2.9 TANK WASHING PLAN

The requirements for tank washing during the ballast voyage are determined primarily by the cargo orders for the next voyage and the cargo previously carried. When the prior cargo is compatible with the next cargo to be carried, tank washing may not be required. At the other extreme, if a clean product voyage follows a black oil cargo then preparation may require extensive tank washing, cleaning and even drying before the tank is ready for the next loading. Other reasons for tank washing include preparations for clean ballast, maintenance cleaning (to prevent sludge accumulation), leak detection, tank or piping repairs and shipyard cleaning. Between 1/4 and 1/3 of the ship's tanks may be washed for the reasons above. In laying out his tank cleaning plan the chief officer considers cargo requirements, ballast, repairs and maintenance. The most efficient program is one which cleans as many tanks as possible for more than one purpose. For example, if the chief officer has the choice of cleaning and de-scaling one of two tanks for a cargo change, one of which has been recently inspected, he should clean the other tank so that it may be inspected during the de-scaling operation. Tanks which were crude oil washed require water washing before they can be filled with 'clean' ballast.

2.9.1 Tank preparation matrix and instructions
A time charterer may be responsible to the cargo owner for the adequate preparation of the ship's tanks to receive cargo, but the owners and master are ultimately responsible to charterer for the condition of the ship's tanks for each voyage. The most important tool for setting out the tank washing program is the tank washing matrix or cargo change matrix. The tank washing matrix is a grid diagram with the 'previous cargoes' arranged in horizontal rows and the 'next cargoes' listed as vertical columns. By finding the intersection between the column corresponding to the last cargo carried and the row headed by the next cargo to be loaded, the chief officer can determine the tank preparation requirements. Tank washing matrices should be provided for both coated and un-coated tanks, as the required preparation is somewhat different. Coated tanks with coating failure of more than 10% of the total surface should be considered and washed as un-coated tanks.

This table is an example of a cargo tank preparation matrix for use on clean product tankers. The recommendations are examples only, and should not be used for guidance when owner's or charterer's tank cleaning instructions are available.

The origin of a tank preparation matrix is the amount of maximum allowable contamination between products which can be permitted while maintaining product quality. Cross contamination between products can affect a number of product criteria such as:
Colour, octane, density, bromine number, lead, sediment, cloud point, flash point, vapour pressure, viscosity, gum, sulphur, thermal stability, odour, pour point, smoke point, carbon content, final boiling point, initial boiling point, water separator index, mercaptan.
It would be difficult for a chief officer to evaluate each of these criteria when changing cargoes. Owner's have done this for him by preparing standard tank washing matrices.

The levels of preparation indicated may include:

No cleaning required Drain lines and pumps Hot water machine wash Ventilate tanks Remove all sludge and scale Mop and ventilate tank Number of washing cycles

Bottom flush and strip dry Cold water machine wash Fresh water rinse Gas free tanks Remove all free water Cleaning impractical Number of washing hours

The table above is an example of a cargo tank preparation matrix for use on black oil tankers. The recommendations are examples only, and should not be used for guidance when the owner's or charterer's tank cleaning instructions are available.

The required preparations may include any one, or a combination of the above procedures. The most stringent preparations are required for refined products, but some mixtures of crude oils can be equally troublesome. Some combinations of crude oils can form nearly impervious residues and some crude oils with special properties, such as nil sulphur, can be contaminated by other crude oil residues.
In every case where a tanker is instructed to load a cargo different from the previous cargo, cleaning instructions must be provided with the orders. The instructions may be a simple as 'charterer's minimum cleaning requirements to be completed', thereby referring the master to the charterer's printed instructions and a tank cleaning matrix for tank preparation. Alternatively, the preparation may be a page of detailed steps. If the master or chief officer has any question about the necessary tank preparation, then the owners must be consulted for clarification. Example tank preparation matrices are included in this section. It is important that the master and chief officer follow the procedures required by the owner/charterer for tank cleaning. If a special circumstance arises which is not covered by the instructions, or which requires special measures, then the situation must be communicated with owners/charterers and the tank(s) cleaned according to their reply.
Tank inspections at the loading port may be performed by independent inspectors (representing the charterer or cargo owner), or by terminal inspectors. Masters should always give serious consideration to the advice of these inspectors and should develop a habit of consulting with them. If a disagreement develops regarding suitability of tanks for loading, then it must be referred to the cargo owner and owner/charterer for resolution. The master may wish to request a surveyor to examine the tanks on the vessel's behalf to determine tank suitability. On more than one occasion, ship's tanks have been rejected by loading terminal representatives because the cargo was not available for loading, or the cargo was so marginally on-specification in the shore tanks that it could not be loaded into any vessel without being put off specification.
Part of the function of the tank cleaning matrix is to ensure that excessive or unnecessary tank washing is not employed. Excessive cleaning wastes time, energy and accelerates tank structure or coating deterioration. In the final decision of the level of cleaning to employ, the chief officer and master must take into account the condition of the ship's tanks and any special considerations relating to the cargo which they are made aware of.

