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Monday
Apr252011

TFE-6

Safety feature impact of tanks and contents

     12-2.2 through 12-2.2.1 deals with the safety features and how they impact various systems. The FLBSS {Flammable Liquid Bulk Storage “Specialist”} must be able to predict the likely behavior of any bulk storage tank type, and its contents. To do this, your tools and methods of controlling a fire or spill are based on what and how your installed safety features will behave and operate. These range from Tank spacing all the way through fire protection systems. Tank monitoring devices as well as facility transfer capabilities are also critical in understanding.

     Tank spacing, impoundment, diking, and tank spacing are critical. Depending on the type of petroleum stored and the tank size, spacing should be as follows; all tanks less than 150’ in diameter shall be 1/6th X the sum of all adjacent tank diameters within the same retainment field, but not less than 3ft between tank walls. Tanks with remote retainment that either are floating roof or class IIIA liquids are calculated by 1/6th X the sum of adjacent tank diameters within the same confinement area. If the same tanks are class I or II liquids the multiplicative factor is 1/4th. With area on-site retainment provided, floating roof and class IIIA liquids also use the 1/4th value in this equation. If retainment is on-site for class I or II liquids, they used 1/3 X the sum of adjacent tank diameters within the same confinement area.

     The control of spills and ground fires is accomplished by way of impoundment dikes. While these have been in previous posts in detail, prioritizing this type of hazard would be to eliminate chemical contact between tank bottom and spillage or fire. The best method of attack for the FLBSS would be a combination of Medium expansion Foam application with subsequent pump off procedures to empty the spill level below the tanks footings. This will minimize corrosive contact to the tank bottom and of course eliminate fire exposure/tank failure.

     Another technique is tank venting. Basically there are two types for flammables stored as bulk commodities, low pressure liquids and high pressure gases. Low pressure liquids are commonly limited in their venting capabilities to comply with stricter environmental standards. However, there is a technique available on an emergency basis to tank farm and FLBSS personnel. Excess pressure can be draw into the plant flare which is commonly always burning off excess gases 24 hrs a day. This system is set up to purposely draw excess pressure off tanks, lines, and production pathways during transfer or pressure overloads. A system of lines open and close to allow these excessive differences to relief, directly to the plant flare where they rise to the appropriate height and are “burned” into the atmosphere. These burning “plant flares” are often seen from a distance. At times they can be quite bright, and at others almost non-existent. During an emergency, the FLBSS can recommend opening the burn discharge of this device to its maximum. This technique may give you additional time for the next stage of your mitigation.

     High pressure gas storage may have emergency individual venting relief valves much like those mounted on transportation vessels, or simply piped into the plant flare. One would expect to see fewer emergency relief valves in bulk storage, as venting will result in environmental issues. The general and probable method of release for pressurized gasses would be a “plant flare” or product transfer to another pressurized tank during an emergency incident. Due to the high blast wave and subsequent massive carnage from a pressurized flammable gas tank detonation, during emergency situations, all options become “on the table”.

     Modern monitoring and detection systems have made massive increases in safety at tank farms. The ability to determine from one central command area, tank pressures, flare strength, line pressures, transfer rates, and loading or unloading operations gives tank farm operators constant and immediate data to make prudent and safe decisions. Before these devices were available, many incidents resulted in disastrous loss of life, property, and product. These systems are usually coordinated in a central command room or “control room” where management supervisors can attend to any situation that may develop. The FLBSS must have access to these “management operators” to assist with the decision making process.

     Fire protection systems have developed along with tank safety, and provide an immense edge to the firefighting world. Present capabilities {and probably future ones} to mount an aggressive fire attack against just one, “6 million gallon tank” of flammable liquids requires multiple fire departments, hundreds of firefighters, thousands of gallons of Foam concentrate, multiple aerial apparatus, and tens of thousands of gallons of water, all operating together in unison. Not an easy scene to coordinate between multiple fire departments. The most successful system available to modern tank farm firefighting is fixed Foam Chambers. These units are designed into or installed as an upgrade, permanently to the fixed site tank. The drawings demonstrate some common and reliable types. Results prove their reliability to remain intact and functioning even after tank walls have begun melting inward. System failure has occurred during initial explosion and when automatic activators have failed to charge the chambers with Foam solution. Properly implemented fixed Foam systems yield the most successful fire suppression systems. In most modern floating tank roofs, smaller “seal” fires are commonly extinguished by Foam chambers 8-10 minutes after ignition.

                                                                                               Haz Mat Mike    



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