STORAGE TANKS & VESSELS - Storage Tanks
Written by Norrie
Monday, 24 May 2010 21:43
Page 2 of 9
3. STORAGE TANKS
Large volumes of harmless liquids involving little or no pressure are usually stored in simple, mild steel, rectangular or cylindrical tanks fitted with a flat or conical roof - known as a 'Fixed Roof Tank'.
Flammable or toxic liquids can also be stored in similar vessels but generally are located well away from residential and process areas due to the safety hazards involved.
When a number of tanks are installed, they are usually referred to as 'Tank Farms' and each tank may contain many thousands of tons (or tonnes) of liquid. The larger the tank, the lower the capital cost per ton (tonne), of liquid stored.
Figure : 34 shows such a tank and the possible combination of features built in. ( Note that not all tanks will have all of the features shown - some types of fittings depend upon the type of liquids the tanks contain ).
The diagram shows all the main features that MAY be found on a storage tank. For example, some tanks may be open and will not need a relief valve. Some will be shallow and will not need an access manhole …etc.
- Roof Access Ladder - As its name implies, is used for safe access to the tank roof.
- Access Manholes -Usually fitted at ground level or on the roof for access to the inside of the tank for cleaning, maintenance and repair.
- Water Drain - For use where water separation occurs in the tank and is to be drained off. Also useful during internal cleaning operations.
- Transfer Pumps - These are used to transfer some or all of the tank contents to a process or to other storage tanks, ships, vehicles .. etc. Depending on their piping arrangement, the pumps may be used for re-circulation (mixing) of the tank contents or pumping liquid into the tank from another source.
- Bund Walls (or Firewalls) - These can be of two types :
- A wall surrounding the tank, high enough to contain the entire tank contents in the event of a burst or severe leak. In this way, flammable, toxic or corrosive substances can be contained safely until recovered.
- An alternative to the above, is having the tank situated in a cylindrical hole in the ground, again large enough to contain all of the tank contents in the event of burst or leakage.
- Relief Valves - Installed where a tank contains pressure and set to relieve excess pressure if it rises to the safe operating limit.
- Inert Gas Blanket - This facility is installed when a tank contains volatile liquid which may produce vapours that become flammable or explosive when mixed with air. The inert gas injection (often Nitrogen), replaces the vapour above the liquid and will not react chemically nor produce an explosive mixture with the tank's contents.
- Foam Injection -In emergency, foam can be sprayed into the tank and over the surface of the liquid. Hazards arising from toxic or flammable vapours can be minimised.
- Vapour Vent - (Often called the 'Breather Valve') - This allows the tank to 'breathe' when emptying or filling. If a vent is not fitted, the tank could over-pressure when filling and cause a rupture in the tank and, when emptying, a vacuum would be pulled possibly causing the tank to collapse (implode). (The vapour vent can consist of simply an open hatch where the liquid being stored has no hazards).
For hazardous materials, the vapour vent may consist of a two-way safety valve. This will operate at a rising, pre-set pressure to vent excess gas from the tank to atmosphere or flare system to prevent rupture of the tank. At a falling pre-set pressure, the valve will operate to admit air, gas or inert gas (called 'Blanket Gas'), into the tank to prevent collapse of the tank if the pressure falls to a vacuum. (Gas or inert gas would be used where ingress of air is undesirable).
- Sampling & Gauging Hatch - As the name implies, this is installed in order to obtain samples of the tank's contents for analysis and is also used for dropping a ' dip-tape ' or ' dip-stick ' into the tank to check the liquid level and also check the automatic level measuring instruments.
- Earth Connections -An ' Earthing-strip ' is connected to tanks (and other equipment) in order to carry away and prevent build up of 'Static Electricity' which tends to form during filling and emptying operations. If static was allowed to build up where flammable liquids are being stored, then we would have a potential fire or explosion hazard. The storage of large volumes of liquid can lead to problems arising from the daily and seasonal variations in the weather. The most important variable, is that of the ambient temperature.
Variations in ambient temperature can lead to :
- Expansion (volume increase), and contraction (volume decrease), of the stored liquid. This is allowed for by leaving sufficient space above the liquid.
- Evaporation of liquid, particularly volatile liquid, in the tank on temperature increase. This will be taken care of by the 'breather' valve.
TYPICAL STORAGE TANK
Figure : 34
The picture on the following page shows a 'Tank Farm' having Fixed and Floating roof tanks.
Last Updated on Monday, 24 May 2010 21:47