compressor used today in
the oil and gas industry is the centrifugal compressor.
 The centrifugal air compressor is a dynamic
compressor which depends on a rotating impeller to
compress the air.
 It allows very high volumetric flow rates and good
compression ratio.

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 The operation of a centrifugal compressor is based on
centrifugal forces, which are outward and the
tangential forces, which are in the same direction of
rotation.
 Gas enters the compressor through the suction port,
which is directed by inlet guide vanes to the center of
the impeller.
 As the impeller rotates, gas rotates with it.
How Centrifugal Compressor Works?

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Centrifugal compressors compress gas by the mechanical action of
rapidly rotating impellers or bladed rotors that impart velocity and
pressure to the flowing gas. Velocity is further converted into
pressure in stationary diffusers.

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Centrifugal Compressors
Impeller assebled on Rotor

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Fluid (gas)
An impeller is made of two plates separated by curved blades

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The movement of gas inside the impeller

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 This circular motion generates forces, which lift and
move the gas away from the center of the impeller.
 The forced gas enters the diffuser or volute passage at
a higher velocity and higher pressure.
 Suction is created at the eye (center) of the impeller
to draw more gas into the compressor.
 The impeller blades act like airplane wings, generating
a lift force on the gas that helps to force it from low
pressure, at the eye, to higher pressure at the outside
edge of the impeller.

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 The centrifugal and lifting forces combined give the
gas two velocity components:
• Radial velocity is the speed of the gas moving
outward from the center towards the outside edge.
• Tangential velocity is the speed of the gas moving in
a straight line at right angle, to the radial velocity.
It is in the same direction as the outside edge is
moving.
 This combination of velocities increases the kinetic
energy of gas motion.

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 As gas leaves the outside edge of the impeller, it
enters the diffuser.
 The diffuser is designed such that the flow area
increases as the gas leaves the impeller.
 This increased area allows the gas to slow down.
 As the velocity decreases, kinetic energy of the gas is
converted into pressure.
 The combination of centrifugal and tangential forces
together with the change in velocities produces the
final gas discharge pressure.

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 When the gas is at the tips of the impeller blades, it is
at maximum velocity.
 As the gas leaves the impeller, it is forced (through)
into a passage way called the diffuser.
 When the gas enters the diffuser, the impeller is not
acting directly on the gas.
 Due to the shape of the diffuser, the flow path of the
gas through the diffuser is in a larger spiral (the
following figure).

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 Since the flow path is wider and there is no direct
action by the impeller blades, the velocity of the gas
decreases and its pressure increases.
 The diffuser converts the velocity into pressure.
 Gas passes from the diffuser into volute. In the
volute, the conversion from velocity to pressure
continues.
 A centrifugal compressor, by doing work on a gas
imparts both pressure and velocity to the gas, and
then the velocity of the gas is converted into pressure
within the compressor.

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Diffuser
Impeller

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The flow of gas from the impeller to diffuser and volute

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Gas Flow through a Compressor
Volute
Collector
Volute
Diffuser
Impeller
Inlet
Radial
Diffuser
Volute
Collector Stream Line
Spacer

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 In a centrifugal
compressor, there is
a direct relationship
between impeller
speed, velocity,
pressure, and flow.
As the impeller speed
increases, velocity
increases. As velocity
increases, pressure
increases. As
pressure increases,
flow increases.

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 Centrifugal compressors may be single-stage or
multistage, and the stages may be contained in one
casing or several different casings.

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Centrifugal Compressor Components

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Centrifugal compressors consist of three main parts:
 The first is a rotating impeller, which imparts work to
the gas by increasing its angular momentum.
 The second component is the diffuser section, the
diffuser converts the kinetic energy into the static
pressure by decelerating the fluid.

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 The third component is a volute or collector, used for
collecting the gas from diffuser and delivering to the
outlet pipe. A volute has two functions: collection and
diffusion.
 The volute collect and transport the fluid to the
downstream system. It also raises the static pressure by
converting kinetic energy to potential energy (static
pressure).

