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 Types of pumps

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مُساهمةموضوع: Types of pumps   06/02/11, 08:37 am

Types of pumps

Centrifugal


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


Shown above is a cross section through a vertically mounted centrifugal pump.
Water is
led to the suction eye of the rotating impeller. The water gains energy
by the centrifugal action of the pump and is discharged to the volute
outlet casing. The volute is created by increasing the area of the
outlet port and is greatest at outlet from the pump. By this design the
kinetic energy of the water is converted to pressure energy.

Sealing
is provided by a mechanical seal (one half of which is shown above and
in more detail below) or by packed gland. For the former cooling water
is supplied from the discharge side of the pump. For the latter cooling
is provided by the allowance of slight leakage, lubrication is by a
grease filled manual lubricator.

Mechanical seal
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]



<A name=gland>Packed gland seal
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


The
pump unit shown above relies on the driving motor bearings for
alignement. For larger pumps a leaded bronze or brass bush may be
fitted positioned just below the seal.For the largest pumps, especially
those fitted with an inducer the shaft may be extended below the
impeller fixing and a second bearing fitted

Impellers
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


The
kinetic energy of the fluid flowing through the impeller is converted
to pressure energy by the shape of the volute casing. For high pressure
pumps such as boiler feed pumps a diffuser ring is fitted in the casing
which converts a greater portion of the pressure energy allowing
greater pressures to be generated.

A scroll
type inducer may be fitted to the inlet which improves the efficiency
of unit and allows the pump to operate with low suction pressures.


<A name=wear>Wear rings
For
efficient operation it is important to ensure that leakage from the
high to low pressure side is kept to a minimum. This is achieved by the
use of wearing rings. Traditionally these are fitted to the casing,to
increase the longevity of the impeller wear ring tyres may be fitted.


The
clearance given for wear rings is often a source of contention
especially when dealing with on-ship made rings. A clearance of 1/1000
of the diameter of the bore is often quoted although this may be very
difficult to achieve in practice.



Axial force
Without
careful design an axial force is created by the action of the impeller.
This is due to the low pressure acting on the suction eye whilst the
rest of the impeller is subjected to discharge pressure.

One
solution is shown above where radial blades are cast into the back
(stuffing box side) of the impeller. These blades are commonly called
pump-out vanes, and are meant to increase the centrifugal force of the
fluid trapped behind the impeller. This causes the fluid to be "thrown"
outwards, reducing the pressure behind the impeller for the same reason
that the impeller causes a reduction of pressure at the suction eye.

Another
method which may be found in conjunction with the pump-out vanes are
the balancing holes. These are holes drilled near the center of the
impeller, connecting the space in the back of the impeller with the
suction eye. This reliefs the pressure behind the impeller by allowing
the high pressure fluid trapped there to flow to the low pressure
region at the suction eye. In order for this to be effective, there
must be a tight clearance between the impeller and the casing to reduce
the flow of fluid into the back of the impeller.

Alternately dual back to back impellers may be fitted in common with a double casing

Materials suitable for general service Shaft Stainless steel Impeller Aluminium bronze Casing Bronze or cast iron Wear ring Aluminium bronze or brass

<A name=gear>Positive Displacement

  • This class of pump differs from the centrifugal class by several important factors

    • Generally self priming whereas centrifugal generally require a priming means such as a belt driven priming pump
    • Require the fitting of a safety valve to limit maximum pressure- this pump cannot be started against a shut discharge valve.
      Centrifugal
      pumps may be started against a shut or partially shut discharge valve.
      This is especially true for larger pumps where the shutting of the
      discharge reduces starting and running load.

      It should be noted that the partially shutting of the suction valve on both types of pumps leads to damaging cavitation.
    • Positive displacement pumps can handle high differential pressures
    • More suited to low to medium flow rates
    • May operate with higher viscosity fluids then centrifugal types
    </li>
Gear


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]



The
pump shown above is of very common design. It is used for pumping many
types of liquid and gas and is capable of delivering at very high
pressures. This makes it suitable for hydraulic supply.

The tooth
profile is similar involute gear teeth for liquid pumps. For gas pumps
special profiling with very fine tolerances is employed. Scroll



[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
These
pumps are seen in many applications and have a higher capacity then
double row type. Fluid enters the pump and is screwed by the idler
shafts along the outer edge to the discharge port. Axial thrust of the
idlers is absorbed by the integral thrust collar of the driven shaft.
The axial thrust of the driven shaft is absorbed by the thrust bearing.

The scroll sit in a replaceable insert which is sealed to the outer casing by o-rings.


Piston


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


This
type of pump is in common use as a bilge pump or tank stripping pump.
For older vessels steam driven varieties served in almost all systems.

The
design is simple, robust and reliable. Materials are very much
dependent on the usage but bronze is common for larger parts and
stainless steel for piston rods
There
are many other forms of positive displacement pump such as rotary vane
(often found in use as cooling water pumps, Scroll or Screw pumps were
the fluid passes axially along the shaft and Diaphragm Pumps (commonly
used as portable salvage pumps)

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]



This air supply valve assembly normally takes the form of a shuttle valve. Axial Flow


[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
These
tend to fit somewhere between postive displacement and centrifugal.
They tend to be of the very large capacity type and are oftern seen in
use for supply of cooling water for steam ship condensers. This is
particularly true where 'scoops' are employed as the axial flow pump
offers very little resistance to flow when idling.

During
operation cosiderable end trust occurs and a tilting pad thrust bearing
is employed. Guide vanes smooth flow into and out of the impeller.



Inducers


A
type of axial flow pump is sometimes attached to the suction side of a
centrifugal pump. This is called and inducer and is used where the
suction heads are very low or where suction occurs close to the
vaporiation pressure of the fluid being pump. Typical examples are the
main condenser extraction pumps on steam ships and cargo pumps on LNG
and LPG carriers


Cavitation


Disturbances
in the water flow causes rapid localised pressure variations. This can
lead to instantaneous vaporisation and bubble formation. When these
bubbles collapse there is a rapid in rush of water. When this occurs
near to a surface this slug of water can strike at speeds of up to
500m/s and lead to destructive erosion and removal of protective
oixides thereby increaseing rates of corrosion


copied

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Types of pumps
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