Guidelines for Chemical Cargo Handling – Procedures for Butane, Butadiene, VCM, and Oxide Mixtures

Chemical Cargo Handling encompasses the best practices for transporting various hazardous substances to ensure safety and compliance. Proper loading and discharging procedures are critical, especially for chemicals like Butadiene and Vinyl Chloride Monomer (VCM). Each chemical requires specific handling techniques and safety measures to minimize risks during transport.

Understanding the physical properties of each substance is key to managing them effectively. Additionally, special regulations apply when dealing with volatile compounds like Ethylene Oxide, necessitating stringent safety protocols. By adhering to these guidelines, companies can ensure the safe and efficient transport of chemical cargoes.

Introduction

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 pressurized 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 Efficient Cargo Tank Operations – Inerting, Purging and Cooling Techniquescargo tank temperatures I pressure which are required for arrival at the load port.

Cargo loading can be carried out using either of vapor return line, the ship’s reliquefaction plant, or both. Where loading is carried out with a vapor return facility, liquid is taken on board through the liquid header and directed into the appropriate tanks. Vapors generated are returned ashore via the vapor 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 vapor; it may also be constrained by velocity through the ship’s piping system. Where no vapor 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 vapor return is provided will normally be higher than without vapor return.

NOTE THE COMPANY/CHARTERS MUST ALWAYS BE ADVISED PRIOR TO USING A SHORE VAPOR RETURN AS THIS MAY HAVE IMPORTANT IMPLICATIONS AFFECTING THE CHARTER PARTY/ LETTER OF INDEMINITY MUST BE SERVED TO THE TERMINAL BEFORE ACCEPTING VAPOR RETURN, OR IT MAY BEAR OTHER COMMERCIAL CONSIDERATIONS.

All unused manifolds are to be blanked & all bolts secured. When Chemical Composition and Physical Properties of Liquefied Gasesliquefied 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 pipe work. The propane may therefore arrive at the ship’s rail at a temperature of -43,5 °C. When loading without vapor return, the vapor 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 vapor. Depending on the efficiency of the purging operation, significant quantities of incondensibles may be present and without vapor return to shore these incondensibles will have to be vented from the condenser. Care must be taken when venting incondensibles to minimize venting of cargo vapors 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 vapor 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 vapor 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 vapor 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 vapor 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 pipe work is to be drained back to the cargo tanks. The liquid remaining can be cleared by blowing ashore with vapor 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 depressurized, 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. De-ballasting can therefore take place simultaneously with loading subject to regulations. Ship stability and stress are of primary importance during loading and de-ballasting and procedures are in accordance with normal tanker practice.

Loading Procedures

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.

NOTE: «ROLL-OVER» 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 vapor. 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 State 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:

Where:

• VL = maximum loading capacity;
• V = tank volume;
• dr = Relative density of cargo at the reference temperature;
• dl = Relative density of cargo at the ldg temperature & pressure.

NOTE: Reference temperature defined as per paragraph 15.1.4 IMO Code.

Variable 0,98 × 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 vapor 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 reliquefication plant and recirculated 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 de-ballasted. 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. NB Vessels with spherical or cylindrical tanks can only use ballast to correct list.

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. As a precaution against cargo pumps becoming blocked with ice they must be rotated periodically.

VCM/Butadiene Cargoes

Because of their specific Measurement and calculation of cargo on gas carriercharacteristics, VCM and butadiene require special attention during cargo operations. This section of the manual highlights characteristics and procedures of the two cargoes which differ from the procedures included in the other sections of the manual.

Vinyl Chloride Monomer (VCM)

Vinyl Chloride Monomer (VCM) is carried as a liquid in certain LPG ships, which have been converted or built for the purpose.

VCM is a chlorinated hydrocarbon and has flammability limit of 4-33 % by volume and toxicity characteristics similar to many cargoes of hydrocarbon origin. In addition, however, long-term exposure to high concentrations has been linked with a rare form of cancer, the TLV has therefore been set at 2 ppm – a level well below the bottom of the flammable range. VCM has an odor threshold of 250 ppm and therefore smell cannot be relied upon as a means of detection.

It must be noted however, that to date no cases have been recorded relating to exposure to the monomer. The only cases have been linked with production processes, and in particular to cleaning of the autoclaves used in the production of PVC.

Nevertheless, it is policy that exposure levels should not exceed 10 ppm unless suitable protection such as CABA and possibly chemical suits are used.

