Question

(a) Explain why centrifugal pumps cannot handle air or vapors 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 1

Part (a)

Centrifugal pumps rely on centrifugal force to displace fluid from the center to the periphery. For effective operation, they need a continuous flow of liquid at the input center (eye of the impeller). In a centrifugal pump, the centrifugal force generated is insufficient to move air or vapor because the mass of air (density ~1.2 kg/m³) is much lower than that of water (density ~1000 kg/m³). The equation governing centrifugal pump lift in terms of water mass is:

Mw * r * ω^2 > ρw * g * h

where Mw is the mass of water, r is the impeller radius, ω is angular velocity, and ρw is water density. For air, due to its lower density, the centrifugal force Ma * r * ω^2 is too low to overcome the gravitational force needed for lift, resulting in no suction lift.

On the other hand, turbo-blowers, also centrifugal machines, can handle air because the required centrifugal force is feasible for the lower density of air. In turbo-blowers, the centrifugal force:

Ma * r * ω^2 > ρa * g * h

is sufficient for moving air, allowing effective operation with gases.

Part (b)

Ensuring Fire Pump Priming Arrangements Work in Ballast Condition:

Confirm that the NPSH does not exceed 4.5 meters as per regulations to avoid cavitation and ensure effective suction.
Place fire pumps as low as possible in the ship to ensure consistent water availability.
The pump should have a reliable priming device, such as a water ring primer or air eductor type, to remove air from the suction line.
Maintain a sufficient water level in the priming tank to enable efficient priming.
Ensure proper functioning of the spring-loaded valve to maintain suction.
Strictly follow the Planned Maintenance System (PMS) for both the fire pump and the priming device to ensure they are in optimal working condition.

Part (c)

One priming method for a centrifugal pump Priming using Eductor.

There are 2 solenoid valve which opens during the start of the pump (Check the diagram below). 7 bar air keeps flowing through the air eductor. Eductor creates vacuum and takes out air from our pump casing and water floods into the pump casing. When the pressure transmitter senses the discharge pressure above a certain level, the solenoid valve closes.

✓ Verified Model Answer (Text Solution)

Ensuring Fire Pump Priming Arrangements Work in Ballast Condition:

One priming method for a centrifugal pump Priming using Eductor.