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Centrifugal Compressor Components
Compressor components can be divided into:
1. Compressor casing
2. Compressor rotor (shaft, impellers, balancing disc)
3. Compressor seals
4. Compressor bearings

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Main Elements
 Consist of :
• Inlet nozzle
• Inlet guide vanes
• Impellers
• Radial diffuser
• Return channel
• Collector volute
• Discharge nozzle

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Compressor Casing
 Compressor casing contains all internal components
It provides the suction nozzle and discharge nozzle
 There are two casing designs for centrifugal
compressors:
1. The horizontally split casing
2. Vertical split (radial split) casing compressor
 The following figures show these two designs

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A - Casing Split Method
Vertically Split
Good Seal
Difficult Maintenance
Horizontally Split
Bad Seal
Easy Maintenance

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A - Casing Split Method
Vertically Split
Good Seal
Difficult Maintenance
Horizontally Split
Bad Seal
Easy Maintenance

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Compressor Casing
1. The horizontally split casing is in two halves that are
bolted together to form tight enclosure.
 When the top half of the axial (horizontally) split
casing is removed, the entire internal component are
easily accessible.

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Compressor Casing
Axial (horizontal) split case compressor-6 stages

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Compressor Casing
2. Vertically split casing (or radial split casing), the
external casing has two removable end covers which
are bolted to the central (middle) barrel or external
middle casing (see the above figure).
 To reach the entire working component in this type,
the end covers has to be pulled and the internal
component (internal drum) must be removed from
the external casing.

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Compressor Casing
Vertical split (radial split) casing compressor

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 The inlet nozzle accelerates the gas stream and directs
it into the inlet guide vanes which may be fixed or
adjustable.
Centrifugal Compressors - Components

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Suction Plenum
Suction plenum (
‫تهييل‬ ‫و‬ ‫تحضير‬ ‫مكان‬
)
Impeller, difusser,
U-bend, return
channel
Last stage,
Discharge volute,
Balancing drum
BCL series
Barrel compressor, low pressure, seven stages

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Suction Plenum
Casing
Blade
Impeller
casea
pala
girante
girante
pinna Separation wall
Shaft
albero
coclea d’aspiratione
Suction coclea
flangia d’aspirazione
Suction flange
tubazione
Suction pipe

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Suction Plenum
Inlet volute shroud
blade
disk
key
shaft
labyrinth seal
labyrinth seals
suction diagram
First stage sectional view.

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Suction Plenum

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Guide Vanes
 Compressor performance is affected by two factors.
These are:
1.The direction at which the gas enters the impeller
eye
2.The velocity of gas approach to the impeller eye
 If the gas can be made to enter the impeller in the
same direction as the impeller rotation then the
efficiency of the compressor will be increased.

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The Function of the Guide Vanes
 Guide vanes are designed to guide the flow of gas
efficiently into the suction eye of the impeller.
 Guide vanes may be either permanent or replaceable.
Usually, they are ahead of each impeller eye.

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There are two designs of guide vanes:
 Variable guide vanes ahead of the 1st stage in some
designs not all (it is know also as adjustable inlet guide
vanes)
 Fixed guide vanes ahead of the 2nd, 3rd,4th stages

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Fixed guide vanes
Impeller Guide Vanes

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Fixed guide vanes

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Adjustable Inlet Guide Vanes
Adjustable inlet guide vanes

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 Adjustable guide vanes are usually available in single-
stage compressors and are sometimes used in the first
stage of a multi stage compressor.
 These vanes are adjusted automatically to control the
angle of gas flowing into the eye of the impeller.
 This controls the performance of a centrifugal
compressor and keeps it efficient over a wider
operating range.

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Adjustable (variable) guide vanes on the suction of the first stage

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Linkage elements associated with variable-inlet guide vanes

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 The impellers consist of two
discs, referred to as the disc
and shroud, connected by
blades which are shrunk onto
the shaft and held by either
one or two keys.
 The impeller pushes the gas
outwards raising its velocity
and pressure; the outlet
velocity will have a radial and
a tangential component.
Impellers

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Impeller Design
Consist of a series of blades and
attached to the shaft.
As the shaft rotates the centrifugal force generated by the rotating
impeller forces gas to flow outward.

