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Handling of Cargo and Ballast
This Chapter covers the precautions to be taken and the procedures that should be observed on all occasions when handling cargo or ballast whether at terminals or during transfers between vessels. These precautions are additional to those given in Chapters 2 and 6.
SUPERVISION AND CONTROL
The responsibility for safe cargo handling operations is shared between the ship and the terminal and rests jointly with the master and the responsible terminal representative. The manner in which the responsibility is shared should therefore be agreed between them so as to ensure that all aspects of the operations are covered.
Joint Agreement on Readiness to Load or Discharge
Before starting to load or discharge cargo or ballast, the responsible officer and the terminal representative must formally agree that both the tanker and the terminal are ready to do so safely.
The following safeguards must be maintained throughout loading and discharging:
A responsible officer must be on watch and sufficient crew on board to deal with the operation and security of the tanker. A continuous watch of the tank deck must be maintained. If a ship's cargo control room, from which all operations can be controlled, does not have an overall view of the tank deck, then a competent member of the ship's crew must be continuously on watch on the tank deck.
A senior terminal representative must be on duty and communications between him and the responsible officer continuously maintained (see Section 4.5).
A competent member of the terminal Organization should be on continuous duty in the vicinity of the ship to shore connections. Supervision should be aimed at preventing the development of hazardous situations. If, however, such a situation arises, the controlling personnel should have adequate means available to take corrective action. Supervision by systems incorporating television should only be used where they give effective control over the cargo operations and cannot be regarded as satisfactory when cargo operations are at a critical phase or during adverse weather conditions.
The agreed ship to shore communications system must be maintained in good working order.
At the commencement of loading or discharging, and at each change of watch or shift, the responsible officer and the terminal representative' must each confirm that the communications system for the control of loading and discharging is understood by them and by personnel on watch and on duty.
The stand-by requirements for the normal stopping of shore pumps on completion of loading and the emergency stop system for both the tanker and terminal must be fully understood by all personnel concerned.
Checks During Cargo Handling
At the start of and during cargo handling frequent checks should be made by the responsible officer to confirm that cargo is only entering or leaving the designated cargo tanks and that there is no escape of cargo into pumprooms or cofferdams, or through sea and overboard discharge valves.
Tanker and terminal personnel should regularly check the pipeline and hose or metal arm pressures in addition to the estimated quantity of cargo loaded or discharged. Any drop in pressures or any marked discrepancy between tanker and terminal estimates of quantities could indicate pipeline or hose leaks, particularly in submarine pipelines, and require that cargo operations be stopped until investigations have been made.
MEASURING AND SAMPLING'
Depending on the toxicity and/or volatility of the cargo, it may be necessary to prevent or minimize the release of vapor from the cargo tank headspace during measurement and sampling operations. Wherever possible, this should be achieved by use of closed gauging and sampling equipment. Equipment required for the measurement of ullage and temperature within cargo tanks may be either fixed (permanently installed) or portable and samples will normally be drawn using portable equipment. Closed gauging or sampling will be undertaken using the fixed gauging system or by using portable equipment passed through a vapor lock. Such equipment will enable ullages, temperatures, water cuts and interface measurements to be obtained with a minimum of cargo vapors being released This portable equipment, passed through vapor locks, is sometimes referred to as 'restricted gauging equipment'.
When it is not possible to undertake closed gauging and/or sampling operations, open gauging will need to be employed. This will involve the use of equipment passed into the tank via an ullage or sampling port or a sounding pipe and personnel may therefore be exposed to greater concentrations of cargo vapor.
As cargo compartments may be in a pressurized condition, the opening of vapor lock valves, ullage ports or covers and the controlled release of any pressure should be undertaken by authorized, personnel only.
When measuring or sampling, care must be taken to avoid inhaling gas. Personnel should therefore keep their heads well away from the issuing gas and stand at right angles to the direction of the wind. Standing immediately upwind of the ullage port might create a back eddy of vapor towards the operator. In addition, depending on the nature of the cargo being handled, consideration may have to be given to the use of appropriate respiratory protective equipment (see Section
When open gauging procedures are being employed, the tank opening should only be uncovered long enough to complete the operation.
Measuring and Sampling Non-inerted Tanks (Table 7-1 refers)
Static electricity hazards may be present when gauging and sampling non-inerted tanks. An electrostatic charge may be present on the surface of the liquid in the tank, either because it is being pumped or is subjected to agitation. A charge may also be generated on the gauging or sampling equipment or on the person using the equipment.
Reference should be made to Chapter 20 for a full explanation of static electrical hazards. Section 20.3 provides guidance on the safe handling of static accumulator oils and Section 20.5 addresses hazards associated with dipping, ullaging and sampling operations.
Static charge accumulation on unearthed probes introduced into tanks:
Regardless of the volatility of the cargo, in a non-inerted tank there is always the
possibility that the atmosphere may be within the flammable range.
When ullaging, dipping, gauging or sampling all cargoes in non-inerted tanks, irrespective of the volatility classification of the cargo, the following precautions must be observed in order to avoid hazards associated with the possible accumulation of electrical charges on probes, such as metal tapes, lowered into the tank:
Metal tapes or other gauging/sampling devices which could act as electrical conductors throughout their length must be effectively earthed or bonded before introduction into the tank until after removal.
No synthetic tapes or ropes should be used.
(Reference should also be made to Section 4.3(c) regarding operations conducted through full depth sounding pipes).
Static accumulation properties of the cargo:
The precautions to be taken against static electricity during the ullaging, dipping, gauging or sampling of static accumulator oils are to be found in Section . 4.3 and must be rigidly adhered to in order to avoid hazards associated with the accumulation of electrical charge on the cargo.
Measuring and Sampling Inerted Tanks
Ships fitted with inert gas systems will have closed gauging systems for taking measurements during cargo operations. In addition, many vessels will be provided with vapor locks to enable closed gauging and sampling to be undertaken for custody transfer purposes.
Ships equipped with a vapor lock on each cargo tank can measure and sample cargo without reducing the inert gas pressure. The vapor locks are in many cases accompanied by specially adapted measurement devices, including sonic tapes, samplers and temperature tapes. The valves of the vapor lock should not be opened until the instrument is properly attached to the standpipe. Care should be taken to ensure that there is no blowback of vapor.
Sonic tapes, temperature tapes etc. must be used in accordance with good safety practices and the manufacturers' instructions. -The requirements for portable electrical equipment apply to these measurement devices (see Section 2.4).
On ships which are not equipped with vapor locks, special precautions need to be taken for the open measuring and sampling of cargo carried in tanks which are inerted (for inspection of inerted tanks prior to loading, see Section When it is necessary to reduce the pressure in any tank for the purposes of measuring and sampling, the following precautions should be taken:
A minimum positive inert gas pressure should be maintained during measuring and sampling. The low oxygen content of inert gas can rapidly cause asphyxiation and care should therefore be taken to avoid standing in the path of vented gas during measuring and sampling (see Section 2.1). No cargo or ballast operations are to be permitted in cargo compartments while the inert gas pressure is reduced to allow measuring and sampling.
Only one. access point should be opened at a time and for as short a period as possible. In the intervals between the different stages of cargo measurement (e.g. between ullaging and taking temperatures) the relevant access point should be kept firmly closed.
After completing the operation and before commencing the discharge of cargo, all openings should be secured and the cargo tanks re-pressurized with inert gas. (See Chapter 10 for the operation of ship's inert gas system during cargo and ballast handling).
Measuring and sampling which require the inert gas pressure to be reduced and cargo tank access points opened should not be conducted during mooring and unmooring operations or while tugs are alongside. It should be noted that if access points are opened while a vessel is at anchor or moored in an open roadstead any movement of the vessel may result in the tanks breathing. To minimize this risk in such circumstances, care should be taken to maintain sufficient positive pressure within the tank being measured or sampled.
If it is necessary to sound the tanks when approaching the completion of discharge, the inert gas pressure can again be reduced to minimum safe operational level to permit sounding through sighting ports or sounding, pipes. In order to avoid the ingress of air or an excessive release of inert gas it is essential that during this operation tanks, which are still being discharged are not opened.
Measuring and Sampling Cargoes Containing Toxic Substances
Special precautions need to be taken when vessels carry cargoes containing toxic substances in concentrations sufficient to be hazardous.
Loading terminals have a responsibility to advise the Master if the cargo to be loaded contains hazardous concentrations of toxic substances. Similarly, it is the responsibility of the Master to advise the receiving facility that the cargo to be discharged contains toxic substances. This information is required to be exchanged by the Ship/Shore Safety Check List (see Appendix A).
The ship must also advise the terminal and any other tank inspectors or surveyors if the previous cargo contained toxic substances.
Ships carrying cargoes containing toxic substances should adopt closed sampling and gauging procedures or require all personnel undertaking these activities to wear personal protective equipment (see Sections 16.4 and 16.5).
When closed gauging or sampling cannot be undertaken, tests should be made to assess the vapor concentrations in the vicinity of each access point when open in order to ensure that concentrations of vapor do not exceed the Permissible Exposure Limits (PELS) of the toxic substances that may be present. If monitoring indicates the limit could be exceeded, suitable respiratory protection should be worn (see Section 16.4). Access points should be opened only for the shortest possible time.
If effective closed operations cannot be maintained, or if concentrations of vapor are rising because of defective equipment or due to still air conditions, consideration should be given to suspending operations and closing all venting points until defects in equipment are corrected or weather conditions change and improve gas dispersion.
Reference should be made to Chapter 16 for a description -of the toxicity hazards of petroleum and its products.
Closed Gauging for Custody Transfer
The gauging of tanks for custody transfer purposes should be effected by use of a closed gauging system or via vapor locks. For the ullaging system to be acceptable for this purpose, the gauging system should be described in the vessel's tank calibration documentation. Corrections for datum levels, and for list and trim should be checked and approved by the vessel's classification society.
Temperatures can be taken using electronic thermometers deployed into the tank through vapor locks. Such instruments should have the appropriate approval certificates and should also be calibrated.
Samples should be obtained by the use of special sampling devices using the vapor locks.
OPERATION OF PUMPS AND VALVES
The incorrect operation of pumps and valves can produce pressure surges in a pipeline system. These surges may be sufficiently severe to damage the pipeline, hoses or metal arms. One of the most vulnerable parts of the system is the ship to shore connection. Pressure surges are produced upstream of a closing valve and may become excessive if the valve is closed too quickly. They are more likely to be severe where long pipelines and high flow rates are involved.
Where the risk of pressure surges exists, information should be exchanged and written agreement reached between the tanker and the terminal concerning the control of flow rates, the rate of valve closure, and pump speeds. This should include the closure period of remote controlled and automatic shutdown valves. These arrangements should be included in the operational plan (see Sections 5.3 and 5.4).
Butterfly and Non-Return (Check) Valves
Butterfly and pinned back non-return valves in ship and shore cargo systems have been known to slam shut when cargo is flowing through them at high rates, thereby setting up very large surge pressures which can cause line, hose, or metal arm failures and even structural damage to jetties. These failures are usually due to the valve disc not being completely parallel to, or fully withdrawn from, the flow when in the open position. This can create a closing force, which may shear either the valve spindle in the case of butterfly valves, or the hold open pin in the case of pinned back non-return valves. It is therefore important to check that all such valves are fully open when they are passing cargo or ballast.
To avoid pressure surges, valves at the downstream end of a pipeline system should as a general rule, not be closed against the flow of liquid except in an emergency. This should be stressed to all personnel responsible for cargo handling operations both on the tanker and at the terminal.
In general, where pumps are used for cargo transfer, all valves in the transfer system (both ship and shore) should be open before pumping begins, although the discharge valve of a centrifugal pump may be kept closed until the pump is up to speed and the valve then opened slowly. In the case of ships loading by gravity, the final valve to be opened should be that at the shore tank end of the system.
If the flow is to be diverted from one tank to another, either the valve on the second tank must be opened before the valve on the first tank is closed, or pumping should be stopped while the change is being made.
Valves, which control liquid flow should be closed slowly. Th e time taken for power operated valves to move from open to shut and from shut to open should be checked regularly at their normal operating temperatures.
Control of Pumping
Throughout pumping operations no abrupt changes in the rate of flow should be made.
Reciprocating main cargo pumps can set up excessive vibration in metal loading/discharging arms which in turn can cause leaks in couplers and swivel joints, and even mechanical damage to the support structure. Where possible such pumps should not be used. If they are, care must be taken to select the least critical pump speed or, if more than one pump is used, a combination of pump speeds to achieve an acceptable level of vibration. A close watch should be kept on the vibration level throughout the cargo discharge.
Centrifugal pumps should be operated at speeds, which do not cause cavitation. This effect may damage the pump and other equipment on the ship or at the terminal.
