Chapter 7 - Cargo Tank Venting
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Chapter 7 - Cargo Tank Venting

 To protect cargo containment systems from harmful overpressure or underpressure at all times.

7.1 General

7.1.1 All cargo tanks should be provided with a venting system appropriate to the cargo being carried and these systems should be independent of the air pipes and venting systems of all other compartments of the vessel. Tank venting systems should be designed so as to minimize the possibility of cargo vapour accumulating about the decks, entering accommodation, service and machinery spaces and control stations and, in the case of flammable vapours, entering or collecting in spaces or areas containing sources of ignition. Tank venting systems should be arranged to prevent entrance of water into the cargo tanks.

7.1.2 The venting systems should be connected to the top of each cargo tank and, as far as practicable, the cargo vent lines should be self-draining back to the cargo tanks under all normal operational conditions of list and trim. Where it is necessary to drain venting systems above the level of any pressure/vacuum valve, capped or plugged drain cocks should be provided.

7.1.3 Provision should be made to ensure that the liquid head in any tank does not exceed the design head of the tank. Suitable high-level alarms, overflow control systems or spill valves, together with gauging and tank filling procedures, may be accepted for this purpose. Where the means of limiting cargo tank overpressure includes an automatic closing valve, the valve should comply with the appropriate requirements of 15.19 of the IBC Code.

7.1.4 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. 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;

  • .4 pressure loss in vent piping and across valves and fittings; and

  • .5 pressure/vacuum settings of relief devices.

7.1.5 Tank vent piping connected to cargo tanks of corrosion-resistant material or to tanks which are lined or coated to handle special cargoes as required by chapter 15 of the IBC Code, should be similarly lined or coated or constructed of corrosion-resistant material.

7.1.6 The master should be provided with the maximum permissible loading and unloading rates for each tank or group of tanks consistent with the design of the venting systems.

7.2 Types of tank venting systems

7.2.1 An open tank venting system is a system which offers no restriction except for friction losses to the free flow of cargo vapours to and from the cargo tanks during normal operations. An open venting system may consist of individual vents from each tank, or such individual vents may be combined into a common header or headers, with due regard to cargo segregation. In no case should shut-off valves or any other means of stoppage, including spectacle blanks and blank flanges, be fitted either to the individual vents or to the header.

7.2.2 A controlled tank venting system is a system in which pressure- and vacuum-relief valves or pressure/vacuum valves are fitted to each tank to limit the pressure or vacuum in the tank. A controlled venting system may consist of individual vents from each tank or such individual vents on the pressure side only as may be combined into a common header or headers, with due regard to cargo segregation. In no case should shut-off valves or any other means of stoppage, including spectacle blanks and blank flanges, be fitted either above or below pressure- or vacuum-relief valves or pressure/vacuum valves. Provision may be made for bypassing a pressure- or vacuum-relief valve or pressure/vacuum valve under certain operating conditions provided that the requirement of 7.2.6 is maintained and that there is suitable indication to show whether or not the valve is bypassed.

7.2.3 Controlled tank venting systems should consist of a primary and a secondary means of allowing full flow relief of vapour to prevent overpressure or underpressure in the event of failure of one means. Alternatively, the secondary means may consist of pressure sensors fitted in each tank with a monitoring system in the vessel's cargo control room or position from which cargo operations are normally carried out. Such monitoring equipment should also provide an alarm facility which is activated by detection of overpressure or underpressure conditions within a tank.

7.2.4 The outlets of a controlled tank venting system should direct the vapour discharge upwards in the form of unimpeded jets and the position should be arranged at a height of not less than 6 m above the weather deck.

7.2.5 The outlet height referred to in 7.2.4 may be reduced to 3 m above the weather deck provided that high-velocity venting valves of an approved type with an exit velocity of at least 30 m/s are fitted.

7.2.6 Controlled tank venting systems fitted to tanks to be used for cargoes having a flashpoint not exceeding 60C should be provided with devices to prevent the passage of flame into the cargo tanks. The design, testing and locating of the devices should comply with the provisions of the Administration, which should contain at least the standards adopted by the Organization.

7.2.7 In designing venting systems and in the selection of devices to prevent the passage of flame for incorporation into the tank venting system, due attention should be paid to the possibility of the blockage of these systems and fittings by, for example, the freezing of cargo vapour, polymer build-up, atmospheric dust or icing up in adverse weather conditions. In this context it should be noted that flame arresters and flame screens are more susceptible to blockage. Provisions should be made such that the system and fittings may be inspected, operationally checked, cleaned or renewed as applicable.

7.2.8 Pressure tanks should be fitted with pressure relief devices that are so designed as to direct the discharge away from personnel and have a set pressure and capacity which is in accordance with standards acceptable to the Administration taking into account the design pressure referred to in 6.1.5.

7.3 Venting requirements for individual products

Venting requirements for individual products are shown in column g and additional requirements in column o in the table of chapter 17 of the IBC Code.

7.4 Cargo tank gas freeing

7.4.1 The arrangements for gas freeing cargo tanks used for cargoes other than those for which open venting is permitted should be such as to minimize the hazards due to the dispersal of flammable or toxic vapours in the atmosphere and to flammable or toxic vapour mixtures in a cargo tank. Accordingly, gas freeing operations should be carried out such that vapour is initially discharged:

  • .1 through the vent outlets specified in 7.2.4 and 7.2.5; or

  • .2 through outlets at least 2 m above the cargo tank deck level with a vertical exit velocity of at least 30 m/s maintained during the gas freeing operation; or

  • .3 through outlets at least 2 m above the cargo tank deck level with a vertical exit velocity of at least 20 m/s which are protected by suitable devices to prevent the passage of flame.

When the flammable vapour concentration at the outlets has been reduced to 30% of the lower flammable limit and, in the case of a toxic product, the vapour concentration does not present a significant health hazard, gas freeing may thereafter be continued at cargo tank deck level.

7.4.2 The outlets referred to in and may be fixed or portable pipes.

7.4.3 In designing a gas freeing system in conformity with 7.4.1, particularly in order to achieve the required exit velocities of and, due consideration should be given to the following:

  • .1 materials of construction of system;

  • .2 time to gas free;

  • .3 flow characteristics of fans to be used;

  • .4 the pressure losses created by ducting, piping, cargo tank inlets and outlets;

  • .5 the pressure achievable in the fan driving medium (e.g. water or compressed air); and

  • .6 the densities of the cargo vapour/air mixtures for the range of cargoes to be carried.

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