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 The impeller does work on the gas, which in turn
increases the energy of the gas. The energy the gas gains
are in the form of pressure and velocity.
 When the gas reaches the tip of the impeller blades it is
at its maximum velocity and possesses the maximum
amount of energy.
Impellers

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 The part of the centrifugal compressor that moves the
gas is the impeller.
 The gas enters through the eye of the impeller and as
the impeller rotates it moves the gas towards the
outer rim of the impeller.
 Movement of the gas towards the outer rim of the
impeller causes the gas velocity to increase. This
increase in velocity away from the eye creates a low-
pressure area at the-eye of the impeller causing
suction, which allows more gas to enter.
Impellers

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 The impeller does work on the gas, which in turn
increases the energy of the gas. The energy the gas
gains are in the form of pressure and velocity.
 When the gas reaches the tip of the impeller blades it
is at its maximum velocity and possesses the maximum
amount of energy.
Impellers

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Gas Flow through the Impeller
Impellers
Gas Inlet
Shaft
Impeller Hub Blades
Eye
Cover Plates
Volute
Diffuser
Volute
Collector
Casing
Gas
Outlet

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 As the gas leaves the impeller it is thrust into a
passageway called the diffuser. As the flow area in the
diffuser is larger than that in the impeller, the velocity
of the gas begins to decrease. This causes the gas
pressure to increase. The diffuser converts the velocity
of the gas to increased pressure.
 From the radial diffuser the gas passes into the volute
collector and finally the volute where the transition of
energy from velocity to pressure continues.
Impellers

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Impellers

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 The impeller delivers the energy to the gas. Approx.
two thirds of this energy is transformed into pressure in
the impeller itself and the remaining one third is
transformed into pressure in the diffuser. Impellers are
generally closed type. Open impellers are applied only
for overhung compressors. Diffuser is free vortex type
(just two parallel walls) for multistage compressors and
may be of the vaned type for overhung compressors.
Impellers

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The Compressor Rotor
Centrifugal compressor rotor assembly (5 stages)

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Single stage rotor (Impeller fixed to the shaft)

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1. Impellers
 The impeller increases the velocity of a gas by
rotating about the center line and causing the gas to
move from the inlet (impeller suction eye) to the tip
of the impeller (or discharge).
 The difference in the distance from the axis of
rotation of the impeller inlet and the impeller
discharge causes the increase in kinetic energy.

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There are three different
designs of impeller are
commonly used.
Compressor impellers
Open impeller
Used for small heads and small
to large flow in single stage
compressor only
Open Impeller
Semi-open impeller
Used for large flow, usually in
single stage compressors, or as
the first stage in multi-stage
Semi-open Impeller
Closed impeller
Used in multi-stage compressors
Closed Impeller

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MAIN PARTS
Impeller Types
Open
Impeller
Semi-open
Impeller
Closed
Impeller

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Impeller of centrifugal compressor Centrifugal compressor impeller
(Semi-open impeller) (semi-open impeller)

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Semi – Open Impeller
Impellers

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Impellers

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Impellers
Closed
impeller
design

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Impellers
High-pressure, vertically split centrifugal compressor

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Rotor
 The shaft transmits the mechanical energy from the
driver to the compressor.
 The function of the shaft is to carry all impellers,
thrust disk (thrust collar), balance drum and shaft
sleeves and to transmits the required power to drive
all rotating parts.
Centrifugal Compressors – Components
Shafts

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 The material used to manufacture shafts for many
compressors is steel 40 NiCrMo7 which is always
hardened and tempered.
Shafts

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Shaft Sleeves
Shaft sleeves are used to:
1. As spacer between different impellers on the shaft.
2. To afford wearing surfaces for inter-stage seals. In
certain instances shaft sleeves protect the shaft
from corrosive elements in the gas.
3. Sleeves are also used in conjunction with shaft seals
and occasionally for bearing surfaces, as in the case
of shaft made of certain stainless steels.
Shafts

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Shaft sleeve between two impellers
Shafts

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Diffuser
 Diffusers are the stationary passages in the
compressor
 whose primary function is to “diffuse” or slow down
the gas velocity. As the impeller discharges flow into
the diffuser passage, the diffusion process converts
velocity energy into pressure energy. Diffusion can be
achieved by means of parallel-wall diffusers or
volutes, depending on the compressor design.
 Single-shaft, multi-impeller compressors, with or
without side streams, employ parallel-wall diffusers
formed by diaphragms.