Ships, which are in the VCM trade for all or part of the time, have specialized detection and protective equipment. In addition, modified emergency procedures have been developed to deal with the problems, which may arise from the presence of the gas.

a) VCM is a colorless liquid with a characteristic sweet odor. It is highly reactive, though not with water, and may polymerize in the presence of oxygen, heat and light. Its vapors are both toxic and flammable. Aluminium alloys, copper, silver, mercury and magnesium are unsuitable for vinyl chloride service. Steels are, however, chemically compatible.

b) Extract from LMO Regulations I USCG

With VCM – the person in charge of cargo transfer operations shall ensure that:

• Cargo vapors are returned to the cargo tank or shore installation for reclamation or destruction during cargo transfer;
• Continuous monitoring for vapor leaks takes place during all cargo transfer operations. Fixed or portable instruments may be utilized to ensure that personnel are not exposed to VCM vapor concentrations in excess of 1 ppm averaged over any 8 hours period or 5 ppm averaged over any period not exceeding 15 minutes. The method of monitoring and measurement must have an accuracy (with confidence level of 95 %) of not less than plus or minus 50 % from 0,25 through 0,5 ppm, plus or minus 35 % from over 0,5 ppm through 1,0 ppm, and plus or minus 25 % over 1,0 ppm.
• Cargo transfer operation is discontinued or corrective action is initiated by the person in charge to minimise exposure to personnel whenever a VCM vapor concentration exceeds 5 ppm for over 15 minutes, action to reduce the leak can be continued only if the respiratory protection requirements of 29 CFR 1910.93q (g) are met by all personnel in the area of the lea
• Those portions of cargo lines which will be open to the atmosphere after piping is disconnected are free of VCM liquid and that the VCM vapor concentration in the area of the cargo piping disconnect points is not greater than 5 ppm.
• Any restricted gauge fitted on a tank containing VCM is effectively out of service by locking or sealing that device so that it cannot be used; and
• A restricted gauge is neither to be used as a «check» on the required closed gauge nor as a means of sampling.
• Sign bearing the legend: «PROTECTIVE EQUIPMENT REQUIRED» & «AUTHORISED PERSONNEL ONLY» must be posted whenever hazardous operations, such as tank cleaning, are in operation.
• A tank ship undergoing cargo transfer operations must be designated a «regulated area» having access limited to authorized persons and requiring a daily roster of authorized persons who may board the ship.
• Employees engaged in hazardous operations, such as tank cleaning, must be provided and required to wear and use respiratory protection in accordance with the provisions of 29 CFR 1910.93q (g) and protective garments, provided clean and dry for each use, to prevent skin contact with liquid VCM.

Butadiene

Butadiene is an unsaturated hydrocarbon. It is a colorless liquid with a faint, sweetish characteristic odor. The odor threshold is 1 000 ppm & TLV is 10 ppm only. It is a substance suspected of carcinogenic potential in man. It is policy that exposure levels should not exceed 10 ppm unless suitable protection such as CABA and possibly chemical suits are used.

Water is soluble in butadiene, particularly at elevated temperatures. On cooling water-saturated butadiene the solubility of the water decreases and the water will separate out as droplets which will settle as a layer in the bottom of the tank. For instance, on this basis, for a 1 000 cu m tank, 100 cc of free water would require to be drained from the bottom of the tank. On further cooling to below 0 °C this layer of water would increase in depth and freeze.

Butadiene is chemically more reactive than saturated hydrocarbons and may act dangerously with chlorine. It can be chemically reactive with materials of construction. It is chemically incompatible with:

• copper;
• silver;
• mercury;
• magnesium;
• aluminium and monal.

Butadiene streams often contain traces of acetylene which can react to form explosive acetylides with brass and copper.

Loading – VCM & Butadiene Cargoes.

a) All inhibited liquid must be removed before a ballast passage between consecutive cargoes. If a second cargo is to be carried between such consecutive cargoes the reliquefaction plant must be thoroughly drained and purged before loading the second cargo, if compatible. Practical steps are to be taken to ensure that polymers do not accumulate in the ship’s system.

b) Before loading VCM/Butadiene all air is to be excluded from the tanks by purging with Nitrogen. The quality of the nitrogen is to be sampled and demonstrated to have an oxygen content not exceeding 0,1 % by volume.

c) All connections are to be purged with nitrogen.

d) When loading 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 is to be used.

As a precaution against cargo pumps becoming blocked with ice, they must be rotated periodically.

