Question

With reference to the carriage and pumping of liquefied gas cargo:

(a) Sketch a suitable pumping system labelling the component parts. (5)

(b) State

(i) Why submerged hydraulically driven pumps are not used. (3)

(ii) How overheating of pump drive shaft bearings is avoided (3)

(c) State how the risk of fire and explosion in cargo tanks is obviated both in the loaded and discharged condition. (5)

Answer 1

Part (a)

A pumping system for liquefied gas cargo consists of a pump located at the bottom of each cargo tank. An electric motor, situated outside the deck hatch, drives the pump via a long shaft. The shaft housing also serves as a cargo riser, providing cooling and lubrication to the shaft guide bearings. An inducer improves the pump's suction characteristics. Guide vanes and diffuser vanes direct the flow and convert kinetic energy to pressure energy.

Part (b)

(i) Submerged hydraulically driven pumps are unsuitable because the hydraulic oil may freeze at low cargo temperatures, suitable hydraulic fluids for these low temperatures are difficult to find, and leaks pose a risk of cargo contamination.

(ii) Overheating of the pump drive shaft bearings is prevented by using the shaft housing (which also acts as a cargo riser) to cool and lubricate the bearings.

Part (c)

Fire and explosion risks in cargo tanks are mitigated by:

Continuous boil-off gas reliquefaction
Relief valves to release excess pressure
Inert gas blanketing of the cargo hold; and
If cargo hold is considered as secondary barrier, then if primary barrier/ cargo tank leaks then the flammable gas should not get oxygen to from an explosive mixture. So cargo hold is inverted.

✓ Verified Model Answer (Text Solution)

(i) Submerged hydraulically driven pumps are unsuitable because the hydraulic oil may freeze at low cargo temperatures, suitable hydraulic fluids for these low temperatures are difficult to find, and leaks pose a risk of cargo contamination.

Fire and explosion risks in cargo tanks are mitigated by: