Section 5 Cargo tank venting arrangements

5.1.1 Each cargo tank and slop tank or group of cargo tanks or slop tanks connected by means of a vapour collecting pipe system is to be fitted with venting arrangements which will limit the pressure or vacuum in the tank, and are to comply with the requirements of this Section. The vapour collecting system of slop tanks are normally separated and distinct from the cargo tanks, see Pt 5, Ch 13, 3.10 Slop tanks and vessels intended for slops for Type C tankers and Type N tankers 3.10.8.

5.1.2 Cargo tank venting arrangements are to be designed to provide:

1. pressure/vacuum release of small volumes of vapour/air mixtures flowing during a normal voyage;

2. venting of large volumes of vapour/air mixtures during cargo handling and gas freeing operations;

3. Pressure sensors are to be fitted to monitor the overpressure and underpressure of the gas phase in each cargo tank protected by the arrangement required in Pt 5, Ch 13, 5.1 General 5.1.2.(b), with a monitoring system in the ship’s wheelhouse or the position from which cargo operations are normally carried out. Such monitoring equipment is also to provide an alarm facility which is activated by detection of overpressure or underpressure conditions within a tank. The alarm facility shall give a visible and audible alarm at the wheelhouse. If the wheelhouse is not supervised an additional alarm is to be provided at the position from which cargo operations are normally controlled.

5.2.1  Type N-open. Each cargo tank is to be in open connection with the atmosphere through a vapour pipe, or equivalent, of sufficient cross-sectional area. Provision is to be made to prevent collection or the entrance of water into the cargo tanks by approved means or appliances.

5.2.2  Type N-open, with flame arrestor. Arrangements as for Type N-open ships, except that vapour pipes, or equivalent, are to be provided with readily renewable flame arrestors or safety heads of approved type suitable to withstand a long burning proof. Material of wire gauzes is to be resistant to corrosion. Provision is to be made to prevent collection or the entrance of water into the cargo tanks by approved means or appliances.

5.2.3  Type C and N-closed. Each cargo tank or group of cargo tanks connected to a common vapour pipe is to be provided with:

1. means to prevent the tanks being subjected to an overpressure exceeding 115 per cent of the set pressure of the high velocity valve or to a design under pressure of the tanks. In any case, the under pressure is not to exceed 5 kPa during the voyage and any phase of the cargo handling;

2. connections for returning the vapour to shore when loading cargo. These connections are to be provided with a positive means of closing, see Pt 5, Ch 13, 3.4 Terminal fittings at cargo loading stations;

3. an approved type high velocity vent capable of resisting a long burning proof. Vapour should be discharged in an upward vertical direction.

4. at each cargo tank connected to a common vapour pipe, a device should be fitted at the inlet of the vapour pipe to prevent the passage of flames and being capable of resisting a detonation in the common vapour pipe. The required device is to be of an approved type.

5. a vacuum valve of an approved type and provided with a wire gauze capable of resisting a deflagration.

6. a device of approved type to depressurise the cargo tanks in a safe manner. This device is consisting of a flame arrestor capable of resisting a long burning proof and a valve. The position of the valve i.e. open or closed is to be clearly indicated.

7. high velocity vents to be arranged not less than 2 m above deck and their outlets should also be arranged at a distance of at least 6 m from the accommodation and service spaces positioned outside the cargo area. Openings below 2 m can be accepted provided no handling equipment will be fitted in a radius of 1 m and the area will be clearly marked as dangerous. In no case is the height of the opening to be less than 0,5 m above deck.

8. a pressure gauge on each cargo tank suitable for under and over-pressure measurement. The over and under-pressure readings are to be visible at a position were loading or discharging can be stopped or otherwise from a position in the vicinity of the control of the water spray system. The maximum allowable under and over-pressure for the cargo tank is to be indicated on each pressure gauge. The pressure gauges readings are to be visible under all weather conditions.

