Liquefied gases are normally carried as boiling liquids at either:
- ambient temperature (fully pressurised ships), or
- atmospheric pressure (fully refrigerated ships), or
- intermediate temperatures and pressures (semi-pressurised ships, often referred to as semi-refrigerated).
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS)
Particularly hazardous cargoes such as ethylene oxide and propylene oxide may be carried below their boiling points to reduce boil-off and increase safety. In such cases the tank pressure is maintained above atmospheric with nitrogen padding.
Any heat input to the cargo will vaporise some of the liquid and gradually increase the tank pressure. Pressure vessels are designed to accommodate this increase, but on fully or semi-refrigerated ships the boil-off is condensed by the reliquefaction system and returned to the cargo tanks as a boiling liquid. On LNG vessels cargo tank pressure is almost always controlled by burning the boil-off in the main propulsion system or in rare cases (e.g. emergency) by venting it to atmosphere.
If the pressure above a boiling liquid is increased, vaporisation from the surface is reduced, and vice versa.
1. High and Low Pressure Effects
Pressures above or below the design range can damage a system, and operating personnel should be fully aware of any pressure limitation for each part of the cargo system; pressures should always be kept between the specified maximum and minimum.
2. Pressure Surge
High surge pressures (shock pressures or ‘liquid hammers’) can be created if valves are opened or shut too quickly, and the pressure may be sufficient to cause hose or pipeline failure.
3. Pressurised Systems
In pressurised systems, with the cargo at ambient temperature, there is normally no external frosting to indicate the presence of liquid or vapour anywhere in the system. Checks should be made for the presence of high pressure vapour or liquid by gauges and test cocks before opening valves etc. It is possible for vapour trapped in a system to condense in cold weather, causing a slight reduction in pressure. If the cargo is inhibited, this condensed liquid will be uninhibited and the precautions given in paragraphs 1.4.2,1.4.3 and 1.8.4 should be observed.
4. Reciprocating Compressors
If vapour trapped in a reciprocating compressor condenses, it can dilute the lubricating oil in the crankcase which could cause bearing failure, overheating or possibly an explosion. The crankcase heating equipment, if fitted, should be used to reduce the possibility of cargo condensing and should be operated before the compressor is started. Liquid condensed in the compressor may also cause mechanical damage.
5. Cargo Tank Pressures
Cargo tank pressure should normally be maintained above atmospheric pressure to prevent the ingress of air and the possible formation of flammable mixtures. Positive pressures should be maintained if the tank contains any cargo vapour or inert gas. However, many pressure vessels are designed to withstand vacuum and it is possible to reduce tank pressure below atmospheric without drawing in air, for example during inerting and gas freeing (but see paragraph 4.6.4).
Cargo operations such as cooldown, warm-up, loading and discharge may affect pressures in hold or interbarrier spaces. Pressures can also be affected by climatic changes and the variation in temperature between day and night. Pressure in cargo tanks and hold or interbarrier spaces should be closely monitored, especially during cargo operations, and the equipment provided should be used to make the necessary adjustments. Particular care is necessary with membrane or semi-membrane systems which are vulnerable to damage from vacuum or incorrect differential pressures because of the thin barrier material.
ICS TANKER SAFETY GUIDE (LIQUEFIED GAS)
Pressures in cargo tanks may be maintained above atmospheric by:
- equalising pressures between tanks which contain the same cargo, or
- circulating cargo liquid or vapour, or both, between tanks containing the same cargo, or
- circulating cargo within a tank by use of the cargo pumps, or
- allowing the cargo to warm up.
6. Liquid Gas Samples
Liquid gas samples should not be placed in containers which cannot withstand the pressure created by the sample at the highest ambient temperature expected. Sufficient ullage should be left in the container to ensure that it does not become liquid full at the highest temperature anticipated (see paragraph 4.18.1). Liquid gas samples should be stored within the cargo area.
Within a range of tank filling levels, the pitching and rolling of the ship and the liquid free-surface can create high impact pressure on the tank surface. This effect is called ‘sloshing’ and can cause structural damage. Filling levels within this range must therefore be avoided. However, some cargoes may be carried safely within the range specified for a particular system if the sloshing forces are permissible; guidance should be sought from the shipowner, the designer and the Classification Society.
8. Pressure Relief Valves
Pressure relief valves depend on accurate setting of opening and closing pressures for effective operation .
9. Cargo Heat Exchangers
Heat exchangers should be pressure tested prior to use. This is especially important after a long period of idleness and before a ship is delivered on time charter . In addition to testing the tubes for tightness, the seawater low temperature cut-out must be tested to ensure that the cargo inlet valve to the heater closes, thereby avoiding damage to the tubes from freezing should the outlet temperature of the seawater fall below 5″C In use, seawater flow through the heater must be established before product flow commences.
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Source: ICS TANKER SAFETY GUIDE (LIQUEFIED GAS)