Figure 2.5 The polymerisation of vinyl chloride

The difference between the vapour pressure of an inhibitor and its cargo has an important bearing on the effectiveness of the inhibitor. Generally, inhibitors have a vapour pressure lower than the cargo in which they sit. Accordingly, the greatest pro­tection is provided in the liquid. This leaves the gases in the vapour space relatively unprotected. It follows therefore that condensation in the vapour space can suffer from increased rates of polymerisation and problems have been known to occur in these areas.

Inhibitors can be toxic. Those most commonly used are hydroquinone (HQ) and TBC. Health and safety data for these products is included in 9.1. As will be noted, care should be taken when handling inhibitors and cargoes with inhibitor added.

Ships' personnel should ensure that an Inhibitor Information Form is received from the cargo shipper before departure from the loading port. This certificate should provide the information shown in the figure below:—

Figure 2.5(a) Inhibitor information form

In addition, the quantity of inhibitor required for effective inhibition and the toxic properties of the inhibitor should be advised.

A similar but more difficult reaction to control is known as dimerisation. This cannot be stopped by inhibitors or any other means. The only way to avoid or slow down dimerisation is to keep the cargo as cool as possible and such cooling is recom­mended, especially during longer voyages.

2.7 HYDRATE FORMATION

Propane and butane may form hydrates under certain conditions of temperature and pressure in the presence of free water (see Reference 2.14). This water may be present in LPG as an impurity or may be extracted from cargo tank bulkheads if rust is present. Rust which has been dehydrated in this way by LPG loses its powers of adhesion to tank surfaces and may settle to the tank bottom as a fine powder.

LPG hydrates are white crystalline solids which may block filters and reliquefaction regulating valves. Furthermore they may damage cargo pumps.

Hydrate inhibitors such as methanol or ethanol may be added at suitable points in the system but nothing whatsoever should be added without the consent of the shipper and ship operator. It should be noted that in some countries the use of methanol is banned. In addition, some chemical gases may be put off specification by the addition of methanol. Care must be taken if a hydrate inhibitor is added to a polymerisable cargo as the polymer inhibition mechanism may be negated.

Since methanol is toxic, care should be taken regarding its safe handling.

2.8 LUBRICATION

The property of a fluid which restricts one layer of the fluid moving over an adjacent layer is called viscosity. Viscosity is important in determining the lubricating properties of liquid. The majority of liquefied gases have poor lubricating properties by com­parison with lubricating oils or even water and this is shown in Table 2.4(a).

Table 2.4(a) Factors affecting lubrication

Liquid (temperature)	Lub oil (at +70°C)	Water (at +100°C)	Propane (at-45°C)
Viscosity (centi poise)	28.2	0.282	0.216
Specific Heat (k cal/kg °C)	0.7	1.0	0.5
Latent Heat of Vaporisation (k cal/kg)	35	539	101

 

Liquefied hydrocarbon gases can dissolve in lubricating oil and, for certain applications, such admixture can result in inadequate lubrication of pump seals and compressors. The solvent action of liquefied gases on grease can cause the degreasing of mechanical parts with similar loss of lubrication in fittings such as valves.

In addition to low viscosity, liquefied gas has relatively poor cooling properties and liquids are not able to carry heat away from a shaft bearing very effectively. Any excessive heat will result in a relatively rapid rise in temperature of the bearing. (Specific heat of propane is about half that of water). Under these circumstances, the liquid will vaporise when its vapour pressure exceeds the product pressure in the bearing. The vapour will expel liquid from the bearing and result in bearing failure due to overheating. This is the cause of compressor lubricating problems referred to in 4.6.1.

It should also be noted that the lubricating oil used in a compressor must be com­patible with the grade of cargo being carried (see 7.6.1).

2.9 PHYSICAL PROPERTIES

The physical properties of a liquefied gas depend on its molecular structure. Some compounds have the same molecular formula, but the ways in which the atoms are arranged within the molecule may be different. These different compounds of the same basic substance are called isomers. They have the same molecular mass but differing physical and chemical properties. Examples are n-butane and iso-butane, shown in Figure 2.1. The principal physical properties of the main liquefied gases are listed in Table 2.5. From this data the different physical properties of the isomers of butane and butylene should be noted.

The most important physical property of a liquefied gas is its saturated vapour pressure/temperature relationship. This property, which will be studied in detail later, governs the design of the tank containment system best suited to each cargo and has a strong influence on economic considerations.

2.10 STATES OF MATTER

2.10.1 Solids, liquids and gases

Most substances can exist in either the solid, liquid or vapour state. In changing from solid to liquid (fusion) or from liquid to vapour (vaporisation), heat must be given to the substance. Similarly, in changing from vapour to liquid (condensation) or from liquid to solid (solidification), the substance must give up heat. The heat given to or given up by the substance in changing state is called latent heat. For a given mass of the substance the latent heats of fusion and solidification are the same. Similarly, the latent heats of vaporisation and of condensation are the same, although of different values from the latent heat of fusion or solidification.

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