This article contains Special Requirements for ships carrying liquefied gas, regarding materials of the tanks, gases, moisture control, pumps, vents and a lot more.
- Materials of construction
- Independent tanks
- Refrigeration systems
- Deck cargo piping
- Exclusion of air from vapour spaces
- Moisture control
- Permanently installed toxic gas detectors
- Flame screens on vent outlets
- Maximum allowable quantity of cargo per tank
- Submerged electric cargo pumps
- Cargo containment system
- Cargo piping systems
- Instrumentation – safety devices
- Personnel protection
- Filling limits for cargo tanks
- Diethyl ether and vinyl ethyl ether
- Ethylene oxide
- Isopropylamine, monoethylamine
- Methyl acetylene-propadiene mixtures
- Propylene oxide and mixtures of ethylene oxide/propylene oxide with ethylene oxide content of not more than 30 % by weight
- Vinyl chloride
The provisions of this article are applicable where reference is made in column “i” in the table of Summary of Minimum RequirementsSummary of Minimum Requirements for LNG and LPG tankers. These are requirements additional to the general requirements of this Rules.
Materials of construction
Materials which may be exposed to cargo during normal operations are to be resistant to the corrosive action of the gases. In addition, the following materials of construction for cargo tanks, and associated pipelines, valves, fittings and other items of equipment are not to be used for certain products as specified in column “i” in the table of Summary of Minimum RequirementsSummary of Minimum Requirements for LNG and LPG tankers by referring to this paragraph:
- Mercury, copper and copper bearing alloys, and zinc.
- Copper, silver, mercury, magnesium and other acetylide-forming metals.
- Aluminium and aluminum bearing alloys.
- Copper, copper alloys, zinc and galvanized steel.
- Aluminium, copper and alloys of either.
- Copper and copper bearing alloys with greater than 1 % copper.
Products should be carried in independent tanks only.
Products should be carried in type C independent tanks and the provisions of “Cargo Temperature Control and Cargo Vent Systems”Pressure and Temperature Control apply. The design pressure of the cargo tank shall take into account any padding pressure and/or vapour discharge unloading pressure.
Only the indirect system described in “Cargo Temperature Control and Cargo Vent Systems”Refrigeration systems is to be used.
For ships engaged in the carriage of products which readily form dangerous peroxides, recondensed cargo is not be allowed to form stagnant pockets of uninhibited liquid. This may be achieved either by:
- using the indirect system described in “Cargo Temperature Control and Cargo Vent Systems”Refrigeration systems with the condenser inside the cargo tank;
- or using the direct system or combined system described in “Cargo Temperature Control and Cargo Vent Systems”Refrigeration systems;
- and respectively or the indirect system described in “Cargo Temperature Control and Cargo Vent Systems”Refrigeration systems with the condenser outside the cargo tank, and designing the condensate system to avoid any places in which liquid could collect and be retained. Where this is impossible inhibited liquid is to be added upstream of such a place.
If the ship is to carry consecutively products specified above with a ballast passage between, all uninhibited liquid should be removed prior to the ballast voyage. If a second cargo is to be carried between such consecutive cargoes the reliquefaction system should be thoroughly drained and purged before loading the second cargo. Purging should be carried out using either inert gas or vapour from the second cargo, if compatible. Practical steps should be taken to ensure that polymers or peroxides do not accumulate in the ship’s system.
Deck cargo piping
100 % radiography of all butt welded joints in cargo piping exceeding 75 mm in diameter is required.
Exclusion of air from vapour spaces
Air is to be removed from the cargo tanks and associated piping before loading and then subsequently excluded by:
- introducing inert gas to maintain a positive pressure. Storage or production capacity of the inert gas is to be sufficient to meet normal operating requirements and relief valve leakage. The oxygen content of inert gas must at no time be greater than 0,2 % by volume;
- or control of cargo temperature such that a positive pressure is maintained at all time.
For gases which are non-flammable and may become corrosive or react dangerously with water, moisture control is to be provided to ensure that cargo tanks are dry before loading and that during discharge, dry air or cargo vapour is introduced to prevent negative pressures. For the purposes of this paragraph, dry air is air which has a dewpoint of -45 °C or below at atmospheric pressure.