HANDLING STATIC ACCUMULATOR CARGOES
Precautions against static electricity may be necessary when the cargo being handled is an accumulator of static electricity. Reference should be made to Chapter 20 for a full explanation of the hazards of static electricity.
a) Clean oils are, in general, accumulators of static electricity because of their low conductivity. Static accumulator oils include:
- Natural gasolines - Naphtas 0 Naphthas
- Kerosenes - Heating oils 0 Heating oils
- White spirits - Heavy gas oils
- Motor and aviation gasolines - Clean diesel oils 0 Clean diesel oils
- Jet fuels - Lubricating oils
Under certain conditions, cargo handling operations involving static accumulator oils will require adherence to the anti-static precautions described in Section
Other hazards associated with static electricity may occur during cargo handling operations. It is essential that the precautions detailed in Section 2.2 are also rigorously followed to prevent hazards associated with the accumulation of static electricity on measurement and sampling equipment,
(b) As explained in Chapter 20, black oils have sufficient conductivity to prevent the
accumulation of static electricity. Consequently, black oil cargoes such as:
- Crude oils - Residual fuel oils
- Black diesel oils - Asphalts (bitumens)
are not classed as static accumulators and the anti-static precautions described in Section 4.3 will not necessarily be required when handling such oils.
Hazards associated with static electricity may still occur with such black oil cargoes. It is important that the precautions detailed in Section 2.2 are followed to prevent the accumulation of static electricity on measurement and sampling equipment,
Cargo Tank Operation Lowering of equipment with ropes Loading clean oils Tank washing
When hazard can occur: or tapes of synthetic material
Electrostatic Hazard Rubbing together of synthetic Flow of static accumulator liquids Water mist droplets
(Chapter 20) polymers (Chapter 20.5.2) (Sections 20.3, 20.5.3) (Section 20.4.2, 20.5.5)
Precautions Necessary (Sections 4.3(b), 9.2.4(i)) (Section 4.3(b)) (Sections 9.2.3(b), 9.2.4(i))
For dipping, ullaging and
(i) metallic equipment not Use of ropes or tapes made of Not permitted at any time Not permitted during washing
earthed or bonded: synthetic materials for lowering and for 5 hours thereafter
equipment into cargo tanks not
permitted at any time
(ii) metallic equipment which is Not permitted during loading No restrictions
earthed and bonded from and for 30 minutes thereafter
before introduction until “
(iii) non-conducting equipment No restrictions No restrictions
with no metallic parts: “
Exceptions Permitted if: Sounding pipe is used (a) sounding pipe is used
(b) tank is continuously
mechanically ventilated, when
5 hours can be reduced to 1 hr
Table 7-1 Non-Inerted Tanks
Summary of Precautions Against Electrostatic Hazards when Gauging and Sampling
Precautions Against Static Electricity Hazards
When a tank is maintained in an inert condition no anti-static precautions are necessary.
If the tank is in a non-inert condition, specific precautions will be required with regard to safe flow rates and ullaging, sampling and gauging procedures when handling static accumulator oils as follows:
(a) During the initial stages of loading into each individual tank the flow rate in its branch line should not exceed a linear velocity of 1 m sec.
When the bottom structure is covered and after all splashing and surface turbulence has ceased, the rate can be increased to the lesser of the ship or shore pipeline and pumping systems maximum flow rates, consistent with the proper control of the operation. It should be noted that static accumulator cargoes should not be handled at flow rates exceeding 7m/sec. However, attention is drawn to the possible existence of national regulations with respect to lesser maximum permitted flow rates.
To assist in calculating the volumetric loading rate which corresponds to a linear velocity in a branch line of 1 m/sec, the following table can be used to relate the volumetric flow rate to the pipeline diameter:
Pipeline Diameter* Approximate Flow Rate
mm Cubic Meters/Hour
Table Rates Corresponding to 1 MetrelSecond
*Note that the diameters given are nominal diameters which are not necessarily the same as the actual internal diameters.
(b) During loading, and for 30 minutes after the completion of loading, metallic equipment for dipping, ullaging or sampling must not be introduced into or remain in the tank. Examples of equipment include manual steel ullage tapes, portable gauging devices mounted on deck standpipes, metal sampling apparatus and metal sounding rods. Non-conducting equipment with no metal parts may, in general, be used at any time. However, ropes or tapes used for lowering equipment into tanks must not be made from synthetic materials (see Section 2.2).After the 30 min utes waiting period, metallic equipment may also be used for dipping, ullaging and sampling but it is essential that it is effectively bonded and securely earthed to the structure of the ship before it is introduced into the tank and that it remains earthed until after it has been removed. (see also Section 20.5.3).
carried out through sounding pipes are permissible at any time because it is
not possible for any significant charge to accumulate on the surface of the
liquid within a correctly designed and installed sounding pipe. A sounding pipe is defined as a conducting
pipe which extends the full depth of the tank and which is effectively bonded
and earthed to the tank structure at its-extremities. The pipe should be slotted in order to prevent any pressure differential between the inside of the pipe and the tank and to ensure that true level indications are obtained.
If the sounding facilities are provided, for example, through a deck standpipe that does not extend the full depth of the tank, all the static precautions detailed in Section 4.3 a) and b) above should be strictly adhered to.
A permanently fitted metal float level gauge does not present a static electricity hazard provided the metal float has electrical continuity through the tape to the structure of the ship and the metal guide wires are intact. Other wire guided gauging systems may be used provided the metal guide wires are intact.
(c) Micropore filters, usually made of paper, cellulose or glass fiber are
known to be capable of generating high static charge levels. If a
micropore filter is fitted in the shore pipeline system the loading rate
should be adjusted to ensure that at least 30 seconds elapse between
the time the cargo leaves the filter and the time it enters any cargo
Discharge of Static Accumulator Oils
As air and/or gas bubbles in a liquid can generate static electricity, stripping pumps and eductors should be operated in order to avoid as far as possible the entrainment of air or gas.
Discharge into Shore Installations
In addition to the requirements set out in the preceding paragraphs, when discharging static accumulator oils into shore tanks the initial flow rate should be restricted to 1 meter/second unless or until the shore tank inlet is covered sufficiently to limit turbulence.
For a side entrance (horizontal entrance) the inlet is considered adequately covered if the distance between the top of the. inlet and the free surface exceeds 0.6 meter. An inlet pointing downwards is considered sufficiently covered if the distance between the lower end of the pipe and the free surface exceeds twice the inlet diameter. An inlet pointing upward may require a considerably greater distance to limit turbulence. In floating roof tanks, the low initial flow rate should be maintained until the roof is floating. Similar requirements apply to fixed roof tanks with inner floats.
Commencement of Deballasting
Deballasting to shore tanks must only be commenced with the agreement of the terminal and after it has signified that the shore system is ready to receive the ballast,
Allowance for Stress
Ballast must be discharged in such a way as to avoid the ship's hull being subjected to excessive stress.
Deballasting of a Ship Fitted with an Inert Gas System
Ships fitted with an inert gas system must replace the ballast discharged from cargo tanks with inert gas so as to maintain the oxygen content of the tank atmosphere at not more than 8% by volume.
Ballast carried in segregated tanks may be retained on board in order to restrict the freeboard if this is necessary because of weather conditions or to keep within the envelope restrictions of the terminal metal loading arms or shore gangway. Care must be taken, however, not to exceed the maximum draught for the berth and to include the ballast weights in the hull stress calculations.
LOADING OF CARGO
The loading plan of combination carriers and double hull tankers must take into account the ship's stability instructions and the need to avoid excessive free surface with consequent loss of stability (see Sections 8.1 and 12.4.2).
Inert Gas Procedures
The inert gas plant should be closed down and the inert gas pressure in the tanks to be loaded reduced prior to the commencement of loading unless simultaneous loading of cargo and discharge of ballast from cargo tanks is to take place.
For effective closed loading, cargo must be loaded with the ullage, sounding and sighting ports securely closed. The gas displaced by the incoming cargo must be vented to the atmosphere via the mast riser(s) or through high velocity or constant velocity valves, either of which will ensure that the gases are taken clear of -the cargo deck. Devices fitted to mast risers or vent stacks to prevent the passage of flame must be regularly checked to confirm they are clean, in good condition and correctly installed.
To undertake closed loading, the vessel should be equipped with ullaging equipment and independent overfill alarms (see Section 3) which allow the tank contents to be monitored without opening tank apertures. On vessels without inert gas systems this equipment should comply with the precautions highlighted in Sections 4 and 20.5.
Vessels operating with inert gas are considered always to be capable of closed loading.
Commencement of Loading Alongside a Terminal
When all necessary terminal and tanker valves in the loading system are open, and the ship has signified its readiness, loading can commence. Whenever possible the initial flow should be by gravity, the shore pumps not being started until the system has-been checked and the ship advises that cargo is being received in the correct tank(s). When the pumps have been started the ship/shore connections must be checked for tightness until the agreed flow rate or pressure has been reached.
Commencement of Loading at Buoy Berths
Before commencing to load at an ofshore berth, the ship should confirm its full understanding of the communications system, which will be used to control the operation. A secondary communications system should be provided ready to be brought into immediate action in the event of failure- of the primary system.
After an initial slow loading rate to test the system, the flow rate may be brought up to the agreed maximum. A close watch should be kept on the sea in the vicinity of the seabed manifold so those leaks may be detected. During darkness, where safe and practical, a bright light should be shone on the water in the vicinity of the hoses.
Commencement of Loading Through a Stern Line
Before commencing loading through a stern loading line the dangerous area extending not less than 3 meters from the manifold valve should be clearly marked and no unauthorized personnel should be allowed within this area during the entire loading operation.
A close watch must be maintained for any leakage and all openings, air inlets and doors to enclosed spaces should be kept tightly closed.
Fire fighting equipment must be laid out and ready for use in the vicinity of the stern loading manifold.
Commencement of Loading Through a Bow Line
Vessels involved in bow loading will necessarily be designed for use at particular terminals (normally single point moorings) for which detailed operating and safety procedures will be specified.
In general, however, the following checks should be carried our prior to loading:
The mooring system should be inspected for security of connection and to ensure that any wear is within acceptable operational limits.
The cargo hose connection should be carefully inspected for correct alignment and security of coupling. Where possible, a water pressure test of the coupling seals should be made.
Any emergency release systems provided for the mooring and cargo connection should be operational. Tests of these systems should take place prior to mooring.
Mooring load monitoring systems should be activated and tested.
All primary and secondary means of communication with the loading terminal should be tested, including any telemetry control system.
A continuous watch by a responsible crew member should be maintained on the bow throughout loading. During darkness the illumination on and around the vessel's bow should permit an effective visual watch to be maintained on the mooring point, mooring system, cargo hose connection, loading hoses and the area of water around the bow.
Emergency Shutdown Plan
An emergency shutdown procedure should be agreed between the ship and the terminal and recorded on an appropriate form.
The agreement should designate those circumstances in which operations must be stopped immediately.
Due regard should be given to the possible dangers associated with any emergency shutdown procedure (see Sections 6.5.5 and
Fluctuation of Loading Rate
The loading rate should not be substantially changed without informing the ship.
Cessation of Loading by the Terminal
Many terminals require a 'standby' period for stopping pumps and this should be understood and noted as discussed in Section 8 of the Ship/Shore Safety Check List Guidelines before loading commences (see Appendix A).
Topping Off on Board the Tanker
The ship should advise the terminal when the final tanks are to be topped off and request the terminal, in adequate time, to reduce the loading rate sufficiently to permit effective control of the flow on board the ship. After topping off individual tanks, master valves should be shut, where possible, to provide two valve segregation of loaded tanks. Ullages should be checked from time to time to ensure that overflows do not occur as a result of leaking valves or incorrect operations.
The number of valves to be closed during the topping off period should be reduced to a minimum.
The tanker should not close all its valves against the flow of oil.
Before topping off. operations commence at an offshore berth, the ship/shore communications system must be tested.
Where possible the completion of loading should be done by gravity. If pumps have to be used to the end, their delivery rate during the 'standby' time should be regulated so that shore control valves can be closed as soon as requested by the ship. Shore control valves should be closed before the ship's valves.
Checks After Loading
After the completion of loading, a responsible officer should check that all valves in the cargo system are closed, that all appropriate tank openings are closed and that pressure/vacuum relief valves are correctly set.
Loading Very High Vapor Pressure Cargoes
Consideration should be given to the need for special precautions during loading when the True Vapor Pressure (TVP) of the cargo is expected to exceed the following:
For natural gasoline type cargoes (e.g. pentanes plus): 0.75 bar
For crude oils, with or without added gas: 1.0 bar.
For some intermediate cargoes (for example flash stabilized condensates, some distillation overhead products, and crude oils with abnormally low methane and ethane contents) TVP limits may lie between these two values.
When cargo temperature, crude oil stabilization conditions and Reid Vapor Pressures are known, true vapor pressures can be calculated for checking with the above criteria. The necessary information should be supplied by the terminal.
Precautions which should be applied may include:
Permitting only closed loading methods (see Section
Avoiding loading when the wind speed is less than 5 knots.
The use of very low initial flow rates into tanks.
The use of very low topping off rates.
Avoiding a partial vacuum in the loading line.
Avoiding loading hot oil which has been lying in shore lines exposed to the sun; if this is unavoidable, loading this oil in tanks which vent well clear of the superstructure (e.g. forward tanks).
Providing additional supervision to see that gas dispersion is monitored and to ensure compliance with all safety requirements.
Loading Heated Products
Unless the ship is specially designed for carrying very hot cargoes (e.g. a bitumen carrier), cargo heated to a high temperature can damage a tanker's structure, protective coatings and equipment such as valves, pumps and gaskets.
Some classification societies have rules regarding the maximum temperature at which cargo may be loaded and masters should consult their owners whenever the cargo to be loaded has a temperature in excess of 60'C.
The following precautions may help to alleviate the effects of loading a hot cargo:
Spreading the cargo throughout the ship as evenly as possible in order to dissipate excess heat and to avoid local heat stress.
Adjusting the loading rate in an attempt-to achieve a more reasonable temperature.
Taking great care to ensure that tanks and pipelines are completely free of water before receiving any cargo that has a temperature above the boiling point of water.
Loading Overall (Loading from the Top)
There may be specific port or terminal regulations relating to loading overall.
Volatile petroleum, or non-volatile petroleum having a temperature higher than its flashpoint minus 10'C, must never be loaded or transferred overall into a non-gas free tank.
Non-volatile petroleum having a temperature lower than its flashpoint minus 10'C may be loaded overall in the following circumstances:
If the tank concerned is gas free and provided no contamination by volatile petroleum can occur.
It prior agreement is reached between the master and the terminal representative.
The free end of the hose should be lashed inside the tank coaming to prevent
Ballast or slops must not be loaded or transferred overall into a tank, which contains a flammable gas mixture.
LOADING AT TERMINALS HAVING VAPOUR EMISSION CONTROL SYSTEMS
The fundamental concept of a vapor emission control system is relatively simple. When tankers are loading at a terminal, the vapors are collected as they are displaced by the incoming cargo or ballast and are transferred ashore for treatment or disposal. However, the operational and safety implications are significant because the ship and terminal are connected by a common stream of vapors, thereby introducing into the operation a number of additional hazards, which have to be effectively controlled. Detailed guidance on technical issues associated with vapor emission control and treatment systems is available from a number of sources. IMO has developed international standards for the design, construction and operation of vapor collection systems on tankers and vapor emission control systems at-terminals and OCIMF has promulgated guidance on vapor manifold arrangements.
The primary hazards associated with the use of vapor emission control (VEC) systems are summarized below.
Misconnection of Liquid and Vapor Lines
To guard against the possible misconnection of the ship's vapor manifold to a terminal liquid loading line, the vapor connection should be clearly identified by painting the outboard 1 meter section with yellow and red bands and by stenciling the word 'VAPOUR” in black letters upon it. In addition, a cylindrical stud should be permanently attached to each presentation flange face at the 12 o'clock position on the flange bolt circle. The stud should project 25.4mm (1 inch) perpendicular to the flange face, and should be 12.7mm (1/2 inch) in diameter, in order to prevent the connection of standard liquid transfer hoses. Blank flanges, inboard ends of reducers and hoses for the vapor line will have an extra hole to accommodate the lug on the presentation flange (see Figure 7-2).
Full details of vapor manifold arrangements, materials and fittings are contained in the OCIMF publication 'Recommendations for Oil Tanker Manifolds and Associated Equipment'.
Vapor Over/Under Pressure
Although all 'closed' cargo operations require in-tank pressures to be effectively monitored and controlled, the connection to a vapor emission control system results in. pressures within the ship's vapor spaces being directly influenced by any changes that may occur within the terminal's system. It is therefore important to ensure that the individual cargo tank PN protection devices are fully operational and that loading rates do not exceed maximum allowable rates. In addition, pressures within vapor collection piping systems should be continuously monitored by sensors that incorporate high and low pressure alarm functions connected to audible and visual alarms.
Cargo Tank Over-fill
The risk of overfilling a cargo tank when utilizing a VEC system is no different to that when loading under normal 'closed' conditions. However, owing to the reliance placed on closed gauging systems, it is important that they are fully operational and that back-up is provided in the form of an independent overfill alarm arrangement. The alarm should provide audible and visual indication and should be set at a level, which will enable operations to be shut down prior to the tank being overfilled. Under normal operations, the cargo tank should not be filled higher than the level at which the overfill alarm is set.
Individual overfill alarms should be tested at the tank to ensure their proper operation prior to commencing loading unless the system is provided with an electronic self-testing capability which monitors the condition of the alarm circuitry and sensor and confirms the instrument set point.
Sampling and Gauging
A cargo tank should never be opened to the atmosphere for gauging or sampling purposes while the ship is connected to the shore vapor recovery system unless loading to the tank is stopped, the tank is isolated from any other tank being loaded and precautions are taken to reduce any pressure within the cargo tank vapor space.
The inter-connection of ship and shore vapor streams, which may or may not be within the flammable range, introduces significant additional hazards which are not normally present when loading. Unless adequate protective devices are installed and operational procedures are adhered to, a fire or explosion occurring in the vapor space of a cargo tank onboard could transfer rapidly to the terminal, and vice versa.
A detonation arrestor should be fitted in close proximity to the terminal vapour connection at the jetty head in order to provide primary protection against the transfer or propagation of a flame from ship to shore or from shore to the ship.
The design of the terminal vapor collection and treatment system will determine whether or not flammable vapors can be safely handled and, if they cannot, will include provisions for either inerting, enriching or diluting the vapor stream and continuously monitoring its composition.
Liquid Condensate in the Vapor Line
The ship's systems should be provided with means to effectively drain and collect any liquid Condensate that may accumulate within vapor pipelines. Any build-up of liquid in the vapor line could impede the free passage of vapors and thus increase in-line pressures and also could result in the generation of significant electrostatic charges on the liquid's surface. It is important that drains are installed at the low points in the ship's piping system and that they are routinely checked to ensure that no liquid is present.
The precautions contained in Section 4 with regard to initial loading rates and measuring and sampling procedures should be followed. In addition, to prevent the build up of electrostatic charges within the vapor collection system, all pipework should be electrically bonded to the hull and should be electrically continuous. The bonding arrangements should be inspected periodically to check their condition.
The terminal vapor connections should be electrically insulated from the tanker vapor connection by the use of an insulating flange or a single section of insulating hose.
It is important that ship and shore personnel in charge of transfer operations complete a structured training programme covering the particular vapor emission control system installed either onboard their ship or within their terminal. Ship's personnel should be provided with general information on typical terminal arrangements and procedures and shore personnel should be made aware of typical tanker equipment and operating procedures.
The introduction of VEC's reinforces the importance of good cooperation and communications between the ship and shore. Pre-transfer discussions should provide both parties with an understanding of each others' operating parameters. Details such as maximum transfer rates, maximum allowable pressure drops in the vapor collection system and alarm and shutdown conditions and procedures, must be agreed before operations commence. (see Appendix A - Ship/Shore Safety Checklist).
A summary of the terminal's vapor emission control system should be included in the terminal information booklet.
MONITORING OF VOID AND BALLAST SPACES
Void and ballast spaces located within the cargo tank block should be routinely monitored to check that no leakage has occurred from adjacent cargo tanks. Monitoring should include regular atmosphere checks for hydrocarbon content and regular sounding/ullaging of the empty spaces.
The guidance given in Chapter 8 'Double Hull Operations' should be followed to the extent that it may apply to single hull tankers, particularly with regard to routine monitoring procedures (Section 8.2); actions to be taken in the event of cargo leakage being detected (Section 8.5) and the handling of ballast after a leak (Section 8.9).
DISCHARGE OF CARGO
9.i Combination Carriers and Double
The discharge plan of these ships must particularly take into account the ship's stability instructions and the need to avoid excessive free surface and consequent loss of stability (see Sections 8.1 and 12.4.2).
Inert Gas Procedures
Ships using an inert gas system must have the system fully operational and producing good quality (i.e. low oxygen content) inert gas at the commencement of discharge.
The inert gas system must be fully operational and working satisfactorily throughout the discharge of cargo or deballasting. Chapter 10 gives details on the operation of the IGS.
Cargo discharge must not be started until:
All relevant cargo tanks, including slop tanks, are common with the IG main.
All other cargo tank openings, including vent valves, are securely closed.
The inert gas main is isolated from the atmosphere and, if a cross connection is fitted, also from the cargo main.
The inert gas plant is operating.
The deck isolating valve is open.
A low but positive inert gas pressure after completion of discharge will permit the draining of the manifold driptray into a tank and, if required, allow manual dipping of each tank.
Vessels correctly operating their inert gas systems may be considered to is conducting closed discharging operations.
Discharge should normally take place on non-inerted vessels with all ullage, sounding and sighting ports closed. Air should be admitted to the tanks by the dedicated venting system. Where the design of the vessel does not allow admittance of air via the vapor system at a satisfactory rate, air may be admitted via a sighting or ullage port providing it is fitted with a permanent flame screen.
When cargo is being run between tanks during discharge operations care should be taken to ensure that vapors are vented to deck via the deck apertures protected by flame screens.
Pressurizing of Cargo Tanks
When high vapor pressure petroleum (e.g. natural gasoline and certain crude oils) reaches a low level in cargo tanks, the head of liquid is sometimes insufficient to keep cargo pumps primed. If an inert gas system is installed in the tanker this system can be used for pressurizing cargo tanks in order to improve pump performance.
Crude Oil Washing
If the ship needs to crude oil wash ail or some of its tanks during discharge, the responsible officer should incorporate a crude oil washing plan in the required discharge plan set out in. Section 5.5.
A full description of the requirements relating to crude oil washing is given in Section 9.4.
Commencement of Discharge Alongside a Terminal
Shore valves must be fully open to receiving tanks before the tanker's manifold valves are opened. If there is a possibility that, owing to the elevation of the shore tanks above the level of the ship's manifold, pressure might exist in the shore line and no non-return (check) valves are fitted in the shore line, the ship must be informed and the tanker's manifold valves should not be opened until an adequate pressure has been developed by the pumps.
Discharge should start at a slow rate, and only be increased to the agreed rate once both parties are satisfied that the flow of oil to and from designated tanks is confirmed.
Commencement of Discharge at an Offshore Terminal
Before commencing discharge at an offshore terminal, communications between ship and shore should be tested and fully understood. The ship must not open its manifold valves or start its pumps until a clear signal has been received from the shore that the terminal is ready.
Discharge must be started slowly until the system has been tested and then gradually brought up to the maximum agreed flow rate or pressure. A close watch should be kept on the sea in the vicinity of the hoses to detect leaks. During darkness a bright light should, where safe and practicable, be shone on the water in the vicinity of the hoses.
Commencement of Discharge Through a Stern Discharge Line
Before commencing discharge through a stern discharge line, a dangerous area extending not less than 3 meters from the manifold valve should be clearly marked and no unauthorized personnel should be allowed within this area during the entire discharge operation.
A close watch must be maintained for any leakage and all openings, air inlets and doors to enclosed spaces should be kept tightly closed.
Fire fighting equipment must be laid out and ready for use in the vicinity of the stern discharge manifold.
Fluctuations in Discharge Rate
During discharge the flow of cargo should be controlled by the tanker in accordance with the agreement reached with the terminal.
The discharge rate should not be substantially changed without informing the terminal.
Stripping and Draining of Cargo Tanks
If, during the discharge of the main bulk of cargo, a slop tank or other selected tank is used to receive the drainings of tanks being stripped, personnel should be alert to the fact that the ullage in the receiving tank will be decreasing. In these circumstances great care should be taken to avoid an overflow and proper precautions taken in respect of any vapors emitted.
Simultaneous Ballast and Cargo Handling
If ballasting of cargo tanks is carried out simultaneously with the discharge of cargo, vapors may be emitted from the tanks being ballasted, in which case proper precautions should be taken. Crude oil washing ships should, if required, control atmospheric emissions by using the methods described in Section 9.4.8.
FAILURE OF THE INERT GAS SYSTEM DURING DISCHARGE
In the event of the failure of the inert gas system to deliver the required quality and quantity of inert gas, or to maintain a positive pressure in the cargo tanks and slop tanks, action must be taken immediately to prevent any air being drawn into the tanks. All cargo and/or ballast discharge must be stopped, the inert gas deck isolating valve closed, the vent valve between it and the gas pressure regulating valve (if provided) opened, and immediate action taken to repair the inert gas system.
Masters are reminded that national and local regulations may require the failure of an inert gas system to be reported to the harbour authority, terminal operator and to the port and flag state administrations.
