Pumps pumping systems

M a r i n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G18 September 2020

How Pump Works?
Pumps are defined as machines which supply Energy to a liquid in order to move it from one place to
another, which is at higher energy levels. Pumps enable liquids to :
1. Flow from a region of low pressure to a region of high pressure.
2. Flow from a low level to a higher level.
3. Flow at a faster rate.
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Pumps operate by some mechanism (typically reciprocating or rotary), and consume energy to perform
mechanical work by moving the fluid. Pumps operate via many energy sources, including manual
operation, electricity, engines, or wind power, come in many sizes, from microscopic for use in medical
applications to large industrial pumps.

General pumping system
𝑯𝑯𝑯𝑯 = 𝑯𝑯𝑯𝑯𝑯𝑯 + 𝑯𝑯𝑯𝑯𝑯𝑯 ∓ 𝑯𝑯𝑯𝑯 − 𝑯𝑯𝑯𝑯𝑯𝑯 + 𝑯𝑯𝑯𝑯𝑯𝑯 + 𝑯𝑯𝑯𝑯𝑯𝑯 + 𝑯𝑯𝑯𝑯 + 𝑯𝑯𝑯𝑯𝑯𝑯 + 𝑯𝑯𝑯𝑯𝑯𝑯𝑯𝑯
Total Head losses
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Total System Head losses
Where:
Hes => pressure head acting on the liquid surface at the suction inlet.
Hfs => loss in pressure head due to friction resistance at the suction side.
Hvs => loss in pressure head due to velocity of the liquid in the suction pipe, it is negligible at low velocity.
Hs => height of the liquid free surface above the center line of the pump (negative when the level is below the pump)
Hvap => loss in pressure head due to vapor pressure of the liquid at the working temperature.
Hfd => pressure head loss due to friction resistance in the discharge pipe.
Hd => pressure head losses due to the height of the discharge tank
Hed => pressure head acting on the liquid surface at the discharge outlet.
Hvd => loss in pressure head due to velocity of the liquid in the discharge pipe, it is negligible at low velocity.
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System demand & Pump supply
∴ 𝑯𝑯𝑯𝑯 = 𝑯𝑯𝑯𝑯𝑯𝑯 ∓ 𝑯𝑯𝑯𝑯 + 𝑯𝑯𝑯𝑯𝑯𝑯 + +𝑯𝑯𝑯𝑯 + +𝑯𝑯𝑯𝑯𝑯𝑯𝑯𝑯
K
HQ
P t
a
ρ××
=
Where:
Pa = power absorbed in kilo watt
Q = quantity delivered in liters/second
Ht = total head losses in meter
ρ = density of liquid in gm/ml (1 for fresh water)
K = Constant (101.9368)
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• The input power to the pump required from the prime mover is
• For an electrically driven pump, the power consumed is
Power & Efficiency
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Pa
Pc
Motor
losses
Pi
Pump
losses
M

System Characteristic curve
Q: Flow Rate
H: Head
h
Q = Zero
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Net Positive Suction Head- NPSH
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𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵 = 𝑯𝑯𝑯𝑯𝑯𝑯 ± 𝑯𝑯𝑯𝑯 − 𝑯𝑯𝑯𝑯𝑯𝑯 − 𝑯𝑯𝑯𝑯𝑯𝑯 − 𝑯𝑯𝑯𝑯𝑯𝑯𝑯𝑯
Where:
Hes => pressure head represent the barometric pressure
Hfs => loss in pressure head due to friction resistance at the suction side.
Hs => height of the liquid free surface above the center line of the pump (Suction lift)
Hvap => loss in pressure head due to vapor pressure of the liquid at the working temperature.

NPSH
∴ 𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝒂𝒂𝒂𝒂 = 𝑯𝑯𝑯𝑯𝑯𝑯 ± 𝑯𝑯𝑯𝑯 − 𝑯𝑯𝑯𝑯𝑯𝑯 − 𝑯𝑯𝑯𝑯𝑯𝑯𝑯𝑯
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Q: Flow Rate
H: Head
𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵
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NPSH

