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PART III
FUNCTION AND DESIGN CONSIDERATIONS

11. This section addresses itself to inert flue gas systems; the design of systems other than inert flue gas systems should take into account, whenever applicable, the general principles outlined in this section.

Description of an Inert Flue Gas System

12. (1) A typical arrangement for an inert flue gas system is shown in Figure 6.

(2) Flue gas isolating valves are located at the boiler uptake points, through which pass hot, dirty gases to the scrubber and demister; here the gas is cooled and cleaned before being piped to blowers, which deliver the gas through the deck water seal, the non-return valve and the deck isolating valve to the cargo tanks.

(3) A gas pressure regulating valve is fitted downstream of the blowers to regulate the flow of gases to the cargo tank.

(4) A liquid-filled pressure vacuum breaker is fitted to prevent excessive pressure or vacuum from causing structural damage to cargo tanks.

(5) A vent is fitted between the deck isolating/non-return valve and the gas pressure regulating valve to vent any leakage when the plant is shut down.

(6) For delivering inert gas to the cargo tanks during cargo discharge, de-ballasting, tank cleaning, and for topping up the pressure of gas in the tank during other phases of the voyage, an inert gas deck main runs forward from the deck isolating valve for the length of the cargo deck; from this inert gas main, inert gas branch lines lead to the top of each cargo tank.

Function of Inert Gas Scrubber

13. (1) The scrubber cools the flue gas and removes most of the sulphur dioxide and particulate soot; all three actions are achieved by direct contact between the flue gas and large quantities of sea water.

(2) Before entering the bottom of the scrubbing tower, the gas is cooled by either passing through a water spray, or bubbling through a water seal; such a seal may also serve as the additional safety device to prevent any leakage of gas from the boiler uptake when the scrubber is opened up for inspection or maintenance.

(3) In the scrubbing tower itself the gas moves upwards through downward flowing water; for maximum contact between gas and water, several layers made up of one or more of the following arrangements may be fitted:

1. spray nozzles;
   
2. trays of "packed" stones or plastic chippings;
   
3. perforated "impingement" plates;
   
4. venturi nozzles and slots.

(4) At the top of the scrubbing tower or downstream of it, water droplets are removed by one or more demisters which may be polypropylene mattresses or cyclone dryers; designs of individual manufacturers vary considerably.

Design Considerations for Inert Gas Scrubber

14. (1) The scrubber should be of a design related to the type of tanker cargoes and combustion control equipment of the inert gas supply source; it should be capable of dealing with the quantity of inert gas required by Schedule VII, at the designed pressure differential of the system.

(2) The performance of the scrubber at full gas flow should be such as to remove solids effectively and at least 90 per cent of sulphur dioxide; in product carriers, more stringent requirements may be needed for product quality.

(3) The internal parts of the scrubber should be constructed of corrosion resistant materials because of the corrosive effect of the gas; alternatively, the internal parts may be lined with rubber, glass fibre epoxy resin or other equivalent material, in which case the flue gases may require cooling before they are introduced into the lined sections of the scrubber.

(4) Adequate openings and sight glasses should be provided in the shell for inspection, cleaning and observational purposes; the sight glasses should be reinforced to withstand impact and be heat resistant; this condition may be achieved by double glazing.

(5) The design should be such that, under normal conditions of trim and list, the scrubber efficiency will not fall more than 3 per cent, nor will the temperature rise at the gas outlet exceed the desired gas outlet temperature by more than 3 degrees Celsius.

(6) The location of the scrubber above the load waterline should be such that the drainage of the effluent is not impaired when the ship is fully loaded.

Function of Inert Gas Blowers

15. (1) Blowers deliver the scrubbed flue gas to the cargo tanks; at least two blowers are required, which together shall be capable of delivering inert gas to the cargo tanks at a rate of at least 125 per cent of the maximum rate of discharge capacity of the ship, expressed as a volume.

(2) In practice, installations vary from those that have one large blower and one small blower, whose combined total capacity complies with subsection (1), to those in which each blower can meet this requirement.

(3) The advantage claimed for the former in subsection (2), is that it is convenient to use a small capacity blower when topping up the gas pressure in the cargo tanks at sea.