9. means for draining liquid in the vent piping system, from places where it may accumulate, should be provided. The high velocity valves and piping are to be so arranged that, under no circumstances, liquid can accumulate in or near the high velocity valves.

10. If explosion protection is not required as per the ADN, Table C, Column 17 the following relaxations can be given:

    • High velocity vents as per Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.3.(c) are not required to be of an approved long burning proof type.
    • Vacuum valves as per Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.3.(e) are not required to be provided with a wire gauge capable of resisting a deflagration.
    • Detonation devices as per Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.3.(d) are not required to be fitted.
    • Depressurising devices are not to be provided with a flame arrestor capable of resisting a long burning proof as per Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.3.(f).

11. Flammable cargoes are banned from the list of dangerous goods for tankers in compliance with the above Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.3.(j).

5.2.4  Type G. The requirements of Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.5 to Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.19 apply.

5.2.5 All cargo tanks are to be provided with a pressure relief system appropriate to the design of the cargo containment system and the cargo being carried. Hold spaces and cargo piping which may be subjected to pressures beyond their design capabilities are also to be provided with a suitable safety relief system. The pressure safety relief system is to be connected to a vent piping system to minimise the possibility of cargo vapour accumulating about the deck or entering accommodation and machinery spaces, or any other space where it may create a dangerous condition.

5.2.6 Each cargo tank with a volume greater than 20 m³ is to be fitted with at least two pressure relief valves of approximately equal capacity, suitably designed and constructed for the prescribed service. For cargo tanks with a volume less than 20 m³, a single relief valve may be fitted.

5.2.7 The pressure relief valves should be connected to the highest part of the cargo tank above deck level.

5.2.8 In the vent piping system means, for draining liquid, from places where it may accumulate, should be provided. The pressure relief valves and piping are to be so arranged that, under no circumstances, can liquid accumulate in or near the pressure relief valves.

5.2.9 Pressure relief valves are to be prototype tested to ensure that the valves have the capacity required.

5.2.10 In general, the opening pressure of the safety valves should not be higher than the vapour pressure which has been used in the design of the tank. However, where two or more pressure relief valves are fitted, valves comprising not more than 50 per cent of the total relieving capacity may be set at a pressure up to 5 per cent above MARVS. (Maximum Allowable Relief Valve Setting of a cargo tank).

5.2.11 The setting of the pressure relief valves is in no case exceeding the maximum design pressure of the independent cargo tanks.

5.2.12 The pressure relief valves are to have a (combined) relieving capacity for each cargo tank as per 8.3 Additional pressure relieving system for liquid level control and 8.5 Size of valves of the Rules for Ships for Liquefied Gases.

5.2.13 Each tank is to be provided with a pressure gauge suitable for under and over-pressure measurement. The over and under-pressure readings are to be visible at a position were loading or discharging can be stopped. The maximum allowable under and over-pressure for the cargo tank is to be indicated on each pressure gauge. The pressure gauges are to be visible under all weather conditions.

5.2.14 For refrigerating systems, see 7.2 Refrigeration systemsof the Rules for Ships for Liquefied Gases.

5.2.15 Each cargo tank in which cooled cargo will be carried is to be provided with a safety device which will prevent inadmissible over and under pressure in the cargo tanks.

5.2.16 For the carriage of cooled cargo, the opening pressure of the safety device is to be established by the arrangement of the cargo tank. For cargoes which are required to be cooled the safety device is to be set not less than 25 kPa above the calculated pressure as intended in 7.2 Refrigeration systems of the Rules for Ships for Liquefied Gases.

5.2.17 Openings for the relief of gases from the overpressure devices are to be arranged not less than 2 m above deck and their openings should also be arranged of at least 6 m from accommodation and service spaces situated outside the cargo zone. A height of less than 2 m can be accepted provided no handling equipment will be fitted in a radius of 1 m of the discharge opening of the overpressure device and the area will be clearly marked as dangerous.

