ISSUE 1 focused on Tanker Safety Series elaborating: Hot Water Washing of Cargo and Slop Tanks

 IN CONTINUATION….

 ISSUE 2

GMS Trader and Tanker Expert, Kiran Thorat highlights – Purging and Gas Freeing Operation for Man Entry.

During the Hot Water Washing process, the atmosphere of cargo tanks and slop tanks are kept inert, ensuring that the oxygen content inside these tanks remains below 8% by volume.

Cargo tanks and slop tanks on tanker ships must achieve a Gas Free for Hot Work (GFHW) condition prior to arriving at recycling facilities. Following the completion of hot water washing, the cargo tanks are subsequently purged and prepared for Gas Free for Man Entry (GFME).

Risk Assessment:

Vessel’s staff should conduct a comprehensive risk assessment before initiating purging and gas freeing operation. The tank atmosphere must be periodically monitored during and after the completion of these operations. Consequently, gas measuring instruments should be in proper working condition and calibrated regularly. These instruments should be capable of measuring the concentrations of oxygen, carbon monoxide, hydrogen sulfide, and flammable gases. A typical instrument being used is the: Multi Gas Meter.

Purging:

This process is carried out to lower the hydrocarbon concentration in the tank atmosphere below the critical dilution line or to less than 2% by volume. The oxygen content of the inert gas used for purging should be below 5% by volume. Purging can be performed using either the dilution or displacement method.

• Dilution Method: Inert gas is introduced at its maximum flow rate, generating significant turbulence within the tank. This turbulence facilitates the gradual reduction in the concentration of hydrocarbon vapors.

• Displacement Method: Inert gas is introduced at the top of the tank, causing the heavier hydrocarbon vapors to be displaced through a designated piping system.     
  
    

Gas Freeing:

•  Standard Operating Procedures are developed for safely carrying out gas freeing operation over the period by tanker industry. The objective is to reduce the concentration of hydrocarbon vapors to below 1% of the Lower Flammability Limit (LFL) and to increase the oxygen concentration to 21% by volume.        

• Before initiating forced fresh air ventilation, the cargo tank in question must be fully isolated from other cargo tanks. Ventilation should only begin after confirming that the hydrocarbon concentration is below 2% by volume and the oxygen concentration is below 8% by volume.

• Ventilation is done with the fixed equipment such as Inert Gas Blowers installed on the ship and the portable Hydro Blowers which are run through the vessel’s seawater pump through the fire line and then into the tank cleaning line. This ensures the efficiency of gas freeing operation is increased and time is saved.

• Forced fresh air ventilation should be maintained until the concentration of flammable gases decreases to below 1% of the LFL and the oxygen concentration reaches 21% by volume.   
   

To summarize Gas Freeing is a crucial operation that should be closely supervised by the senior management of the onboard staff. Adherence to local regulations, guidelines and compliance is essential for Gas Freeing to avoid any penalties.

For more information or any clarification, please write to [email protected]