Text MARINE PUMPS

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• Lesson 15
• Text MARINE PUMPS

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• The pumps employed on board ship can be divided
  into two main categories

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• (i) positive displacement pumps(????)
• and (ii) centrifugal pumps(???)?

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• They can be broken down into three classes
• (1) reciprocating(???),
• (2) rotary(???)
• and (3) centrifugal.

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• Under reciprocating are the direct acting(???),
  power, and crank-fly-wheel types.

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• Under rotary are the gear, vane(???),
  cam-and-piston and screw types(???).

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• Under centrifugal are the volute(???),
  diffuser(???,???), mixed-flow, axial-flow and
  turbine or regenerative types(???).

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• Displacement pumps are those where the volume of
  the pump chamber is alternately(???) increased to
  draw the liquid in from the suction pipe and then
  decreased to force the liquid out into the
  delivery pipe.

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• Centrifugal pumps are those wherein an
  impeller(??) rotating at high speed throws(???)
  the liquid by centrifugal force from the center
  to the periphery of the impeller where the liquid
  is discharged through the delivery outlet.

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• Gear pumps consist of two or more meshing(??)
  gears spur(???), single or double helical teeth
  (???) enclosed in a close-fitted housing.

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• These are used extensively for pumping fuel oil,
  lubricating oil and hydraulic oil.

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• When gears are unmeshed on the inlet side, a
  pressure drop occurs that helps the atmospheric
  pressure acting on the surface of the oil in the
  reservoir to force it up the pipe to fill the
  void(??),

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• the oil is then transferred around the periphery
  of the pump housing.

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• As the gear teeth mesh again on the outlet side,
  they form a seal that prevents oil from backing
  up to the inlet, the oil in the void is then be
  forced out into the discharge line.

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• All vane pumps have a rotor driven within a ring
  (camring??????) by a drive shaft coupled to a
  prime mover.

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• A cylindrical rotor with sliding vanes,
  generally in radial slots(?), rotates within the
  ring.

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• In a simple vane pump(??????), the centre of the
  rotor is offset(??) from the centre of the ring,
  leaving a crescent(????) shaped chamber between
  rotor and the ring.

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• As the rotor turns, the vanes are forced outward
  against the inner surface of the cam ring by
  centrifugal force.

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• This outward radial movement of the vanes and
  turning of the rotor causes the chamber between
  the vanes to increase as the vanes pass further
  away from the inlet port.

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• This increase in volume results in a lowering of
  pressure until the atmospheric pressure is
  sufficient to force oil from the reservoir into
  the inlet chamber.

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• Oil from the inlet is swept away by the vanes
  toward the outlet port through a decreasing
  (volume) series of chambers until it is forced
  through the outlet port.

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• A vane pump of balanced design has an
  elliptical(???) cam ring so that two pumping
  chambers are formed.

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• This pump is in hydraulically balance, since the
  two intake ports and two outlet ports are
  diametrically opposed(?????) to each other.

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• Therefore the side loads exerted on the rotor
  cancel out(??), thereby increasing bearing life
  and permitting high operating pressure.

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• According to their piston arrangements,
  rotary-piston pumps are classified either axial
  or radial type.

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• The pistons in an axial pump operate along the
  axis of the pump drive shaft. The cylinder block
  has a series of cylinder bores with pistons that
  move in and out.

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• Rotation of the drive shaft causes the pistons
  and the cylinder block to rotate at the same
  speed.

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• As the block rotates, each piston moves in and
  out of its cylinder, the length of stroke depend
  on the angle of the cylinder block with reference
  to the drive plate or swash plate(???) according
  to the designs.

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• As each piston moves outward, oil is drawn into
  the cylinder bore through the valve plate(???).

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• On the return stroke, the oil is forced out
  through the valve plate under pressure.

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• Reciprocating pumps and rotary pumps are
  positive-displacement type because they discharge
  a near constant amount of liquid regardless of
  pressure.