Example (1)
(PB= 1bar), (Lsuc Suction head above pump=23 m), (Vp= 0.17 bar, at 25 ˚C), (hf =7m) – fresh water liquid
By aid of simple sketch describe above example & Calculate the NPSH and the gauge reading at pump
suction
PB : Barometric pressure.
LSuc: Suction head [above pump (+) / under pump (-)].
Vp: Vapor pressure.
hf: Friction losses in the piping system leading to pump suction.
24.3m
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(PB= 1bar), (Lsuc Suction head above pump=23 m),
(Vp= 0.75 bar at 90 ˚C), (hf =7m) – fresh water liquid -
By aid of simple sketch describe above example & Calculate the NPSH and the gauge reading at pump
suction
PB : Barometric pressure.
LSuc: Suction head [above pump (+) / under pump (-)].
Vp: Vapor pressure.
hf: Friction losses in the piping system leading to pump suction.
18.64 m
Example (2)
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Application
𝑯𝑯𝟏𝟏
𝑯𝑯𝟐𝟐
𝑯𝑯𝟑𝟑
𝑯𝑯𝟒𝟒
pp
𝑸𝑸𝟏𝟏
𝑸𝑸𝟐𝟐
𝑸𝑸𝟑𝟑
𝑸𝑸𝟒𝟒
??
NPSHav
Q: Flow Rate
H: Head
Filter
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Pump types
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Positive displacement
pumps

Positive displacement pumps
The displacement pumping action is
achieved by the reduction or increase
in volume of a space causing the liquid
(or gas) to be physically moved. The
method employed is either a piston in a
cylinder using a reciprocating motion, or
a rotating unit using vanes, gears or
screws.
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Reciprocating pump
The pump is a double-acting, that is liquid is
admitted to either side of the piston where it
is alternately drawn in and discharged. As the
piston moves upwards, suction takes place
below the piston and liquid is drawn in, the
valve arrangement ensuring that the
discharge valve cannot open on the suction
stroke. Above the piston, liquid is discharged
and the suction valve remains closed. As the
piston travels down, the operations of suction
and discharge occur now on opposite sides.
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Reciprocating pump
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Diaphragm pump
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Screw pump
These Screws are working like an endless piston
which constantly moves forward
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Screw pump
The liquid enters the outer suction manifolds and passes
through the meshing worm wheels, which are gear driven
from the motor to the central discharge manifold. Such
pumps are quite and reliable and are particularly suited to
pumping all fluids in particular oil, but it should be free form
the abrasive material. The pump can deal with large volume
of air whilst running smoothly and maintain discharge
pressure. It will be suited to tank draining and intermittent
fluid supply such as may occur in lubricating oil supply
systems engine, with vessel rolling.
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Counter Screw Pump
• Timing gears are fitted to some screw
pumps to insure correct clearance is
maintained at all times between the screws,
thereby preventing overheating and possible
seizure.
• Modern designs of screws preclude the use
of timing gears, ensure efficient simple
operation, eliminate turbulence and
vibration.
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Counter Screw Pump
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Triple Screw Pump
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M a r i n e E n g i n e e r i n g K n o w l e d g e U E 2 3 1 | Y A S S E R B . A . F A R A G18 September 2020 278
Triple Screw Pump
https://youtu.be/nvK-jL3SzxQ

Gear pump
Diesel engines and gearbox lubrication
systems are normally supplied by gear
pumps which are independently driven
for large slow speed and stand by duties
but usually shaft driven for medium and
high speed engines. Gear pumps are also
used for fuel and oil transfer, boiler
combustion systems and other duties.
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Gear pump
• The liquid being pumped is forced out after
being carried around between the gear teeth
and housing, as the teeth mesh together. It is
certain that the centrifugal effect contributes
to the pumping action. There is no side thrust
with straight gear teeth.
• Side thrust produced by single helical gears
causes' severe wear and in one pump opened for
examination, bronze bearing bushes exhibited
wear to a depth 3 mm. despite the excessive
clearance duet o wear it was noted that the
pump continued to be effective when repaired
as far as possible.
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Attached grar pump
• Gear pump can be used in attached with a
reversible diesel engine. This pump should be
fitted with control valves to control the
direction of flow in case of reversed
direction of the engine rotation.
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Screw pump Vs. Gear pump
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Internal Gear Pump
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https://youtu.be/TtlIvEovEtQ
Internal gear pumps in cast iron, for a wide range of viscous, non-corrosive liquids and are specifically designed for
numerous applications and those involving high viscosity liquids. It is suitable for pumping oil, asphalt, chocolate, paint,
lacquer, molasses, soap, other industrial viscous liquids, additives, polyol, viscose, sulphate soap, maltose, grease,
pitch, base oil, bitumen, polyester