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Diffuser
Downstream of the impeller in the flow path, it is the diffuser's
responsibility to convert the kinetic energy (high velocity) of the gas
into pressure by gradually slowing (diffusing) the gas velocity.
Return
Channel
Diffuser
Impeller
Inlet
Diffuser
Channel

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Diffuser
Return
Channel
Diffuser
Impeller
Inlet
Diffuser
Channel

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Diffuser

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Diffuser
The diffusers of centrifugal compressors can be divided to two classes:
vaneless and vaned diffusers. Vaneless diffusers have a wider ﬂow range
but lower pressure recovery and efficiency, whereas vaned diffusers have
higher pressure recovery and efficiency, but narrower ﬂow range.
Vaneless diffuser
Impeller
Vaned diffuser
impeller

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Diffuser
 Vaneless diffusers were the most widely-used style
in industrial centrifugal until the late 1980’s when
some Original Equipment Manufacturers (OEMs)
began applying a vaned diffuser design.
 Vaneless diffusers contain no vanes. Conversely,
vanned diffusers contain one or more rows of vanes.
In general, vaneless diffusers offer the widest flow
range because there are no vanes to interfere with
the gas as it passes through the diffuser. However,
the static pressure recovery in vaneless diffusers is
not as high as in their vaned counterparts.

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Vaned Diffuser
Diffuser

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Diffuser
Vaned diffusers may improve stage
efficiency (up to 3-5%) but generally
decrease compressor flow range

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U Bend and Return Channel
U-bend and return channel lead gas to the next stage of a multistage
compressor. Return channel is bladed and has the purpose to de-swirl the gas
leading it to next stage impeller suction annulus axially No transformation of
kinetic energy in to pressure takes place in the return channel.

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Diaphragms
 Compressor diaphragms are stationary inserts that
must direct the process gas into successive impellers.
It should be noted that diffusers, return bends, and
return passages all constitute volumes created by the
walls of the diaphragms and casing.
Return passages
To the next stage
Diaphragm
Compressor casing
Inlet guide vanes
For the next stage

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Diaphragms
 Diaphragms are used in multi-stage centrifugal
compressor as partition between the different stages
(impellers) to create a passage for the gas from one
stage to the next one.

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Impeller
Diffuser Cross-Over
Return
Channel
Eye Seal
Diaphragm
Seal
CASING
Diaphragms

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Diaphragms
Diaphragm in the lower half of the casing

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Purpose of the discharge volute is to convey the gas from last
stage diffuser exhaust to the discharge pipe by recovering into
pressure its kinetic energy with minimum losses.
Discharge Volute

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Balancing Drum
 The balance drum is mounted on the shaft after the end
impeller. It serves to balance the total thrust produced
by the impellers. Having end impeller delivery pressure
on one side of the drum, compressor inlet pressure is
applied to the other by an external connection. In this
way, gas pressures at both ends of the rotor are roughly
balanced.

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Balancing Drum
• On the disc side, the impeller is exposed to discharge pressure
and on the other side partly to the same pressure and partly to
suction pressure. Thus a thrust force is created towards suction.

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Balancing Drum
 Thrust force is always developed in the impeller with a resultant
force in the direction of the inlet.
thrust force thrust neutralization
Direction
of Thrust
Direction
of Thrust
Direction
of Thrust
Flow
Suction
Pressure
Suction
Pressure
Flow
Discharge
Pressure
Suction
Pressure

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Balancing Drum
Axial load
Last impeller
Balancing Drum
Suction
Pressure
Discharge
Pressure
Hub Side Disk Side
Axial Force

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Balancing Drum
 The purpose of the balancing drum is to counteract the
net thrust which the impellers exert to move the rotor
axially towards the gas inlet end of the compressor
(low pressure end).

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Balancing Drum
 This counteraction is accomplished by venting the
outboard side of the drum via a balance pipe to the
compressor suction end.
 Since the inboard side of the piston is exposed to
compressor discharge pressure, then a pressure
differential is created across the piston, which acts to
balance the impeller thrust.
 The impeller thrust is slightly greater than that of the
balancing piston and the difference is taken up by the
thrust bearing.

