PREVIOUS | TABLE OF CONTENTS | NEXT

PART VI
PRODUCT CARRIERS

The basic principles of inerting are exactly the same for a product carrier as for a crude oil tanker; however, there are differences in operation as outlined below.

43. One area of difference relates to the carriage of products having a flashpoint exceeding 60°C (closed cup test), as determined by an approved flashpoint apparatus.

1. Subsection 47(1), Fire Protection, Detection and Extinguishing Equipment Regulations, implies that section 49 and Schedule VII, Fire Protection, Detection and Extinguishing Equipment Regulations, do not apply to tankers carrying petroleum products having a flashpoint exceeding 60°C; in other words, product carriers may carry bitumens, lubricating oils, heavy fuel oils, high flashpoint jet fuels and some diesel fuels, gas oils and special boiling point liquids, without inert gas systems having to be fitted, or, if fitted, without tanks containing such cargoes as have to be kept in the inert condition.
   
   
2. If cargoes with a flashpoint exceeding 60 degrees, whether heated or otherwise, are carried at temperatures near or above their flashpoint (some bitumen cut-backs and fuel oils), a flammable atmosphere can occur; when cargoes with a flashpoint exceeding 60°C are carried at a temperature higher than 5 degrees Celsius below their flashpoint they should be carried in an inerted condition.
   
   
3. When a non-volatile cargo is carried in a tank that has not been previously gas freed, then that tank shall be maintained in an inert condition.

Product Contamination by Other Cargoes

44. Contamination of a product may affect its odour, acidity or flashpoint specifications, and may occur in several ways; the types of contamination relevant to ships with an inert gas main (or other gas line) inter-connecting all cargo tanks are:

1. Liquid contamination due to overfilling a tank.
   
   
2. Vapour contamination through the inert gas main; this is largely a problem of preventing vapour from low flashpoint cargoes, typically gasolines, from contaminating the various high flashpoint cargoes listed in subsection 43(1) plus aviation gasolines and most hydrocarbon solvents; this problem can overcome by
   1. (a) removing vapours of low flashpoint cargoes prior to loading; and
   2. (b) preventing ingress of vapours from low flashpoint cargoes during loading and during the loaded voyage;
   
   When hydrocarbon solvents are being carried where quality specifications are stringent, and where it is necessary to keep individual tanks positively isolated from the inert gas main after a cargo has been loaded, pressure sensors should be fitted for monitoring the pressure in each tank; when it is necessary to top up the relevant tanks, the inert gas main should first be purged of cargo vapour.

Contamination of Cargoes by Inert Gas

45. For a well-designed and operated flue gas system, experience suggests that petroleum cargoes traditionally carried on product tankers do not suffer contamination from the flue gas itself, as opposed to contamination from other cargoes; however, unacceptable contamination from the flue gas may be encountered if proper control is not exercised over fuel quality, efficiency of combustion, scrubbing and filtering; the more critical petrochemical cargoes on product carriers can be contaminated by flue gas.

Contamination of Cargoes by Water

46. All lubricating oils and jet fuels are acutely water critical; current practice requires full line draining and mopping up of any water in tanks before loading; water contamination may occur on inerted ships due to:

1. water carry-over from the scrubber and/or deck water seals due to inadequacies in design or maintenance of the various drying arrangements; and
   
2. condensation of water from warm, fully saturated flue gas delivered to the tanks.

Additional Purging and Gas-freeing

47. Gas-freeing is required on non-inerted product carriers more frequently than on crude carriers, both because of the greater need for tank entry and inspection, especially in port, and for venting vapours of previous cargoes; any gas-free operation on inerted product carriers has to be preceded by a purging operation, but gas-freeing for purely quality reasons may be replaced by purging only; in addition purging may be required on the basis outlined in subsection 43(3) above.

It should be recognized that:

1. There are increased risks of air leaking into inert tanks and of inert gas leaking into a tank being entered.
   
2. Purging is not a prerequisite of gas-freeing when the hydrocarbon gas content of a tank is below 2 per cent by volume.
   
3. The operation of gas-freeing for product purity, and where tank entry is not contemplated, does not require the atmosphere to have an oxygen content of 21 per cent by volume.

PREVIOUS | TABLE OF CONTENTS | NEXT