Lobe Pump
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Rotary Vane pump
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Positive displacement pumps Properties
1. The chambers of the displacement pump are alternatively filled and emptied. A positive amount
of liquid passes through the pump. They mechanically displaces the liquid inside pump.
2. They have limmitted flow rate as it depends on the pump speed and size.
3. Pump’s sealing is critical for this type of pump to operate efficiently.
4. They can develop high pressures to overcome high system’s head as it has tight clearances.
5. It’s efficiency greatly affected with liquid’s viscosity as viscous liquids provide better sealing
inside the pump, however it may require more power to displace such liquid.
6. They can be used as a transfer pump for viscous liquids like HFO or LUB, Sludge pump and as a
bilge pump.
7. They do not require a priming device. Some times, they used as a priming device for other types
of pumps.
8. It must be fitted with a relieve valve on its discharge line to limit the system and pump head.
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Positive displacement pump Q-H Curve
Q: Flow Rate
H: Head
Ideal
Real
Slippage
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Roto-dynamic pumps

Centrifugal Pump
Rotation of the impeller causes any liquid
contained in it to flow towards the
periphery because of the centrifugal
force generated. The center or eye of the
impeller is thus evacuated and liquid from
the suction line then flows in to fill the
void. Assume there are no losses through
the volute casing and the flow pattern is
laminar, A1V1 = A2V2 = Constant
In a centrifugal pump liquid enters the centre or eye of the impeller and flows radially out between
the vanes, its velocity being increased by the impeller rotation. A diffuser or volute is then used to
convert most of the kinetic energy in the liquid into pressure.
Discharge
Volute Casing
Impeller
Suction
Cut water
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Centrifugal Pump
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Components
• Impeller: Bronze, Aluminum bronze, Stainless steel
Open, Semi open or Closed type.
• Shaft: Stainless Steel
• Casing: Cost Iron, Cost Steel or Gun metal (
Depends on Liquid medium ).
Volute Casing => More Quantity
Diffuser => Higher Head ( Boiler Feed Pump)
• Casing Ring – Wear Ring – Cover Ring:
Cupper ,Protects the Impeller and Casing from
Wear and maintains the Clearance between Shaft
and Casing
• Bearings: Horizontal => Very big
Double entry => Small
Vertical => Very Small
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Impeller types
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Impeller Shapes
Liquid
ClosedType Semi-ClosedType OpenType
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Axial thrust
This is the oldest method for balancing axial thrust and involves reducing the pressure in a chamber equipped with a
throttling gap, usually down to the pressure level encountered at the impeller inlet. The pressure is balanced via
balancing holes in the impeller.
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Clearances
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Wear ring
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Wear ring
• Wear rings act as a seal between the high-pressure and
low-pressure regions within a pump.
• Leakage past the wear rings (QL) recirculates within
the impeller as shown in Figure.
• The operators only see the flow coming out of the pump
(Q).
• The total energy consumption of the pump, however, is
a function of the total flow through the impeller Q + QL
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Single and double eye
IMPELLER
SEALING RING
DRIVE SHAFT DRIVE SHAFTKEYKEY
Q
H
Single entry
Double entry
NPSHrequired
Axial thrust balancing by double-
entry impeller arrangement
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Wear ring
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Wear ring
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Volute Casing
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Characteristic Curve
Losses:
1. Friction losses in bearings, glands, surfaces of
impeller and casing
2. Head losses due to shock at entry and exit to
impeller vanes and eddies formed by vanes edges.
3. Leakage loss in thrust balance devices, gland
sealing, clearance between cut water and casing
and bearing seals.
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Applications
NPSH Available MUST BE > NPSH Required
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Head (m)
Flow (m3/hr)
Pump characteristic
NPSH required
NPSH available
System head losses