For crude oils, the following guidance is provided:

General crude oil trading
Normally, cleaning is not necessary between different types of crude oils, or between successive voyages. All tanks should be re-stripped to the slop tank after the vessel has departed and reached warmer waters. This step alone will recover most of the residual cargo in the tanks. Crude oil residues may be the most difficult to wash from the cargo tanks. They combine the high hydrocarbon vapour characteristics of gasoline cargoes with a potential for leaving sediment and wax. Sediment must not be allowed to accumulate in the tanks. If heavy sediment was noted on the previous discharge report, the tanks should be bottom washed and checked for proper drainage. On trades involving crudes with heavy sediment, frequent washing of all cargo tanks will be necessary to control the accumulation in tank bottoms. Alternatively, lighter crudes without sediment will require little or no tank washing between cargoes. Tanks which have been crude oil washed require little water washing to prepare them for clean ballast and limited ventilation before entry. When it is important to minimise washing water volumes (ie. when slop disposal ashore is required), then all tanks scheduled for water washing should be crude oil washed at the discharge port.

Salt water contamination of crude oils
Salt is a contaminate of crude oils. It is important to have crude tanks as dry as possible before loading. This is particularly true if the specific gravity of the oil is high, ie. near to that of water. Salt water is difficult to separate from heavy crude oils and it can cause significant problems in the refinery. Some crude oils requiring particular care in this respect are:

Preparation of tanks for these and similar crude oils should be conducted as follows:
a Hot water wash tanks to remove all loose scale, sludge and sediment.
b Flush the entire cargo system with fresh water.
c Strip tanks dry prior to loading and drain all lines and pumps.
d Blow lines with air or inert gas.
e Give all tanks a fresh water wash.
f Drain and blow down lines a second time.
If shore lines are to be displaced to the ship, they should all be received into a single tank. Any slops from the previous voyage should be segregated.
After loading, check each tank for water and record the results. If excessive water is found, give a letter of protest to the loading terminal.

Crude oils with API's near to that of water (API = 10.0)
a Drain all tanks as thoroughly as possible to remove the remains of the previous cargo. A bottom flush or bottom wash is also recommended. Tanks containing heavy deposits of sediment and scale should be gun-cleaned with water.
b Remove all scale /sediment. If tank entry is involved, tanks must be gas freed and rendered 'safe for men and fire'. (See section 2.15 for tank entry procedures.)
c After discharge of ballast at the loading port, and the thorough stripping of tanks and cargo lines, drain all cargo pumps and pipeline systems.
d Purge pumps and lines with compressed air or inert gas, giving due consideration to the relevant safety regulations in force.
e Test all heating coils to make sure they are in good operational condition.

Cutback asphalt
a Since water is a critical contaminant of this cargo, it is essential that no water remains in the cargo system. The entire cargo system to be used for asphalt should be given a line and tank flush with fuel oil prior to loading. If detailed charterer's instructions are not provided, then proceed as follows:
b Arrive with tanks free of residues and gas free. Strip the tanks, lines and pumps as dry as possible. Cargo systems (lines, pumps, and strainers), must be drained, then blown dry. All cargo valves must be opened and drained to the cargo tanks. The tanks must be free of pumpable water.
c At the loading terminal, set the lines for loading and load 200 to 400 barrels of flushing oil into each cargo tank. Then transfer the oil from tank to tank , using the ship's cargo pumps, until all tanks have been flushed. Dispose of oil (ashore or co-mingle with other fuel oil), according to charterer's or owner's instructions.

