Transcript Document 7206039

• Lesson 15
• Text MARINE PUMPS
• The pumps employed on board ship can
be divided into two main categories:
• (i) positive displacement pumps(容积式泵)
• and (ii) centrifugal pumps(离心泵)。
• They can be broken down into three
classes:
• (1) reciprocating(往复式),
• (2) rotary(回转式)
• and (3) centrifugal.
• Under reciprocating are the direct
acting(直动式), power, and crank-flywheel types.
• Under rotary are the gear, vane(叶片式),
cam-and-piston and screw types(螺杆式).
• Under centrifugal are the volute(涡壳式),
diffuser(扩散式,导轮式), mixed-flow,
axial-flow and turbine or regenerative
types(旋涡式).
• Displacement pumps are those where the
volume of the pump chamber is
alternately(交替地) increased to draw the
liquid in from the suction pipe and then
decreased to force the liquid out into the
delivery pipe.
• Centrifugal pumps are those wherein an
impeller(叶轮) rotating at high speed throws(甩、
扔) the liquid by centrifugal force from the
center to the periphery of the impeller where the
liquid is discharged through the delivery outlet.
• Gear pumps consist of two or more
meshing(啮合) gears [spur(直齿轮), single or
double helical teeth (螺旋齿)] enclosed in a
close-fitted housing.
• These are used extensively for pumping
fuel oil, lubricating oil and hydraulic oil.
• 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(空间),
• the oil is then transferred around the
periphery of the pump housing.
• 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.
• All vane pumps have a rotor driven within
a ring (camring叶片泵定子环) by a drive
shaft coupled to a prime mover.
• A cylindrical rotor with sliding vanes,
generally in radial slots(槽), rotates within
the ring.
• In a simple vane pump(单作用叶片泵), the
centre of the rotor is offset(偏移) from the
centre of the ring, leaving a crescent(新月形的)
shaped chamber between rotor and the ring.
• As the rotor turns, the vanes are forced
outward against the inner surface of the
cam ring by centrifugal force.
• 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.
• 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.
• 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.
• A vane pump of balanced design has an
elliptical(椭圆的) cam ring so that two
pumping chambers are formed.
• This pump is in hydraulically balance,
since the two intake ports and two outlet
ports are diametrically opposed(完全相
对的) to each other.
• Therefore the side loads exerted on the
rotor cancel out(抵消), thereby increasing
bearing life and permitting high
operating pressure.
• According to their piston arrangements,
rotary-piston pumps are classified either
axial or radial type.
• 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.
• Rotation of the drive shaft causes the
pistons and the cylinder block to rotate at
the same speed.
• 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(倾斜盘)
according to the designs.
• As each piston moves outward, oil is
drawn into the cylinder bore through the
valve plate(配油盘).
• On the return stroke, the oil is forced out
through the valve plate under pressure.
• Reciprocating pumps and rotary pumps
are positive-displacement type because
they discharge a near constant amount of
liquid regardless of pressure.
• If the discharge valve is closed, they build
up a bursting pressure or stall(使…停转)
their motor(电机堵转).
• Centrifugal pumps are nonpositive and a
fairly low pressure unit. Here a close
discharge valve will not cause excessive
pressure to build up.
• The simple centrifugal pump consists of a
rotating impeller within a stationary(固
定的) casing.
• The fully shrouded(封闭), single entry
impeller is the type most widely used.
• The impeller is mounted on a shaft driven
by a prime mover.
• It consists of a number of vanes backward
from the direction of the rotation between
the shroudings.
• Water enters the rotating impeller
through the entry, known as ‘eye(吸入
口)', at the centre.
• Due to the circular motion given it is thrown by
centrifugal force to the open periphery of the
impeller where it leaves tangentially(沿着切线
方向) and enters the space in the casing, and
then directed to the outlet branch.
• Unlike the reciprocating displacement pump,
the centrifugal pump does not have a positive
suction action(自吸能力) and must be primed
by flooding before it will draw water from a
lower level.
• An air pump (真空泵) or a central
priming system may be used for this
purpose.
• 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.
• Centrifugal pumps, however, can be
designed with a number of impellers in
series(串联) to attain a high final delivery
pressure.
• They are popular because they are very
simple in construction, and can produce
smooth, constant, nonpulsating( 无脉动)
discharge.
READING MATERIAL
• A. INSTALLATION AND
MAINTENANCE OF CENTRIFUGAL
PUMPS
Installation (installation on site)
•
•
1. Foundation
The foundation of marine pumps is a
steel base frame (plane parallel
alignment(水平平面的校正) is essential).
• Ensure the foundation has set hard
before mounting the pump set. The
surface must be smooth and flat.