Comparison between Centrifugal and positive displacement pumps with respect to L.O duties
Q
H
Totalhead
Quantity
Centrifugal H/Q
Positive
displacement
H/Q
resultant pressure drop
Centrifugal Q increase
Q increase
Centrifugal resultant pressure drop
Fall in system resistance
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• High Flow rate.
• Relatively Low discharge head
• Needs priming
• Simple and easy to maintain
• Low cost
• Perform better with high Oil temperature
• Discharge pressure could be increased by
means of multi-staging or using a diffuser.
• It can be used in systems where high flow
rate is required like : Fire System, Ballast
System, Cooling Water Pump and as a cargo
pump onboard tankers
Centrifugal Pump properties
Q
H
n=1000 RPM
n=800 RPM
Operating points
Statichead
System characteristic
𝑚𝑚3
ℎ𝑟𝑟
mlc
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Performance Improvements
306

Diffuser
307

Multi Staging – in series
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Multi Staging – in series
• Centrifugal pumps in series are used to overcome
larger system head loss than one pump can
handle alone.
• For two identical pumps in series the head will be
twice the head of a single pump at the same flow
rate - as indicated in point 2. With a constant
flowrate the combined head moves from 1 to 2.
• Note! In practice the combined head and flow rate
moves along the system curve to point 3.
• point 3 is where the system operates with both
pumps running.
• point 1 is where the system operates with one
pump running
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Head
Flow
rate
One pump
Two pumps in-series
1
2
3
h2
h1
q1 q3
Operating point
New operating pointh3

Multi Staging – in Parallel
When the system characteristic curve is
considered with the curve for pumps in
parallel, the operating point at the
intersection of the two curves represents a
higher volumetric flow rate than for a single
pump and a greater system head loss. As
shown in Figure, a greater system head loss
occurs with the increased fluid velocity
resulting from the increased volumetric flow
rate. Because of the greater system head, the
volumetric flow rate is actually less than twice
the flowrate achieved by using a single pump.
Head
Flow rate
One pump
Two pumps in-parllel
1
2
3
q2
h1
q1 q3
Operating point
New operating point
h3
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Multi Staging – in Parallel
Head
Flow rate
One pump
Two pumps in-parllel
1
2
3
q2
h1
q1 q3
Operating point
New operating point
h3
• Centrifugal pumps in parallel are used to overcome
larger volume flows than one pump can handle alone.
• for two identical pumps in parallel, and the head is
kept constant, the flowrate doubles as indicated with
point 2 compared to a single pump
• Note! In practice the combined head and volume flow
moves along the system curve as indicated from 1 to 3.
• point 3 is where the system operates with both pumps
running
• point 1 is where the system operates with one pump
running
• In practice, if one of the pumps in parallel or series
stops, the operation point moves along the system
resistance curve from point 3 to point 1 - the head and
flow rate are decreased.
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Emergency Bilge Pump
• The function of this pump is to drain compartments adjacent
to a damaged (holed) compartment.
• The pump is capable of working when completely submerged.
• The pump is a standard centrifugal pump with reciprocating
or rotary air pumps.
• The motor is enclosed in an air bell so that even with the
compartment full of water the compressed air in the bell
prevents water gaining access to the motor.
• The motor is usually dc operated by a separate remote
controlled electric circuit which is part of the vessels
emergency essential electric circuit.
• The pump is designed to operate for long periods without
attention and is also suitable for use as an emergency fire
pump.
• This design is particularly suited for use in large passenger
vessels giving outputs of about 60 kg/s.
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Priming

Priming means filling the pump casing with liquid in order to operate. Centrifugal pumps must be
primed prior its operation while positive displacement pumps are self-priming.
When starting a centrifugal pump the suction valve is opened and the discharge valve left shut: then the
motor is started and the priming unit will prime the suction line. Once the pump is primed the delivery
valve can be slowly opened and the quantity of liquid can be regulated by opening or closing the delivery
valve. When stopping the pump the delivery valve is closed and the motor stopped.
The centrifugal pump can be primed by one of the following methods:
• The pump to be submerged in the suction tank
• High head tank or any means of head pressure applied on the pump suction.
• Ejector
• Positive displacement priming pump
• Central priming system.
Priming Methods
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Primer
• When a centrifugal pump is operating, the liquid leaving the
impeller produces a drop in pressure at the entry or eye of the
impeller .
• This causes liquid from the suction pipe to flow into the pump.
In turn, there is a movement of the liquid to be pumped. The
latter is normally subject to atmospheric pressure .
• A centrifugal pump will maintain a suction lift of four metres
or more once it has been primed, because of the water passing
through.
• The water in a pump acts like a piston for water in the suction
pipe and an empty pump will not operate.
• A pump which is required to initiate suction from a liquid level
below itself, must be fitted with an air pump.