Crude condensate
Ras Tanura loadings - special problems
Some crude oil condensate which has a Naphtha base can form chemical compositions when loaded into cargo tanks that previously contained crude oil. This occasionally results in the presence of large quantities of un-pumpable asphalt residues at the discharge port. These residues are derived from the interaction of the condensate with the ROB from the previous crude cargo. Asphalt cannot be removed by ordinary cleaning methods, so the cleaning is very expensive. Therefore, naphtha base condensate should never be shipped in unclean tanks containing crude oil ROB.
When condensate is loaded on top of crude oil, or if being topped up with crude oil, the concentration of the lighter component should not exceed 10% by volume of the total crude/condensate mixture. This arrangement must be agreed between owners/shippers/ charterers prior to loading, because:

a A higher concentration may cause a significant fallout of heavy, high boiling hydrocarbons, making it impossible to drain the tanks at the cargo port.
b If a richer mixture is shipped, the high vapour pressure of the cargo may prevent the use of crude oil washing in the discharge port, as the P/V valves will lift. Higher tank pressures will prevent IGS from properly flowing to the tank. Such cargoes should only be carried in SBT vessels.

Crude oils to be used in the manufacture of lube oils or asphalt
a Gun clean all tanks with hot water - one full cycle.
b Thoroughly flush all pumps and pipelines with water and pump as dry as possible.
c Remove all sediment, loose scale and sludge from tank bottoms. If tank entry is required, tank atmosphere requirements for 'safe for men; safe for hot work' must be met.
d Rinse the entire cargo system with fresh water. If time allows, give all tanks a fresh water rinse.
e Strip all tanks as dry as possible. Drain all water from pumps and pipelines prior to loading. Remove cargo piping drain plugs and pump drain plugs during pre-loading inspection at the loading berth.

Fuel oil/'black oil' cargoes
Thorough draining is required when changing from crude oil to fuel oil, or from a fuel oil with a higher number to one of a lower number, such as from No.6 oil to No.4 oil.
Draining is not required when changing from a fuel oil with a lower number to one with an equal or higher number.
Accumulation of sediment noted on a discharge report is an indication of the need for bottom washing of black oil cargo tanks.
Absence of cargo preparation orders
If cargo loading orders have not been received at the time the ballast voyage begins, the ship should perform only the minimum necessary preparations, including:
• Flush tank bottoms with water and strip flushings to the slop tank.
• Wash pumps and pipelines to the slop tank.
• Wash necessary tanks for clean ballast (if required).
• Load clean ballast (if required).
• Decant dirty ballast, stripping residues to slop tank in accordance with MARPOL requirements.
• Flush pumps and pipelines to the sea.

2.9.2 The pumpman and deck crew
The tank washing operation requires the closest cooperation between the chief officer, pumpman, and the deck crew to operate efficiently and obtain the required result. The best results will be obtained if the chief officer prepares a preliminary plan and discusses the plan with the pumpman and boatswain. The plan should be detailed enough to indicate the times of starting and finishing the washing operation in each tank and the sequence of tanks to be washed.
It is not unusual for the pumpman to have completed more tank washing operations on the vessel in question than the chief officer. In that case, his assistance is invaluable with respect to knowing how well the stripping pumps or eductors work, how well the tanks will drain and which tank washing machines may present problems. Problem tanks should be scheduled for daylight hours; easy ones can be done at night. The deck crew may be able to offer suggestions on handling of the portable tank washing machines, such as where they have been known to become entangled in tank structure when the ship is rolling. Including the crew in the planning of the job makes them more interested in seeing it done well. With proper planning and scheduling, an experienced crew is capable of conducting all of the mechanical aspects of the tank washing operation with minimal, general oversight by the chief officer.
When the chief officer has reviewed his plan with the pumpman and crew, he adds the final details, then discusses it with the master. The final plan should include both a diagram of the program and a detailed schedule. On the diagram, mark the time each tank will start, the times of program changes, or portable machine 'drops' and the time it will finish. Allow sufficient time between tanks for stripping. Indicate the time when tanks may be sounded to measure residues (not less than 30 minutes after finish washing and only with a grounded probe). When the master has approved the plan, copies are distributed to the watch officers, chief engineer, engine control room, pumpman, boatswain and one copy is posted on the crew's (company), bulletin board.
With the program set out in advance, the chief officer is free to concentrate on verifying the safe conduct of the operation, checking the operation of fixed or portable machines and ensuring that the next tank is set up for washing before the previous tank is finished. His primary objective is the safety of the crew. This is particularly important when washing with portable machines. The portable machines are heavy, and there are many opportunities for the crew to create hazardous static electricity conditions if their actions are not properly monitored and correct procedures followed. When washing with portable machines, the crew is exposed to the dangers of broken feet and hands, hot water burns and tank explosions. The crew must be carefully instructed in safe work practices and provided with the necessary protective equipment of prevent injury. All personnel on board should be notified that tank washing operations are to commence and that all safety precautions as when loading must be observed.
A basic tank diagram chalkboard should be set up on the main deck (one may be conveniently painted on the side of the superstructure near the pumproom). The tank washing plan is marked on the chalkboard. The deck watch marks the chalk board to reflect the current progress of the tank washing.
If the tank washing program is extensive, or the ballast voyage short, the tank washing operation should be started immediately after dropping the outbound pilot and continue nonstop until it is completed. Three or four men are required in each hose handling operation, so the watch and crew rotation will need some careful consideration. Handling the heavy machines and hoses repeatedly for eight to twelve hours at a time is demanding work.