• Position the pump on the steel base frame
or foundation, then align(找平) and bolt
in position.
2. Alignment of Pump and
Driver
• After fitting and bolting on the driver,
centre (校正…的中心) it in the flange.
3. Connecting the Piping
• Never use the pump as an anchorage
point(固定点) for the piping.
• The pipes must be supported very near
the pump and connected to the pump
without transmitting any stresses or
strains(张力) to it.
• Make sure the pump does not bear the
weight of the piping. The nominal(名义上
的) bore of the pipes should be the same
as or greater than that of the pump
nozzles(管口).
• Thermal expansion of the piping must be
compensated for by suitable means, so as
not to impose any additional load on the
pump.
• 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(均匀) of flow to the impeller.
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(据此).
5. Coupling Guard(防护装置)
• Safety regulations specify that the pump
can only be operated if the coupling has a
guard. The coupling guard must be fitted
before startup.
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.
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.
• 8. The pump must never run dry. Avoid
prolonged operation against a closed
shut-off valve(截止阀).
• 9. The temperature of the ball and roller
bearings may rise to 50oC above ambient
temperature but must not exceed 80oC.
• The shut-off valves in the various supply
lines must not be closed during operation.
• 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.
• 11. A sealant reservoir for the mechanical
seal should always be filled with water
containing a washing up liquid.
• Rapid water level fluctuation(波动)
points to a damaged mechanical seal or
shaft seal ring.
• 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.
• 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.
• 14. If work is interrupted, ensure that the
set is still completely disconnected beforestarting. Drain the pump and flush it out
if necessary.
• Use the general arrangement drawing,
which shows the part numbers, and the
parts list to help you when working on
the pump.
• 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(重新填充) before
startup.
• The pump must also be repacked every
time the bearings are dismantled.
• 16. Wash and clean the bearings, bearing
housing and bearing cover-plate using petrol,
benzol (安息油,苯) or similar, then completely
remove all traces of washing agent(清洗剂).
• Pack the spaces between the balls/rollers
with 30% to 50% grease and the bearing
coverplate with about 30% grease.
• 17. Check the mechanical seal every day
for overheating. A leaking seal should be
removed, checked and replaced, if
necessary.
• B. BILGE AND BALLAST SYSTEM
• The bilge system(舱底污水系统) and the
ballast system(压舱水系统) each have
particular functions to perform, but are
in many ways interconnected.
Bilge system
• The bilge main is arranged to drain any
watertight compartment(水密舱室) other
than(除了) ballast, oil or water tanks and
to discharge the contents overboard.
• The number of pumps and their capacity
depend upon the size, type and service of
the vessel.
• All bilge suctions must be fitted with
suitable strainers, which in the
machinery space would be mud boxes(泥
箱) positioned at floorplate(地板,楼板,防
滑铁板) level for easy access(接近).
• The emergency bilge suction or bilge
injection valve is used to prevent flooding
of the ship.
• It is a direct suction from the machinery
space bilge which is connected to the
largest capacity pump or pumps.
• An emergency bilge pump is required for
passenger ship but may also be fitted as
an extra (stand-by) on cargo ships.
• It must be an independent unit capable of
operating even if submerged(淹没).
• A centrifugal pump with a priming device
is usually used, driven by an electric
motor housed in an air bell(空气罩).
• The power supply is arranged from the
emergency generator.
• The various pumps and lines are
interconnected to some extent so that
each pump can act as an alternative or
standby for another.
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(平衡)
the vessel.
• Combined or separate mains for suction
and discharge may be provided.
• Where a tank or cargo space can be used
for ballast or dry cargo then either a
ballast or bilge connection will be
required.
• The system must therefore be arranged
so that only the appropriate pipeline is in
service;
• the other must be securely blanked or
closed off.
• Where tanks are arranged for either oil or ballast
a change-over chest(转换阀箱) must be fitted in
the pipeline so that only the ballast main or the
oil transfer main is connected to the tank.
C. Domestic water systems
• Domestic water systems(生活用水系统)
usually comprise a fresh water system for
washing and drinking and a salt water
system for sanitary(卫生的、清洁的)
purposes.
• Both use a basically similar arrangement
of an automatic pump supplying the
liquid to a tank which is pressurized by
compressed air.
• The compressed air provides the head(压
头) or pressure to supply the water where
required.
• The pump is started automatically by a
pressure switch which operates as the
water level falls to a predetermined level.
• The fresh water system has, in addition, a
heater which is heated usually with steam.
• Fresh water supplied for drinking purpose
must meet purity standards.
• Water produced from most
evaporator/distillers(蒸馏器) will not meet these
standards and must be treated to ensure it is
biologically pure and neutral(中性的) or
slightly alkaline(碱性的).