Care should be taken to ensure that the cargo is sufficiently inhibited to prevent polymerization at all times during the voyage. Ships should be provided with a certificate from the manufacturer stating:
- name and amount of inhibitor added;
- date inhibitor was added and the normally expected duration of its effectiveness;
- any temperature limitations affecting the inhibitor;
- the action to be taken should the length of the voyage exceed the effective lifetime of the inhibitors.
Permanently installed toxic gas detectors
Gas sampling lines are not to be led into or through gas safe spaces. Alarms referred to in Cargo Tank Instrumentation on Gas TankersTemperature indicating devices are to be activated when the vapour concentration reaches the threshold limiting value.
The alternative of using portable equipment in accordance with Cargo Tank Instrumentation on Gas TankersTemperature indicating devices should not be permitted.
Flame screens on vent outlets
Cargo tank vent outlets are to be provided with readily renewable and effective flame screens or safety heads of an approved type when carrying a cargo referenced to this section. Due attention is to be paid in the design of flame screens and vent heads to the possibility of the blockage of these devices by the freezing of cargo vapour or by icing up in adverse weather conditions. Ordinary protection screens are to be fitted after removal of the flame screens.
Maximum allowable quantity of cargo per tank
When carrying a cargo referenced to this section, the quantity of the cargo should not exceed 3 000 m3 in any one tank.
Submerged electric cargo pumps
The vapour space of cargo tanks equipped with submerged electric motor pumps should be inerted to a positive pressure prior to loading, during carriage and during unloading of flammable liquids.
Anhydrous ammonia may cause stress corrosion cracking in containment and process systems made of carbon manganese steel or nickel steel. To minimize the risk of this occurring, measures detailed below are to be taken as appropriate.
Where carbon manganese steel is used, cargo tanks, process pressure vessels and cargo piping are to be made of fine grained steel with a specified minimum yield strength not exceeding 355 N/mm2 and with an actual yield strength not exceeding 440 N/mm2. One of the following constructional or operational measures shall also be taken:
- lower strength material with a specified minimum tensile strength not exceeding 410 N/mm2 shall be used;
- or cargo tanks, etc., are to be post weld stress relief heat treated;
- or carriage temperature is to be maintained preferably at a temperature close to the product’s boiling point of -33 °C but in no case at a temperature above -20 °C;
- or the ammonia shall contain not less than 0,1 % w/w water.
If carbon manganese steel with higher yield properties are used other than those specified above, the completed cargo tanks, piping etc. are to be given a post weld stress relief heat treatment.
Process pressure vessels and piping of the condensate part of the refrigeration system are to be given a post weld stress relief heat treatment when made of materials mentioned above.
The tensile and yield properties of the welding consumables shall exceed those of the tank or piping material by the smallest practical amount.
Nickel steel containing more than 5 % nickel and carbon manganese steel not complying with the requirements voiced above are particularly susceptible to ammonia stress corrosion cracking and shall not be used for containment and piping systems for the carriage of this product.
Nickel steel containing not more than 5 % nickel may be used provided the carriage temperature complies with the requirements specified above.
In order to minimize the risk of ammonia stress corrosion cracking, it is advisable to keep the dissolved oxygen content below 2,5 ppm w/w. This can best be achieved by reducing the average oxygen content in the tanks prior to the introduction of liquid ammonia to less than the values given as a function of the carriage temperature T in Table 1.
|Table 1. Average oxygen content|
|T [°C]||O2 [% v/v]|
|-30 and below||0,90|
Oxygen percentages for intermediate temperatures may be obtained by direct interpolation.
Cargo containment system
The capacity of each tank is not to exceed 600 m3 and the total capacity of all cargo tanks is not to exceed 1 200 m3. The tank design vapour pressure is not to be less than 13,5 bar (see also “Cargo Temperature Control and Cargo Vent Systems”Pressure and Temperature Control and “Independent tanks”).
Parts of tanks protruding above the upper deck are to be provided with protection against thermal radiation taking into account total engulfment by fire (i.e. the insulation of such parts of tanks is to be resistant to fire).
Each tank is to be provided with two pressure relief valves. A bursting disc of appropriate material is to be installed between the tank and the safety relief valves. The rupture pressure of the bursting disc is to be 1 bar lower than the opening pressure of the safety relief valve, which is to be set at the design pressure of the tank but not less than 13,5 bar gauge. The space between the bursting disc and the relief valve is to be connected through an excess flow valve to a pressure gauge and a gas detection system. Provisions are to be made to keep this space at or near the atmospheric pressure during normal operation.
