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Ex 3: Drying of cargo tanks and operations of related system

The purpose of drying cargo tanks is to remove humid air, which would be present during a dry docking or tank inspection. As part of the process of preparing the cargo tanks ready to carry the next LNG cargo, they must be dried out to avoid the formation of ice when the cargo tanks are cooled down.

Dry-air, which is supplied by the inert gas generator in dry-air mode, is used to expel the moist cargo tank atmosphere via the displacement method of exchange, known as a ‘piston effect’. This can be achieved by introducing the dry air to the bottom of the cargo tank via the filling lines and venting at the top via the vapour lines and slowly displacing the cargo tank’s humid air with dry air. Alternatively, the dry air can be introduced at the top of the cargo tank via the vapour lines and expelled out of the bottom via the filling lines.

To decide upon which method to adopt depends upon the relative humidity of the cargo tank atmosphere, which will vary due to seasonal variations of humidity associated with the climate and location. However, it is assumed that in the summer months, the relative humidity is high, and in ‘Winter’ months, the relative humidity is low and thus dry.

Therefore, for summer months the dry air is introduced to the cargo tank bottom and the lighter warm humid air can be vented at the top of the cargo tank, whereas in winter months, the dry air is introduced at the top of the cargo tank and vented at the bottom.
The actual dew point should be measured to assess the relative humidity and thus the best method to achieve effective cargo tank atmosphere air exchange.

This exercise is designed to help users understand the procedures required to introduce dry air into cargo tanks using an inert gas generator, with the goal of achieving tanks with zero moisture. The process involves introducing dry air with a low dew point into the tanks to replace the moist air inside.

Key Objectives:

  • The exercise aims to ensure the user understands the steps and parameters necessary to successfully complete the drying operation.
  • The user will be guided through various milestones, including starting and operating multi-gas detectors and dew point meters, which are essential for monitoring the atmosphere inside the tanks during the drying process.

Exercise Breakdown:

  1. System Preparation:
  • The user is shown how to correctly install the necessary equipment, including gas detectors and dew point meters.
  • The procedure for preparing the inert gas generator in dry air mode is also explained.
  1. Monitoring the Process:
  • Once the inert gas generator is started and dry air is introduced into the system, the user is instructed to continuously monitor the tank atmosphere using portable gas-measuring equipment.
  • The user will track the progress of the drying process, ensuring the moisture is being effectively replaced by dry air.
  1. Completion and Assessment:
  • The exercise concludes when the indicators show that the dry air has successfully replaced the moist air inside the tanks.
  • The user must stop the supply of dry air once the required dew point has been achieved for all cargo tanks and systems.

This exercise provides hands-on experience in operating the inert gas generator, monitoring the tank atmosphere, and ensuring successful moisture removal to maintain safe conditions within the cargo tanks.

Drying Intro and line up

Drying-Starting IGG in Dry air mode

Drying-Operation and Assessment

Milestones:

  • Learning and Training Phase – Pre-Requisites for the start of operation:
  1. Start the Portable Multi-Gas Detector and Dew Point Meter
  2. Install the Spool Piece to Connect the Inert Gas Line to the Liquid Line
  3. Set the Cargo Tank Vapour Valves
  4. Set the Cargo Tank Liquid Valves
  5. Set the Vapour Line to No. 1 Vent Mast Regulating Valve
  6. Connect the Dry Air Line to the Liquid Line by Opening CL701.
  7. Prepare the Inert Gas Generator (IGG) in Dry Air Mode
  • Assessment Phase – Operational flow and Assessment:
  1. Start the Inert Gas Generator (IGG) in Dry Air Mode
  2. Achieve a Cargo Tank Dew Point Below -20°C
  3. Maintain Cargo Tank Pressure Between 5 kPa and 7 kPa
  4. Check All Vapour Sampling Points for Dew Point in All Tanks
  5. Stop the Dry Air Supply Upon Reaching the Required Dew Point

Exercise Actions and Warnings:

  1. Deviation from Exercise Steps:
    • Avoid opening unnecessary valves or closing required ones after starting the drying process.
    • If such actions occur, a caution message will instruct you to undo the previous action.
    • Failure to correct the deviation will terminate the exercise.
  2. Maintaining IGG Outlet Flow:
    • Keep the IGG outlet flow and flow to the atmosphere between 15,500 kg/h and 16,500 kg/h.
    • If a high-priority alarm is triggered, restore the flow within 1 minute, or the exercise will fail and terminate.