(4) The advantage claimed for the latter in subsection (2), is that, if either blower is defective the other one is capable of maintaining a positive gas pressure in the cargo tanks without extending the duration of the cargo discharge.

Design Considerations for Inert Gas Blowers

16. (1) The blower casing should be constructed of corrosion-resistant material or of mild steel (but then its internal surfaces should be stove-coated, or lined with rubber or glass fibre epoxy resin or other equivalent material to protect it from the corrosive effect of the gas).

(2) The impellers should be manufactured of a corrosion-resistant material; aluminum, bronze impellers should be stress relieved after welding; all impellers should be tested by overspeeding to 20 per cent above the design running speed of the electric motor, or 10 per cent above the speed at which the overspeed trip of the turbine would operate, whichever is applicable.

(3) Substantial drains, fitted with adequate water seals, should be provided in the casing to prevent damage by an accumulation of water; the drains should be in accordance with the provisions of subsection 27(4).

(4) Means, such as fresh water washing, should be provided to remove the build-up of deposits that would cause vibration during blower operation.

(5) The casing should be adequately ribbed to prevent panting and should be so designed and arranged as to facilitate the removal of the rotor without disturbing major parts of the inlet and outlet gas connections.

(6) Sufficient openings in the casing should be provided to permit inspection.

(7) Where separate shafts are provided for the prime mover and the blower, a flexible coupling between these shafts should be provided.

(8) When roller or ball bearings are used, due regard should be paid to the problem of brinelling and the method of lubrication; the choice of type of lubrication, i.e. oil or grease, should have regard to the diameter and rotational speed of the shaft; if sleeve bearings are fitted then resilient mountings are not recommended.

(9) The blower pressure/volume characteristics should be matched to the maximum system requirements; the characteristics should be such that, in the event of the discharge of any combination of cargo tanks at the discharge rate indicated in subsection 5(13), a minimum pressure of 200 millimetres water gauge is maintained in any cargo tank after allowance is made for pressure losses due to:

1. the scrubber tower and demister;
   
2. the piping conveying the hot gas to the scrubbing tower;
   
3. the distribution piping downstream of the scrubber;
   
4. the deck water seal;
   
5. the length and diameter of the inert gas distribution system.

(10) When both blowers are not of equal capacity, the pressure-volume characteristics and inlet and outlet piping should be so matched that, if both blowers can be run in parallel, they are able to develop their designed outputs; the arrangements should be such as to prevent the blower on load from motoring the blower that is stopped or has tripped out.

(11) If the prime mover is an electric motor, then it should be of sufficient power to ensure that it will not overload under any possible operating conditions of the blower; the overload power requirement should be based on the blower inlet conditions of -5 degrees Celsius at -400 millimetres water gauge and outlet conditions of 0 degrees Celsius and atmospheric pressure; arrangements should be provided, if necessary, to maintain the windings in a dry condition during the inoperative period.

Function of Non-Return Devices

17. The deck water seal and mechanical non-return valve together provide the means of automatically preventing the backflow of cargo gases from the cargo tanks to the machinery spaces, or other safe area in which the inert gas plant is located.

Deck Water Seal

18. (1) The deck water seal is the principal barrier; a water seal is fitted that permits inert gas to be delivered to the deck main but prevents any backflow of cargo gas, even when the inert gas plant is shut down; it is vital that a supply of water is maintained to the seal at all times, particularly when the inert gas plant is shut down; in addition, drains should lead directly overboard and not pass through the machinery spaces; one of three principal types of design may be adopted.

Wet type

(2) This is the simplest type of water seal; when the inert gas plant is operating, the gas bubbles through the water from the submerged inert gas inlet pipe, but if the tank pressure exceeds the pressure in the inert gas inlet line, the water is pressed up into this inlet pipe, thus preventing backflow; the drawback to this type of water seal is that water droplets may be carried over with the inert gas, which, although not impairing the quality of the inert gas, could increase corrosion; a demister should, therefore, be fitted in the gas outlet from the water seal to reduce any carry-over; Figure 7 shows an example of this type.

Figure 7 Deck water seal - wet type ^

Semi-dry type

(3) Instead of bubbling through the water trap, the inert gas flow draws the sealing water into a separate holding chamber by venturi action, thus avoiding or at least reducing the amount of water being carried over; otherwise this seal is functionally the same as the wet type; Figure 8 shows an example of this type.

