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Table 2.2 Chemical properties of liquefied gases

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    Methane   Ethane   Propane   Butane   Ethylene   Propylene   Butylene   Butadiene   Isoprene   Ammonia   Vinyl chloride   Ethylene oxide   Propylene oxide   Chlorine  
Flammable   x   x   x   X   X   x   X   X   x   X   x   x   X      
Toxic                               X       x   x   x   X   x  
Polymerisable                               x   x       x   x          

 

REACTIVE WITH

Magnesium                               X   X           X   X      
Mercury                               X   X   X       X   X   X  
Zinc                                       X               x  
Copper                               X   X   X       X   X      
Aluminium                               X   X   X   X   X   X   x  
Mild carbon steel   X3               X1                                      
Stainless steel                                               X2          
Iron                                               X   X      
PTFE*                                       X                  
PVC+                                       X                  
Polyethylene   X3   X   X   X           X                              
Ethanol                                                       x  
Methanol                                                       x  

 

Notes: Study can be made to the data sheets in the Reference 2.1 or to the IGC Code for further details on chemical reactivity.

1 Stainless steel containing 9 per cent nickel is the usual containment material for ethylene.

2 Refer to IGC Code - Section 17.16.3

3 Not suitable with liquid methane due to brittle fracture.

*PTFE:- polytetrafluoroethylene (jointing material) +PVC:- polyvinyl chloride (electric cable insulation)


Table 2.3(a) Chemical compatibilities of liquefied gases                  X = incompatible

    Methane Ethane Propane Butane Ethylene Propylene Butylene Butadiene Isoprene Ammonia Vinyl chloride Ethylene oxide Propylene oxide Chlorine Water vapour Oxygen or Air Carbon dioxide
Methane                                                     X            
Ethane                                                     X            
Propane                                                     X            
Butane                                                     X            
Ethylene                                                     X            
Propylene                                                     X            
Butylene                                                     X            
Butadiene                                                     X X X    
Isoprene                                                     X X X    
Ammonia                                             X X X         X
Vinyl chloride                                                     X     X    
Ethylene oxide                                     X                     X    
Propylene oxide                                     X                            
Chlorine X X X X X X X X X X X             X        
Water vapour                             X X                 X            
Oxygen or Air                             X X     X X                    
Carbon dioxide                                     X                            

 

Note: Reference should be made to the Data Sheets in Ref. 2.1 for details of chemical compatibility.


Table 2.3(b) Previous cargo compatibilities of liquefied gases



TANK CLEANING TABLE

NEXT CARGO
    Butane   Butadiene   Butylene   C4-Raff*   Ethylene   Propane   Propylene   Propylene Oxide   Propane Propylene mix   Vinyl Chloride   C4-Crude*  
02 Content   <0.5% <0.2% <0.3% <0.3% <0.3% <0.5% <0.3% <0.1% <0.3% <0.1% <0.3%
Dew-point   <-10°C <-10°C <-10°C <-10°C < 50°C <-40°C <-25°C <-40°c <-40°C <-20°C <-10°C
LAST CARGO                                              
Ammonia  

Loading cargoes after ammonia is often subject to specific terminal requirements
Butane       N2 <5% N2I <5% ET V,N2 S V,N2 V,N2 ET V,N2 ET
Butadiene   ET     N2I <25% N2I <25% V,N2 ET V,N2 V,N2 V,N2 V,N2 ET
Bulylene   ET N2 <5%     ET V,N2 ET V,N2 V,N2 V,N2 V,N2 ET
C4-Raff*   ET N2 <5% N2I <25%     V,N2 ET V,N2 V,N2 V,N2 V,N2 ET
Ethylene   S Heat N2 <5% N2I <5% S     S N2 <3000ppm V,N2 ET Heat N2 <1000ppm S Heat
Propane   ET N2 <5% N2I <5% ET N2 <1000ppm     N2 <5% V,N2 ET N2 <1000ppm S
Propylene   ET N2 <5% N2I <5% ET N2 <1000ppm ET     V,N2 ET N2 <1000ppm S
Propylene Oxide   W,V,N2I W,V,N2 W,V,N2I W,V,N2I W,V,N2 W,V,N2I W,V,N2     W,V,N2 W,V,N2 W,V,N2
Propane Propylene mix   ET N2 <5% N2I <5% ET V,N2 S N2 <25% V,N2     N2 <1000ppm S
Vinyl Chloride   V,N2I V,N2 V,N2I V,N2I V,N2 V,N2I V,N2 V,N2 V,N2     V,N2
Butane & Propane wet S N2 <5% N2I <5% ET V,N2 ET V,N2 V,N2 S V,N2    
C3/C4* ET N2 N2I ET V,N2 S V,N2 V,N2 V,N2 V,N2    

 

*These cargoes are mixtures of various liquefied gases and are not listed in the IGC Code.