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 Centrifugal compressor rotors are normally built with a
balancing drum installed at the discharge end.
 When the compressor is running, there is axial force
acting on the impeller forcing it towards the suction
side.
 In multi-stage centrifugal compressor, if the impellers
are arranged in one direction, the final resultant force
which is acting on the shaft will be very high.
Balancing Drum

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 To cut down this axial force due to pumping, the
designers use balance drum.
 The balance drum is just cylindrical drum (or disc)
fixed to the compressor shaft and rotate with it.
 One side of the balance drum is subjected to suction
pressure and the other side is subjected to discharge
pressure.
Balancing Drum

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 The pressure difference at the balancing drum causes
(creates) axial force equal to and in opposite direction
to the axial load which is acting on the rotor due to
pumping action (see the following figure on the next
page).
 These two forces balance each other because they are
equal and exerted in apposite directions.
 As a result the shaft does not have much axial force in
either direction (see the following figure).
Balancing Drum

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Balancing drum and balance line
Balancing Drum
Centrifugal
Compressors –
Components
SUCTION
BALANCE
LINE DISCHARGE
GAGE
BALANCING
DRUM
DISCHARGE
PRESSURE
SUCTION
PRESSURE
BALANCING
DRUM
LABYRINTH SEAL
IMPELLER
SUCTIO
PRESSURE
DISCHARGE
PRESSURE

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Balance drum
Last stage
impeller
Balancing Drum

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Balance drum
Last stage
impeller
Balancing drum
Balancing Drum

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Balance drum
Multi-stage centrifugal
compressor- 6 stages with
balancing drum
Centrifugal Compressors – Components
Balancing Drum

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 Due to the pressure rise across successive compression
stages, seals are required at the impeller eye and rotor
shaft to prevent gas backflow from the discharge to
inlet end of the casing.
 Internal labyrinths, are used along the gas path to
minimize leakage between stages, along the balance
piston, and ahead of the seals separating compressor
internals from compressor bearings. Internal leakage
from regions of higher pressure to regions of lower
pressure contributes to compressor inefficiency.
Inter-stage Seals

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Inter-stage Seals

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Inter-stage Seals

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Labyrinth Seal
1. Labyrinth seal does not stop gas leakage 100%.
2. It minimizes the leakage to lower limits. For this
reason it is used inside the compressor to minimize
the leakage from one stage to another i.e. from high
pressure area to low pressure area.
3. The leakage rate depends upon the labyrinth seal
clearance and the pressure difference on both sides of
the seal.
Inter-stage Seals

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4. Labyrinth seal materials must be:
 Softer than the rotor material.
 Able to resist chemical attack from the gas being
pumped (compatible).
 Can’t create any spark if rubbing happen.
Inter-stage Seals

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Each both ends of centrifugal compressor has two sealing
areas, They are two shaft seals on both ends of the shaft
are used to separate the gas stream from the bearing
area (chamber) and from the surrounding environment.
Particular care must be taken when the gas is
combustible or toxic.
Inter-stage Seals

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Shaft seals are used to achieve one of the following:
1. Reduce or prevent in-leakage of gas (example:
preventing air or oil vapour from entering the gas
stream) i.e. from outside the compressor casing as high
pressure area to inside the compressor casing as low
pressure area.
2. Reduce or prevent out-leakage of gas from high
pressure areas to the atmosphere (discharge end of the
compressor). Example: If the gas stream is chlorine,
which is very toxic.
3. Reduce or prevent both in-leakage and out-leakage.

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Shaft Seals
The primary purpose of compressor shaft seals is to
avoid:
 Gas leakage into the atmosphere. Both safety and loss
of revenue are at issue here.
 The American Petroleum Institute, in its Standard API-
617, describes five generic (
‫عام‬
) types of seals:
labyrinth, restrictive carbon rings seal, mechanical
contact, liquid film, and “dry” gas seals. Mechanical
contact and liquid film seals employ liquid as the
sealing medium. Process gas or a buffered inert gas
medium alone is used in the other three seal types.

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Shaft Seals
Labyrinth seals

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Shaft Seals – Labyrinth Seal
Labyrinth seals
Labyrinth Seals
Generally used in air or nitrogen applications
Or where slight leakage to the atmosphere
Is tolerable.