Figure 8 Deck water seal - semi dry type  ^

Dry type

(4) In this type, the water is drained when the inert gas plant is in operation (gas flowing to the tanks), and filled with water when the inert gas plant is either shut down or the tank pressure exceeds the inert gas blower discharge pressure; filling and drainage are performed by automatically operated valves controlled by the levels of the water seal and drop tanks and by the operation of the blowers; the advantage of this type is that it prevents water carry-over; the drawback could be the risk of failure of the automatically controlled valves that may render the water seal ineffective; Figure 9 shows an example of this type.

Figure 9 Deck water seal - dry type ^

Deck Mechanical Non-return Valve and Deck Isolating Valve

(5) As a further precaution against any backflow of gas from the cargo tanks and any backflow of liquid that may enter the inert gas main if the cargo tanks are overfilled, a mechanical non-return valve, or equivalent is required; this should be fitted forward of the deck water seal and should operate automatically at all times.

(6) The valve should be provided with a positive means of closure or, alternatively, a separate deck isolating valve fitted forward of the non-return valve, so that the inert gas deck main may be isolated from the non-return devices; the separate isolating valve has the advantage of facilitating maintenance work on the non-return valve.

Inert Gas Vent Valve

(7) The valve should be opened when the inert gas plant is shut down to prevent leakage past the non-return devices from building up any pressure in the inert gas line between the gas pressure regulating valve and these non-return devices.

Design Considerations for Non-return Devices

19. (1) The material used in the construction of the non-return devices should be resistant to fire and to corrosive attack from acids formed by the gas; alternatively, low carbon steel protected by a rubber lining or coated with glass fibre epoxy resin or equivalent material may be used; particular attention should be paid to the gas inlet pipe to the water seal.

(2) The deck water seal should resist backflow of not less than the pressure setting of the pressure/vacuum breaking device on the inert gas distribution system; it should be so designed as to prevent the backflow of gases under any foreseeable operating conditions.

(3) "Where the deck seals are of dry type or semi-dry type, it is to be arranged such that the automatic sealing, equivalent to a wet type is achieved in approximately 6 seconds.

(4) A regulating flow of clean water through the deck seal reservoir should maintain the water in the deck seal.

(5) Sight glasses and inspection openings should be provided on the deck seal to permit satisfactory observation of the water level during its operation and to facilitate a thorough survey; the sight glasses should be reinforced to withstand impact.

(6) Any drains from the non-return devices should incorporate a water seal in accordance with subsection 27(7), and comply generally with section 28.

Inert Gas Distribution System

20. (1) The inert gas distribution system, together with the cargo tank venting system, where applicable, has to provide:

1. means of delivering inert gas to the cargo tanks during discharge, de-ballasting and tank cleaning operations, and for topping up the pressure of gas in the tank;
   
2. means of venting tank gases to atmosphere during cargo loading and ballasting;
   
3. additional inlet or outlet points for inerting, purging and gas-freeing;
   
4. means of isolating individual tanks from the inert gas main for gas-freeing (see paragraph 24(2)(d);
   
5. means of protecting tanks from excessive pressure or vacuum.

(2) A large variety of designs and operational procedures may be used to meet these interrelated requirements; section 21 considers some of the major design options and their more important operational consequences; Part V gives further advice on operational precautions.

Design Considerations for Valves
and Pipework in Inert Gas Systems

21. (1) The flue gas uptake point should be such that the gas is not too hot for the scrubber or does not cause hard deposits on the flue gas isolating valves; it should not be so close to the uptake outlet that air can be drawn into the system; when boilers are fitted with rotary air heaters, the offtake point should be before the air heater inlet.

(2) The materials used for flue gas isolating valves should take into account the temperature of gas at the offtake; cast iron is acceptable for temperatures below 220 degrees Celsius; valves exposed to a temperature exceeding 220 degrees Celsius should be made from a material not only compatible with the temperature, but also resistant to the corrosive effect of stagnant flue gases.

(3) Flue gas isolating valves should be provided with facilities to keep the seatings clear of soot, unless the valve is designed to close with a seat cleaning action; flue gas isolating valves should also be provided with air sealing arrangements.

(4) If expansion bellows are considered necessary, they should have a smooth internal sleeve and preferably be mounted so that the gas flows through them vertically; they should be constructed of material resistant to stagnant damp acidic soot.

