#4 Marine pumps Flashcards
Positive Displacement
moves a fluid by repeatedly enclosing a fixed volume and moving it mechanically through the system
all outlet valve must be open in order to avoid pressure build up
failure to do so will result in weakest point of system to break down
broken pipe
damaging the pump, valve
Positive displacement pumps
Reciprocating, rotary, combination pump
Rotary pump
gear, screw, vane
Combination pump
heleshaw
Dynamic pressure pump
centrifugal, axial flow pump
suction stroke
when piston goes down, pressure within the cylinder is reduced
pressure outside the cylinder is now higher than internal cylinder pressure relatively causing suction valve to open and fluid to flow in
this is known as the suction stroke
Fluid will keep on filling the cylinder until the piston has reached the end of the stroke
Discharge stroke
The discharge valve in the mean time stays shut due to the positioning of the valve
2. Fluid will keep on filling the cylinder until the piston has reached
3.Fluid cannot be compressed. When the piston
goes up, the cylinder fluid pressure increases.
4. It shuts the suction valve and opens the
discharge valve, causing the fluid to flow out.
this is known as the discharge stroke
cycle
When a piston does a suction stroke and a
discharge stroke
GEAR PUMPS
- As the gears rotate they separate on the intake side of the pump, creating a void and suction which is filled by fluid.
- The fluid is carried by the gears to the discharge side of the pump,
where the meshing of the gears displaces the fluid. - The mechanical clearances between the gears and casing are few microns (micrometer).
- The tight clearances, along with the speed of rotation, effectively prevent the fluid from leaking backwards.
5.The rigid design of the gears and housing allow for very high pressures and the ability to pump highly viscous fluids
Dynamic Pressure
not positive displacement pumps.
cooling water system and ballast system.
Large volume of fluid flow is involved with great speed or velocity. There will be slight pressure increase in the pipe when the discharge valves are shut but generally no damage is done to the piping system.
Working of a Centrifugal Pump
1.The impeller is rotated at high speed by electric motor and it generates centrifugal force, which acts radially outwards from the centre of the impeller.
The water inlet pipe is connected to the “eye” of the impeller or the centre.
Thus the fluid in the pipe line is pulled or sucked into the impeller and thrown out by centrifugal force.
As the fluid comes out of the impeller vanes it strikes the curved volute casing and changes into pressure energy once again at the pump Discharge.
The increasing area of cross- section of the volute casing helps to accommodate the increasing volume of liquid and give it path or momentum to flow
Centrifugal pumps - no pressure pulsations
there is a continuous stream of fluid
coming in and going out. Therefore there is a steady flow and no pulsations like a
reciprocating pump. Thus the volume handled by such a pump is large as compared to
positive displacement pumps. This is good for circulating sea water through the engine room
and other similar piping systems.
Centrifugal Pumps
rotodynamic pump that uses a rotating impeller to increase the
velocity of a fluid
Centrifugal pumps are commonly used to move liquids through a piping
system such as sea water piping, fresh water piping and fresh water hydrophore system
The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber, from where it exits into the downstream piping system. Centrifugal pumps are used for large discharge through smaller heads
Suction and Discharge Head
It is then said to have a “positive head”
Priming
All pumps must be initially filled (or flooded) with fluid before they can work. (i.e. any air present in the casing should be purged). This process of filling the pump and clearing the air is known as “priming” and this can be achieved by pouring fluid into a small hole, provided for this purpose, in the pump casing. Throughout its operation the pump must be maintained in this flooded state
If for some reason air is sucked into the pump then the process of transferring the fluid will be hampered. Because the pump will not be able to push the air along with the fluid. Hence this will be a state where the pump has lost suction and it must be primed again in order to resume the pumping. Air can get into the system through leaking glands, pipe cracks, etc.