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• If the discharge valve is closed, they build up a
  bursting pressure or stall(???) their
  motor(????).

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• Centrifugal pumps are nonpositive and a fairly
  low pressure unit. Here a close discharge valve
  will not cause excessive pressure to build up.

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• The simple centrifugal pump consists of a
  rotating impeller within a stationary(???)
  casing.

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• The fully shrouded(??), single entry impeller is
  the type most widely used.

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• The impeller is mounted on a shaft driven by a
  prime mover.

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• It consists of a number of vanes backward from
  the direction of the rotation between the
  shroudings.

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• Water enters the rotating impeller through the
  entry, known as eye(???)', at the centre.

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• Due to the circular motion given it is thrown by
  centrifugal force to the open periphery of the
  impeller where it leaves tangentially(??????) and
  enters the space in the casing, and then directed
  to the outlet branch.

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• Unlike the reciprocating displacement pump, the
  centrifugal pump does not have a positive suction
  action(????) and must be primed by flooding
  before it will draw water from a lower level.

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• An air pump (???) or a central priming system may
  be used for this purpose.

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• In general, reciprocating pumps are suitable for
  delivering small quantities at high pressure,
  rotary pumps are used for moderate quantities at
  moderate pressure, and centrifugal pumps are more
  suitable for large quantities at low pressure.

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• Centrifugal pumps, however, can be designed with
  a number of impellers in series(??) to attain a
  high final delivery pressure.

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• They are popular because they are very simple in
  construction, and can produce smooth, constant,
  nonpulsating( ???) discharge.

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READING MATERIAL

• A. INSTALLATION AND MAINTENANCE OF CENTRIFUGAL
  PUMPS

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Installation (installation on site)

• 1. Foundation
• The foundation of marine pumps is a steel
  base frame (plane parallel alignment(???????) is
  essential).

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• Ensure the foundation has set hard before
  mounting the pump set. The surface must be smooth
  and flat.

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• Position the pump on the steel base frame or
  foundation, then align(??) and bolt in position.

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2. Alignment of Pump and Driver

• After fitting and bolting on the driver, centre
  (?????) it in the flange.

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3. Connecting the Piping

• Never use the pump as an anchorage point(???) for
  the piping.

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• The pipes must be supported very near the pump
  and connected to the pump without transmitting
  any stresses or strains(??) to it.

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• Make sure the pump does not bear the weight of
  the piping. The nominal(????) bore of the pipes
  should be the same as or greater than that of the
  pump nozzles(??).

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• Thermal expansion of the piping must be
  compensated for by suitable means, so as not to
  impose any additional load on the pump.

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• The suction pipe must not have any valves,
  fittings etc., which might interfere with the
  flow, at a distance of not less than 5 times the
  suction pipe bore from the pump, to ensure
  uniformity(??) of flow to the impeller.

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4. Auxiliary Connections

• The size and locations of connections for leak
  fluid and venting, if applicable, are shown in
  the dimension sheet and the lines should be
  connected up accordingly(??).

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5. Coupling Guard(????)

• Safety regulations specify that the pump can only
  be operated if the coupling has a guard. The
  coupling guard must be fitted before startup.

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6. Final Check

• It must be possible to rotate the rotor easily by
  manually turning the coupling through 360o.
  Ensure all auxiliary lines are correctly
  connected up.

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Maintenance

• 7. The pump must run quietly, smoothly and free
  from vibrations at all times. Rough running can
  cause damage to the bearings and shaft seal.

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• 8. The pump must never run dry. Avoid prolonged
  operation against a closed shut-off valve(???).

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• 9. The temperature of the ball and roller
  bearings may rise to 50oC above ambient
  temperature but must not exceed 80oC.

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• The shut-off valves in the various supply lines
  must not be closed during operation.

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• 10. Standby pumps must be started-up from time to
  time to ensure that they are in a state of
  constant readiness also rotate the shaft by hand
  at frequent intervals.

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• 11. A sealant reservoir for the mechanical seal
  should always be filled with water containing a
  washing up liquid.