(5) The pipework between the flue gas isolating valve and the scrubber should be made from heavy gauge steel, resistant to corrosion and arranged without unnecessary bends and branches so as to prevent the accumulation of damp acidic soot.

(6) The inlet piping to the scrubber should be so arranged as to permit positive isolation from the flue gases before the scrubber is gas freed for entry for maintenance purposes; this may be accomplished by the removal of a suitable length of pipe section and blanking, by spectacle flanges or by a water seal that would prevent any leakage of gas from the boiler when the flue gas isolating valve is shut and the scrubber is opened for inspection and maintenance; in the event that the drainage of the water seal is itself required for inspection purposes, then isolation should be achieved either by removal of the suitable lengths of pipe sections and blanking, or by the use of spectacle flanges.

(7) The gas outlet piping from the scrubber to the blowers and recirculating lines should be made from steel suitably coated internally.

(8) Suitable isolating arrangements should be incorporated in the inlet and outlet blower to permit safe overhaul and maintenance, while the inert gas system uses the other blower.

(9) The gas regulating valve should be provided with means to indicate whether the valve is open or shut; where the valve is used to regulate the flow of inert gas, it should be controlled by the inert gas pressure sensed between the deck isolating valve and the cargo tanks.

(10) Deck lines should be steel and so arranged as to be self draining; they should be firmly attached to the ship’s structure, with suitable arrangements to take into account movement due to heavy weather, thermal expansion and flexing of the ship.

(11) The diameter of the inert gas main, valves and branch pipes should take account of the system requirements detailed in subsection 16(9); to avoid excessive pressure drop, the inert gas velocity should not exceed 40 m/s in any section of the distribution system, when the inert gas system is operating at its maximum capacity; if the inert gas main is used for venting during loading, other factors may need to be taken into consideration as developed in the Hull Construction Regulations, for cargo tank venting systems.

(12) All pressure and vacuum relief openings should be fitted with flame screens that have easy access for cleaning and renewal; the flame screens should be at the inlets and outlets of any relief device, and be sufficiently robust to withstand the pressure of gas generated at maximum loading and during ballasting operations, while presenting minimum resistance.

Gas Pressure Regulating Valves and
Recirculating Arrangements

22. (1) Pressure control arrangements should be fitted to fulfill two functions:

1. to prevent automatically any backflow of gas in the event either of a failure of the inert gas blower, scrubber pump, etc., or that the inert gas plant is operating correctly, but the deck water seal and mechanical non-return valve have failed, and the pressure of gas in the tank exceeds the blower discharge pressure, e.g. during simultaneous stripping and ballasting operations;
   
2. to regulate the flow of inert gas to the inert gas deck main.

(2) A typical arrangement, by which the flow of inert gas can be regulated, is described for systems with automatic pressure control and a gas recirculating line; these installations permit control of inert gas pressure in the deck main without the need to adjust the inert gas blower speed; gas not required in the cargo tanks is recirculated to the scrubber or vented to the atmosphere; gas pressure regulating valves are fitted in both the main and recirculating lines; one is controlled by a gas pressure transmitter and regulator, while the other may be controlled either in a similar manner or by a weight-operated valve; the pressure transmitter is sited downstream of the deck isolating valve; this enables maintenance of a positive pressure in the cargo tanks during discharge; it does not necessarily ensure, however, that the scrubber is not overloaded during inerting and purging operations.

Figure 10 Typical automatic pressure control system ^

Alternative methods of regulating gas may be considered.

Arrangements for Inerting, Purging and Gas-freeing

23. (1) The principles of dilution and displacement have already been described in section 9; their application to specific installations depends on a variety of factors, including:

1. the results of laboratory tests;
   
2. whether or not purging of hydrocarbon gas is required in every tank on every voyage; and
   
3. the method of venting cargo tank vapours.

(2) Several arrangements are possible; one feature that should be common to all is the location of the inlet and outlet points such that efficient gas replacement can take place throughout the tank.

(3) There are three principal arrangements:

it will be noted that all three arrangements can be used for inerting, purging and gas-freeing and that a particular ship design may incorporate more than one arrangement.