Primer
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Water Ring Pump
Suction portDischarge port
Impeller
Casing
317

Ejector
318

Ejector
• The ejector design is simple and is used for stripping.
• This ejector has no revolving or reciprocating parts and is
thereby especially easy to maintain.
A1
V1
P1
A2
V2
P2
Vacuum
319

Central Priming System
System advantages:
1. Saving in total power
2. Reduced capital cost
3. Simplified maintenance
4. Automatic operation
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Axial Flow Pumps

These tend to fit somewhere between positive displacement and centrifugal. They tend to be of the
very large capacity type. The axial flow pump is used where large quantities of water at a low head
are required, for example in condenser circulating.
Axial Flow Pump
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UPPER GEARBOX
DRIVE SHAFT
EXTENSION PIPE
TUNNEL LOWER GEARBOX
PROPELLER
An axial-flow pump uses a screw propeller to axially accelerate the liquid. The outlet passages and guide
vanes are arranged to convert the velocity increase of the liquid into a pressure.
Axial Flow Pump
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Characteristics
• The Pump is efficient, simple in design and is
available in wide range of capacities.
• It can if required, be reversible in operation (a
friction clutch between motor and pump would
be required) and
• ideally suited to scoop intake for condensers
as it offers very little resistance when idling.
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Characteristic Curve
325

Application
326

Comparison between different pump’s types
327
Centrifugal
Axial
Positive
displacement
100%
Head
Discharge

Pump shaft sealing

Packing
329

Mechanical Seal
330

Comparison
331

Cavitation

Vapor pressure
𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝒂𝒂𝒂𝒂 = 𝑯𝑯𝑯𝑯𝑯𝑯 ± 𝑯𝑯𝑯𝑯 − 𝑯𝑯𝑯𝑯𝑯𝑯 − 𝑯𝑯𝑯𝑯𝑯𝑯𝑯𝑯
Hvap 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, hvap. The value of hvap 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.
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Basis of
Comparison
Evaporation Boiling
Meaning It is when the liquid or state changes into a vapour
Boiling is steaming or bubbling up under the influence
of heat
Occurrence It occurs at the surface of the liquid
Boiling occurs throughout the liquid because of the
addition of a lot of heat
Temperature Evaporations needs a little change in temperature
It requires a temperature which is greater than the
boiling point
Nature Evaporation is a natural process It is an unnatural process
Time It takes a longer time to complete Boiling requires a shorter period of time
Energy It requires little to no energy A lot of energy adds in this process

Water vapor pressure curve
0
10
20
30
40
50
60
70
80
90
100
110
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050
Temperature(C)
Pressure ( mmHg )
Atm pressure @ 760 mm Hg = 1.0133 bar
Add
heat
Lower
pressure
Liquid
Vapor
Temprature
( C )
Pressure
( mm Hg )
Pressure
( bar )
Max
Elevation
(m)
0 4.6 0.0061 10.27
5 6.5 0.0087 10.24
10 9.2 0.0123 10.21
15 12.8 0.0171 10.16
25 23.8 0.0317 10.01
30 31.8 0.0424 9.90
35 41.2 0.0549 9.77
40 55.3 0.0737 9.58
45 71.9 0.0959 9.36
50 92.5 0.1233 9.08
55 118 0.1573 8.73
60 149.4 0.1992 8.30
65 187.5 0.2500 7.78
70 233.7 0.3116 7.16
75 289.1 0.3854 6.40
80 355.1 0.4734 5.51
85 433.6 0.5781 4.44
90 525.8 0.7010 3.18
95 633.9 0.8451 1.71
100 760 1.0133 0.00
105 906.1 1.2080 -1.99
110 1074.6 1.4327 -4.28
Vapor Pressure
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1. pitting,
2. noise,
3. vibration,
4. pump damage and fall off in
pump performance.
LOW
PRESSURE
DRIVE SHAFT
IMPELLER
HIGH PRESSURE HIGH PRESSURE
SHAFT
SEAL
SEALING
RING
In the suction area of the pump, high local speeds of the fluid
occur. This gives rise to low pressures at these points. Due to
the reduction in pressure, the liquid may vaporize causing
bubbles to form. The bubbles then collapse when they reach a
high-pressure area. This happens very quickly and can cause
very high-pressure hammer blows, which result in:
Centrifugal Pump Cavitation
335

