Tank Blanketing or padding

Tank blanketing, or padding, refers to applying a cover of gas over the surface of a stores commodity; usually a liquid. 

• Purpose

- Prevent volatile vapours from escaping
 If the liquid is volatile it is used to protect or contain the stored product and prevent it from harming  personnel, equipment, or the environment. It can maintain the atmosphere above a flammable or combustible liquid to reduce ignition potential.
- Prevent oxidization (oxygen entering)
If the liquid can be oxidized it protects it from oxidation or contamination through exposure to air or moisture.
- Prevent corrosion
It prevents outside air, moisture, and other contaminents from entering and causing corrosion.
It can also reduce the moisture content
- Maintaining constant pressure
If for some reason the tank suddenly cools, the vapours inside the tank will suddenly condense causing the the tank pressure to decrease.

The blanketing is typically set for as low as possible ( minimize the consumption of 2 in of H20) to minimize the wastage of blanketing gas.

• Gases 

- Gas such as nitrogen is supplied in a very pure and dry state.
- LLP Fuel gas 

• Approach

In order to effectively perform any of these functions the blanketing system must be capable of pressurizing the vapor space and accurately maintaining that pressure.

PURGING

One method involves continuous purging, whereby blanketing gas introduced into the tank as a continuous flow exits through a vent or other opening. This method is wasteful of the blanketing gas and not always effective in maintaining an inert atmosphere.

PRV

Another approach employs a simple, direct-operated pressure-reducing valve (PRV) to blanket the tank. However, these devices are the best suited to a continuous flow rate. When used to blanket a tank they must throttle over a wide flow range. Ranging from a shut-off to a full flow condition varies the controlled pressure significantly due to droop and lockup conditions.

CONTROLS/VALVES TO MAINTAIN SET PRESSURE

One of the more effective techniques relies on a blanketing system with the necessary controls and valving to sense and maintain the set pressure within the tank to as close as ± 0.25 inch w.c. [water column] (± 0.009 psi). Set pressures of 0.5 inch w.c. are possible and common.

Such a system will directly sense the tank pressure, control the inlet blanketing gas pressure if required and, through a main control valve, throttle blanketing gas into the tank.

The system may also provide a purge in the sensing and main supply piping, plus reverse flow protection to keep product out of the blanketing system. The installation may also require a pressure switch to monitor tank pressure and perform alarm or control functions in response to over- or under-pressure conditions.

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