Arrangement I

(4) In this, the simplest arrangement, gases are both introduced and vented from the top of the tank; gas replacement is by the dilution method; the incoming gas should always enter the tank in such a way as to achieve maximum penetration and thorough mixing throughout the tank; gases can be vented through a vent stack on each tank or through a common vent main. (see Figure 11)

Figure 11 Dilution (I) Figure 12 Dilution (II) ^

Inerting, purging and gas-freeing by dilution method

Arrangement II

(5) Gas is introduced at the bottom of the tank and vented from the top; gas replacement is by the dilution method; this arrangement introduces the gas through a connection between the inert gas deck main (just forward of the mechanical non-return valve) and the bottom cargo lines (see figure 12); a special fixed gas-freeing fan may also be fitted; exhaust gas may be vented through individual vent stacks or, if valves are fitted to isolate each cargo tank from the inert gas main, through this main to the mast riser or high velocity vent.

Arrangement III

(6) Gas is introduced at the top of the tank and discharged from the bottom; this arrangement permits the displacement method (see Figure 13), although the dilution method may predominate, if the density difference between the incoming and existing gases is small or the gas inlet velocity is high (see Figure 14); the inert gas inlet point is often led horizontally into a tank hatch to minimize turbulence at the interface; the outlet point is often a specially fitted purge pipe extending from within 1 metre of the bottom plating to 2 metres above deck level (to minimize the amount of vapour at deck level).

Figure 13 Displacement (III)  ^

Figure 14 Dilution (III) ^

Inerting, purging and gas-freeing by displacement or dilution methods
Isolation of-Cargo Tanks from the Inert Gas Deck Main

14. (1) For gas-freeing and tank entry, some valve or blanking arrangement is always fitted to isolate individual cargo tanks from the inert gas deck main.

(2) The following factors should be considered in choosing a suitable arrangement:

1. protection against gas leakage or incorrect operation during tank entry;
   
2. ease and safety of use;
   
3. facility to use the inert gas main for routine gas-freeing operations;
   
4. facility to isolate tanks for short periods for the regulation of tank pressures and manual ullaging;
   
5. protection against structural damage due to cargo pumping and ballasting operations, when a cargo tank is inadvertently isolated from the inert gas main.

(3) In no case should the arrangement prevent the proper venting of the tank.

(4) Figure 15 shows some examples of arrangements in use.

Figure 15 Examples of methods for isolating tanks from inert gas main ^

Liquid-filled Pressure-vacuum Breakers

25. (1) One liquid-filled pressure-vacuum breaker, or more, should be fitted, unless pressure-vacuum valves are fitted that have the capacity to prevent excessive pressure or vacuum.

(2) These devices require little maintenance, but will operate at the required pressure only if they are filled to the correct level with liquid of the correct density; either a suitable oil or a freshwater/glycol mixture should be used to prevent freezing in cold weather; evaporation, ingress of seawater, condensation and corrosion should be taken into consideration and adequately compensated for; in heavy weather, the pressure surge, caused by the motion of liquid in the cargo tanks, may cause the liquid of the pressure-vacuum breaker to be blown out (see Figure 16).

Figure 16 Principles of liquid filled pressure-vacuum breakers ^

(3) The designer should ensure that the characteristics of the deck water seal, pressure-vacuum breakers and pressure-vacuum valves and the pressure settings of the high and low inert gas deck pressure alarms are compatible; it is also desirable to check that all pressure-vacuum devices are operating at their designed pressure settings.

Instrumentation and Alarms

26. (1) Certain fixed and portable instruments are required for the safe and effective operation of an inert gas system; all instruments should be graduated to a consistent system of units.

(2) Clear instructions should be provided for operating, calibrating and testing all instruments and alarms; suitable calibration facilities should be provided.

(3) All required instrumentation and alarm equipment should be designed to withstand supply voltage variation, ambient temperature changes, vibration, humidity, shock, impact and corrosion normally encountered on board ships.

(4) The arrangement of scrubber instrumentation and alarm should be as follows in subsections (5) through (10).

(5) The water flow to the scrubber should be monitored either by a flow meter or by pressure gauges; an alarm should be initiated when the water flow drops below the designed flow requirements by a predetermined amount, and the inert gas blowers should be stopped automatically in the event of a further reduction in the flow; the precise setting of the alarm and shutdown limits should be related to individual scrubber designs and materials.