Cavitation in pumps
90 % of pumps problems are due to CAVITATION
336

Cavitation in pumps
𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝒓𝒓𝒓𝒓𝒓𝒓
𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝒂𝒂𝒂𝒂
Q: Flow Rate
H: Head
Cavitation
𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝒂𝒂𝒂𝒂 𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝑵𝒓𝒓𝒓𝒓𝒓𝒓>
337

Cavitation in propellers
338

Cavitation in SUBMARINES !
339

Video
340

Cargo Systems

Conventional Oil Tanker
342

Barrel-type cargo pump
• The pump with double eye inlet.
• The pipe connections in bottom half of casing has two
external bearings above the impeller, the upper one takes all
the hydraulic thrust and the lower act as a radial load
bearing.
• This pump has some advantages over its counterparts:
1. Impeller can be sited lower in the pump room thus
improving suction conditions and reducing stripping
time,
2. Removal of impeller without disturbing pipe joints.
3. Easier access to beatings and shaft seal without
removal of rotating elements.
343

Inducer
• Inducers are sometimes fitted to centrifugal pump impeller
shafts at suction.
• Their purpose is to ensure the supply of fluid to the impeller
is at sufficient pressure to avoid cavitation at impeller
suction (less NPSHreq), i.e. it enables the pump to operate
with a lower net positive supply head (NPSHav).
344

Submerged pump
The Submerged electric motor driven pump rests on a
spring cartridge which closes when the pump is raised and
seals off the tank from the column
• Chemical, LPG, or multi-product tanker: a separate pump
is sited in each tank.
• Pumps driven through line shafting coupled to hydraulic
motor on deck (deep well, single or multistage or
submerged pumps electrically or hydraulically driven)
• The Submersible pumps eliminate line shaft bearings, and
gland problems but expensive problems could occur due
to hydraulic fluid leakage into the cargo and vice-versa.
345

LPG
 Petroleum hydrocarbon products such as Propane and Butane, and mixtures of both have been
categorised by the oil industry as LPG.
 The most important property of LPG is that it is suitable for being pressurised into liquid form and
transported..
 At least one of the following conditions need to be complied with, for transportation of LPG:
• The gas should be pressurised at ambient temperature.
• The gas should be fully refrigerated at its boiling point. Boiling point of LPG rangers from -30
degree Celsius to -48 degree Celsius. This condition is called fully-refrigerated condition.
• The gas must be semi-refrigerated to a reduced temperature and pressurised
 Other gases such as ammonia, ethylene and propylene are also transported in liquefied form in LPG
carriers. Ethylene, however, has a lower boiling point (-140 degree Celsius) than other LPGs. Hence it
must be carried in semi-refrigerated or fully-refrigerated conditions.
346

LNG
 Natural gas from which impurities like sulphur and carbon-dioxide have been removed, is called Liquefied Natural
Gas.
 After removal of impurities, it is cooled to its boiling point (-162 degree Celsius), at or almost at atmospheric pressure.
 Note here, that unlike LPG, LNG is cooled to low temperatures but not pressurised much above atmospheric
pressure. This is what makes the design of LNG carriers slightly different from LPG carriers.
 LNG, at this condition is transported as liquid methane.
347

NG transport
LNG consists mainly of methane (CH4), with minor amounts of other
hydrocarbons (ethane, propane, butane and pentane). By liquefying the
methane gas, LNG takes up only 1/600th of the volume of natural gas in its
gaseous state, which means the gas can be distributed around the world
more efficiently. By comparison, compressed natural gas (CNG) takes up
around 1/100th of the volume of natural gas in its gaseous state, depending
on the actual pressure.
348

Tanks types
1. Integral Tanks
• These are the tanks that form a primary structural part of the ship and are influenced by the loads
coming onto the hull structure.
• They are mainly used for cases when LPG is to be carried at conditions close to atmospheric condition,
for example – Butane. That is because, in this case, there are no requirements for expansion or
contraction of the tank structure.
2. Independent tanks
 These tanks are self-supporting in nature, and they do not form an integral part of the hull structure.
Hence, they do not contribute to the overall strength of the hull girder.
 According to IGC Code, Chapter 4, independent tanks are categorised into three types:
1. Type ‘A’ tanks
2. Type ‘B’ tanks
3. Type ‘C’ tanks
349