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Labyrinth seal
Shaft Seals – Labyrinth Seal

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 In this seal there are several carbon rings around the
shaft.
Shaft Seals – Restrictive Seal

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 The rings are held in position around the shaft by
stationary ring cups. These rings do not contact the
shaft.
 The carbon ring elements can either be solid, or
segmented.
 When using a segmented carbon ring, it is usually
enclosed with a garter spring to maintain the ring.
Furthermore, segmented carbon rings are generally
used where it is desired to have a split element to ease
maintenance and / or replacement.

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The carbon rings
Garter Spring = ‫الرباط‬ ‫سوستة‬

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Shaft Seals
Mechanical Seals
Oil or Water

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It is suitable for high pressures.
 In this type of shaft seal, seal oil is injected between
two sleeves.
 One of these two sleeves is the rotating shaft sleeve
(fixed to the shaft) and rotate with it ,the other
sleeve is stationary.
 These two sleeves have and operate at close
clearance together.
Shaft Seals – Liquid Film Seal

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Liquid film shaft seal

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1. This type of seal has inner and outer sleeves (A, B),
these two sleeves together represent the stationary
sleeve. They do not rotate when the shaft rotates.
2. The shaft itself is also surrounded by a sleeve (shaft
sleeve), As the shaft rotates, the shaft sleeve rotates
with it.
3. Between these two sleeves - shaft sleeve and the
stationary sleeves (inner and outer sleeves A, B) there
is close running clearance just enough to prevent
rubbing.

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4. If the gas leaks from the compressor, it leaks between
these two sleeves (the rotating and stationary
sleeves).
5. To prevent any leakage between the sleeves, high-
pressure oil is forced in between the sleeves. As the
shaft sleeve rotates, it is surrounded by a film of oil.
6. For the oil to seal the shaft sleeve against leakage,
the pressure of the oil must be slightly higher than the
pressure of the gas.

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7. The pressure in the compressor may vary, so the seal
oil pressure must also vary accordingly.
8. A head tank is used with the seal for controlling the
pressure of the oil as shown in the following figure.

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9. No matter how much the pressure in the compressor
changes, the pressure of the oil will always be more
by the amount of head.
10. As the oil circulates, several gallons per day leak into
the pressure side of the compressor. However, most
of the oil is recovered via the oil outlets.
11. In this type of seal, it requires a continuous supply of
clean oil at high pressure; it also requires a system
for circulating the oil and for maintaining oil
pressure.

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Gas
Compressor rotor
Oil Mech. seal
Oil Mech. seal
Heater
Oil Tank
Contaminated
Oil to disposal
Gas to flare
OIL PUMPS
FILTERS
Drain pot
Drain pot

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Part 7
HEATER
SEAL OIL COOLER
FILTERS
OIL PUMPS
SEAL OIL
HEAD TANK
REFRENCE
LINE
SEAL OIL TANK
Gas to flare
Drain
pot
Drain
pot
Oil to disposal
Opened

124
Part 7
GAS TO FLARE
Drain
pot
CLEAN
SEAL OIL
CLEAN
OIL TO
RESERVOIR
COMPRESSOR GAS
CONTAMINATED
OIL TO DISPOSAL
Liquid Film Seal

125
Part 7
Shaft Seals – Dry Gas Seals

126
Part 7
Arrangement

127
Part 7
 Centrifugal compressor can be arranged in many
different styles:
• SINGLE STAGE OVERHUNG TYPE
• BEAM TYPE SINGLE STAGE
• MULTISTAGE WITH STRAIGHT – THROUGH FLOW PATH
Centrifugal Compressors - Arrangement

128
Part 7
Single Stage
 Flow enters axially and
exits in a tangential
direction.

129
Part 7
Beam Type
 The impeller is located
between two bearings,
as for the multistage.
 The flow enters and
leaves in a tangential
direction with the
nozzles located in the
horizontal plane.

130
Part 7
Multi-Stage
• The flow path is straight
through the compressor,
moving through each
impeller in turn.
• This type of centrifugal
compressor is probably the
most common of any found
in process service, with
applications ranging from
air to gas.