(6) The water level in the scrubber shall be monitored by a high-water-level alarm; this alarm should be given when pre-determined limits are reached and the scrubber pump shut down when the level rises above set limits; these limits should be set having regard to the scrubber design and flooding of the scrubber inlet piping from the boiler uptakes.

(7) The inert gas temperature at the discharge side of the gas blowers shall be monitored; an alarm should be given when the temperature reaches 65 degrees Celsius, with automatic shut down of the inert gas blowers if the temperature reaches 75 degrees Celsius.

(8) If a precooler at the scrubber inlet is necessary to protect coating materials in the scrubber, the arrangements for giving an alarm in subsection (7) should apply to the outlet temperature from the precooler.

(9) To monitor the scrubber efficiency, it is recommended that the cooler water inlet and outlet temperatures and the scrubber differential pressures be indicated.

(10) All sensing probes, floats and sensors required to be in contact with the water and gas in the scrubber should be made from materials resistant to acidic attack.

(11) For the deck water seal, an alarm should be given when the water level falls by a pre-determined amount, but before the seal is rendered ineffective; for certain types of deck water seals, such as the dry type, it may be necessary to suppress the water level alarm when inert gas is being supplied to the inert gas distribution system.

(12) The pressure of the inert gas in the inert gas main shall be monitored; an alarm should be given when the pressure reaches the set limit; the set limit should take into account the design of cargo tanks, mechanical non-return valve and deck water seal.

(13) The arrangement for oxygen analyser, recorder and indicating equipment should be as follows in subsections (14) through (22).

(14) The sampling point for the oxygen analyser and recorder unit should be located in the pipework after the blowers and before the gas pressure regulating valve, at the chosen position, turbulent flow conditions should prevail at all outputs of the blowers; the sample point should be easily accessible and provided with suitable air or steam cleaning connections.

(15) The sampling probe should incorporate a dust filter in accordance with the instrument manufacturer’s advice; the probe and filter should be able to be withdrawn and cleaned or renewed as necessary.

(16) The sensing pipe from the sampling probe to the oxygen analyser should be so arranged that any condensation in the sensing pipeline does not prevent the gas sample from reaching the oxygen analyser; joints in the pipeline should be kept to a minimum to prevent the ingress of air.

(17) Any coolers required in the sensing pipes should be installed at the coldest point in the system; alternatively, in certain instances It may be prudent to heat the sensing pipes to prevent condensation.

(18) The position of the analyser should be chosen so that it is protected from heat and adverse ambient conditions, but it should be placed as close as practicable to the sampling point to reduce to a minimum the time between the extraction of a sample and its analysis.

(19) The recording unit and repeater indication should not be located in positions subject to heat and undue vibration.

(20) The resistance of the connecting cables between the analyser and the recorder should be in accordance with the instrument manufacturers’ instructions.

(21) The oxygen analyser should have an accuracy of ± 1 per cent of the full-scale deflection of the indicator.

(22) Depending on the principle of measurement, fixed zero and/or span calibration arrangements should be provided in the vicinity of the oxygen analyser, and fitted with suitable connections for portable analysers.

(23) A sampling point should be provided between the automatic gas pressure regulating valve and the deck water seal for use with portable instruments.

(24) The inert gas pressure sensor and recorder should obtain the signal from a point in the inert gas main between the deck isolating/non-return valve and the cargo tanks.

(25) When the pressure in the inert gas main forward of the non-return devices falls below 50 millimetres water gauge, means shall be provided to sound an alarm or to shut down the main cargo pumps automatically.

(26) The alarms required by paragraph 19(1)(g), Schedule VII, should be given on the navigating bridge and in the machinery space.

(27) Portable instruments shall be provided for measuring oxygen and flammable concentration; with regard to the hydrocarbon vapour meter, it should be borne in mind that meters working on the catalytic filament principle are unsuitable for measuring hydrocarbon concentration in oxygen deficient atmospheres; furthermore, meters using this principle cannot measure concentrations of hydrocarbon vapours above the lower flammable limit; it is, therefore, advisable to use meters that are not affected by oxygen deficiency, and which are capable of measuring hydrocarbon concentration in and above the flammable range; the catalytic filament meter is suitable for measuring below the lower flammable limit, provided sufficient oxygen is present.