Outlets from pressure relief valves are to be arranged in such a way as to minimize the hazards on board the ship as well as to the environment. Leakage from the relief valves is to be led through the absorption plant to reduce the gas concentration as far as possible. Leakage is to be led to the absorption tower. The relief valve exhaust line is to be arranged at the forward end of the ship to discharge outboard at deck level with an arrangement to select either port or starboard side, with a mechanical interlock to ensure that one line is always open.
The Administration and Port Administration may require that chlorine is carried in refrigerated state at a maximum pressure specified by these Administrations.
Cargo piping systems
Cargo discharge shall be performed by means of compressed chlorine vapour from shore, dry air or another acceptable gas or fully submerged pumps. The pressure in the vapour space of the tank during discharging should not exceed 10,5 bar gauge. Cargo discharge compressors on board ships will not be accepted by the Society.
The design pressure of the cargo piping system is to be not less than 21 bar gauge. The internal diameter of the cargo pipes is not to exceed 100 mm. Only pipe bends will be accepted for compensation of pipe line thermal movement. The use of flanged joints is to be restricted to a minimum, and when used the flanges are to be of the welding neck type with tongue and groove.
Relief valves of the cargo piping system shall discharge to the absorption plant (see also “Cargo Temperature Control and Cargo Vent Systems”Pressure relief system).
The cargo tanks and cargo piping systems are to be made of steel suitable for the cargo and for a temperature of -40 °C, even if a higher transport temperature is intended to be used. The tanks are to be thermally stress relieved. Mechanical stress relief will not be accepted as an equivalent.
Instrumentation – safety devices
The ship shall be provided with a chlorine absorbing plant with connexions to the cargo piping system and the cargo tanks. The absorbing plant shall be capable of neutralizing at least 2 % of the maximum cargo capacity at a reasonable absorption rate.
During the gas freeing of cargo tanks, vapours should not be discharged to the atmosphere.
A gas detecting system shall be provided capable of monitoring chlorine concentrations of at least 1 ppm by volume. Suction points should be located:
- near the bottom of the hold spaces;
- in the pipes from the safety relief valves;
- at the outlet from the gas absorbing plant;
- at the inlet to the ventilation systems for the accommodation and machinery spaces and control stations;
- on deck at the forward end, in the middle and at the aft end of the cargo area. (Only required during cargo handling and gas freeing operations).
The gas detection system shall be provided with audible and visual alarm with a set point of 5 ppm.
Each cargo tank shall be fitted with a high pressure alarm giving audible alarm at a pressure equal to 10,5 bar gauge.
In addition to the requirements given in “Personal protection of crew on Gas Carriers”Personal protective equipment the following requirements should be met:
- The enclosed space required by “Personal protection of crew on Gas Carriers”Personnel protection requirements for individual products shall be easily and quickly accessible from the open deck and accommodation spaces and shall be capable of being rapidly closed gas-tight. Access to this space from the deck and from the accommodation spaces shall be by means of an airlock. The space shall be so designed as to accommodate the entire crew of the ship and be provided with a source of uncontaminated air for a period of not less than four hours. One of the decontamination showers required by “Personal protection of crew on Gas Carriers”Personnel protection requirements for individual products shall be located near the airlock to the space.
- A compressor and the necessary equipment for filling the air bottles shall be provided.
- One set of oxygen therapy equipment should be carried in the space referred above.
Filling limits for cargo tanks
The requirements of “Filling Limits for Cargo Tanks on Liquefied Gas Tankers”Cargo tank limits do not apply when it is intended to carry chlorine. The chlorine content of the gas in the vapour space of the cargo tank after loading should be greater than 80 % by volume. When determining the filling limits of the cargo tanks for the transport of chlorine, the effect of the refrigeration plant is not to be considered.
Diethyl ether and vinyl ethyl ether
The cargo is to be discharged only by deep-well pumps or by hydraulically operated submerged pumps. These pumps are to be of a type designed to avoid liquid pressure against the shaft gland. Inert gas displacement may be used for discharging cargo from type C independent tanks provided the cargo system is designed for the expected pressure.
For the carriage of ethylene oxide the requirements of “Propylene oxide and mixtures of ethylene oxide/propylene oxide with ethylene oxide content of not more than 30 % by weight” apply analogously with the additions and modifications as given in this paragraph.
