#4 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
Reciprocating: 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
Reciprocating: Discharge stroke
The discharge valve in the mean time stays shut due to the positioning of the valve
Fluid cannot be compressed.
When the piston goes up, the cylinder fluid pressure increases.
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
Single acting pump
single suction and discharge stroke in one cycle
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
Differences: Positive Displacement Pump
Recirpocating pump
Gear Pump
Self-Priming
High Discharge Pressure
Low Discharge Volume flow
Can be used as metering pumps as strokes and volume easily determined
Differences: Dynamic Pressure
Centrifugal pump
Non-Self Priming
High Discharge Pressure
Low Discharge Volume flow
Can be used as metering pumps as strokes and volume easily determined
Priming
pump to be initially filled with fluid before they can work
must be airless for the pump to work
Self priming: once primed, it will continue pumping even if air enters the system
Non-self priming: cannot sustain priming once
