Safety considerations for Atmospheric Storage Tanks
The type of storage tank used for specified product is principally determined by safety and environmental requirement. Operation cost and cost effectiveness are the main factors in selecting the type of storage tank.
Design and safety concern has come to a great concern as reported case of fires and explosion for the storage tank has been increasing over the years and these accident cause injuries and fatalities. Spills and tank fires not only causing environment pollution, there would also be severe financial consequences and significant impact on the future business due to the industry reputation.
When flammable materials are being stored, fire is the greatest hazard normally addressed in the design of the storage system. Design items that should be addressed in this area are given below.
- Protection against electrostatic charges which can cause ignition. This may include the bonding and grounding of the tank, piping, and other ancillary equipment and the use of bottom or dip-pipe loading to minimize material splashing in the tank.
- Fire fighting facilities applicable to the type of tank protected. This can include fire loops with hydrants and monitors in the storage area, foam systems for the individual tanks, and deluge spray systems to keep the exposed surfaces of tanks cool in case of fire in an adjacent tank.
Foam systems usually consist of a foam storage tank, an incoming firewater line, a mixing fixture, foam / water piping up the side of the tank, and foam/water applicator nozzles. The systems for fixed roof tanks are designed to create a foam layer over the flammable material in the tank.
The systems for floating roof tanks are designed to cover the space immediately over the seal area, but if an internal floating roof is constructed of lightweight materials, the foam system should be designed as if the tank were a cone roof type.
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- Adequate spacing between tanks.
- Install flame arresters on atmospheric vents to prevent impinging fire on the outside of the tank from reaching the vapor space inside the tank, Deflagration and Detonation Flame Arresters.
- Do not use air to mix flammable materials.
- Provide fire resistant insulation for critical vessels, piping, outlet Valves on tanks, Valve actuators, instruments lines, and key electrical facilities.
- Provide remote controlled, automatic, and fire-actuated Valves to stop loss of tank contents during an emergency; provide fire protection to these Valves. Valves should be close-coupled to the tank, and must be resistant to corrosion or other deleterious effects of spilled fluids.
Overpressure and Underpressure
Internal deflagration is a concern because of the presence of a flammable organic/air mixture in the presence of an ignition source. This mixture can occur during filling, emptying, or mixing in tanks that contain vapors of organics near their flash point. The mixture may also occur in stored products containing impurities or light gases such as hydrogen in petroleum fractions as a result of an upset in an upstream process unit.
Fixed-roof tanks can be constructed as "weak-seam roof tanks" which are designed so that the roof-to-shell connection will fail preferentially to any other joint and the excess pressure will be safely relieved if the normal venting capacity should prove inadequate (API Std 2000). The pressure-venting capabilities can be defeated by erroneous construction. A peripheral railing and walkway, if attached to the top of the wall and to the outer portion of the roof, make the wall-to-roof joint too strong relative to the strength of the wall-tofloor joint. The result is that overpressure may cause the bottom to cup up and tear loose from the wall, instead of tearing off the roof. This is a critical concern for tanks with a diameter less than 10 m (30 feet). Weak seam tanks for storing toxic materials are generally discouraged since a tank rupture would release the material to the atmosphere.
Additional pressure relief devices, directing the hazardous material to a safe area, are used to protect the tank.
Roof sections that could be propelled during an explosion must be restrained with a roof hinge, or cable and springs.
Underpressure (vacuum) in fixed roof tanks can be caused when material is rapidly withdrawn or when a sudden drop in temperature or pressure, usually caused by weather conditions, reduces the volume of the vapor in the tank. The underpressure protection should be sized to handle the maximum withdrawal rate plus the maximum temperature/ volume reduction occurring simultaneously (see API Std 2000 and NFPA 30). The vacuum relief device should be located at, or near, the highest point in the tank. In addition, differential pressure measurement relative to local ambient conditions must be provided.
Excessive Vapor generation
Excessive vapor generation is the result of a deviation of temperature or routing of products more volatile than the design fluid. For tanks provided with internal heaters, adequate level should be maintained above the surface of the heater so as not to overheat the tank contents and cause vapor generation or reach the autoignition temperature. Adequate venting capacity should be provided for excess vapor generation or coil rupture.
The polymerization of materials in a tank can yield sudden high overpressure combined with elevated temperatures in the tank. In this situation standard pressure Relief Valves may not be enough, both because very large two-phase flows may be involved, and because solid, polymerized materials may plug the Relief Valve. In these cases rupture discs with ducting leading to the atmosphere may be used, with the relief effluent being directed to a safe area of the plant. If the polymerization of the tank can yield potentially hazardous materials, the safe area may include an isolation or containment tank or sump for the hazardous material. Additionally, the discharge piping should be anchored, and the pipe elbows braced to counteract the thrust placed on them by a discharge of this type.
