Positive displ pp is suitable for oil duty due to its self priming quality but has speed restriction due to high power requirement and also excessive pr head. So, it requires a bypass valve to control the flow, if a constant AC motor is given.
A centrifugal pump with an extended shaft open end impeller serves pupose of self priming.
When l.o temp increases, its viscosity decreases... hence the pump will be able to discharge more but with a pressure drop. This can be seen in the graph and it shows that centrifugal pr drop is less compared to that of postive displ pp. It also shows that there is huge increase in quantity using a centrifugal pump while positive displ pp shows a marginal increase.
POSITIVE DISPLACEMENT PUMP
Works on the principle of constant quantity. They can create their own suction lift and able to pump air or vapour, without loss of suction head. Efficiency highly depends on the close clearances btw parts.
CAPACITY
For zero head, it gives max volumetric efficiency. But as head increases, the quantity(capacity) decreases which is due to the slip btw clearances pf gear or screws and the casing slip.
Capacity is directly proportional to speed and slip almost remains constant
POWER
Power requirement depends on frictional resistance to overcome by the parts and the pumping liquid
Power input increases with increase in speed and head.
HEAD
Theoretically head is infinite but practically, limited by the torque availability and strength of the pump parts.
Why centrifugal pump for lube oil pump?
From the curve, we can see that with increase in temp, the positive displacement pump pressure drop is high and the quantity increase is just marginal. The main engine lube oil prime purpose is to cool the engine, for which if there is an increase in temperature, the centrifugal pump pressure drop will be very less and the quantity increases very much. For this purpose, centrifugal pump is used so that we can increase in temperature the quantity also has to be increased.
AXIAL PUMP
Axial pump requires a relief valve because While starting, if the discharge valve is throttled, the head increases 3times and power 2times which trips the motor. Actually water hammering and vibr occurs and relief valve lifts.
Beyond its normal duty, power req decreases with capacity increases marginally and head falls sharply.
While in a centrifugal pump at beyond its normal duty, there is a sharp increase in power with fall in H&Q.
Application
Axial pump is ideally suited where high throughput and low head is reqd. It draws less power when run at high speed, compared to the centrifugal pump.
DISADVANTAGES
It creates a vortex at suction and hence requires a vortex destroyer in the suction.
Material
Casing & Guide vanes - Gun metal
Shaft stainless steel
Impeller Aluminium Bronze - Cu76%,Tin22%Al-2%
MIX FLOW PUMP
Application - LNG,LPG and chemical carrier. efficiency of the pump is very high as it takes less power compared to centrifugal pump or displacement pump.
WHY INDUCER?
For a given discharge, the NPSH reqd is reduced by fitting a inducer at the mouth of the shaft. It enhances the suction characteristics by collapsing any vapor/cavitation, formed in the suction, in the region btw inducer and the eye of the impeller. It reduces the NPSH reqd as inducer can handle air/vapor to a certain extent.
WHAT IS RIGID SHAFT AND FLEXIBLE SHAFT
RIGID shaft has its operating speed less than critical speed.
FLEXIBLE shaft has its operating speed more than the critical speed so that it can absorb the vibr arising out of crossing critical speed.
A centrifugal pump with an extended shaft open end impeller serves pupose of self priming.
When l.o temp increases, its viscosity decreases... hence the pump will be able to discharge more but with a pressure drop. This can be seen in the graph and it shows that centrifugal pr drop is less compared to that of postive displ pp. It also shows that there is huge increase in quantity using a centrifugal pump while positive displ pp shows a marginal increase.
POSITIVE DISPLACEMENT PUMP
Works on the principle of constant quantity. They can create their own suction lift and able to pump air or vapour, without loss of suction head. Efficiency highly depends on the close clearances btw parts.
CAPACITY
For zero head, it gives max volumetric efficiency. But as head increases, the quantity(capacity) decreases which is due to the slip btw clearances pf gear or screws and the casing slip.
Capacity is directly proportional to speed and slip almost remains constant
POWER
Power requirement depends on frictional resistance to overcome by the parts and the pumping liquid
Power input increases with increase in speed and head.
HEAD
Theoretically head is infinite but practically, limited by the torque availability and strength of the pump parts.
Why centrifugal pump for lube oil pump?
AXIAL PUMP
Axial pump requires a relief valve because While starting, if the discharge valve is throttled, the head increases 3times and power 2times which trips the motor. Actually water hammering and vibr occurs and relief valve lifts.
Beyond its normal duty, power req decreases with capacity increases marginally and head falls sharply.
While in a centrifugal pump at beyond its normal duty, there is a sharp increase in power with fall in H&Q.
Application
Axial pump is ideally suited where high throughput and low head is reqd. It draws less power when run at high speed, compared to the centrifugal pump.
DISADVANTAGES
It creates a vortex at suction and hence requires a vortex destroyer in the suction.
Material
Casing & Guide vanes - Gun metal
Shaft stainless steel
Impeller Aluminium Bronze - Cu76%,Tin22%Al-2%
MIX FLOW PUMP
Application - LNG,LPG and chemical carrier. efficiency of the pump is very high as it takes less power compared to centrifugal pump or displacement pump.
WHY INDUCER?
For a given discharge, the NPSH reqd is reduced by fitting a inducer at the mouth of the shaft. It enhances the suction characteristics by collapsing any vapor/cavitation, formed in the suction, in the region btw inducer and the eye of the impeller. It reduces the NPSH reqd as inducer can handle air/vapor to a certain extent.
WHAT IS RIGID SHAFT AND FLEXIBLE SHAFT
RIGID shaft has its operating speed less than critical speed.
FLEXIBLE shaft has its operating speed more than the critical speed so that it can absorb the vibr arising out of crossing critical speed.
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