2.9.3 Tank washing machines
Portable machines
When tankers were smaller and crews were larger, all tankers were cleaned with portable tank washing machines. Portable machines are connected to the end of a special tank washing hose. The machines have a bronze body and the hose couplings are bronze. The hoses are marked at intervals of five feet or two meters. A natural fibre line must be attached to the machine for support and ease of handling, even though the hose is fully capable of supporting the machine unaided. The other end of the hose is connected to a hydrant on the deck tank washing line or fire line. The water should be started and the hose flushed through before the machine is placed in the tank. The machine is then lowered into the tank through a tank washing opening and supported by a 'saddle' placed over the opening. The hose is clamped into the saddle and the machine tether line is made up on the saddle clamp to support the machine.
Tank washing openings are strategically placed in the main deck to permit good coverage of the tank's inner surface without permitting the machines to become entangled in the tank's internal structure. The tank cleaning openings are normally covered by bolted circular covers with gaskets. The bolts are removed from all the plates to be used, but the plates should be kept in place until a machine is ready to be inserted into the opening and the plate replaced promptly after the machine is removed. The distance the machine is lowered into the tank depends on the size of the tank and the amount of washing required. For heavy cleaning, the machine may be placed at three or four successive levels ('drops'), depending on the complexity of the tank structure. Each drop should be just above one of the main structural members, so that the upper surface of these side longitudinals or transverse members are thoroughly cleaned. If the cargo tank is known to have a heavy accumulation of sediment, the machines should be lowered to the bottom first and washed for ten minutes to clear the limber holes of sediment, then raised to begin the normal wash.
At each level, the machines are operated through a full washing cycle. The cycle time is determined by the washing pressure. When all machines are ready, the engine room is asked to start up the tank washing pump and if necessary the tank washing heater. Sea water is pumped at high pressure from the engine room, through the heater (if required), into the deck washing or fire line and into the tank washing hoses. The force of the water passing through an impeller in the machine causes the offset nozzles to rotate in a vertical plane and the whole machine to rotate in a horizontal plane. The jets of water from the nozzles trace a pattern on the inside of the tank which provides effective coverage of the entire surface within range of the jets. The maximum range of the jets is normally nine to ten meters. The washing pattern described by the dual nozzle rotation resembles a ball of twine.
Cycle times for the washing machines are a function of nozzle size and washing water pressure at the machine. The pressure at the machine will be lower than the pressure at the pumproom or in the engine room. The only way to accurately measure the pressure is to have a portable gauge fitted to the wash water supply line on a spare tank washing outlet near the machine hose connections. A typical product carrier tank washing machine with 12 mm nozzles will have a cycle time of 38 minutes at 5 kg/cm sq. (75 psig) and a cycle time of 28 minutes at 12 kg/cm sq. (175 psig).
Higher pressures will produce higher flow rates. The same product carrier machine will use 20 tons of water per hour at 5 kg/cm sq. and 30 tons per hour at 12 kg/cm sq.

Example performance curves of a portable tank washing machine.

