The Ultimate Guide to Cargo Operations for Tankers
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| MASTER DRY DOCK OPERATIONS For Marine Engineers For Deck Officers Why inert operation Highly flammable hydrocarbon (HC) gases are usually present in the cargo tank atmosphere on oil tankers. HC cannot burn in an atmosphere containing less than approximately 11% oxygen by volume This oxygen control is done by inerting, a process of replacing the tank atmosphere by an inert gas whose O content is less than 8%. Thus, inerting is carried out for replacing the tank atmosphere by introducing inert gas to ensure oxygen content is 5% or less by Volume. The main purpose of using inert gas on ships is to • Inert the atmosphere of empty cargo tanks 109. l Marine Insight Inert Gas System on Deck Inerting Cargo Operations for Tanker I 110. • Inert the atmosphere of the cargo tank during cargo discharge • Purge tanks before gas freeing • Top up the pressure in the cargo tanks when necessary during other stages of the voyage Method of inerting tank atmosphere The most common methods used for inerting a tank atmosphere used onboard ships are Dilution Displacement Dilution In this method, the inert gas is forced into the tanks at high velocity so that the IG reaches even the bottom part of the tank. When the inert gas enters, it mixes with the original tank atmosphere to form a homogeneous mixture throughout the tank so that, as the process continues, the concentration of the original gas decreases progressively. 111. l Marine Insight Cargo Operations for Tanker I 112. In this method, a limit must be placed on the number of tanks that can be inerted simultaneously to get the desired results. 113. l Marine Insight 2. Displacement This method works on the principle that the inert gas is slightly lighter than hydrocarbon gas. Hence, the inert gas is made to enter from the top of the tank the heavier hydrocarbon gas escapes from the bottom through the piping arrangement. The displacement method uses the inert gas at a very low velocity to enable a stable horizontal interface between the incoming and escaping gas. This method allows several tanks to be inerted or purged simultaneously. Whichever method is employed, whether inerting or purging, it is vital that oxygen or gas measurements are taken at several heights and horizontal positions within the tank to check the efficiency of the operation. Cargo Operations for Tanker I 114. • After opening the deck main isolation valve, the inert gas generator must be started • The control of the generator is to betaken in cargo control room • Once the line is set to deliver inert gas into tanks, the inert gas delivery to deck should be started • Monitor the oxygen content of the inert gas and set the pressure in the tanks to mm WG A mixture of inert gas and petroleum gas, when vented and mixed with air, can become flammable. The safety precautions which are normally taken when the petroleum gas is vented from a tank, therefore, should not be relaxed. Inerting is done to reduce the content of oxygen in the tanks. All tanks must be inerted to make them free of cargo residues. Wherever Purge pipes are fitted in the line, they should be provided with proper flame screens. The following procedure is to be followed • All openings to cargo tanks to be closed and branch inert gas valves to be opened • Open the gas freeing port lids on the PV valves 115. l Marine Insight Keep monitoring the oxygen content of the inert gas, which is delivered by the sampling from sample points provided on the line. The tank O levels to be monitored continuously to ascertain if the oxygen content is reducing to 8% or less Cargo Operations for Tanker I 116. • Goon with purging by inert gas until the hydrocarbon content reduces to the required/desired level Download 6.36 Mb. Share with your friends:
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