Alarms & Remarks:

Alarm Condition Alarm Time Alarm Level Alarm Title Alarm Description Action
Supply Started Remark Drying Of Cargo Tanks Started Drying of the cargo tanks has started.
On Drying completed Remark Drying of tanks completed Drying of cargo tanks is completed, stop the deivery of inert gas.
started delivery of Dry Air Remark Open Valve CG702 Open the valve CG702, To vent out the Normal air.
Stopped delivery of Dry Air Remark Close Valve CG702 Drying of tanks is stopped, close the valve CG702.
started delivery of Dry Air Remark Maintain IGG Oulet Flow And Outlet Flow To Atmosphere Maintain IGG outlet flow and outlet flow to atmosphere between 15500 kg/h to 16500 kg/h.
when un necessary valve opened or required valve closed Caution Deviation Detected Deviation from the exercise is detected, undo your last action. Fails if that valve is not closed or opened within 1 minute

Step By Step Procedure:

  • Learning and Training Phase – Pre-Requisites for the start of operation:
  1. Start the Portable Multi-Gas Detector and Dew Point Meter:
    • Navigate to MENU > Miscellaneous Equipment.
    • Press and hold the power button for 2 seconds on both the Portable Multi-Gas Detector and Dew Point Meter until they start operating.
  2. Install the Spool Piece to Connect the Inert Gas Line to the Liquid Line:
    • Navigate to MENU > Cargo System.
    • Click to connect the spool piece located above valve CL701, linking the IGG outlet and the cargo tank’s liquid lines.
  3. Set the Cargo Tank Vapour Valves:
    • Open the following valves:
      CG100, CG101, CG200, CG201, CG300, CG301, CG400, CG401.
  4. Set the Cargo Tank Liquid Valves:
    • Open the following valves 100%:
      CL100, CL107, CL200, CL207, CL300, CL307, CL400, CL407.
  5. Set the Vapour Line to No. 1 Vent Mast Regulating Valve:
    • Open valve CG701.
  6. Connect the Dry Air Line to the Liquid Line:
    • Open valve CL701.
  7. Prepare the Inert Gas Generator in Dry Air Mode:
    • Navigate to MENU > IGG System.
    • Configure the following settings:
      • Generator Mode: Set to “Dry Air.”
      • Operating Percentage: Set to “100%.”
      • Fuel Pump: Set to “Off.”
      • IG Chiller Unit: Set to “On.”
  • Assessment Phase – Operational flow and Assessment:
  1. Start the Inert Gas Generator in Dry Air Mode:
      • Wait until the outlet temperature of the IG Chiller Unit is between 2°C and 3°C.
      • Click Start to activate the IGG.
      • Once started, air flows through the Demister Vessel and the dryer unit, reducing the dew point to below -30°C.
      1. Cargo Tank Dew Point < -20°C:
        • Monitor the dew point using the Dew Point Meter.
          • Once the dew point reaches below -20°C at the vapour line sampling point, this milestone is completed.
  1. Maintain Cargo Tank Pressure Between 5 kPa and 7 kPa:
    • Adjust the tank pressure by regulating the vent mast valves.
  2. Check All Vapour Sampling Points for Dew Point in All Tanks:
    • Open the vapour line sampling points for all tanks and confirm the dew point is -25°C or lower.
  3. Stop the Dry Air Supply:
    • Once all tanks reach a dew point of -25°C, stop the IGG delivery by clicking STOP under the delivery option on the IGG System page.
    • Close the vent mast regulating valve CG702.

 

Gas Properties Display on the Detector:

  1. Dry Air Properties:
    • CH₄: 0% LEL
    • O₂: 20.9% vol

Milestones:

  • Learning and Training Phase – Pre-Requisites for the start of operation:
  1. Start the Portable Multi-Gas Detector and Dew Point Meter
  2. Install the Spool Piece to Connect the Inert Gas Line to the Liquid Line
  3. Set the Cargo Tank Vapour Valves
  4. Set the Cargo Tank Liquid Valves
  5. Set the Vapour Line to No. 1 Vent Mast Regulating Valve
  6. Connect the Dry Air Line to the Liquid Line by Opening CL701.
  7. Prepare the Inert Gas Generator (IGG) in Dry Air Mode
  • Assessment Phase – Operational flow and Assessment:
  1. Start the Inert Gas Generator (IGG) in Dry Air Mode
  2. Achieve a Cargo Tank Dew Point Below -20°C
  3. Maintain Cargo Tank Pressure Between 5 kPa and 7 kPa
  4. Check All Vapour Sampling Points for Dew Point in All Tanks
  5. Stop the Dry Air Supply Upon Reaching the Required Dew Point

Exercise Actions and Warnings:

  1. Deviation from Exercise Steps:
    • Avoid opening unnecessary valves or closing required ones after starting the drying process.
    • If such actions occur, a caution message will instruct you to undo the previous action.
    • Failure to correct the deviation will terminate the exercise.
  2. Maintaining IGG Outlet Flow:
    • Keep the IGG outlet flow and flow to the atmosphere between 15,500 kg/h and 16,500 kg/h.
    • If a high-priority alarm is triggered, restore the flow within 1 minute, or the exercise will fail and terminate.

