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Purging The Tank  

Gas blanketing and loading under a nitrogen blanket

With certain products, the empty space in the tank container during loading and discharge and the ullage space during transit is filled with an inert gas, usually nitrogen. For pressure loading and discharge, inert gas is used for the propellant medium by which the pressure is supplied. Before loading it is normally necessary to purge the tank of atmospheric air.

Safety Note: Inert gases are normally non-toxic, but inhalation in confined spaces can cause asphyxiation. Tanks which have been purged of air should be fitted with a temporary seal and marked to indicate that the tank contains inert gas.

Preparing the Tank

Products carried under a gas blanket require the normal pre-trip checks to ensure that the tank container is suitable for the purpose. There should be an additional pressure check for tightness of all valves and fittings to ensure that no loss of inert gas will occur during

Two methods of purging the tank of atmospheric air are available to the tank user depending upon the equipment available.
Method One – By pressure

1. Close manlid and valves

2. Connect nitrogen supply to airline, or bottom outlet.

3. Inject nitrogen until pressure in tank is 1.5 bar (21psi)

4. Release pressure in tank through top outlet. (if top outlet is used to inject nitrogen, or if there is no top outlet, the pressure should be released by opening the airline or bottom outlet. The tank now contains 40% air and the amount of nitrogen used is 1.5 times the volume of the tank.

Repeat steps 3 and 4 if necessary:

- After 2 purges the air content is 16%. Oxygen content 4%. Nitrogen consumed 3 volumes.
- After 3 purges the air content is 6%. Oxygen content 1.5%. Nitrogen consumed 4.5 volumes.
- After 4 purges the air content is 2.5%. Oxygen content 0.5%. Nitrogen consumed 6 volumes.

5. Close all valves on tank
Method Two – Mixing and Venting

1. Close manlid

2. Connect nitrogen supply to top outlet or bottom outlet.

3. Open outlet not used in steps 2 above. (If top outlet is not fitted, open airline connection).

4. Inject nitrogen until number of air changes required have been completed using a flow meter to measure the nitrogen used.

- After 1 volume change the air content is 37%. Oxygen content 10%. Nitrogen consumed 1 volume.
- After 2 volume changes the air content is 13%. Oxygen content 3%. Nitrogen consumed 2 volumes.
- After 3 volume changes the air content is 5%. Oxygen content 3%. Nitrogen consumed 3 volumes.
- After 4 volume changes the air content is 2%. Oxygen content 0.4%. Nitrogen consumed 4 volumes.

Note: The effectiveness of this method depends upon the thorough mixing of the air in the tank with the incoming nitrogen. In practice it is not possible to achieve the theoretical percentages shown above and it is recommended that an oxygen analyser is used to confirm that the air content is sufficiently reduced.

5. Close all valves on tank
Loading Under A Nitrogen Blanket 
Loading under a nitrogen blanket.
Read in conjunction with the appropriate loading section with particular attention to the ‘before’ and ‘after’ instructions.

1. Purge the tank.

2. Connect the product loading hose to the top or bottom hose connection

3. Pressurise hose.

4. Open valves and commence loading.

5. As the cargo fills the tank the nitrogen pressure will rise
6. when the nitrogen pressure required for the cargo to be carried is reached, commence bleeding off the excess pressure by slightly cracking open the airline valve.

7. The product quality must be metered at the pump or bulk storage location. When the complete cargo is loaded, close the tank inlet valve or valves.

8. Check that the correct nitrogen pressure has been maintained.

9. Drain and remove the hose.

10. Replace all blanketing plates and dust caps.

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