Pyrophoric iron sulphide deposits, formed when hydrogen sulphide gas, reacts with rusted surfaces in the absence of oxygen, may be present in the cargo tanks and these deposits can heat to incandescence when coming into contact with air. Therefore, in the case of tankers engaged in the carriage of crude oil, the failed inert gas system must be repaired and restarted, or an alternative source of inert gas provided, before discharge of cargo or ballast is resumed.
In the case of product carriers the formation of pyrophors is usually inhibited by tank coatings. If, therefore, it is considered totally impracticable to repair the inert gas system, discharge may be resumed with the written agreement of all interested parties, provided an external source of inert gas is provided or-the following precautions are taken:
Devices to prevent the passage of flame or flame screens (as appropriate) are in place and are checked to ensure that they are in a satisfactory condition.
Valves on the vent mast risers are opened.
No free fall of water or slops is permitted.
No dipping, ullaging, sampling or other equipment is introduced into the tank unless essential for the safety of the operation. If it is necessary for such equipment to be introduced into the tank it should be done only after at least 30 minutes have elapsed since the injection of inert gas has ceased. All metal components of any equipment to. be introduced into the tank should be securely earthed. This restriction should be applied until a period of five hours has elapsed since the injection of inert gas has ceased.
See also Section 10.12 and Chapter 23.
PIPELINE AND HOSE CLEARING
The procedure for clearing the pipelines and hoses or arms between the shore valve and ship's manifold will depend on the facilities available and whether these include a slop tank or other receptacle. The relative heights of the ship and shore manifolds may also influence procedures.
Some terminals require the ship to displace with water the contents of the hoses or arms, and perhaps also the shore pipelines, on completion of cargo operations. Prior to commencing the displacement the ship and terminal should reach agreement on the procedures to be adopted.
On completion of loading, the ship's cargo deck lines should be drained into appropriate cargo tanks to ensure that thermal expansion of the contents of the lines cannot cause leakage or distortion. The hoses or arms and perhaps a part of the pipeline system between the shore valve and the ship's manifold are also usually drained into the ship's tanks. Sufficient ullage must be left in the final tanks to accept the drainings of the hoses or arms and ship or shore lines.
On completion of discharge the ship's cargo deck lines should be drained into an appropriate tank and then be discharged ashore or into a slop tank.
When draining is complete and before hoses or arms are disconnected, the ship's manifold Valves and shore valves should be shut and the drain cocks at the vessel's manifold should be opened to drain into fixed drain tanks or portable drip trays. Cargo manifolds and arms or hoses should be securely blanked after being disconnected. The contents of portable or fixed drip trays should be transferred to a slop tank or other safe receptacle.
Clearing of Hoses and Arms
If lines, hoses or arms have to be cleared to the shore using compressed air or inert gas, the following precautions should be strictly observed in order to avoid the possible creation of a hazardous static electrical charge or mechanical damage to tanks and equipment:
The procedure to be adopted must be agreed between ship and terminal. a There must be adequate ullage in the reception tank.
To ensure that the amount of compressed air or inert gas is kept to a minimum the operation must be stopped when the line has been cleared.
The inlet to the receiving tank should be 4ocated well above any water that may be in the bottom of the tank.
The line clearing operation must be continuously supervised by a responsible person.
Ship's Cargo Pipelines
When compressed air or inert gas is used to clear ship's pipelines (e.g. when evacuating the liquid column above a deep well pump), similar hazards to those identified in Section 11.3 may arise. Therefore when compressed air or inert gas is to be used for such pipeline clearing the precautions detailed in Section 11.3 must be observed. Line clearing operations must be undertaken in accordance with the operating procedures previously established for the particular ship.
TRANSFERS BETWEEN VESSELS
Ship to Ship Transfers
In ship to ship transfers both tankers should comply fully with the safety precautions required for normal cargo operations. If the safety precautions are not being observed on either vessel, the operations must not be started or, if in progress, must be stopped.
Ship to ship transfers undertaken in port or at sea may be subject to approval by the port or local marine authority and certain conditions relating to the conduct of the operation may be attached to such approval.
A full description of the safety aspects of transfer operations is contained in the ICS/OCIMF “Ship to Ship Transfer Guide (Petroleum)', to which reference should be made before starting the transfer.
Ship to Barge Transfers
In tanker/barge transfers of petroleum, only authorized and properly equipped barges should be used. Precautions similar to those set out for ship to ship cargo transfers in the ICS/ OCIMF 'Ship to Ship Transfer Guide (Petroleum)' should be followed. If the safety precautions are not being observed on either the barge or the tanker the operations must not be started or, if in progress, must be stopped.
Masters of ships should be aware that barge crews may not be conversant with the 'Ship to Ship Transfer Guide (Petroleum)'.
The rate of pumping from ship to barge must be controlled according to the size and nature of the receiving barge. Well understood communications procedures must be established and maintained, particularly when the freeboard of the ship is high in relation to that of the barge.
If there is a large difference in freeboard between the ship and the barge, the barge crew must make allowance for the contents of the hose on completion of the transfer.
Arrangements should be made to release the barge in an emergency, having regard to other shipping or property in the vicinity. If the tanker is at anchor, it may be appropriate for the barge to drop anchor clear of the tanker, where it could remain secured to wait for assistance.
Barges should be cleared from the ship's side as soon as possible after they have completed the loading or discharging of volatile petroleum.
Ship to Ship Transfers Using Vapor Balancing
Specific operational guidance should be developed to address the particular hazards associated with vapor emission control activities during ship to ship transfer operations where vapor balancing techniques should only be undertaken between inerted vessels and, as a minimum, the following recommendations should be followed:
Before commencing cargo transfer:
Equipment should be provided on at least one of the vessels to enable the oxygen content of the vapor stream to be monitored. This should draw samples continuously from a location close to the vapor manifold connection and should include the facility for audible and visual alarms should the oxygen content of the vapor stream exceed 8% by volume. The oxygen analyzer and associated alarms should be tested for proper function prior to each cargo transfer operation.
The oxygen content of the vapor space of each tank connected to the IG main of both ships should be checked and confirmed to be less than 8% by volume.
The vapor transfer hose should be purged of air and inerted prior to commencing transfer of vapors.
The vapor manifold valves should not be opened until the pressure in the vapor system of the receiving vessel exceeds that of the vessel discharging cargo.
During the cargo transfer:
Transfer operations should be terminated if the oxygen
content of the vapor stream exceeds 8% by volume and
should only be resumed once the oxygen content in the
tanks of the receiving ship has been reduced to 8% or less by
- The IG pressure on both ships should be monitored and each
ship advised of the other's pressure on a regular basis at the
same time as transferred volumes are compared.
