Failure Of Inert Gas 2n36

Follow-up Action on Product Tankers on failure of Inert gas Tank coatings usually inhibit the formation of pyrophors in the cargo tanks of product tankers. If it is considered totally impracticable to repair the inert gas system, discharge may therefore be resumed with the written agreement of all interested parties, provided that an external source of inert gas is provided or detailed procedures are established to ensure the safety of operations. The following precautions should be taken: 1. The manual referred to in Section 7.1.12 above should be consulted. ( I G Manual) 2. Devices to prevent the age of flame or flame screens (as appropriate) are in place and

are checked to ensure that they are in a satisfactory condition. 3. Valves 4.

on the vent mast risers are opened.

No free fall of water or slops is permitted.

No dipping, ullaging, sampling or other equipment is introduced into the tank unless essential for the safety of the operation. If it is necessary for such equipment to be introduced into the tank, it should be done after at least 30 minutes have elapsed since the injection of inert gas has ceased. (See Section 7.1.6.8 for static electricity precautions relating to inert gas and Section 11.8 for static electricity precautions when dipping, ullaging and sampling.) 6. All metal components of any equipment to be introduced into the tank should be securely earthed. This restriction should be applied until a period of five hours has elapsed since the injection of inert gas has ceased. 5.

Action to be Taken on Failure of the Inert Gas System In the event that the inert gas system fails to deliver the required quality and quantity of inert gas, or to maintain a positive pressure in the cargo tanks and slop tanks, action must be taken immediately to prevent any air being drawn into the tanks. All cargo and or ballast discharge from inerted tanks must be stopped, the inert gas deck isolating valve closed, the vent valve between it and the gas pressure regulating valve (if provided) opened, and immediate action taken to repair the inert gas system. Masters are reminded that national and local regulations may require the failure of an inert gas system to be reported to the harbour authority, terminal operator and to the port and flag state istrations.

Follow-up Action on Crude Oil Tankers Pyrophoric iron sulphide deposits (pyrophors), formed when hydrogen sulphide gas reacts with rusted surfaces in the absence of oxygen, may be present in the cargo tanks of crude oil tankers and these deposits can heat to incandescence when coming into with air. In the case of tankers engaged in the carriage of crude oil, the failed inert gas system must therefore be repaired and restarted, or an alternative source of inert gas provided, before discharge from inerted tanks is resumed. (See also Section 2.6.3.)

Venting Arrangements Venting capacity is based on the maximum volume of cargo entering a tank plus a 25% margin to for gas evolution (vapour growth). When loading cargoes having a very high vapour pressure, gas evolution may be excessive and the allowance of 25% may prove to be insufficient. Actions to consider in order to ensure that the capacity of the venting system is not exceeded include a close monitoring of vapour line

pressures on inerted ships and limiting loading rates on non-inerted ships throughout the loading period or during crude oil washing during discharge operations. It should be noted that the vapour growth increases when the liquid levels in the tank are above 80%. On inerted ships, close attention should be given to monitoring inert gas system pressures, particularly when topping-off during loading operations or on commencing crude oil washing during discharge operations. When calculating loading rates, a maximum venting line velocity of 36 metres per second should be considered. This flow rate should be calculated for each diameter of line used. The volume throughputs may be aggregated where a common vent riser is used, but the maximum flow rate should not be exceeded anywhere within the system. Static accumulator oil

An oil with an electrical conductivity of less than 50 picoSiemens/metre (pS/m), so that it is capable of retaining a significant electrostatic charge. (such as kerosene) Liquids are considered to be non-conductors when they have conductivities less than 50 pS/m (pico Siemens/metre). Such liquids are often referred to as static accumulators. Petroleum products, such as clean oils (distillates), frequently fall into this category with a conductivity typically below 10 pS/m. Chemical solvents and highly refined fuels can have conductivities of less than 1 pS/m. The following additional precautions should be taken against static electricity during ullaging, dipping, gauging or sampling of static accumulator oils: _ Banning the use of all metallic equipment for dipping, ullaging and sampling during loading and for 30 minutes after completion of loading. After the 30 minute waiting period, metallic equipment may be used for dipping, ullaging and sampling, but it must be effectively bonded and securely earthed to the structure of the ship before it is introduced into the tank, and must remain earthed until after removal. _ Banning the use of all non-metallic containers of more than 1 litre capacity for dipping, ullaging and sampling during loading and for 30 minutes after completion of loading. Non-metallic containers of less than 1 litre capacity may be used for sampling in tanks at any time, provided that they have no conducting components and that they are not rubbed prior to sampling. Cleaning with a high conductivity proprietary cleaner, a solvent such as 70:30% IPA:toluene mix, or soapy water, is recommended to reduce charge generation. To prevent charging, the container should not be rubbed dry after washing.

3.2.2 Bonding The most important countermeasure that must be taken to prevent an electrostatic hazard is to bond all metallic objects together to eliminate the risk of discharges between objects that might be charged and electrically insulated. To avoid discharges from conductors to earth, it is normal practice to include bonding to earth (‘earthing’ or ‘grounding’). On ships, bonding to earth is effectively accomplished by connecting metallic objects to the metal structure of the ship, which is naturally earthed through the sea. Some examples of objects which might be electrically insulated in hazardous situations and which must therefore be bonded are: _ Ship/shore hose couplings and flanges, except for the insulating flange or single length of nonconducting hose required to provide electrical isolation between the ship and shore. (See Section 17.5.) _ Portable tank washing machines. _ Manual ullaging and sampling equipment with conducting components. _ The float of a permanently fitted ullaging device if its design does not provide an earthing path through the metal tape. The best method of ensuring bonding and earthing will usually be a metallic connection between the conductors. Alternative means of bonding are available and have proved effective in some

applications, for example semi-conductive (dissipative) pipes and ‘O’ rings, rather than embedded metallic layers, for GRP pipes and their metal couplings.