Figure 4.16 Typical rotor for an oil-free screw compressor
Capacity control of screw compressors can be achieved in a number of ways. The most common is the use of a sliding valve which effectively reduces the working length of the rotors. This is more efficient than suction throttling. Screw compressors consume more power than reciprocating compressors.
4.6.3 Compressor suction liquid separator
It is necessary to protect cargo vapour compressors against the possibility of liquid being drawn in. Such a situation can seriously damage compressors since liquid is incompressible. It is normal practice, therefore, to install a liquid separator on the compressor suction line coming in from the cargo tanks. The purpose of this vessel is to reduce vapour velocity and, as a result, to allow any entrained liquid to be easily removed from the vapour stream. In case of over-filling, the separator is fitted with high-level sensors which set off an alarm and trip the compressor.
4.6.4 Purge gas condenser
Many reliquefaction plants are fitted with a heat exchanger mounted above the cargo condenser. These units are of the shell and tube types. The purpose of this heat exchanger is to condense any cargo vapours which remain mixed with incondensible gases (such as nitrogen). These cargo vapours may have failed to condense in the main condenser. For example, commercial propane which may have two per cent ethane in the liquid will have perhaps 14 per cent ethane in the vapour; ethane being the more volatile component. On a semi-pressurised LPG carrier, the presence of ethane can cause difficulties in a conventional sea water-cooled condenser.
Figure 4.17 Typical purge gas condenser system
Figure 4.17 shows a typical purge gas condenser system. The uncondensed gases in the main condenser are displaced into the shell of the purge condenser. Here they are subjected to the same pressure that exists in the main condenser but to a lower condensing temperature. This is equivalent to the outlet temperature from the expansion valve, since the whole or part of this liquid passes through the tube side of the purge condenser. This lower condensing temperature allows cargo vapours to be condensed and incondensible gases are purged from the top of the purge gas condenser by a pressure control system.
4.6.5 LNG boil-off and vapour-handling systems
The older LNG ships use steam turbine-driven compressors to handle boil-off vapours. Newer designs incorporate electrically driven equipment. Boil-off vapours are produced during cool-down, loading and during the loaded and ballast voyages. Normally, a low-duty compressor handles the boil-off whilst on passage: a high-duty compressor handles cargo vapours produced during cool-down and loading, returning these vapours to shore.
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When a ship is at sea, the low-duty compressor collects the boil-off gas from a header connected to each cargo tank. It then passes the boil-off through a steam heater and into the engine room (see 7.6.2). The pipeline is jacketed from the point at which it enters the engine room or the accommodation up to the boiler front. The annular space (between the gas pipeline and its jacket) is either pressurised with nitrogen or exhaust-ventilated with air giving at least 30 changes per hour. The gas pipeline must be purged with inert gas before and after gas-burning operations.
There are a number of automatic protective devices built into the system to ensure safe operation and these must be regularly inspected and maintained. Protective systems include continuous monitoring for leakage and automatic shut-down in the event of system malfunction or leak detection. These systems are described in some detail in the IGC Code.
The compressors are provided with surge controls and other protective devices.
LNG is the only liquefied gas product allowed by the Gas Codes to be burnt in the ship's boilers. The other gases, having densities heavier than air, are considered to be hazardous for this purpose.
4.7 INERT GAS AND NITROGEN SYSTEMS
As covered in 2.5, gas carriers use various forms of inert gas and these are listed below:
• Inert gas from combustion-type generators
• Nitrogen from shipboard production systems, and
• Pure nitrogen taken from the shore (either by road tanker or barge)
Unlike oil tanker inert gas systems, which have their design and operation established by extensive regulations and guidelines, the fitting of inert gas systems to gas carriers is subject to limited advice in the Gas Codes, special consideration by administrations and the particular demands of the trade. In general, for gas carriers, the production of combustion generated inert gas will be covered in newbuilding specifications at about one per cent oxygen
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LNG ships were once provided with storage facilities for liquid nitrogen but newer designs include a nitrogen generation plant. However, up to now, the quantity of nitrogen produced on board has not been of sufficient volume for tank-inerting operations. It is fitted mainly for interbarrier space inerting. Where cargo tank inerting is required on LNG ships, nitrogen from the shore, or combustion-generated inert gas is used.
As can be seen from the foregoing, most ships, barring only the smallest pressurised gas carriers, have the capability of generating their own inert gas. Furthermore, all LNG ships have the capability of producing nitrogen for hold space and interbarrier space inertion — this is a necessary specification as the carbon dioxide in inert gas would freeze when in close proximity to the cargo. The methods of producing the inert gases, as listed at the beginning of this section, are covered below.
4.7.1 Inert gas generators
The Gas Codes require continuous oxygen monitoring in the inert gas stream and the oxygen content should normally be no more than about one per cent. High oxygen content can trigger an alarm; however, the generator is not normally shut down on this alarm but the gas is diverted to atmosphere via a vent riser.
The main advantages of the on board inert gas generator are as follows:
• The cost of inert gas is less than the purchase of liquid nitrogen
• The inert gas plant capacity is available either at sea or in port
The disadvantages of the combustion-type generator centre on the quality of gas produced. Combustion must always be carefully adjusted to avoid the production of toxic carbon monoxide and soot. Also, even under good operating conditions, the volume of oxygen in the inert gas may be unsuitable for use with the chemical gases, as detailed in Chapter Two. Accordingly, given that an oxygen-critical gas is to be loaded, as a preliminary operation, pure nitrogen must be taken from the shore.
Inert gas produced by the careful combustion of diesel or gas oil, results in a reduced oxygen content in the products of combustion. In the inert gas generator, the resulting gases are further treated to give an inert gas of acceptable standard. Apart from plant operation, the final quality of the inert gas also depends on the fuel used and generally fuel of low sulphur content is preferred. In this regard, experience often dictates that gas oil should be used in preference to marine diesel oil but bunker prices also have a bearing on the final choice.
A typical analysis of inert gas from a modern generator is shown in Table 2.4. The quality of the inert gas produced, however, is very dependent on the conditions under which the generator is operated and, in this respect, the manufacturer's guidance
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