After successfully completing this
lesson you'll be familiar with different
types of rotor dynamic pumps the
component parts of rotor dynamic pumps
maintenance of rotor dynamic pumps the
lesson will end with a test and you're
there are three different types of rotor
dynamic pumps axial flow pumps
centrifugal pumps and mixed flow pumps
click on an item to jump to it an axial
flow pump uses a screw propeller to
actually accelerate the liquid the
outlet passages and guide vanes are
arranged to convert the velocity
increase of the liquid into a pressure
as distinct from the centrifugal pump
the axial flow pop absorbs the maximum
power at zero flow a mechanical seal
prevents leakage where the shaft leaves
the casing a thrust bearing of the
tilting pad type is fitted on the drive
shaft the prime mover maybe an electric
motor or a steam turban the axial flow
pump is used where large quantities of
water at a low head are required for
example in condenser circulating the
efficiency is equivalent to a low lift
centrifugal pump and the higher speeds
possible enable a smaller driving motor
to be used the axial flow pump is also
suitable for supplementary use and a
condenser scoop circulating system since
the pump will offer little resistance to
flow when idling with scoop circulation
the normal movement of the ship will
draw in water the pump would be in use
only when the ship was moving slowly or
stopped the pump is reversible and this
in conjunction with high-capacity flow
makes it suitable for trimming and
healing duties as well the pump casing
is of gunmetal for condenser cooling
duties and cast iron for healing and
trimming pumps the impellers are of
aluminium bronze and guide vanes of
gunmetal are
changed immediately after the impeller
the pump shaft being of stainless steel
it's time for a quick quiz which part of
the pump is the screw propeller click on
the screw propeller if you're not sure
go to the previous screen to refresh
your memory correct in this part of the
lesson we'll take a closer look at the
centrifugal pumps centrifugal pump
duties are usually for the movement of
large volumes of liquid at low pressures
although higher pressures can be
achieved with multi staging click on an
item to jump to it a centrifugal pump
can be further defined as a machine
which uses several energy
transformations in order to increase the
pressure of a liquid the energy input
into the pump is typically the fuel
source energy used to power the driver
the remaining energy transformations
take place inside the pump itself the
rotating pump shaft is attached to the
pump impeller which rotates in a volumes
housing a particular feature of
centrifugal pumps is that the power
absorbed is a minimum at zero flow and
therefore can be started up against
closed valve by increasing the size of
the impeller and or the speed of pump
rotation we can achieve larger pumping
rates the fluid flow causes a vacuum to
be formed in the pump suction which will
draw fluid into the impeller suction
thus fluid flow will occur from the
suction to discharge the liquid enters
the center or I of the impeller actually
changes direction and flows regularly
out between the veins when the
sure falls below the vapor pressure of
the liquid at a given temperature
boiling occurs and small bubbles of a
performed these bubbles will grow in the
low-pressure area and implode when
they're transported to an area of
pressure above vapor pressure the term
given to this local vaporization of the
fluid is cavitation the collapsing of
the bubbles is the area of cavitation
we're concerned with as extremely high
pressures are produced which causes
noise and erosion of the metal surface
click on an item to jump to it
centrifugal pumps all those suitable for
most general marine duties suffer in one
very important respect they are not self
priming and require some means of
removing air from the suction pipe line
and filling it with a liquid where the
liquid to be pumped is at a higher level
than the pump opening an air release
cock near the pump section will enable
the air to be forced out as the pipeline
fills up under the action of gravity
this is often referred to as flooding
the pump
alternatively an air pumping unit can be
provided to individual pumps or as a
central priming system connected to
several pumps the water ring or liquid
ring primer can be arranged as an
individual unit mounted on the pump and
driven by it or as a motor-driven unit
mounted separately and serving several
pops known as a central priming system
where several pumps require a priming
aid for example a cargo pumping system
or a number of engine pump rooms
essential priming system is often used
this reduces the number of priming pumps
saving spares maintenance and cost with
this system essential priming unit
consisting of two or more liquid ring
primer pumps is arranged to pull a
vacuum on a central tank the tank has
connections to float chambers in each of
the suction lines for the system comes
isolated by either manually operated or
solenoid operated valves the priming
pumps are controlled by the vacuum of
the central tank cutting in and out as
required according to demand as a system
pump is required the priming connection
is opened manually or automatically
until good suction is achieved the
illustration shows a typical central
priming system including tank valves
gauges and switches the selection of
centrifugal pumps depends mainly upon
duty and the space available click on an
item to jump to it
when assessing the amount of power
needed to operate a centrifugal pump you
must always take into account the
various losses friction loss and
bearings and glands services of impeller
encasing some impellers are highly
polished to minimize friction loss head
losses and pumps due to shock at entry
and exit to impeller veins and Eddie's
formed by vain edges leakage loss in
thrust balance devices gland ceiling and
clearances between cut water and casing
and bearing seals a characteristic curve
for a centrifugal pump is obtained by
operating the pump at rated speed with a
suction open and that is charged valve
shut that is charged valve is then
opened in stages to obtain different
discharge rates and total head
corresponding to them the data can then
be represented graphically as a curve
the illustration shows the
characteristic curves for three
different types of pumps losses can be
caused by failure to deliver capacity
reduction and excessive vibration
this is a vertical single stage
single-entry centrifugal pump for
general marine use the mainframe and
casing together with a motor support
bracket how's the pumping element
assembly the volume casing is split in
two halves along a vertical plane since
the suction and discharge nozzles are
provided in the rear half of the casing
the rotating element can be taken out by
removing only the front half casing
without disturbing the rest of the pump
the pumping element is made up of a top
cover a pump shaft an impeller a bearing
bush and a seating arrangement around
the shaft the seating arrangement maybe
a pact gland or mechanical seal and the
bearing lubrication system will vary
according to the type of seal
replaceable wear rings are fitted in the
casing around the top and bottom faces
of the impeller the motor support
bracket has two large apertures to
provide access to the pumping element
and a coupling spacer is fitted between
the motor and pump shaft to enable the
removal of the pumping element without
disturbing the motor or vice versa
to connect the motor to the impeller the
shaft has to pass through an aperture in
the casing to allow the shaft to rotate
freely in the casing aperture there
needs to be a gap but this gap needs to
be closed off to stop air from being
drawn in from atmosphere or liquid from
leaking out during operation there are
two common methods packing or mechanical
seal the role of the pump its speed and
the type of liquid being pumped all play
a part in deciding which application
works best
it's time for a quick quiz where is the
packing material located
click on the packing material if you're
not sure go to the previous screen to
refresh your memory
correct the incoming liquid enters the
double impeller from the top and bottom
and passes into the valued casing for
discharge in the same way as
before a double entry pump has a lower
net positive suction head required
characteristic which will have
advantages in poor section conditions
it should be noted that different
impeller sizes could be fitted into a
basic pumping element
this enables various discharge head
characteristics to be provided for the
same basic pump frame the larger to
control the axial movement of the
rotating assembly a balanced piston is
arranged to counteract the effect of the
thrust of the impellers especially in
the multistage pumps when the pump is
due for overhaul it will be necessary to
dismantle it to its component parts to
examine them for where the following
procedures are intended as a general
guide only and your attention should be
drawn to the manufacturer's operational
instructions regarding specific pump
requirements before commencing to
dismantle the pump in this type the
pressure is developed partly by
centrifugal action and partly by the
veins and as the name implies the flow
is both axial and radial through the
impeller