Loading LPG cargo and related safety factors
Special care for LPG cargo: Before the loading operation begins, the pre-operational ship / shore procedures must be thoroughly discussed and subsequently carried out. The appropriate information exchange is required and the relevant ship / shore safety check list must be completed.
Particular attention must be paid to the conditions and setting of cargo relief valves, reliquefaction plant, gas detection system, alarms and controls and to the maximum loading rate, taking into account restrictions in ship / shore systems, etc. Lines are to be pressurised to the maximum working pressure, and checked for leaks.
The terminal must provide the necessary information on the cargo, including inhibitor certificates where inhibited cargoes are loaded. Any other special precautions for specific cargoes are to be made known to ship personnel, e.g. the lower setting of the compressor discharge temperature cut-out switch required for some chemical cargoes. Variable setting pressure valves and gas detection sample valves must be correctly set.
Either the terminal or the shippers will advise the required cargo tank temperatures / pressure which are required for arrival at the load port.
Cargo loading can be carried out using either a vapour return line, the ship’s reliquefaction plant, or both. Where loading is carried out with a vapour return facility, liquid is taken on board through the liquid header and directed into the appropriate tanks. Vapours generated are returned ashore via the vapour return line using the compressor or jetty blower. Under these conditions the loading rate is independent of the ship’s reliquefaction plant and governed by the rate at which the terminal can handle the vapour; it may also be constrained by velocity through the ship’s piping system.
Where no vapour return is provided then the loading rate is governed by the capacity of the ship’s reliquefaction plant. In most cases the terminal reliquefaction plant capacity will be much larger than that on board the vessel and, as a result, loading rates where vapour return is provided will normally be higher than without vapour return.
Fig:Fully refrigerated LPG carrier at sea
All unused manifolds are to be blanked.
When liquefied gas is being loaded from terminal to ship, it is necessary to consider the location, pressure, temperature and volume of the stored product on shore as well as the pumping requirements of the transfer facility. Ship loading will normally be from fully refrigerated storage where the tanks typically operate at a pressure of approximately 60 mbar; this pressure will allow the propane at the bottom of a full shore tank to sustain a temperature perhaps 1°C above the atmospheric boiling point, i.e. -44°C as against -45°C.
When LPG is pumped to the jetty head, the pumping energy required for transfer is dissipated in the liquid as heat, to which must be added the heat flow into the liquid through the pipework. The propane may therefore arrive at the ship’s rail at a temperature of -43.5°C. When loading without vapour return, the vapour which is displaced by the incoming liquid must be liquefied by the reliquefaction plant; the capacity required for this, plus the heat loss through the insulation, may leave little or no capacity for cooling of the cargo during loading.
The early stages of loading are critical, particularly where significant distances exist between storage tank and jetty. Ship’s tank pressures must be regularly observed and on no account should relief valves be allowed to lift. Loading rates must be reduced and, if necessary, stopped when difficulties are experienced in maintaining acceptable tank pressures. Ship’s tank pressure rise in the early stages of loading can also be controlled to a certain extent by taking liquid into the cargo tank via the top sprays so condensing part of the cargo vapour.
Depending on the efficiency of the purging operation, significant quantities of incondensibles may be present and without vapour return to shore these incondensibles will have to be vented from the condenser. Care must be taken when venting incondensibles to minimise venting of cargo vapours to the atmosphere. As the incondensibles are vented, the condenser pressure will drop and the vent valve should be throttled and eventually closed.
A close watch must be kept on ship’s cargo tank pressure, temperatures, liquid levels, inter-barrier space pressures etc., throughout the loading operation. Monitoring of liquid levels may present difficulties when the reliquefaction plant is in operation. This is because the liquid in the tank is boiling and as a result vapour bubbles within the liquid increasing the volume of the liquid, thus giving false reading with float-type ullage gauges. An accurate level monitoring can be achieved by temporarily suppressing boiling, i.e. by closing the vapour suction from the tank.
Towards the end of the loading operation, loading rates must be reduced to an appropriate rate as previously agreed with shore staff in order to accurately “top-off” tanks.
At this time the vapour suction valve is also to be closed to prevent the surface of the liquid boiling as explained above. This boiling and the resulting increase in liquid volume due to the vapour bubbles may give a false reading on the level gauge and result in a lesser amount of cargo being loaded.
On completion of the loading operation, ship’s pipework is to be drained back to the cargo tanks. The liquid remaining can be cleared by blowing ashore with vapour using the ship’s compressor or by nitrogen injected into the loading arm to blow the liquid into the ship’s tanks.
Once liquid has been cleared and lines depressurised, manifold valves should be closed and the hose or loading arm disconnected from the manifold flange.
The ballast handling arrangements are quite independent of the cargo system. Deballasting can therefore take place simultaneously with loading subject to regulations. Ship stability and stress are of primary importance during loading and deballasting and procedures are in accordance with normal tanker practice.
The Company should be immediately informed if the vessel receives any request from charterer to comingle the LPG cargo. A letter of indemnity for co-mingling (as per P&I wording) would need to be requested from the charterer. Reference also should be made to SIGTTO’s publication “Co-mingling of LPG – Essential Best Practice for the Industry” for safety precautions to be taken in such situations.
(a) The reliquefaction plant is usually to be kept running and the tank pressure maintained as low as possible throughout the loading.
(b) It may be possible to load two cargoes simultaneously by segregating the two cargo systems, the two systems being separated by spool pieces.
