CARGO TANK VENTING SYSTEM

The venting system of ships carrying oil must comply at least with the requirements of Solas 74/78 as amended, Chapter II-2, Reg 59 and 62 regardless the year of built of the vessel.
Each cargo tanks must be provided with “ independent ” P/V or H/V valves to take into account the effects of temperature variations ( breathing system ) and “ independent or common ” venting arrangements designed and operated as to ensure the passage of large volumes of vapour mixtures during loading, discharging or ballast handling. The two functions can be combined in one single system, as in the case of independent high velocity valves.
Both the tanks breathing system and the venting of large volumes of vapours are to be backed up by an alternate system capable of assuring the completeness of the functions ( breathing and venting of large volume of vapour ) in case of a failure of the primary system.
The requirement for a pressure sensor to be provided with a monitoring and alarm system for all cargo tanks can be accepted as an alternative to the duplications of the system only in case the vessel is fitted with a centralized cargo control room from where the entire cargo operations can be monitored and an alarm for high or low pressure can immediately be picked up and the respective tank can be identified.

For chemical carriers, venting during loading and discharging operations of those product requiring a “ controlled venting system” has to be carried out through automatic devices (P/V or High Velocity valves ) capable of

• Maintaining an adequate vapour pressure in the tank ( or vacuum during discharging for non-inerted tanks ).
• Assuring that the tank design maximum pressure and vacuum are never exceeded .

P/V and H/V valves must be bench tested for maximum pressure and vacuum setting and venting capacity at least every 60 months and a certificate issued by the classification society.

Information must be available and posted in the cargo control room to confirm cargo tanks venting capabilities and maximum admissible cargo loading rate for each tank.