For effective tank washing the number of machines used at one time must not put more water into the tanks than the stripping pump is capable of removing. By referring to the discharge rate curves for the machines on his ship, the chief officer can determine the maximum number of machines his stripping pumps can keep up with.
Complete coverage of the tank surface may not be possible with machines lowered through the dedicated tank washing openings. It may be necessary to lower a machine through the tank hatch, or to actually enter the tank and tie off a machine in a corner of the tank to remove accumulations in hidden corners. Confined space entry procedures must be used!
It is a good idea to leave the main cargo suction/fill valve open while the tank is being washed, to drain any residual cargo from the branch line into the tank. If clean ballast is to be loaded, each tank should receive a short flush from the sea immediately before or when starting the washing operation.

Fixed machines
Tank ships larger than 60,000 tonnes are difficult to wash effectively with portable machines. The number of deck openings required to provide adequate coverage and the depth of the tanks would require a long and strenuous effort which today's smaller crews are not capable of supporting. Tank washing capability for these larger tankers is provided by fixed tank washing machines, sometimes referred to as 'guns'. Fixed tank washing machines are mounted on the deck of the ship with control machinery outside the cargo tank and the rotating parts and spray nozzle inside the tank. Fixed washing machines are capable of higher washing pressures and longer effective reach than the portable machines. On crude carriers, they are carefully placed to provide a washing pattern which covers the maximum amount of the tank surface by direct impingement of the washing stream. In accordance with IMO COW system requirements (section 4.2.8), the shadow areas of the tanks shielded from direct impingement cannot exceed 10% of the tanks horizontal surfaces and 15% of vertical surfaces. The washing sequence of the cargo tank is programmed into each machine, depending of its location in the tank. A tank cleaning operation with fixed machines requires only that the machines be set to the correct starting position and operated in the correct order, to provide a complete cleaning of the tank. The fixed machines can be used to crude oil wash (COW), or to water wash the cargo tanks, according to the purpose of the washing. Because of the static electricity hazards of high-capacity tank washing machines, they must only be operated in properly inerted tanks (see section 2.10.3).

2.9.4 Washing pressures, temperatures and times
The tank washing operation consumes energy and time and erodes the tank coating or tank structure. It is important that the washing be thorough enough to properly prepare the tank(s) for the next cargo, but not excessive. Washing tanks with too high pressure or temperature, or for excessive time will not improve the result and may cause coating or steel deterioration which, if repeated often enough, eventually becomes significant damage.
Maximum tank washing pressures are often dictated by the capacity of the tank washing pump. Higher pressures will reduce machine cycle times and the overall tank washing time. High pressures will produce a more thorough cleaning. Washing pressure of 12.6 kg/cm sq (180 psi), is most desirable for washing crude or fuel oils from un-coated tanks. For washing clean product tanks, pressures of 9 to 10 kg/cm sq (130 -140 psig) are suitable. Washing pressure must be maintained below the maximum allowed for the tank coating material in coated tanks. To properly measure the tank washing pressure, a gauge must be fitted in the tank washing line near to the machine connection. Chief officers should refer to the owner's operating instructions to determine maximum washing pressures. If no guidance is provided, it should be requested from the owners.
Unless there is a special requirement, tanks should be washed with cold (ambient) sea water. Cold water washing will normally leave behind only a waxy skin, which provides a protective film for the steel. Tepid water (between 30 and 65 °C), is of no practical benefit since it has the disadvantages of both hot and cold water without providing the benefits of either.
Maximum tank washing temperatures are often dictated by the capacity of the tank washing heater and the number of machines in use. Higher temperatures will increase the effectiveness of difficult washing situations. Washing temperature must be maintained below the maximum allowed for the tank coating material in coated tanks. Modern coatings which have been correctly applied to properly prepared surfaces should have no washing water temperature limitation and some charterers will not accept any. If maximum permitted temperature information is not available for a ship with coated tanks, then use an upper limit of 120 °F (49 °C). Special caution is required if a coated tank to be washed has carried a product (such as benzene or toluene), which can soften the coating. Such tanks should not be cleaned until the coating has 'cured' after the discharge of the cargo. To properly measure the tank washing temperature, a thermometer may be fitted in the tank washing line near to the machine connection, or a bucket filled from the washing line and the temperature measured by dip thermometer. Chief officers should refer to the owner's operating instructions to determine maximum washing temperatures. If no guidance is provided, it should be requested from the owners.
Tank washing times must be calculated based on the washing pressure at each machine. For portable machines, the tank should be washed until the machine with the lowest pressure has been able to complete the full cycle. The portable machines may then be lowered to the next 'drop' level. Only by permitting the machines to complete their full cycle is complete washing ensured. Portable machines are usually left at each drop for a complete cycle, but may be operated at the lowest drop for two cycles to remove heavy residue or scale accumulation from the tank bottom.
It is important to check the position indicators on fixed machines to ensure that they have completed their cycle before stopping the flow and starting the next machine.