Tank types
1. Type ‘A’ tanks
• These tanks are designed using the traditional method
of ship structural design.
• LPG at near-atmospheric conditions or LNG can be
carried in these tanks.
• The design pressure of Type A tanks is less than 700
mbar.
• The IGC Code specifies that Type ‘A’ tanks must have a
secondary barrier to contain any leakage for at least
15 days.
• The secondary barrier must be a complete barrier of
such capacity that it is sufficient to contain the entire
tank volume at any heel angle.
350

Type ‘A’ tanks
• The above figure shows how the aluminium tank structure is not integrated
to the inner hull of the methane carrier by means of any metal contact.
• The inner hull plating and aluminium tank plating are separated by layers
consisting of timber, glass fibre, and balsa panels for insulation from
external temperatures.
• The balsa panels are held together by plywood on both faces which are
sealed using PVC foam seals. An inert space of 2 or 3 mm separates the
inner glass fibre layer from the aluminium tank plate. This space is provided
for insulation and also allows expansion and contraction of the tank
structure. This type of non-welded integration makes this tank structurally
independent in nature.
351

Type ‘B’ tanks
• The most common arrangement of Type ‘B’ tank is Kvaerner-Moss
Spherical Tank.
• The tank structure is spherical in shape, and it is so positioned in the ship’s
hull that only half or a greater portion of the sphere is under the main deck
level. The outer surface of the tank plating is provided with external
insulation, and the portion of the tank above the main deck level is
protected by a weather protective layer. A vertical tubular support is led
from the top of the tank to the bottom, which houses the piping and the
access rungs.
• As evident from the layout, any leakage in the tank would cause the spill to
accumulate on the drip tray below the tank. The drip pan and the
equatorial region of the tank are equipped with temperature sensors to
detect the presence of LNG. This acts as a partial secondary barrier for the
tank.
• LNG is usually carried in this type of tanks. A flexible foundation allows
free expansion and contraction according to thermal conditions, and such
dimensional changes do not interact with the primary hull structure.
352

Type ‘B’ tanks
The following are the advantages of Kvaerner-Moss
Spherical tanks:
 It enables space between the inner and outer hull
which can be used for ballast and provided
protection to cargo in case of side-ward collision
damages.
 The spherical shape allows even distribution of
stress, therefore reducing the risk of fracture or
failure.
 Since ‘Leak before Failure’ concept is used in the
design, it presumes and ensures that the primary
barrier (tank shell) will fail progressively and not
catastrophically. This allows crack generation to
occur before it propagates and causes ultimate
failure
353

Type ‘C’ tanks
• These tanks are designed as cryogenic pressure vessels, using conventional
pressure vessel codes, and the dominant design criteria is the vapour pressure.
The design pressure for these tanks is in ranges above 2000 mbar.
• The most common shapes for these tanks are cylindrical and bi-lobe. Though
Type ‘C’ tanks are used in both, LPG and LNG carriers, it is the dominant design in
LNG carriers.
• Note, in Figure, that the space between the two cylinders is rendered useless. Due
to this, the use of cylindrical tanks is a poor use of the hull volume. In order to
circumvent this, the pressure vessels are made to intersect, or bilobe tanks are
used.
• The hold space is filled with inert gas or dry air. Sensors placed in the hold space
can detect the change in composition of the inert gas or dry air due to fuel vapour,
and leakages can hence be detected and prevented. Bilobe tanks at the forward
end of the ship are tapered at the end.
354

Membrane Tanks
• Unlike independent tanks, membrane tanks are non-
self-supporting structures.
• Their primary barrier consists of a thin layer of
membrane (0.7 to 1.5 mm thick).
• The membrane is supported to the inner hull structure
through an insulation that can range up to 10 mm
thickness as per IMO IGC Code.
• Due to their non-self-supporting nature, the inner hull
bears the loads imparted onto the tank. This way, the
expansions and contractions due to thermal fluctuations
are compensated by not allowing the stress to be taken
up by the membrane itself.
• Membrane tanks are primarily used for LNG cargo.
355