Deck tanks should not be used for the carriage of ethylene oxide.
Stainless steels types 416 and 442 as well as cast iron are not to be used in ethylene oxide cargo containment and piping systems. Before loading, tanks should be thoroughly and effectively cleaned to remove all traces of previous cargoes from tanks and associated pipework, except where the immediate prior cargo has been:
- ethylene oxide;
- propylene oxide or mixtures of these products.
Particular care should be taken in the case of ammonia in tanks made of steel other than stainless steel.
Ethylene oxide is to be discharged only by deepwell pumps or inert gas displacement. The arrangement of pumps is to comply with this. Ethylene oxide should be carried refrigerated only and maintained at temperatures of less than 30 °C.
Pressure relief valves are to be set at a pressure of not less than 5,5 bar gauge. The maximum set pressure is to be specially considered by the Society. The protective padding of nitrogen gas as required by this should be such that the nitrogen concentration in the vapour space of the cargo tank will at no time be less than 45 % by volume.
Before loading and at all times when cargo tank contains ethylene oxide liquid or vapour, the cargo tank should be inerted with nitrogen. The water spray system required by this and that required by “Fire protection and Fire extinction on Liquefied Gas Carriers”Water spray system is to operate automatically in a fire involving the cargo containment system.
A jettisoning arrangement is to be provided to allow the emergency discharge of ethylene oxide in the event of uncontrollable self-reaction.
Separate piping systems are to be provided as defined in Section 1, C.32.
Methyl acetylene-propadiene mixtures
Methyl acetylene-propadiene mixtures should be suitably stabilized for transport. Additionally, upper limits of temperature and pressure during the refrigeration should be specified for the mixtures.
Examples of acceptable, stabilized compositions are:
1 Composition 1:
- maximum methyl acetylene to propadiene molar ratio of 3 to 1;
- maximum combined concentration of methyl acetylene and propadiene of 65 mole %;
- minimum combined concentration of propane, butane, and isobutane of 24 mole %, of which at least one-third (on a molar basis) must be butanes and one-third propane;
- and maximum combined concentration of propylene and butadiene of 10 mole %.
2 Composition 2:
- maximum methyl acetylene and propadiene combined concentration of 30 mole %;
- maximum methyl acetylene concentration of 20 mole %;
- maximum propadiene concentration of 20 mole %;
- maximum propylene concentration of 45 mole %;
- maximum butadiene and butylenes combined concentration of 2 mole %;
- minimum saturated C4 hydrocarbon concentration of 4 mole %;
- and minimum propane concentration of 25 mole %.
Other compositions may be accepted provided the stability of the mixture is demonstrated to the satisfaction of the Society.
A ship carrying methyl acetylene-propadiene mixtures shall preferably have an indirect refrigeration system as specified in “Cargo Temperature Control and Cargo Vent Systems”Refrigeration systems. Alternatively, a ship not provided with indirect refrigeration may utilize direct vapour compression refrigeration subject to pressure and temperature limitations depending on the composition.
For the example compositions given above, the following features are to be provided:
1 A vapour compressor that does not raise the temperature and pressure of the vapour above 60 °C and 17,5 bar gauge during its operation, and that does not allow vapour to stagnate in the compressor while it continues to run.
2 Discharge piping from each compressor stage or each cylinder in the same stage of a reciprocating compressor shall have:
- two temperature actuated shut-down switches set to operate at 60 °C or less;
- a pressure actuated shut-down switch set to operate at 17,5 bar gauge or less;
- and a safety relief valve set to relieve at 18,0 bar gauge or less.
3 The relief valve required by above shall vent to a mast meeting the requirements of “Cargo Temperature Control and Cargo Vent Systems”Pressure relief system and shall not relieve into the compressor suction line.
4 An alarm that sounds in the cargo control position and in the navigating bridge when a high pressure switch or a high temperature switch operates.
The piping system, including the cargo refrigeration system, for tanks to be loaded with methyl acetylene-propadiene mixtures should be completely separate from piping and refrigeration systems for other tanks. If the piping system for the tanks to be loaded with methyl acetylene-propadiene mixture is not independent, the required piping separation should be accomplished by the removal of spool pieces, valves or other pipe sections and the installation of blank flanges at these locations.