Cooling Down – VCM & Butadiene Cargoes.

a) When carrying VCM and butadiene cargo temperatures are to be controlled so as to maintain a positive pressure in the tanks.

b) VCM and butadiene cargoes are carried with inhibitors to prevent polymerization.

c) During cargo loading, the reliquefaction plant can normally be operated without inter-cooling, but if the gas temperatures are approaching their upper limit, VCM 90 °C; butadiene 60 °C; inter-cooling will be necessary. If incondensibles cause an unacceptable increase in condenser pressure, these are to be returned with the condensate into the tank, or discharged ashore.

d) During reliquefaction the inhibitor is present only in the liquid phase, i. e. it will not be present in the condenser during a normal open reliquefaction cycle, and the condensate will not contain it either, and as polymerization is accelerated by high temperatures, polymerization will occur downstream from the compressor discharge, which includes the condenser, piping and cargo tanks. Prevention is to supply stabilized liquid, i. e. one containing inhibitor into the condensers using the following procedure.

After cooling down, inhibited cargo liquid can be circulated with a deep well pump via the liquid line to the condenser, returning to the tank via the condensate line – the condenser is to be kept approximately half full during this operation.

At the same time, gas is swept from another tank by a compressor into the condenser to pressurize the inhibited liquid back to the tank. During this operation a careful watch must be maintained on the distribution of condensates back to the tanks to avoid overfilling. Normally it is possible to re-liquefy VCM and butadiene without the need for inter-cooling. On completion of this operation the lines must be emptied into the tanks and blown through with gas.

NOTE 1: The solubility of VCM in lubricating oil increases as temperature rises and pressure drops. Pressure in the compressor crankcase must not exceed 1 bar – achieved by controlling the first stage compressor suction pressure to a maximum of 0,5 bar, and the glycol heating system must be in operation to prevent condensation, even if the compressors are shut down. Temperatures should be maintained at approximately +40 °C.

NOTE 2: Ensure lubricating oil is compatible.

NOTE 3: When compressing VCM and butadiene, the compressor discharge temperatures must not exceed 90 °C and 60 °C respectively. The compressor suction pressure is not to be allowed to exceed 1,5 barg in either case.

Discharge VCM. Prior to discharging, all connections must be purged with nitrogen. Due to its high density VCM must not be discharged with deep well pumps running in series with booster pumps. If, during stripping, it becomes necessary to increase pressure above the liquid residues by introducing gas from another tank, the pressure difference between the two tanks is to be approximately 2,5 bar.

Propylene Oxide & Ethylene Oxide – Propylene Oxide Mixtures

a) Because of their specific characteristics PO and EO/PO mixes require special attention during cargo operations. This section of the manual highlights characteristics and procedures of these cargoes which differ from the procedures for products included in the other sections of the manual.

In the following descriptions the term PO is used for both PO and EO/PO mixes.

b) The transport of PO is only permitted with a valid certificate issued by the responsible authority, (e. g. USCG). The certificate is only issued after inspection of the tanks and plant, for which they will need to be gas-free and clean. During transport the deck spray system must be ready for use at all times.

c) Residues of previous cargoes have to be completely removed before loading PO as they can cause a dangerous reaction. Rust and encrustations, which may contain gas pockets must be removed, and all tank filters and pipes are to be cleaned and thoroughly dried.

d) Cargo holds must be purged with inert gas having an oxygen content < 0,2 %, or nitrogen, until the atmosphere contains < 0,3 % oxygen. The cargo is carried under a padding of 99,9 % nitrogen. The Charterer’s requirements must be noted in this respect.

e) The only other cargoes allowed to be carried simultaneously with PO are propane and butane.

f) PO must only be loaded and unloaded directly, i. e. with this plant it must not:

• Be loaded by the compressors, or
• Heated with the heat exchangers or cooled by the compressors.

This is affected by removing spool pieces so that there is only a liquid line into the tanks and a gas line out. All other lines and plant, is then isolated.

Physical Property

Propylene oxide (PO) has wide flammable limits of 2,8 – 37 % by volume. The TLV of this product is 50 ppm whilst its odour threshold is 200 ppm. Exposure to the liquid or high concentrations of the vapor can lead to eye burns, skin irritation and blistering, vomiting, lack of co-ordination and depression.