Membrane Tanks
The advantages of membrane tanks are as follows:
• They are generally of smaller gross tonnage, that is
the space occupied within the hull is lower for a given
cargo volume.
• Due to the above reason, maximum space in the hold
can be used for cargo containment.
• Since the height of tanks above the main deck is
significantly lesser compared to the cases of Moss
tanks, membrane tanks provide allow visibility from
the navigational bridge. This also allows a lower
wheelhouse.
356

• A typical LNG carrier has four to six tanks located
along the center-line of the vessel.
• Inside each tank there are typically three
submerged pumps.
• There are two main cargo pumps which are used
in cargo discharge operations and a much smaller
pump which is referred to as the spray pump.
• The spray pump is used for either pumping out
liquid LNG to be used as fuel (via a vaporizer), or
for cooling down cargo tanks. It can also be used
for "stripping" out the last of the cargo in discharge
operations.
• All of these pumps are contained within what is
known as the pump tower which hangs from the
top of the tank and runs the entire depth of the
tank. The pump tower also contains the tank
gauging system and the tank filling line, all of
which are located near the bottom of the tank.
Cargo Systems

Deepwell pump
• For liquified gas cargo system, deepwell pumps are or submerged
electrically because of the cargo low temperature.
• The long shaft of the deepwell pump runs in Carbon bearings, the shaft
being protected in way of the bearings by stainless steel sleeves.
• The pump shaft is positioned within the discharge pipe to allow the liquid
cargo to lubricate and cool the bearings.
• The risk of overheated bearings if the pump run dry is reduced by a
pressure cut-out or thermal switch.
• The liquified gas is carried at its boiling temperature to ensure that the
ullage space above the liquid is filled with cargo vapour and air is
excluded.
358

Deepwell pump
• The residue cargo to maintain the tank air free and allows the tank
temperature to be kept at the carrying level and avoid tank structure
from being expanded and contracted.
• The weight of the pump shaft and impeller are opposed by one or more
carrier bearings.
• Lift force of the shaft also requires a downward-acting thrust bearing.
• The number of pump stages is dictated by the discharge head
required.
• The inducer frequently fitted to centrifugal liquified gas pumps at the
pump suction.
• Deepwell pumps in general are driven by hydraulic motors or by a
flameproof electric motors situated at deck level.
• Duplication of pumps in tanks is the safeguard against breakdown of
deepwell pumps in liquid gas carriers.
359

VAC-Strip System
360

Chemical Tanker Cargo System
• The practice of positioning submersible or deepwell pumps
within cargo tanks eliminates pump room dangers.
• The expense of extra suction pipework and the risk of mixing
cargoes with resulting contamination
Three concentric tubes make up:
• the high pressure oil supply pipe to the hydraulic motor, the
return pipe (1,2), and a protective outer cofferdam (3) .
• Working pressure for the hydraulic circuit is up to about 170
bar and return pressure about 3 bar .
• The impeller suction is positioned close to the bottom of the
suction well for good tank drainage but when pumping is
completed the vertical discharge pipe will be left full of
liquid.
• Stopping the pump would allow the liquid to fall back into
the tank and clearing of the tank of cargo or of water used
in tank cleaning would be a constant problem.
361

FRAMO System
362

FRAMO System
• purging connections are fitted to clear the discharge pipe (and the cofferdam if
there is leakage) .
• Discharge pipe purging is effected by closing the deck discharge valve as the
tank clears of liquid, then with the pump left running to prevent cargo fallback
opening the purge connection shown. The compressed air or inert gas at 7 bar
will clear the vertical discharge pipe by pressurising it from the top and forcing
liquid cargo up through the small riser to the deck main.
• The cofferdam is also pressurised before the pump is stopped, to check for
leakage . This safety cofferdam around the hydraulic pipes is connected to the
drainage chamber at the bottom of the pump. Seals above and below the
chamber exclude ingress of low pressure hydraulic oil and liquid cargo from the
tank, respectively . The bottom seal is subject only to pressure from the head
of cargo in the tank, not to pump pressure .
363

FRAMO System
DESIGN PRESSURE:
CARGO 25 BAR
HIGH PRESSURE, HYDRAULIC: 320 BAR
RETURN PRESSURE, HYDRAULIC: 16 BAR
COFFERDAM: 10 BAR
SubmergedBallastWaterPump
364

Pumps pumping systems

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