2.9.5 Handling portable tank washing machines and hoses
Portable tank washing machines and hoses are rugged, precision equipment, which require proper care and handling to provide safe and effective performance. Machines should never be dragged along the decks; the best way to move them from storage to the tank washing area is by use of a hand truck. They should not be used in tanks with complex structure while the ship is rolling. The rolling may cause the machines to strike tank framing; this could damage the machine, damage tank coating and cause dangerous sparks. The portable machine should be fitted with a natural fibre line of up to 16 mm diameter. This line is used to lower and retrieve the machine from the tank, instead of lowering and pulling it out by the hose. Machines should be flushed with fresh water after use and dried or given a coating of oil according to the manufacturers instructions. The pumpman should be given training to properly maintain the machines and an adequate supply of spare parts to do so.
Tank washing hoses should be drained and stored in a protected area between washing operations. Tank washing hoses couplings should be tightened and disconnected from the washing main using wrenches only, (not hammered on). Regular replacement of the washers will prevent leaking. To prevent static electricity hazards, the hose bonding wire continuity must be verified before each use and the hoses must remain connected to the washing main at all times while the washing machine is in the tank.

2.9.6 Tank coating protection
Modern epoxy tank coatings are tough and resistant to all of the cargoes and tank washing procedures to which they would normally be exposed. However, there are limitations associated with each coating and it is important that these limitations be readily available to the chief officer and that he ask for clarification of any questionable circumstances. For example, certain types of coatings are not approved for carriage of some jet fuels. Both the coating and the cargo will be damaged by loading such fuels into the coated tanks. Coatings are also subject to damage by washing with too high temperature and/or too high pressure water. To prevent such damage, the pressure and temperature limits should be readily available to the chief officer and posted at the tank washing heater/pump in the engine room. The chief officer must remember that a ship's tank coatings represent a significant portion of the overall cost of the ship and require his diligent care to protect them from damage.

2.9.7 Washing with detergents
Detergents should be used only when absolutely necessary. They are normally applied to difficult tank washing situations, where absolute purity of the next cargo is required. They are injected into the tank washing line by a mechanical pump, mix with the washing water and act along with the water jet to remove the previous cargo clingage and residue. When using detergents, the chief officer must:

Detergents are harmful or poisonous to marine life and should not be discharged overboard. The presence of detergent in the wash water increases the potential for static electricity accumulation. To avoid dangerous accumulations of static electricity, used wash water containing detergent should not be recirculated from the slop tank for tank washing.
Many loading terminals will not accept slops containing detergents. For that reason alone, detergents should not be used unless absolutely necessary.

2.9.8 Washing bunker tanks
Washing bunker tanks is not normally required except to remove contaminated bunkers, or to clean the tank for repairs. Bunker tanks are not usually set up with the necessary tank washing openings and the draining of wash water must be accomplished with the bunker transfer pump. For these reasons, bunker tank washing may require even more planning than a normal cargo tank washing. Hoses must be rigged to transfer the wash water from the bunker system to the cargo stripping system. Since the bunker tank opening may be in the engine room, special ventilation or ignition control procedures may be needed to ensure that there is no opportunity for an explosive atmosphere to be created and ignited.

2.9.9 Inert gas system and vent lines
Part of cleaning a tanker for repairs or shipyard period involves washing out all tank vent lines and inert gas lines. Even if cargo has not entered these pipelines as a liquid, cargo vapours are liable to condense in the lines and form accumulations of hydrocarbons. Cargo tank vent lines are fitted with washout connections permitting them to be flushed back to the cargo tank before the cargo tank is washed.
The same flush-out connections are fitted to the IGS line. A tank washing hose should be connected to each flush-out connection and the branch lines flushed either into the cargo tank (before tank washing) or back to the deck non-return valve, from which they must be pumped or drained to a cargo tank.

2.9.10 Steaming tanks
Steaming cargo tanks produces large accumulations of static electricity. For that reason, steaming should only be done in inerted tanks.