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.
Packed gland seal
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
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.
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.
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
This class of pump differs from the centrifugal class by several important factors
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.
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.
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)
This air supply valve assembly normally takes the form of a shuttle valve.
These tend to fit somewhere between positive displacement and centrifugal. They tend
to be of the very large capacity type and are often 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 considerable end trust occurs and a tilting pad thrust bearing is employed. Guide vanes smooth flow into and out of the impeller.
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 vaporization 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
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 oxides thereby increasing rates of corrosion