(28) All metal parts of portable instruments and sampling tubes that must be introduced into tanks should be securely earthed to the ship’s structure while the instruments and sampling tubes are being used; these portable instruments should be intrinsically safe.

(29) Sufficient tubing etc., should be provided to enable fully representative sampling of a cargo tank atmosphere.

(30) Suitable openings should be provided in cargo tanks to enable fully representative samples to be taken from each tank; where tanks are subdivided by complete or partial wash bulkheads, additional openings should be provided for each such subdivision.

Effluent and Drain Piping

27. (1) The effluent piping from scrubbers and deck water seal drain pipes, where fitted, should be corrosion resistant, or made of carbon steel suitably protected internally against the corrosive nature of the fluid.

(2) The scrubber effluent pipe and deck water seal drain pipe, where fitted, should not be led to a common drain pipe and the deck seal drain should be led clear of the engine room and any other gas-safe space.

(3) The effluent lines should, as far as possible, be discharged below the water line under light ballast conditions, or suitable means should be provided to avoid run-off of the effluent along the ship’s side plating in order to prevent accelerated corrosion/erosion of the plating.

(4) Piping made in glass reinforced plastic of acceptable manufacture, and substantial thickness, which is pressure tested and adequately supported, may be acceptable for effluent piping from scrubbers under the conditions given in subsections (5) and (6).

(5) The effluent lines should, as far as possible, be led through cofferdams or ballast tanks and accord with the load line regulations in force.

(6) Where effluent lines are led through machinery spaces the arrangements should include:

1. a valve fitted to a stub piece at the shell and actuated both from inside and outside the machinery space; the valve should have a position indicator; the valve is to be closed at all times when the plant is not in operation as well as in the event of a fire in the machinery space; suitable instructions to this effect are to be given to the master;
   
2. a flap type non-return valve;
   
3. a short length of steel pipe, or spool piece, lined internally and fitted between the valve referred to in (a) above and the non-return valve referred to in (b) above; this is to be fitted with a 12.5 millimetre diameter flanged drain branch pipe and valve;
   
4. a further spool piece fitted inboard of, and adjacent to, the non-return valve referred to in (b) above, similarly fitted with a drain;
   
   Note: the purpose of this arrangement is to enable the valves and non-return valves referred to in (a) and (b) above to be checked for tightness and to facilitate the removal of the non-return valve for examination and replacement.)
   
5. means outside the machinery space for stopping the scrubber pump.

Figure 17 illustrates a suitable arrangement.

Figure 17 A suitable arrangement of effluent piping led through machinery spaces ^

(7) Where effluent lines penetrate water-tight decks or bulkheads, the requirements of the Marine Machinery and Electrical Equipment Regulations, Standard IX shall apply.

(8) A water seal in the shape of a ‘U’ bend, at least 2 m in depth, should be fitted at least 2 m below the equipment to be drained; means should be provided for draining the lowest point of the bend; in addition, the seal should be adequately vented to a point above the water level in the scrubber or deck water seal.

(9) The diameter of the effluent and drain pipes should be adequate for the duties intended and the pipe run should be self-draining from the water seal referred to in subsection 27(8).

Seawater Service

28. (1) It is advisable that the main supply of water to the inert gas scrubber be from an independent pump; the alternative source of supply of water may be from another pump, such as the sanitary, fire, bilge and ballast pumps, provided that the quantity of water required by the inert gas scrubber is readily available, and the requirements of other essential services are not thereby impaired.

(2) The requirement for two separate pumps capable of supplying water to the deck water seal can be met by any of the pumps referred to under alternative source of supply in subsection (1), subject to the same provisions as are recorded in that subsection.

(3) The pumps supplying water to the scrubber and the deck water seal should provide the required throughput of water at light draught conditions; the quantity of water at all other draught conditions should not flood the scrubber or increase the gas flow resistance excessively.

(4) Loops in the piping of the deck water seal to prevent the backflow of hydrocarbon vapour or inert gas should be positioned outside the machinery space and suitably protected against freezing, for example by steam tracing; with reference to the deck water seal arrangement, provisions should be made to prevent a pneumatically controlled system from freezing.

(5) Vacuum breakers provided to prevent the water loops being emptied should vent to a position on the open deck.

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