WELL ANY GUESS, IT SHOULD BE A POSITIVE DISPLACEMENT PUMP RIGGGHHHHHHHHTTTTTTT. OBVIOUSLY IT DOES NOT NEED PRIMING SO BEST OPTION FOR A MAIN ENGINE L.O PUMP. WILL START AND DIRECTLY L.O PRESSURE WILL BE DEVELOP.
BUT HAVE YOU, REALLY SAW ANY POSITIVE DISPLACEMENT PUMP AS A MAIN ENGINE L.O PUMP. I GUESS NO, AND YOU NEVER WILL BE. NOW THE QUESTION IS WHY IS THAT AND IF NOT THEN WHICH TYPE OF PUMP IS USED FOR SUCH AN IMPORTANT WORK.
THE ANSWER IS A CENTRIFUGAL PUMP, BUT HEY HOW COME CENTRIFUGAL PUMP, THEY NEED PRIMING AND WHAT IF WE START IT AND IT DOES NOT DEVELOP PRESSURE. WELL WE USE A SUBMERSIBLE TYPE OF CENTRIFUGAL PUMP SO THAT IT REMAIN PRIMED EVERYTIME.
BUT WHY WE USE A CENTRIFUGL WHEN WE HAVE POSITIVE DISPLACEMENT PUMPS. FOR THAT LETS SEE THE BELOW DIAGRAM.
WE HAVE THE H-Q CURVE FOR CENTRIFUGAL AND POSITIVE DISPLACEMENT PUMP. ALSO WE HAVE FRICTION CURVE FOR OIL IN BOTH HOT AND COLD CONDITION. NOW WHEN THE MAIN ENGINE IS NOT RUNNING, THE TEMPERATURE OF OIL IS LOW BUT AS THE ENGINE RUNS, THE TEMPERATURE OF OIL INCREASES LEADING TO FALL IN FRICTION CURVE. THIS CURVE NOW INTERSECT THE H-Q CURVE OF BOTH THE PUMPS IN DIFFERENT POINTS.
NOW SEE CLOSELY, YOU WILL SEE THERE IS A FALL IN HEAD OF BOTH THE PUMPS BUT THE FALL IN POSITIVE DISPLACEMENT PUMP IS FAR MORE THAN THAT OF CENTRIFUGAL PUMP. BUT THE QUANTITY DISCHARGE OF OIL BY CENTRIFUGAL PUMP IS INCREASED APPRECIABLY WHILE THAT OF POSITIVE DISPLACEMENT FALL. HENCE CENTRIFUGAL PUMP PROVIDE MORE QUANTITY OF OIL THAN POSITIVE DISPLACEMENT PUMP WITH INCREASE IN TEMPERATURE WHICH IS MORE BENEFICIAL THAN POSITIVE DISPLACEMENT PUMP.
HENCE THE CENTRIFUGAL PUMP IS BETTER THAN POSITIVE DISPLACEMENT PUMP FOR MAIN ENGINE L.O PUMP.