Special Requirements from IGC

a) 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 to the design of the flame screens and vent heads to the possibility of the blockage of these devices by the freezing of cargo vapor or by icing up in adverse weather conditions. Ordinary protection screens are to be fitted after removal of the flame screens.

b) Maximum Allowable Quantity of Cargo per Tank. When carrying a cargo referenced to this section, the quantity of the cargo is not to exceed 3 000 Cu mtrs in any one tank, (tank means tank compartment).

c) Propylene Oxide and Mixtures of Ethylene Oxide-Propylene Oxide with Ethylene Oxide content of not more than 30 % by weight:

• Products carried under the provisions of this section must be acetylene-free.
• Unless cargo tanks are properly cleaned, these products are not to be carried in tanks which have contained as one of the three previous cargoes any product known to catalyse polymerization, such as: Ammonia, anhydrous and ammonia solutions; Amines, and amine solutions; Oxidizing substances, e. g. chlorine.
• Before loading, the tanks are to be thoroughly and effectively cleaned to remove all traces of previous cargoes from tanks and associated pipe work, except where the immediate prior cargo has been PO or PO/EO mixtures.
• In all cases, the effectiveness of cleaning procedures for tanks and associated acidic or alkaline materials remain that might create hazardous situations in the presence of these products.
• Tanks must be entered and inspected prior to each initial loading of these products to ensure freedom of contamination, heavy rust deposits and any visible structural defects. When cargo tanks are in continuous service for these products, such inspections are to be performed at intervals of not more than two years.
• Tanks for the carriage of these products must 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 pipe work systems by washing or purging.
• All valves, flanges, fittings, and accessory equipment must be of a type suitable for use with these products and are to be constructed of steel or stainless steel or other material acceptable to the Administration. The chemical composition of all material used is to be submitted to the Administration for approval prior to fabrication. Discs or disc faces, seats and other wearing parts of valves must be made of stainless steel containing not less than 11 % chromium.
• Gaskets must 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 behaviour. The surface presented to the cargo should be POLYTETRAFLUORETHYLENE (PTFE) or materials given 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 Administration.

Spirally wound gaskets are required to be fitted to cargo tank lids by the USCG.

• The following materials are generally found unsatisfactory for gaskets, packing and similar uses in containment systems. These products would require testing before being approved by the Administration:
  • NEOPRENE or NATURAL RUBBER, if it comes in contact with the products;
  • ASBESTOS or binders used with ASBESTOS;
  • Materials containing OXIDES of MAGNESIUM, such as mineral wools.
• Filling and discharge piping is to extend to within 100 mm of the bottom of the tank or any sump pit.
• The products are to be loaded and discharged in such a manner that venting of the tanks to atmosphere does not occur. If vapor return to shore is used during tank loading, the vapor 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 must be maintained above 0,07 bar gauge.
• The cargo is to be discharged only by deep well pumps, hydraulically operated submerged pumps, or Inert Gas Generatorinert gas displacement. Each cargo pump is to be arranged to ensure that the product does not heat significantly, if the discharge line of the pump is shut off or otherwise blocked.
• Tanks carrying these products must 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 must be marked.

For Ethylene Oxide Transfer Only

• Hold spaces are to be monitored for these products.
• Prior to disconnecting shore-lines, the pressure in liquid and vapor lines must be relieved through suitable valves installed at the loading header. Liquid and vapor from these lines is not to be discharged to atmosphere. All connections must be purged with nitrogen.
• Pressure relief valve settings must not be less than 0,2 bar gauge and for type C independent cargo tanks not greater than 7,0 bar gauge for the carriage of propylene 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 separated from the 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 the product is not independent, the required piping separation must be accomplished by the removal of spool pieces, blank flanges, or other pipe sections and the installation of blank flanges at these locations. The required separation applies to all liquid and vapor piping, liquid and vapor vent lines and any other possible connections such as common inert gas supply lines.
• The products are to be transported only in accordance with the cargo handling plans that have been approved by the Administration. Each intended loading arrangement is to be shown on a separate cargo handling plan. Cargo handling plans must 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 is to be kept on board the ship. The Certificate of fitness for the Carriage of Liquefied Gases in Bulk is to be endorsed to include reference to the approved handling plan.
• Before each initial loading of these products, and before every subsequent return to such service, certification verifying 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 connection between a blank flange and pipeline flange must be fitted with wire, and sealed by the responsible person to ensure that inadvertent removal of the blank flange is impossible.

The «responsible person» may be, for example, the ship’s master or the society’s local surveyor.

Read also: Comprehensive Overview of LNG and LPG Cargo Hoses in STS Operations

• The maximum allowable tank filling 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 Administration. A copy of the list is to be permanently kept on board by the master.
• The cargo must 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 gauge in the event of product temperature falling due to ambient conditions or malfunctioning of the refrigeration system. Sufficient nitrogen must 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 vapor space must 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, 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 litres/sq metre/minute. The water spray system is to be capable of both local and remote manual operation and the arrangement should ensure that any spilled cargo is washed away. 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.

For ships trading to the territorial waters of the USA the relevant requirements of the USCG must be complied with, i. e. the water spray system required by 17.20.17 must operate automatically in case of fire.