(c) To avoid thermal shocks it is important to ensure that the temperature of the incoming gas is not at variance with the receiving tank temperature by more than 10 deg. Also, the temperature of the gas must be constant throughout loading to prevent stratification and the possible development of an unstable situation (‘roll-over’). If this occurs the loading operation must be stopped immediately.
If cargo is stored for any length of time and the boil off removed to maintain tank pressure this will cause a slight increase in density and reduction in temperature near the liquid surface. The static head will create a marginally higher temperature and lower density at the tank bottom.
This unstable equilibrium may exist until some disturbance occurs, such as the addition of new liquid. Spontaneous mixing can take place with violent evolution of large quantities of vapour. This phenomenon is called "roll-over".
(d) A deck tank may be loaded simultaneously with cargo tanks, or separately.
(e) Safety Relief Valves must be set in accordance with requirements of Flag Administration or USCG.
The maximum transport pressure of the incoming gas should be at least 0.1 bar below the relief valve settings. If the pressure of the incoming gas is higher than the relief valve settings, flow must be throttled to prevent the valves lifting.
(f) Cargo tanks must not be filled to more than 98% of the tank capacity, measured after necessary adjustments to the temperature of the cargo have been completed.
The maximum volume to which tanks may be filled is calculated using the following formula:-
VL = 0.98 x V x dr/dl
where VL = maximum loading capacity
V = tank volume
Variable 0.98 x dr/dl is obtained from tables for various cargoes at varying loading temperatures and safety valve settings.
If overfilling occurs this must be corrected immediately by transferring the liquid to another tank, or the deck tank using the deep well pump.
A sudden closing of the quick closing valves, caused by overfilling, can cause problems at the terminal because they may not be able to react quickly to the situation.
(g) When cargo is loaded without vapour return, with the reliquefaction plant in operation, the following points apply:-
- Attention must be paid to possible water content of the cargo. This will collect in the filter during reliquefaction and should be drained as necessary.
- During loading, gas displaced from the cargo tanks is led to the reliquefaction plant and re-circulated to the tanks via the spray lines. Non-condensable purge gases collected in the purge condenser should be blown off from the mast.
- In addition to the gas displaced from the tank the reliquefaction plant has to reliquefy flash gas.
Where high cooling capacities are required the capacity of the reliquefaction plant is the limiting factor; for low capacities the only constraint is the available shore pressure.
(h) During loading the vessel has to be deballasted. This must be done in accordance with conditions in the vessel’s Loading Booklet and loading instrument.
Care must be taken to keep the vessel upright at all times, which means keeping the cargo even and the ballast even. If the vessel does develop a list during cargo operations, it must be corrected to bring the vessel upright immediately. This can be down by adjusting the cargo, ballast or both.
N.B. Vessels with spherical or cylindrical tanks can only use ballast to correct list.
Prevention of Hydrate Formation
LPG cargoes may contain traces of water when loaded. It may be permissible in such cases to prevent hydrate formation by adding small quantities of a suitable anti-freeze (e.g. methanol, ethanol) at strategic points in the system.
A careful review of the Cargo Data Sheet is required for if the cargo is inhibited, the anti-freeze may adversely affect the inhibitor.
Nothing whatsoever should be added to the cargo without charterers / shipper’s instruction and a risk assessment carried out onboard the vessel. If the use of the anti-freeze is permitted, it should be introduced at places where expansion occurs because the resultant lowering of temperature and pressure promotes hydrate formation.
For further information on hydrate formation can be found in ICS – Tanker Safety Guide – Liquefied Gas Chapter 1.4.1 and should be followed.
Supercooled or subcooled cargoes are where a gas has been cooled below its boiling point by a number of degrees. An example would be n-butane with a boiling point and normal carriage temperature of -1ºC loaded at -3ºC or less.
On fully refrigerated gas carriers, cargo is normally loaded close to boiling point and cargoes cooled below this are very unusual. Normally, the opposite is the problem, with cargo being warmer than declared resulting in the refrigeration plant being using at maximum capacity. However, officers need to be aware of this risk and be prepared accordingly. Supercooled cargoes can induce a condensation of vapour on top of the cargo, resulting in a reduction in pressure in the cargo tank. This reduction can be rapid and endanger the vessel. This is most likely to occur after leaving the load port when the vessel starts to roll or pitch.
Prior to loading, the Chief Officer is to ascertain the loading temperature of the cargo and compare it with the boiling point as per MSDS / Cargo Quality Data. If the loading temperature is such that supercooled cargo is suspected then the cargo should not be loaded. The Company, charterers and terminal should be notified immediately and further information requested.
However, this condition may not become apparent until completion of the final cargo survey. A vessel that has loaded a cargo that is noted to be cooler than the declared boiling point is to:-
Some cargoes can react with air to form unstable oxygen compounds which could cause an explosion. The IMO Codes require these cargoes to be either inhibited or carried under nitrogen (as in the case of Propylene Oxide) or other inert gas. The general precautions in Chapter 4 of these procedures and paragraph 1.4.2 of the ICS – Tanker Safety Guide – Liquefied Gas, apply and care should be taken to observe the shipper’s instructions
Loading Butane / Butadiene
When loading butane and butadiene it is important to have a dry atmosphere in the tanks to avoid the possibility of ice forming in the cargo. If water is allowed into the tank methanol injection must be used. As a precaution against cargo pumps becoming blocked with ice they must be rotated periodically.
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