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• Rapid water level fluctuation(??) points to a
  damaged mechanical seal or shaft seal ring.

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• 12. Lubricate the pump as described in the design
  and operating instructions, inspect the pump
  carefully during every grease change and replace
  any worn out parts.

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• 13. Before working on the pump and driver, switch
  the set off and ensure it cannot be accidentally
  switched on. Switch off and lock off the main
  switch.

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• 14. If work is interrupted, ensure that the set
  is still completely disconnected before-starting.
  Drain the pump and flush it out if necessary.

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• Use the general arrangement drawing, which shows
  the part numbers, and the parts list to help you
  when working on the pump.

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• 15. If it is likely that handling and storage has
  caused the grease to become contaminated,
  solidify, drain or form condensed water, make
  sure the pump is repacked(????) before startup.

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• The pump must also be repacked every time the
  bearings are dismantled.

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• 16. Wash and clean the bearings, bearing housing
  and bearing cover-plate using petrol, benzol
  (???,?) or similar, then completely remove all
  traces of washing agent(???).

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• Pack the spaces between the balls/rollers with
  30 to 50 grease and the bearing coverplate with
  about 30 grease.

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• 17. Check the mechanical seal every day for
  overheating. A leaking seal should be removed,
  checked and replaced, if necessary.

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• B. BILGE AND BALLAST SYSTEM

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• The bilge system(??????) and the ballast
  system(?????) each have particular functions to
  perform, but are in many ways interconnected.

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Bilge system

• The bilge main is arranged to drain any
  watertight compartment(????) other than(??)
  ballast, oil or water tanks and to discharge the
  contents overboard.

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• The number of pumps and their capacity depend
  upon the size, type and service of the vessel.

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• All bilge suctions must be fitted with suitable
  strainers, which in the machinery space would be
  mud boxes(??) positioned at floorplate(??,??,????)
  level for easy access(??).

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• The emergency bilge suction or bilge injection
  valve is used to prevent flooding of the ship.

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• It is a direct suction from the machinery space
  bilge which is connected to the largest capacity
  pump or pumps.

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• An emergency bilge pump is required for passenger
  ship but may also be fitted as an extra
  (stand-by) on cargo ships.

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• It must be an independent unit capable of
  operating even if submerged(??).

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• A centrifugal pump with a priming device is
  usually used, driven by an electric motor housed
  in an air bell(???).

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• The power supply is arranged from the emergency
  generator.

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• The various pumps and lines are interconnected to
  some extent so that each pump can act as an
  alternative or standby for another.

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Ballast systems

• The ballast system is arranged to ensure that
  water can be drawn from any tank or the sea and
  discharged to any other tank or the sea as
  required to trim(??) the vessel.

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• Combined or separate mains for suction and
  discharge may be provided.

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• Where a tank or cargo space can be used for
  ballast or dry cargo then either a ballast or
  bilge connection will be required.

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• The system must therefore be arranged so that
  only the appropriate pipeline is in service

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• the other must be securely blanked or closed off.

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• Where tanks are arranged for either oil or
  ballast a change-over chest(????) must be fitted
  in the pipeline so that only the ballast main or
  the oil transfer main is connected to the tank.

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C. Domestic water systems

• Domestic water systems(??????) usually comprise a
  fresh water system for washing and drinking and a
  salt water system for sanitary(???????) purposes.

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• Both use a basically similar arrangement of an
  automatic pump supplying the liquid to a tank
  which is pressurized by compressed air.

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• The compressed air provides the head(??) or
  pressure to supply the water where required.

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• The pump is started automatically by a pressure
  switch which operates as the water level falls to
  a predetermined level.

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• The fresh water system has, in addition, a heater
  which is heated usually with steam.

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• Fresh water supplied for drinking purpose must
  meet purity standards.

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• Water produced from most evaporator/distillers(???
  ) will not meet these standards and must be
  treated to ensure it is biologically pure and
  neutral(???) or slightly alkaline(???).