The required separation applies to all liquid and vapour vent lines and any other possible connexions such as common inert gas supply lines.
Materials of construction and ancillary equipment such as insulation must be resistant to the effects of high oxygen concentrations caused by condensation and enrichment at the low temperatures attained in parts of the cargo system. Due consideration should be given to ventilation in such areas where condensation might occur to avoid the stratification of oxygen enriched atmosphere.
Propylene oxide and mixtures of ethylene oxide/propylene oxide with ethylene oxide content of not more than 30 % by weight
Products transported under the provisions of this section should be acetylene free.
Unless cargo tanks are properly cleaned, these products should not be carried in tanks which have contained as one of the three previous cargoes any product known to catalyse polymerization, such as:
- anhydrous ammonia, and ammonia solutions;
- amines and amine solutions;
- oxidizing substances (e.g. chlorine).
Before loading, tanks should be thoroughly and effectively cleaned to remove all traces of previous cargoes from tanks and associated pipework, except where the immediate prior cargo has been propylene oxide or ethylene oxide/propylene oxide mixtures. Particular care should be taken in the case of ammonia in tanks made of steel other than stainless steel.
In all cases, the effectiveness of cleaning procedures for tanks and associated pipework should be checked by suitable testing or inspection to ascertain that no traces of acidic or alkaline materials remain that might create a hazardous situation in the presence of these products.
Tanks should be entered and inspected prior to each initial loading of these products to ensure freedom from contamination, heavy rust deposits and any visible structural defects. When cargo tanks are in continuous service for these products, such inspections should be performed at intervals of not more than two years.
Tanks for the carriage of these products are to be of steel or stainless steel construction.
Tanks which have contained these products may be used for other cargoes after thorough cleaning of tanks and associated pipework systems by washing or purging. All valves, flanges, fittings and accessory equipment are to be of a type suitable for use with these products and are to be constructed of steel or stainless steel in accordance with recognized standards. The chemical composition of all materials used is to be submitted for approval prior to fabrication. Discs or disc faces, seats and other wearing parts of valves are to be made of stainless steel containing not less than 11 % chromium.
Gaskets are to be constructed of materials which do not react with, dissolve in, or lower the auto- ignition temperature of these products and which are fire resistant and possess adequate mechanical behavior. The surface presented to the cargo shall be polytetrafluoroethylene (PTFE) or materials giving a similar degree of safety by their inertness. Spirally wound stainless steel with a filler of PTFE or similar fluorinated polymer may be accepted by the Society. Gaskets of other types and/or materials are to be type approved by the Society.
Insulation and packing if used are to be of a material which does not react with, dissolve in, or lower the auto-ignition temperature of these products.
The following materials are generally found unsatisfactory for gaskets, packing and similar uses in containment systems for these products and require testing before being approved by the Society:
- neoprene or natural rubber if it comes into contact with the products;
- asbestos or binders used with asbestos;
- materials containing oxides of magnesium, such as mineral wools.
Filling and discharge piping are to extend to within 100 mm of the bottom of the tank or any sump.
The products should be loaded and discharged in such a manner that venting of the tanks to atmosphere does not occur. If vapour return to shore is used during tank loading, the vapour return system connected to a containment system for the product is to be independent of all other containment systems.
During discharging operations, the pressure in the cargo tank should be maintained above 0,07 bar gauge. The cargo should be discharged only by deepwell pumps, hydraulically operated submerged pumps, or inert gas displacement. Each cargo pump is to be arranged to ensure that the product does not heat significantly if the discharge line from the pump is shut off or otherwise blocked. Tanks carrying these products are to be vented independently of tanks carrying other products.
Facilities are to be provided for sampling the tank contents without opening the tank to atmosphere.
Cargo hoses used for transfer of these products are to be marked “FOR ALKYLENE OXIDE TRASFER ONLY”.
Hold spaces are to be monitored for these products. Hold spaces surrounding type A and B independent tanks are also to be inerted and monitored for oxygen. The oxygen content of these spaces is to be maintained below 2 %. Portable sampling equipment is satisfactory.
Prior to disconnecting shore-lines, the pressure in liquid and vapour lines to be relieved through suitable valves installed at the loading header. Liquid and vapour from these lines should not be discharged to atmosphere.
