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  Tank Farms

When chemicals are not needed for immediate use, they may be stored in large tanks which are owned by the chemical company, or rented at a transport company's tank farm. Separate tanks are used for different types of products.This chapter presents basic descriptions of fixed-roof tanks (vertical and horizontal); internal, external, and domed external floating roof tanks;pressure tanks; and variable vapor space tanks. In addition, the chapter provides descriptions of perimeter seals and fittings for internal, external, and domed external floating roofs.

Types of Storage Tanks

Fixed-roof tanks

External floating roof tanks;

Internal floating roof tanks;

Domed external floating roof tanks;

Horizontal tanks;

Pressure tanks; and

Variable vapor space tanks.

The first four tank types are cylindrical in shape with the axis oriented perpendicular to the foundation.These tanks are almost exclusively above ground. Horizontal tanks (i.e., with the axis parallel to the foundation) can be used above ground and below ground. Pressure tanks often are horizontally oriented and "bullet" or spherically shaped to maintain structural integrity at high pressures. They are located above ground. Variable vapor space tanks can be cylindrical or spherical in shape. The discussion below contains a detailed description of each of these tank types.

Loss Exposure

Due to the large quanties of flammable luqids being handled, any escape of liquid either due to tank failure or fracture can cause ignition and result in fire and/or explosion. Not only will the contents iginite, it also could cause a spread of combutible liquid over a large area spreading the risk.

Tank failure at the top of a tank will permit vapours to be released while containing the liquid contents. Pressure vacuum valves are provided to accommodate normal pressure fluctuations due to the filling and emptying of the tanks and changes in temperature. Failure of these devices can cause build up of pressure and temperature resulting in rupture and release of flammable liquid. Flames and radiant heat contacting the outer surface of the tank will result in heating of the contents and generation of large quantities of vapour when the contents boil. Flames contacting aboveground tanks quickly increase the rate of vapour production which in turn increases the internal tank pressure. This pressure must be relieved before the design tank pressure is exceeded lest it results in explosion of tank.

Low pressure tanks are shop fabricated and leak tested prior to being shipped as completely assembled units. These tanks are designed for pressures greater than 3.5 kPa up to 100 kPa (gauge). Generally, normal internal operating pressure is 6.9 kPa while 17.2 kPa results in emergency venting conditions. These restrictions recognize that failure of a horizontal low-pressure tank is invariably accompanied by the release of tank contents.

If the internal and external corrosion loss expected during the design life of the tank exceeds that provided for in the original design, a hazardous situation could arise. This happens where, additional metal thickness, protective coating or approved lining has not been provided to meet the situtation.

Tanks may be lined or sprayed with combustible or non-combustible material to provide corrosion protection. As the quantity of the lining material used is small, the fact that it is combustible or non-combustible does not matter. However, this lining could increase the risk of ignition from static electricity .

The air space within the storage tank above the flammable or combustible liquid surface will contain the vapours of the liquid. As the ambient temperature rises, more of the liquid will evaporate. These vapours can then be lost to the environment during venting especially when the tank is being filled. Most fires in this design of tank burn only at the seal area and are usually easily extinguished. However, if fires are not controlled, it can lead to explosion of the tank.

Risk Management

Labelling of Tanks

Each tank must be clearly labeled as to its contents. Lettering size readable from 4.5 metres away or from outside the tank dike area, whichever is greater and from two diagonally opposite tank sides is required. Labeling will assist the firefighters when they arrive on the scene by allowing them to determine the best means to extinguish the fire and the most appropriate extinguishing agent to use. In situations where tank contents are frequently changed, a label indicating "flammable liquid" may be used subject to the approval of the Chief Fire Official. In plants and refineries where qualified personnel, knowledgeable in the various tank contents are on site 24 hours/day and 7 days/week to assist the fire department, a coded identification may be approved.

Overfilling Protection

To prevent overfilling continuous supervision of the filling process by a qualified person or by the use of an overfill protection device conforming to relevant industry standards is suggested. Examples of devices used to prevent overfilling include automatic sensing devices interconnected with shut-off equipment, automatic overfill shut-off devices of a float valve, vent restriction devices, and audible or visual overfill alarm devices.

To prevent overfilling during such liquid transfer operation, the tank owner shall comply with one of the following requirements: during the transfer operation, a person shall be in constant attendance at the receiving tank, gauging the liquid level frequently and be equipped with effective two-way communication to the source to ensure prompt shut-down when the tank is full or to divert the flow to another tank; the receiving tank is equipped with an independent height liquid level alarm, which on detecting a high level sends an audible or visual alarm or both, to a constantly attended location where appropriate action can be taken to prevent overfilling, or the receiving tank is equipped with an independent high liquid level alarm which on detecting a high level will automatically shut-down the liquid flow or divert the flow to another tank also equipped with high level shut-down facilities.

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