Another example of excessive vapor generation occurs in pressure sphere storage tanks where rollover has occurred. This phenomenon is due to stratification of the tank contents causing a dense upper layer which ultimately rolls to the bottom releasing a massive vapor load from the lower (warmer) tank contents. This phenomenon is avoided by carefully keeping the contents mixed and by not loading warm material into the bottom of the sphere using a dip leg. A recent accident attributed to a process similar to rollover is described by Kletz. Warm (+1O0°C) ammonia was loaded into the bottom of a tank of refrigerated (-330°C) ammonia. Kletz suggests that the warm ammonia rose to the surface, evaporated, and the overpressure overwhelmed the Relief Valves, resulting in the tank failing. The tank split from top to bottom, releasing 7000 tonnes of liquid ammonia, forming a pool about 2 feet (0.7 m) deep which caught fire; the fire subsequently spread and resulted in loss of life and many injuries.
Common causes of spills are:
- overfilling due to operator error or high level alarm failure (vehicular as well as stationary tanks)
- withdrawal of water from the tank bottom without operator attention
- mechanical failure of tank support causing collapse of roof
- accumulation of a large volume of water, snow or ice on the tank roof causing collapse and subsequent exposure of liquid surface
An additional cause of spills is specific to floating roof tanks. It is possible for the roof platform to tilt slightly and become wedged into one position. Withdrawal of material from the tank, leaving the roof unsupported, or the addition of material to the tank, forcing fluid up over the roof, may cause the collapse of the floating roof. This is most common when the interior of the tank must be serviced and the roof must be supported on its legs rather than by the tank fluid.
Strategies to avoid spills and minimize damage to other units are:
- Instrumentation for tank high level and flow total alarms and shutoffs should be completely separate from the normal level and flow measurement with separate sensors and control units. Inherently safer design incorporates overflow lines routed to a safe location and secondary containment. Level sensors that depend upon pressure differential to detect level changes should be avoided where changes in the specific gravity of the tank contents are expected.
- Provide safe method of water withdrawal from tanks storing organics and water drainage from the roof the tank.
- Provide secondary containment around tanks to prevent spills from spreading to other areas. This can take the form of dikes, double walled tanks, or tanks in a concrete vault. The containment should be capable of holding the total volume of the largest tank within the containment area plus the rainfall from a specified storm, usually a 25- to 100-year rainfall event. Consideration should also be given to the need to contain f iref ighting water within the secondary containment. The appropriate EPA and state environmental codes should be checked to determine the exact amount of secondary storage as regulatory requirements may vary depending on the chemical and the location. The diked area should be sloped to a low point or sump to allow for the easy removal of liquids. Care should be taken to make sure that the materials stored within a containment area are compatible and that an adverse reaction will not take place if the materials are mixed during an accident.
- Overflow lines should be sized to allow full flow in case of a tank overflow. A general rule of thumb for estimating the size of overflow piping is that it should be sized at least one standard pipe size larger than the inlet pipe, but the exact size will be dependent upon the pressure drop in the pipe. The minimum overflow line size for a self-venting line is D(inches) = 0.92(Q[gpm])°'4. For extremely cold locations, overflow lines should be heated to avoid freezing of condensed atmospheric moisture which can restrict the pipe.
It should be noted that atmospheric tank overflow lines are also a source of vapor releases when volatile fluids are introduced.
Frothover or Boilover
A frothover occurs when the tank temperature increases to the point where water in the tank starts to boil, forming a froth of organics and steam. If froth formation is violent, it may result in frothover of ignitable organics or other fluids, causing a major fire. Frothovers may be caused by:
- mistakenly routing water into a storage tank containing hot oil, creating a steam explosion
- an equipment failure upstream causing water to leak into products being routed to storage
- routing cold light hydrocarbons to hot tanks or hot heavy hydrocarbons to cold tanks
- water in the bottom of mixed or crude oil storage tanks vaporizing during afire
Storage temperatures should be at least 7°C below the boiling point of water to avoid water boilover.
A tank rupture is the sudden loss of tank integrity over a relatively large area of the tank structure, causing a large loss of contents. It can be caused by any of several conditions: overfilling, overpressure due to an internal chemical reaction or material boiling due to a constant exposure to heat, continued impingement of flame over an area of the tank, loss of wall integrity due to corrosion, or loss of wall weld integrity. In a major rupture, such as a tank failure near Pittsburgh, Pennsylvania in January, 1988, the force of the falling material can be so great that large amounts of the material can be pushed up and over the diking and into the environment.
The chances of tank rupture can be reduced by attention to several design features:
- the proper use and sizing of overflow piping and pressure relief safety Valves and rupture disks
- the installation of the appropriate high level alarms and flow shutoffs to prevent overfilling
- the installation of water sprays to protect exposed tank walls during a fire
- the diked area should be sloped to a sump within the diked area
- the proper specification of tank materials and thickness, including corrosion allowances
- the inspection of tank welding during and after construction and the pressure testing of the tank prior to use