CODE   DESCRIPTION  
W   Water wash  
V   Visual Inspection  
N2   Inert with Nitrogen only  
N2I   Inert with Nitrogen or Inert Gas  
ET   Empty Tank: which means as far as the pumps can go  
S   Standard Requirements: cargo tanks and cargo piping to be liquid free and 0.5 bar overpressure (ship-type dependant) prior to loading, but based on terminal or independent cargo surveyor's advice.  

 

Note: Before any inerting starts the tank bottom temperature should be healed to about 0°C Note: A cargo tank should not be opened for inspection until the tank temperature is close to ambient conditions.


2.5 INERT GAS AND NITROGEN

Inert gas is used on gas carriers to inert cargo tanks and to maintain positive pres­sures in hold and interbarrier spaces (see 4.7, 7.2.3, 7.9.3). This is carried out in order to prevent the formation of flammable mixtures. For cargo tanks the inerting operation is a necessary preliminary prior to aerating for inspection or drydock but it can be time-consuming. Inerting is also required before moving from a gas-free condition into the loaded condition. Regarding inerting levels, prior to gassing-up, a tank should have an oxygen content of less than 5 per cent but sometimes a lower figure is required by loading terminals. Prior to aeration, the inerting process should have achieved an hydrocarbon content of below 2 per cent.

In addition to oxygen, another essential element regarding inert gas quality is its dryness. Any moisture contained within the gas can condense at the cold cargo temperatures encountered. Therefore, in order to prevent hydrate formation in the products loaded and to prevent serious condensation and corrosion in tanks and hold spaces, inert gas is thoroughly dried as it leaves the generator.

For inerting holds and interbarrier spaces the shipboard generation (or storage) of inert gas is a requirement of the Gas Codes. This applies to ships fitted with full secondary barriers and for ships having Type 'C' tanks suited to refrigerated products. Of course, this is a lower capacity requirement than that described for cargo tank inertion.

For cargo tank purposes, the shipboard production of inert gas is not a requirement of the Gas Codes. The Gas Codes recognise that when inerting is needed, it should be possible to operate ships by taking an inert gas from the shore. Generally, this is true for ports where trade-switching occurs, but if in-tank maintenance is considered in remote ports supply from ashore can be problematic. Nevertheless, most of the larger gas carriers (and many smaller ships) are fitted with inert gas generators for cargo tank purposes. Gas carriers do not use the ship's main boilers for generating inert gas — it is produced by means of fuel combustion in purpose-built plant. In this regard, the most relevant documentation for the design of inert gas plant are the Gas Codes. However, the content of these Codes is limited and it should be noted that, in practice, the inert gas quality specification is also largely dependant on the unique trades in which these ships are employed.

Accordingly, for all but the smallest of LPG ships, combustion inert gas plant is usually fitted on board and has the primary purpose as described above. Due to great progress made with plants producing nitrogen by the air separating process (see 4.7.2) in recent years, such plants can now produce up to 15,000m3/h of nitrogen and although the production rate might depend much of the oxygen content required, they will probably replace the conventional inert gas combustion plants for ships of small to medium size in the near future. For LNG ships, combustion-type inert gas is often fitted and this is usually in addition to plant able to produce small quantities of nitrogen for inerting holds and interbarrier spaces. (For the older LNG ships nitrogen was often carried in liquid nitrogen tanks). On either LNG or LPG ships, when larger quantities of nitrogen are needed for inerting cargo tanks, it has to be taken from the shore.

As mentioned above, inert gas produced on gas carriers takes two forms. It may be produced by means of a combustion inert gas generator and, in this case, typical components of the gas are shown in Table 2.4. Furthermore, gaseous nitrogen can be produced on board and here again some data is included in the same table. For the shipboard production of nitrogen, this table shows that some oxygen is usually found in the final nitrogen mix. High purity can be obtained but this drastically reduces the production level.

Each type of inert gas (fuel burning, shipboard nitrogen production, or pure nitrogen from the shore) has its own particular use. Throughout this book the term inert gas is used before a gas produced by a combustion inert gas generator. The use of the word


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