131
Part 7
Multi-Stage

132
Part 7
Impellers’ Arrangement
1- Straight Arrangement

133
Part 7
2- In/Out with Intercooler

134
Part 7
3- Back-to-Back

135
Part 7
4- Double Flow

136
Part 7
5- Side-Stream

137
Part 7
Multi-Stage

138
Part 7
Multi-Stage

139
Part 7
Radial-flow horizontally split multistage centrifugal compressor
Multi-Stage

140
Part 7

141
Part 7
Rotor Assembly

142
Part 7
Cooling System
 Gas compression generates heat. For safe and efficient
operation of the facility, compressed gas must be
cooled.
 High gas temperatures make it difficult to lubricate
bearings and other moving parts.
 This is particularly true if the gas is to be recycled
through the compressor because hot suction gas will
overheat the compressor and cause serious damage.
 In many large compressors, gas flow is changed from
the straight through arrangement. Gas is taken out of
the compressor at an intermediate pressure and then
returned to the compressor at another point.

143
Part 7
Cooling System
 Coolers are used to cool gases at various stages in the
process. Precoolers are used to cool the gas before it
enters a compressor. Intercoolers are used in
multistage compressors to cool gas between each stage
of compression. In other words, intercoolers cool the
discharge of the first stage before it enters the suction
to the second stage. Interstage coolers prevent the
overheating of the process gas and equipment. After-
coolers cool the gas downstream from the last stage
once the compression cycle is complete.

144
Part 7
Cooling System
 Some centrifugal compressors consist of two or more
sets of rotors coupled together on a common centre
line.
 In these compressors, gas is discharged from one set
and directed into the next section.
 With these compressor designs, intercoolers are used
at points where gas circulates outside the compressor.
 Gas flows from the compressor through a heat
exchanger for cooling before returning to the
compressor.

145
Part 7
Cooling System
 In large multi stage compressors, there may be several
intercoolers.
 Even with inter-cooling, discharge gas may not be cold
enough.
 In that case, an after-cooler is used. Discharge gas is
passed through an after-cooler before it is delivered to
the pipeline.

146
Part 7
Cooling System
Intercooler
Water IN
Water OUT
Water IN
Aftercooler
Water
OUT
Gas
OUT
Gas IN

148
Part 7
Cooling System
Gas coolers (Intercooler and after-cooler)

149
Part 7
Surge - Definition
 Gas speed is even more
critical in unloaded
condition, when the gas
flow decreases and
therefore could not be
able to overpass
differential pressure to
the diffuser. If this
happens, the flow can go
back into the impeller
bringing compressor to
stall.

150
Part 7
High Pressure High Pressure
High Pressure
Low Pressure Low Pressure Low Pressure
Speed
Speed
Speed

151
Part 7
Surge - Definition
 The surge problem is inherent in dynamic compressors -
centrifugal and axial, as distinguished from positive -
displacement types.
 Surge is defined as the operating point at which the
compressor peak head capability and minimum flow limit
are reached. The compressor loses the ability to
maintain the peak head when surge occurs and the
entire system becomes unstable. Under normal
conditions, the compressor operates to the right of the
surge line. However, as fluctuations in flow rate occur,
or under startup / emergency shutdown, the operating
point will move towards the surge line because flow is
reduced.

152
Part 7
Surge Control Line at Variable speed
of centrifugal compressor
Surge - Definition

155
Part 7
Surge - Definition
 The condition known as surge occurs if the compressor
attempts to raise the pressure of the gas too high when
the flow of gas through the compressor is too low.
 Inside the compressor the gas then starts to flow
backwards.
 When the gas flows backwards the suction pressure is
increased and the compressor can work properly again.
direction.
 The increase in suction pressure is only temporary, so
the compressor starts to surge again.
 This cycle is repeated again and again and takes place,
very quickly.

156
Part 7
Surge - Definition
 What Does Surge Do?
 Surge causes serious damage to compressors
• The consequences of surge are severe. Besides process
disturbance and the eventual process trips and
disruption, surge can damage the compressor:
‐ Damage to seals and bearings is common.
‐ Internal clearance are altered, leading to internal
recycle (internal leakage)
‐ Lowering of compressor efficiency
‐ Destruction of compressor rotor

157
Part 7
Surge Protection
 Surge is generally prevented by recycling gas from the
compressor discharge line to the suction line or in the
case of air compressor, by venting a part of discharge
to the atmosphere. This prevents excessive discharge
pressure build up and allows additional flow through
the machine. If the gas is recycled to the suction line,
it must be cooled to prevent heat build up with in the
compressor.

