Common Types of Bulk Liquid Storage Tanks
There is a wide range of storage tanks found throughout the oil & gas industry. Depending on the tank type or mounting options, a particular tank gauge or measurement solution may be more suitable. Tanks are chosen according to the flash point of liquid stored in the tank. Generally speaking, in refineries, tank farms and terminals where petroleum based liquids are stored, above ground fixed roof tanks or floating roof tanks are predominant. These tanks operate under no (or very little) pressure, distinguishing them from pressure vessels, which in turn have additional requirements that must be considered.
Fixed, Cone Roof Tanks
Fixed (cone, dome or umbrella) roof tanks are the most common and identifiable bulk storage vessels in the oil & gas industry, typically seen with a wrap around staircase. They range in sizes up to 30 meters tall by 100 meters wide and are used to store liquids with very high flash points (e.g. fuel oil, heavy oil, kerosene, diesel oil, water, bitumen, etc.). The addition of a dome roof reduces environmental emissions and provides additional strength to allow slightly higher storage pressures than that of atmosphere. Float and tape tank gauges can be installed ‘at grade’ on the tank-side or on the tank roof. Servo, radar and other gauging technologies are installed on the tank roof. When installed on the tank roof, a gauge is mounted on a flange that is either permanently affixed to the tank roof or integrated into a manhole cover.
Floating Roof Tanks
Some storage tanks need a floating roof in addition to or in lieu of the fixed roof. A sealing device is installed on the peripheral space between the roof and shell plate, which acts as a safety and pollution prevention device by trapping the vapor from low flashpoint products. Floating roof tanks are broadly divided into internal (IFR) and external (EFR) floating roof tanks. IFR tanks are used for liquids with low flashpoints (e.g. gasoline, ethanol, etc.). These tanks are nothing more than cone roof tanks with a floating roof inside the tank, which travels up and down along with the liquid level. At low product levels, the floating roof is supported with legs on which it rests. EFRs are open at the top and do not have a fixed roof. As such, they are suitable for medium flash point liquids (naphtha, kerosene, diesel, crude oil, etc.). When mounting a servo or radar gauge on a floating roof tank, a gauging platform is required in order to mount the gauge over the product. This may be the tank roof on covered internal floating roof tanks or an actual platform that extends out from the tank wall over an open roof tank. Many platforms provide a stilling well. The stilling well provides stability and a “calm” surface to enable an accurate measurement. Servo gauges require a stilling well and Varec recommends the use of a stilling well or a roof reflector for radar measurement. For float and tape gauges, a stilling well is not required as it is more common to see a pan/well inside the floating roof. Alternatively, a weight is placed on the roof that connects to the gauge tape/wire. The actual float and tape gauge level measurement is then an indication of the roof’s position, which can be corrected to the actual liquid level.
Sphere and Bullet Tanks
Flat-bottomed, cylindrical or spherical (single and double shell) storage tanks are typically within the scope of our measurement capabilities. They are used to store liquefied gases with very low flash points (LNG, LPG, Ethylene, butane and ammonia) under pressure or at temperatures under -100 °C. Bullet tanks gain their name due to their shape - long cylindrical tanks with round or flat ends. They are generally 5,000 to 30,000 gallons in size, prefabricated and installed horizontally or vertically. At tank farms, terminals and refineries, they often store products that support the facility’s operations - additives for injection, fuels to run refinery process operations or by-products, such as transmix from terminal product receipts. Bullet tanks can also store liquefied gases under pressure. Due to their smaller size and the products they contain, it is more common to use radar, servo or magnetostrictive technology for gauging the contents of the tank.
Underground Storage Tanks
Underground storage tanks (UST) used to store petroleum based products are regulated to prevent release of petroleum and contamination of groundwater. In the U.S., they are primarily used at automobile filling stations (an application outside our product range), but can also be found at military bases, airports and tank farms. They are prefabricated from steel, aluminum or glass fiber and are generally double walled, bullet shaped tanks that have been buried. Likewise, it is more common to use radar, servo or magnetostrictive technology for gauging the contents of the tank.