Introduction

Most high-volume cargo pumps on modern tankers are centrifugal pumps in the pump room. These are excellent for moving large volumes of cargo but suffer from transporting the fluid from the tank to the pump room and then out of the manifold. They also require a separate stripping system to ensure no damage to these large pumps when insufficient liquid remains in the tank to maintain pump pressure.

An alternative is the FRAMO pump. This submerged pump inside each tank pumps the cargo directly from the tank to the manifold. It has inbuilt stripping capabilities, so it can efficiently remove all the cargo using only one system, saving time and allowing for multiple tanks and grades of cargo to be discharged simultaneously.

What is a Centrifugal Pump?

Centrifugal pumps operate using the principle that liquids are incompressible and can be subjected to centripetal acceleration. When subjected to a rotation force, the molecules will naturally move to the edge of the rotating body, causing low pressure in the centre and high pressure on the outside.

This can be exploited by pumping liquids from one place to another using a rotating impeller in a centrifugal pump. The low pressure created at the impeller's eye will draw liquid from the tank, and the high pressure on the edge of the impeller will move it to the manifold and off the ship. 

The below video shows a centrifugal pump. 

Drawbacks of Conventional Centrifugal Pumps

Centrifugal pumps suffer from an inability to operate at low suction pressures. This is due to them pumping liquids and the effects of deep low pressures on the material state of those liquids.

As the pressure drops in a liquid system, the boiling point of the liquid is reduced in proportion to this. Therefore, the liquid becomes gaseous and forms bubbles. This has two effects on oil cargo. The first is an increased fire danger due to more vapour, and the cargo may polymerise and become worthless. The second is that the bubbles created can cause cavitation damage to the pump and surrounding fittings.

This means that traditional pump room-based centrifugal pumps must be carefully monitored and properly primed before use to ensure that they operate effectively. The ship will also require a separate stripping system; usually, an eductor, to then be able to strip the tanks fully.

FRAMO Pumps

The ship may have submerged FRAMO pumps in each as an alternative to pumping room centrifugal pumps. These pumps have significant advantages over traditional pumps and a designated built-in stripping system to make discharging more efficient. By being a submerged pump, the system benefits from not needing to be primed and maintaining pressure for longer due to its position within the tank.