Tanks are to be designed for the maximum pressure expected to be encountered during loading, carriage or unloading of cargo. Tanks for the carriage of propylene oxide with a design vapour pressure of less than 0,6 bar and tanks for the carriage of ethylene oxide/propylene oxide mixtures with a design vapour pressure of less than 1,2 bar are to have a cooling system to maintain the cargo below the reference temperature. For reference temperature see “Filling Limits for Cargo Tanks on Liquefied Gas Tankers”Cargo tank limits .
Pressure relief valve settings are not to be less than 0,2 bar gauge, and for type C independent tanks not greater than 7,0 bar gauge for the carriage of propylene oxide and not greater than 5,3 bar gauge for the carriage of ethylene oxide/propylene oxide mixtures.
The piping system for tanks to be loaded with these products is to be completely separate from piping systems for all other tanks, including empty tanks, and from all cargo compressors. If the piping system for the tanks to be loaded with these products is not independent as defined in Section 1, C.20. the required piping separation must be accomplished by the removal of spool pieces, valves, or other pipe sections and the installation of blank flanges at these locations. The required separation applies to all liquid and vapour piping, liquid and vapour vent lines and any other possible connexions such as common inert gas supply lines.
The products may be transported only in accordance with cargo handling plans that have been approved by the Society. Each intended loading arrangement is to be shown on a separate cargo handling plan. Cargo handling plans are to show the entire cargo piping system and the locations for installation of blank flanges needed to meet the above piping separation requirements. A copy of each approved cargo handling plan shall be kept on board the ship. The Certificate of Fitness for the Carriage of Liquefied Gases in Bulk should be endorsed to include reference to the approved cargo handling plans.
Before each initial loading of these products and before every subsequent return to such service, certification verifying that the required piping separation has been achieved is to be obtained from a responsible person, acceptable to the Port Administration, and carried on board the ship. Each connexion between a blank flange and pipeline flange is to be fitted with a wire and seal by the responsible person to ensure that inadvertent removal of the blank flange is impossible.
Note: the “responsible person” may be e.g. the ship’s master or the Society’s local Surveyor.
The maximum allowable tank loading limits for each cargo tank are to be indicated for each loading temperature which may be applied and for the applicable maximum reference temperature, on a list to be approved by the Society. A copy of the list is to be permanently kept on board by the master.
The cargo should be carried under a suitable protective padding of nitrogen gas. An automatic nitrogen make-up system is to be installed to prevent the tank pressure falling below 0,07 bar in the event of product temperature fall due to ambient conditions or malfunctioning of refrigeration system. Sufficient nitrogen is to be available on board to satisfy the demand of the automatic pressure control. Nitrogen of commercially pure quality (99,9 % by volume) is to be used for padding. A battery of nitrogen bottles connected to the cargo tanks through a pressure reduction valve satisfies the intention of the expression “automatic” in this context.
The cargo tank vapour space should be tested prior to and after loading to ensure that the oxygen content is 2 % by volume or less.
A water spray system of sufficient capacity is to be provided to blanket effectively the area surrounding the loading manifold, the exposed deck piping associated with product handling and the tank domes. The arrangement of piping and nozzles is to be such as to give a uniform distribution rate of [10 ℓ/m2 per minute]. The water spray system is to be capable of both local and remote manual operation and the arrangement is to ensure that any spilled cargo is washed away.
Remote manual operation shall be arranged such that remote starting of pumps supplying the water spray system and remote operation of any normally closed valves in the system can be carried out from a suitable location outside the cargo area, adjacent to the accommodation spaces and readily accessible and operable in the event of fire in the areas protected. Additionally, a water hose with pressure to the nozzle, when ambient temperatures permit, shall be connected ready for immediate use during loading and unloading operations.
Note: For ships trading to the territorial waters of the United States of America the relevant requirements of the U.S. Coast Guard should be complied with, i.e. the water spray system required by above must operate automatically in case of a fire.
In cases where polymerization of vinyl chloride is prevented by addition of an inhibitor “Inhibition” is applicable. In cases where no or insufficient inhibitor has been added, any inert gas used for the purposes of “Exclusion of air from vapour spaces” should contain not more oxygen than 0,1 %. Before loading is started, inert gas samples from the tanks and piping should be analysed. When vinyl chloride is carried, a positive pressure should always be maintained in the tanks, also during ballast voyages between successive carriages.