Alarms & Remarks:

Alarm Condition Alarm Time Alarm Level Alarm Title Alarm Description Action
Supply Started Remark Drying Of Cargo Tanks Started Drying of the cargo tanks has started.
On Drying completed Remark Drying of tanks completed Drying of cargo tanks is completed, stop the deivery of inert gas.
started delivery of Dry Air Remark Open Valve CG702 Open the valve CG702, To vent out the Normal air.
Stopped delivery of Dry Air Remark Close Valve CG702 Drying of tanks is stopped, close the valve CG702.
started delivery of Dry Air Remark Maintain IGG Oulet Flow And Outlet Flow To Atmosphere Maintain IGG outlet flow and outlet flow to atmosphere between 15500 kg/h to 16500 kg/h.
when un necessary valve opened or required valve closed Caution Deviation Detected Deviation from the exercise is detected, undo your last action. Fails if that valve is not closed or opened within 1 minute

Step By Step Procedure:

  • Learning and Training Phase – Pre-Requisites for the start of operation:
  1. Start the Portable Multi-Gas Detector and Dew Point Meter:
    • Navigate to MENU > Miscellaneous Equipment.
    • Press and hold the power button for 2 seconds on both the Portable Multi-Gas Detector and Dew Point Meter until they start operating.
  2. Install the Spool Piece to Connect the Inert Gas Line to the Liquid Line:
    • Navigate to MENU > Cargo System.
    • Click to connect the spool piece located above valve CL701, linking the IGG outlet and the cargo tank’s liquid lines.
  3. Set the Cargo Tank Vapour Valves:
    • Open the following valves:
      CG100, CG101, CG200, CG201, CG300, CG301, CG400, CG401.
  4. Set the Cargo Tank Liquid Valves:
    • Open the following valves 100%:
      CL100, CL107, CL200, CL207, CL300, CL307, CL400, CL407.
  5. Set the Vapour Line to No. 1 Vent Mast Regulating Valve:
    • Open valve CG701.
  6. Connect the Dry Air Line to the Liquid Line:
    • Open valve CL701.
  7. Prepare the Inert Gas Generator in Dry Air Mode:
    • Navigate to MENU > IGG System.
    • Configure the following settings:
      • Generator Mode: Set to “Dry Air.”
      • Operating Percentage: Set to “100%.”
      • Fuel Pump: Set to “Off.”
      • IG Chiller Unit: Set to “On.”

 

  • Assessment Phase – Operational flow and Assessment:
  1. Start the Inert Gas Generator in Dry Air Mode:
    • Wait until the outlet temperature of the IG Chiller Unit is between 2°C and 3°C.
    • Click Start to activate the IGG.
    • Once started, air flows through the Demister Vessel and the dryer unit, reducing the dew point to below -30°C.
  1. Cargo Tank Dew Point < -20°C:
    • Monitor the dew point using the Dew Point Meter.
      • Once the dew point reaches below -20°C at the vapour line sampling point, this milestone is completed.
  2. Maintain Cargo Tank Pressure Between 5 kPa and 7 kPa:
    • Adjust the tank pressure by regulating the vent mast valves.
  3. Check All Vapour Sampling Points for Dew Point in All Tanks:
    • Open the vapour line sampling points for all tanks and confirm the dew point is -25°C or lower.
  4. Stop the Dry Air Supply:
    • Once all tanks reach a dew point of -25°C, stop the IGG delivery by clicking STOP under the delivery option on the IGG System page.
    • Close the vent mast regulating valve CG702.

 

Gas Properties Display on the Detector:

  1. Dry Air Properties:
    • CH₄: 0% LEL
    • O₂: 20.9% vol
    • CO: 0 ppm
    • CO₂: 0% vol
    • Dew Point: -25°C
    • Air Temperature: -25°C
    • Relative Humidity: 0%
  2. Warm Vapour Properties:
    • CH₄: 0% LEL
    • O₂: 20.9% vol
    • CO: 0 ppm
    • CO₂: 0% vol
    • Dew Point: 18°C
    • Air Temperature: 30°C
    • Relative Humidity: 39%
  3. Mixed Gas Properties:
    • Values will vary between the ranges above, depending on the proportions of dry air and normal air.

Proper Line Up For Drying: