What types of cargo heaters can be found on board of gas carriers?
22 May '17, 06:24
Seawater is used to heat the cargo when it is required to discharge into either ambient temperature or semi-refrigerated shore storage tanks. Two types are described:
Type "A" Seawater passes through a large number of tubes inside the heater as the product being warmed zig-zags its way through.
Type "B" The product passes through a number of tubes in the heater, whilst the seawater zig-zags its way through the heater.
In both types, the heater discharge temperature can be lowered by mixing cold product with the warmed product. The cargo heaters should not be used if the seawater temperature is below 10° C. The seawater outlet temperature should not be allowed to fall below 5° C. If there is any risk of this happening, the rate of discharge should be slowed down.
Type "B" is considered to be more reliable than Type "A" for the following reasons:
If water passing through one or more of the tubes in Type "A" is impeded, there is risk of it freezing and the tube bursting. In Type "B", however, any risk of freezing would be confined to the water in immediate contact with the tubes containing the product. Ice might form on the outside of these tubes and become an insulating layer. The heater would lose efficiency due to the formation of ice, but it would not cause the tubes containing the product to fracture.
The ends of the water tubes in Type "A" are fixed and, not being free to move, any uneven expansion between the tubes and the shell could cause the tubes to be strained or even fractured at the tube plate. In Type "B", the tubes containing the product are free to expand and contract.
Associated with the cargo heater is the Booster Pump. It is used to increase the discharge pressure of the vessel's cargo pumps. It can be mounted on deck provided the motor driving it is adequately flame-proofed. Otherwise, it is located in the L.P.G. room, driven by a motor in the motor room. The pump should be provided with a by-pass so that the cargo heater can be used without it.
22 May '17, 06:34