Wednesday, December 8, 2010

PIGGING ON CHEMICAL TANKERS - CAPT AJIT VADAKAYIL

PIPELINE PIGGING CHEMICAL TERMINALS







A pig is a rubber sphere or cylinder pushed trough the shore pipeline by compressed air ( 40 to 100 psi ) or propelled by another grade of liquid.  The pig catcher is a pig receiver which is mounted outboard of the ships manifold, from which the pig may be removed.

At some terminals the shore pipeline from shore tanks to ships manifold is pigged. Its is possible that a large amount of cargo enters the tank at a very fast rate and high pressure. The amount which comes in must be discussed in the preloading meeting . Precautions must betaken by chief officer to prevent a spill on board.

Clearing line without pig—

The manifold valve is closed at the completion of the loading operation and is not opened until the commencement of the line clearing operation.  At the beginning of the line clearing operation, the pump stack or tank valve is partly opened. Then the manifold valve is opened and manually throttled (opened and closed repeatedly) to control the operation.  The valve operator needs to listen for the sound of flowing cargo or gas in the line, and close the valve when a flow of gas is heard. An experienced operator should be in control of the valve during the line clearing operation.

The operator in control of the manifold valve must maintain direct communication with dock personnel during the entire line clearing operation.

The manifold valve is throttled in short bursts during the line clearing operation to facilitate the clearing process and to control the pressure in the tank.  The manifold valve should never be fully opened.  Between bursts of compressed gas, the pressure in tank should be allowed to equalize/dissipate.  The type of manifold valve being used has a significant impact on the quality of control during the line clearing operation. Ball and gate valves are more effective than butterfly valves when attempting to manually control the flow rate of compressed gas.

Lifting hoses, tapping lines, and feeling for “hot” cargo are methods used to indicate how well the line clearing operation is proceeding.  The pressure in the line must be allowed to ‘bleed off’ prior to disconnecting hoses.
In no case should the P/V vents be restricted or closed while line clearing to the vessel.

Line Clearing with pig--

Procedures must be in place to ensure the correct pig (type and size) is used.
The system should be lined up with the vessel manifold valve closed.  Responsible personnel should verify the lineup and ensure that all safeguards are operational.

The pig should be introduced at the pig launcher.  Care must be taken to ensure that the pig is properly positioned in the launcher.  The pressure should be equalized between the line and the pig launcher.  

The vessel should open it’s manifold valve part way.  Compressed gas should be introduced into the launcher, behind the pig, to begin moving the pig through the line. Personnel should monitor the flow of cargo and the movement of the pig throughout the operation and communicate this information to each other.  Once the pig enters the pig trap, the shore valve and vessel manifold valve should be closed immediately.  The line and hoses from shore to vessel manifold should then be cleared directly as described previously.  

      CAPT AJIT VADAKAYIL ( 28 YEARS IN COMMAND )
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