5.2.18 Suitable protection screens are to be fitted on vent outlets to prevent the ingress of foreign objects.

5.2.19 The design and testing of the devices mentioned in Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.2 to Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.17 are to comply with the requirements of the relevant National Authorities.

5.3.1 The design vapour pressure P _o is the maximum gauge pressure at the top of the tank which has been used in the design of the tank.

5.3.2 For cargo tanks where there is no temperature control and where the pressure of the cargo is indicated only by the ambient temperature, P _o is not be less than the gauge vapour pressure of the cargo at a temperature of 40 °C.

5.3.3 In all cases, including Pt 5, Ch 13, 4.3 Design loads 4.3.2, P _o should not be less than MARVS.

5.4.1 The design temperature for selection of materials is the minimum temperature at which cargo may be loaded or transported to the cargo tanks. Provisions to the Society's satisfaction are to be made to ensure that the tank or cargo temperature cannot be lowered below the design temperature.

5.5.1 Cargo tank venting systems should be designed and operated so as to ensure that neither pressure nor vacuum created in the cargo tanks during loading or unloading exceeds tank design parameters, see Pt 5, Ch 13, 5.2 Pressure/vacuum and venting systems for various tanker types 5.2.3.(a) for maximum allowable cargo tank pressures. The main factors to be considered in the sizing of a tank venting system are as follows:

1. design loading and unloading rate;

2. gas evolution during loading: this should be taken account of by multiplying the maximum loading rate by a factor of at least 1.25;

3. density of the cargo vapour mixture, based on a mixture of 50 vol. per cent vapour and 50 vol. per cent air;

4. pressure loss in the vent piping and across valves and fittings. Detonation devices and flame arrestors are to be considered in the clogged condition responsible for an additional pressure loss of not less than 30 per cent ;

5. pressure/vacuum settings of relief devices;

6. dimensions of the cargo tank venting system.

5.5.2 The maximum permissible loading and unloading rates for each tank or group of tanks consistent with the design of the venting system shall be carried on board.

5.5.3 The maximum permissible loading and unloading rates for each tank or group of tanks are to be limited in order to avoid static electricity, but should in any case not exceed 7 m/sec in the cargo pipes.

5.6.1 Unless the entire cargo system is designed to withstand the full gauge vapour pressure of the cargo under conditions of the upper ambient design temperatures, maintenance of the cargo tank pressure below the MARVS should be provided by one or more of the following means:

1. A system which regulates the pressure in the cargo tank by the use of mechanical refrigeration.

2. A system that accepts warming up and rise of pressure. The insulation together with the design pressure of the cargo tank are to be capable to guarantee an appropriate safety in respect of the period of operation and working temperature. The safety of at least three times the period of operation is to be guaranteed.

3. Any other system acceptable to the Society.

For requirements of the above systems, see Pt 5, Ch 13, 1.1 Application 1.1.8.

5.6.2 The above systems should be constructed, fitted and tested to the satisfaction of the Society. Materials used in the construction should be suitable for use with the cargoes to be carried. For normal service, the upper ambient temperature should be:

• Air temperature : + 30 °C.
• Water temperature : + 20 °C.

5.6.3 For certain highly dangerous cargoes specified in Table C of the ADN , the cargo containment system should be capable of withstanding the full vapour pressure of the cargo under conditions of the upper ambient design temperatures irrespective of any system provided for dealing with boil-off gas, see Pt 5, Ch 13, 1.3 Materials 1.3.9 for reference.

5.7.1 All tankers are to be equipped with at least one portable instrument for measuring the percentage of LEL of hydrocarbon concentrations in air, together with a sufficient set of spares.

5.7.2 All tankers are to be equipped with at least one portable oxygen analyser.

5.7.3 A sufficient set of spares of the above portable instruments is to be carried on board.

5.7.4 Suitable means are to be provided for the calibration of gas measurement instruments.