Figure 4.6 Typical deepwell pump
consists of a double mechanical seal flushed with lubricating oil. This stops cargo leakages to atmosphere. The accurate alignment of the motor coupling, thrust bearing and mechanical oil seal is important.
Furthermore, the length of the drive shaft can be a problem and the longer it becomes the more support is needed. Accordingly, it is often found that the largest types of ships are fitted with submerged pumps.
Submerged motor pumps
Submerged motor pumps are installed at the bottom of cargo tanks and enable very low pump-down levels to be achieved. They are fitted on all LNG carriers and on some of the larger LPG carriers.
The pump and electric motor are integrally mounted on the same shaft so eliminating the need for a mechanical seal or coupling. Power is supplied to the motor through specially sheathed cables. Electrical cabling is passed through a hazardous area junction box in the tank dome and then, by flexible cables to the motor terminals. The older mineral insulated copper sheathed cable used inside cargo tanks has been superseded in modern ships by flexible stainless steel armoured insulated power cables.
These pumps are cooled and lubricated by cargo flow and are, therefore, prone to damage due to loss of flow. Accordingly, the pump is protected from dry running by safety devices such as an under-current relay, a low discharge pressure switch, or a low tank level switch. Figure 4.7(a) shows a typical submerged pump/motor assembly for an LPG carrier and Figure 4.7(b) shows a similar pump but in this case designed for LNG.
Figure 4.7(a) Submerged motor pump for LPG
Submerged pumps need to be designed for the particular grades of cargo found on the ship's Certificate of Fitness. For example contrary to the hydrocarbon gases, ammonia is an electric conductor and can also be a particularly corrosive cargo for
Figure 4.7(b) Typcial LNG submerged motor pump assembly
some materials such as copper wires and electrical insulation. Pump design must take this into account. To preserve the electric motor, pumps used for ammonia have the electric stator enclosed in a 'can'.
Booster pumps
Booster pumps are usually of the centrifugal type. They may be vertically or horizontally mounted on deck in the appropriate discharge line. In these positions, they will be driven by an increased safety (E Exe) (see 4.8) electric motor. Alternatively, they may be in the cargo compressor room. When fitted in the compressor room, they are driven through a gas-tight bulkhead by an electric motor installed in the electric motor room. Figures 4.8 and 4.9 show examples of these types of pump. The particular pumps shown are fitted with double mechanical seals. The seal flushing system should be well maintained to ensure continuing reliability.
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