Loading PO/PO – EO Mixes

The following conditions must be observed before loading PO:

a) PO must not be loaded into tanks which contained during one of the last three voyages either ammonia, amines, caustic solutions or other products above to react with PO A proof of the last ten cargoes must be kept on board.

b) Washing water used in the tanks must not contain solvents or additives which have not been authorized by a competent technical supervision authority. The last washing has to be done with clear, pure water. Samples of the last purge water are to be handed to the responsible person or to the terminal before loading.

c) The ship must have her own independent nitrogen system for padding the tank after loading. There is to be sufficient nitrogen available on board to guarantee a minimum tank pressure of 5 psi, (0,35 bar).

d) Sealing material has to be approved and controlled by the competent technical supervision authority. All tanks and pipes are to be clean, dry and prepared for inspection after arrival at the loading port. Before loading the tanks and pipes have to be inspected and approved by the responsible person of the shore terminal or by the marine inspector.

e) All air must be purged from the tanks and pipes with pure nitrogen or inert gas, so that the oxygen content is < 0,3 %, and the carbon dioxide content < 1 %.

On board generated CO₂ must not be used.

The quality of the inert gas must be confirmed by laboratory analysis and approved by the responsible person of the shore terminal.

f) All open connections or pipes and valves must be closed or equipped with blind flanges. Loading hoses must be approved and marked by the responsible authorities.

They must not be used for any other product.

g) The pipe systems to be used for loading cargo, including the return lines for liquids and gases must be entirely segregated from the main cargo handling system on board ship.

h) Cargo tanks must not be filled to more than 98 % of the tank capacity, relative to the density of the gas at the set pressures of the safety valves.

For PO/EO mixtures the pressure must be not greater than 5,3 barg.

The maximum volume to which tanks may be filled is calculated using the following formula:

Where:

• VL = Maximum volume to which tank may be loaded.
• V = Volume of the tank.
• Pr = Relative density of cargo at the reference temperature.
• Pl = Relative density of cargo at the loading temperature and pressure.

NOTE: Reference temperature defined as per paragraph 15.1.4 IMO Code.

On completion of cargo loading, and after blowing liquid residues from the piping system ashore with nitrogen, the system must be purged with nitrogen from a shore supply or the on board bottles. Finally the tank nitrogen padding is introduced.

Transport of PO/PO – EO Mixes

PO must be transported under a padding of nitrogen, and because it has a relatively high boiling temperature of +34 °C at atmospheric pressure, pressure in the tanks should be at least 0,35 bar above atmospheric. Pressure is to remain constant throughout the loaded voyage by feeding nitrogen as required. A nitrogen supply system is installed for this purpose.

Safety valve setting for transportation of PO/PO mixes must be in accordance with gas plant supplier’s instruction manual, usually USCG requirements.

Discharging of PO/PO – EO Mixes

During discharging operations the following conditions must be observed:

a) The tanks and piping system must be segregated from all other cargo systems on board ship.

b) Over-pressure in the tank must not fall below 0,07 bar. Unloaded liquid is replaced with inert gas or nitrogen from shore.

c) Cargo is discharged using deep well pumps and possibly the boosters. Total discharge can be achieved by displacement with nitrogen and by using the 2” stripping line. Draining and purging of lines requires a high purge gas speed, which is best obtained by using the nitrogen accumulated in the tank.

d) Liquid residues in the deck lines are to be drained into the collectors and discharged ashore.

e) When changing cargo grades, tanks must be totally discharged, water washed and ventilated for inspection.

The inert gas from the stripping phase can be retained in the tanks because washing must be done in an inerted atmosphere. This is to offset the possible formation of static charges during washing, due to the relatively high boiling temperature of the cargo.

f) Tank washing is to be carried out in accordance with the approved Annex II Procedures and Arrangements manual, a copy of which is retained on board the ship.

In the context of the requirements of MARPOL Annex II, provided the tanks have been properly stripped and ventilated dry, any water introduced into the tank for preparing the tank to receive the next cargo can be regarded as being clean, and not subject to the discharge restrictions included in Annex II.

The water for tank washing is supplied from ashore or from an on board soft water generator and will normally be at ambient temperature. Pressure at the spray line connection is to be at least 3 barg above tank pressure. (At 3 bar the flow rate will be approximately 28 cu.mtrs/ hr/tank). Water can be pumped from the bottom of the tanks by deep well pumps into adjacent tanks, or recirculated, and finally ashore, or discharged to sea. This procedure is to be repeated several times. After completely drying and ventilating tanks and pipe systems, the system shall be inspected and the appropriate certificates issued.