- No air should be permitted to enter the cargo tanks of the
Reference should also be made to Section 7, which provides general guidance on vapor recovery operations.
Ship to Ship Transfers Using Terminal Facilities
Where a tanker at a berth is transferring cargo to a tanker at another berth through the shore manifolds and pipelines, the two tankers and the terminal should comply with all regulations relating to ship to shore transfers, including written operating arrangements and communications procedures. The co-operation of the terminal in establishing these details is essential.
The ballasting operation should be discussed and agreed in writing between ship and terminal representatives. In drawing up this agreement, the following should be borne in mind:
The responsible officer must calculate the hull stresses imposed by ballast weights to ensure that they do riot exceed permitted limits.
Before ballasting of tanks containing hydrocarbon vapor is carried out alongside a terminal, the responsible officer must consult with the terminal representative and all safety checks and precautions applicable to the loading of volatile petroleum observed. Closed loading procedures should be followed.
During ballasting of cargo tanks, which contain hydrocarbon vapor, gas is expelled which may be within the flammable range on mixing with air. This gas should therefore be vented through the recognized vent lines as during loading.
When ballasting tanks that previously- contained cargoes which required closed operations to be adopted, the ballast should also be loaded 'closed' by following the procedures in Section
Ballast must not be loaded overall into tanks containing hydrocarbon vapor.
The agreement of the terminal representative must be obtained before the simultaneous handling of cargo and ballast, other than segregated ballast, takes place.
Operation of Cargo Pumps
When starting to ballast, cargo pumps should be operated so that no oil is allowed to escape overboard when the sea suction valve is opened. See the ICS/OCIMF publication 'Prevention of Oil Spillages Through Cargo Pumproom Sea Valves'.
Sequence of Valve Operations
The following procedures should be adopted when loading ball6st into non-inerted tanks which contain hydrocarbon vapor:
The tank valves should be the first valves opened.
The initial flow of ballast should be restricted so that the entrance velocity is less than 1 meter/second until the longitudinals are covered or, if there are no longitudinals, until the depth of the ballast in the tank is at least 1.5 meters.
These precautions are required to avoid a, geyser effect, which may lead to the build up of an electrostatic charge in a mist or spray cloud near the point where the ballast enters the tank. When a sufficient charge exists the possibility of a discharge and ignition cannot be excluded.
TANK OVER PRESSURISATION AND UNDER PRESSURISATION
Over pressurization of cargo and ballast tanks is caused by compression of the ullage space due to the inadequate release of vapor or by the overfilling of the tank. The consequences may result in serious deformation of the tank structure and its peripheral bulkheads or catastrophic failure which can seriously affect the structural integrity of the vessel and could lead to fire, explosion and pollution.
Structural damage can also be caused by not allowing inert gas, vapor or air into a tank whilst liquid is being discharged. The resulting under pressure caused in the tank can result in deformation of the ship's structure, which could result in fire, explosion or pollution.
To guard against over and under pressurization of tanks, owners/operators should give consideration to fitting:
Individual pressure sensors with an alarm on each tank.
Individual full flow pressure/release devices to each tank.
Tank Over Pressurization - Causes
Over pressurization usually occurs during ballasting, loading or internal transfer of cargo or ballast. It can be caused by one of the following reasons:
Overfilling of the tank with liquid.
Incorrect setting of the tank's vapor or inert gas isolating valve to the vapor line or inert gas line.
Failure of the isolating valve to the vapor line or inert gas line.
Failure or seizure of the venting valve.
Choked flame arresters/screens
Loading the tank at a rate, which exceeds the maximum venting capacity.
Ice forming on vents during cold weather conditions.
Ice on the surface of the ballast.
Restriction in the vapor lines caused by wax, residues or scale.
Tank Over Pressurization - Precautions
The major safeguard against tank over pressurization is the adherence to good operating procedures. These should include:
On ships without an inert gas system, establishing a procedure to control the setting 0 the isolating valves on the vent lines. The procedure should include a method o recording the, current position of the isolating valves and a method for preventing then from being incorrectly or casually operated.
A method of recording the status of all valves in the system and preventing them from being incorrectly or casually operated.
A system for setting the valves in the correct position for the operation and monitoring that they remain correctly set.
Restricting the operation of the valves to authorized personnel only.
On vessels with inert gas systems where isolating valves are fitted to the branch line to each tank, SOLAS requires these valves be 'provided with locking arrangements Which shall be under the control of the responsible ship's officer' This statement should be taken to mean that the valves must be locked to prevent any change in the valve setting being possible without application to the responsible officer to obtain the means of releasing the locking system on the valve.
Failure or seizure of isolating valves, pressure/vacuum valves or high velocity vents can be guarded against by regular maintenance procedures, pre-operational testing and by operator awareness to detect failure during operation.
To protect against over pressurization through filling tanks too quickly, all ships should have maximum filling rates for each individual tank available for reference onboard by ship’s personnel.
Tank vents should be checked to ensure that they are clear when the operation commences and during freezing weather conditions they should be inspected at regular intervals throughout the operation.
Tank Under Pressurization - Causes
The causes of under pressurization are similar to those of over pressurization, particularly:
Incorrect setting of the tank isolating valve sited between the tank and the vapor line o inert gas line.
Failure of an isolating valve on the vapor line or inert gas line.
The inert gas fan not being run due to breakdown or failure to operate it.
Failure in one of the inert gas supply valves.
A choked flame screen on the vapor inlet line.
Ice forming on the ballast vents during cold weather conditions.
4.5 Tank Under Pressurization - Precautions
The precautions to guard against under pressurization are the same as those relating to over pressurization (see Section
Where under pressurization of a tank or tanks is suspected the situation requires corrective action.
The methods of reducing a partial vacuum in a tank are either to raise the liquid level in the tank by running or pumping cargo or ballast into the affected tank from another tank or to admit inert gas or air into the tank ullage space.
On a ship with an inert gas system, there is a possibility that the quality of the inert gas may be compromised by air leaking past the seals in the tank access locations.
Admitting inert gas at a high velocity to return the tank to a positive pressure could cause an electrostatic hazard, and the precautions identified in Section 20.5.6 should be observed.
On ships without an inert gas system where it is not possible to reduce the partial vacuum other than by admitting air into 'the tank, care should be exercised to ensure that foreign objects with a possible ignition capability, such as rust scale, are not drawn into the tank.
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