158
Part 7
Surge Control
What Does Surge Do?
Surge causes serious damage to compressors.
With the gas moving very rapidly backwards and forwards
in the compressor three things happen:
1. Because the gas is re-circulating within the compressor
it gets hotter and hotter. The heat of the gas can
damage the internal parts of the compressor or can
cause the lubricating oil to lose its lubricating
properties.

159
Part 7
Surge Control
2. Surging also causes violent changes of thrust.
First there is a normal thrust force on the compressor's
thrust bearing, then there is none.
• As the gas flow in the compressor changes direction
the rotor of the compressor is slammed back onto
the thrust bearing.
• This is more load than the thrust bearing can take
and the thrust bearing might break.
3.The rapid change in the flow direction of the gas in the
compressor causes vibrations in the compressor.
• When the vibrations reach a certain level they can
cause parts of the compressor to break.

160
Part 7
Surge Control
Controlling Surge
A centrifugal compressor can be brought out of surge
by any of the following:
1. Reducing the speed of the compressor.
• This method can only be used if the compressor has
a variable speed prime mover.
2. Increasing the flow through the compressor.
• Using this method depends on the process which the
compressor forms part of.
• In some processes it may not be possible to increase
the gas flow.

161
Part 7
Surge Control
3. Reducing the compression ratio either by increasing
the suction pressure or decreasing the discharge
pressure.
This method also depends on the process which the
compressor forms part of. In some processes it may not
be possible to alter the suction pressure or the
discharge pressure.
• Most large centrifugal compressors are fitted with
an automatic anti-surge system. Figure shows an
anti-surge system.

162
Part 7
Surge Control
 An anti surge system looks almost the same as a
recycle loop used for capacity control.
 Surge is prevented by recycling some of the discharge
gas back to the compressor suction.
 This increases the suction pressure and reduces the
compression ratio, so the compressor does not surge.

163
Part 7
Surge Control
Anti - Surge System

164
Part 7
Surge Control
 The control box on the anti-surge system receives
more information, usually from more than one
instrument, than the recycle loop does.
 The control box looks at the information it receives
and uses it to check if the compressor is near to its
surge point.
 If the surge point is near, the control box signals the
control valve in the recycle line to open.

165
Part 7
Possible Causes of Surge
 Restriction in suction or discharge.
 Process changes in pressure, temperature or gas
composition.
 Inadvertent loss in speed.
 Compressor fouling.
 Improper distribution of flow between parallel
operated compressor.

166
Part 7
Effects of Surge
 Rapid flow and pressure oscillations cause process
instabilities.
 Rising temperatures inside the compressor.
 Mechanical damage
• Radial bearing load during the initial phase of
surging.
• Thrust bearing load due to loading and unloading.
• Seal rubbing.
• Stationary and rotating part contact if thrust
bearing is overloaded.

167
Part 7
Dynamic Compressors
Advantages
1. High horsepower per unit of space and weight.
2. Easily adapted to combined cycle and cogeneration
for high fuel efficiency.
3. Easily automated for remote operations.
4. Skid mounted self-contained.
5. Low initial costs per horsepower.
6. Lower maintenance cost than reciprocating.
7. High availability factor.
8. Larger capacity available per unit.

168
Part 7
Dynamic Compressors
9. Provides pulseless operation.
10. High volumetric flow rate.
11. Oil free discharge.
12. Simple and robust design.
13. Modern impellers can produce a pressure ratio as
high as 8:1.

169
Part 7
Dynamic Compressors
Disadvantages
1. Lower compressor efficiency.
2. Limited flexibility for capacity.
3. Higher fuel rate than reciprocating units.
4. Larger horsepower outage disrupts process or pipeline
capabilities.

170
Part 7
API 617
 Axial and Centrifugal
Compressors and
Expander-compressors for
Petroleum, Chemical and
Gas Industry Services
API Standards

171
Part 7
API 617
 Axial and Centrifugal
Compressors and
Expander-compressors
for Petroleum,
Chemical and Gas
Industry Services
API Standards

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