Answer EKG Question 41

Q41. A. Explain why centrifugal pumps cannot handle air or vapours to effect priming yet turbo- blowers operating on the same principle can.
B. If a vessel is fully laden, how may it be ascertained that the fire pump priming arrangements would operate satisfactorily in the ballast condition.

C. Explain a method of priming suitable for a centrifugal pump.

Answer:

A centrifugal pump cannot pump a gas; therefore, the differential pressure necessary for flow will not be created if the impeller is having air or vapour. Prior to start-up, the pump's Casing should be filled with liquid and vented of all gases.
In applications where the pump must operate with a suction lift, a vacuum must initially be created to draw the liquid, into the eye of the impeller.
Priming a centrifugal pump :
a. The pump can be connected through vents to a central priming system. This type of a system is fitted with either vacuum pumps or  air ejectors, and is generally used to prime multiple pumps on the vessel.
b. The pump can be fitted with an integral or independent vacuum pump.
c. A special centrifugal pump that is designed to be self-priming can be used. This type of a pump is typically fitted with a suction chamber that retains liquid when the unit is not operating. 

As the velocity of the outer tips of the impeller of a centrifugal pump is relatively low, the suction efforts of the pump, when empty rarely exceeds 12mm water gauge and hence must be primed with water and cannot exhaust the contained air in the manner that a displacement pump can.
Self priming action of a attached vacuum pump:-

The vacuum pump has been designed so that it can be mounted at the side of centrifugal pump to make them self priming. It consists of a vacuum pump, clutch actuating device and separate tank. The vacuum pump operate only while priming the main pump and automatically stops as soon as it enters into normal operation.

When motor is started the spring load by the hydraulic pressure piston pushes through the lever the clutch facing against the coupling and through the frictional force the vacuum pump begins operation.
When the vacuum pump starts, the air from the main pump's casing top passes through the auto valve and after mixing with some water from the separate tank, is sucked by the vacuum pump and discharged into the separating tank, in the separate tank air is separated from water and discharged outside and thus priming is affected.
When priming has been complicated and the pressure of the main pump has raisin, the water pressure led from the hydraulic pressure piston, pushes the lever, disconnects the clutch and brings the vacuum pump to a stand still. Also auto valve closed automatically.
It is important to keep separate tank to be filled with water, pour machine oil through a small hole inside the friction clutch and see if it can slide axially when the lever is pulled. Check the vacuum pump can move freely when coupling turned by hand.