FIXED ROOF TANK
Fixed Roof Tanks may be of cone roof or dome roof type construction.
Dome roofs are usually supported from the shell by framing consisting of rafters and girders pressed to the roof radius.
Cone roofs are usually supported from the shell by rafters or trusses.
Large cone roof tanks can also be built with columns and girders to support the rafters and roof plates.
Due to environmental concerns and to reduce vapour losses there is tendency to install more and more internal fixed decks in fixed roof tanks either under construction or as a retrofit.
This combines the advantage of both fixed and fixed roof design.
Breathing loss is the expulsion of vapor from a tank through vapor expansion and contraction, which are the results of changes in temperature and atmospheric pressure. This loss occurs without any liquid level change in the tank. Working loss is a result of an increase in the liquid level in the tank. Fixed roof tanks are either freely vented or equipped with a pressure/vacuum vent. The latter allows the tanks to operate at a slight internal pressure or vacuum to prevent the release of vapors during very small changes in temperature, pressure, or liquid level. It is also possible to inert tanks with small gas over pressure.
Fixed roof tank emissions vary as a function of vessel capacity, vapor pressure of the stored liquid, utilization rate of the tank, and atmospheric conditions at the tank location.
According to type (shape) of fixed roof:
Fixed Roofs (Dome Or Cone) With Internal Fixed Roof.
Different hypothesis are assumed for the use of fixed roof tank model:
Fixed roof tanks must be substantially liquid and vapour tight and must operate approximately at atmospheric pressure.
The potential emission sources for above-ground horizontal tanks are the same as those for vertical fixed roof tanks. Emissions from underground storage tanks are associated mainly with changes in the liquid level in the tank. In that case breathing loss can be neglected (losses are minimal for underground tanks because the surrounding earth limits the diurnal temperature change).
In order to limit working losses, vapor balancing is commonly used. As the storage tank is filled, the vapors expelled from the storage tank are directed to the emptying tanker truck. The truck then transports the vapors to a station where a vapor recovery or control system is used to control emissions. In that case working loss can be neglected.
Storage tanks containing organic liquids, non organic liquids, vapors and can be found in many industries. Most storage tanks are designed.
These tanks can have different sizes, ranging from 2 to 60 m diameter or more. They are generally installed inside containment basins in order to contain spills in case of rupture of the tank.
Industries where storage tanks can be found are: petroleum producing and refining, petrochemical and chemical manufacturing, bulk storage and transfer operations, other industries consuming or producing liquids and vapors.
Types of storage tanks
Basically there are eight types of tanks used to store liquids:
The first four tank types are cylindrical in shape with the axis oriented perpendicular to the sub grade. These tanks are almost exclusively above ground. Horizontal tanks can be used above and below ground. Pressure tanks often are horizontally oriented and 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.
External fixed roof tanks
Internal fixed roof tanks
Domed external fixed roof tanks
Variable vapor space tanks
LNG (Liquefied Natural Gas) tanks
A containment basin of a product should be built around the tanks are made of brick or concrete and the lining should be impervious to liquid stored to prevent spills that can cause fire, property damage or contaminate the environment. The minimum capacity of the basin volume should be equal to the capacity of the largest tank plus 10% of the sum of the capacities of others. To prevent a spill or other emergency the walls of the containment basin must be resistant to the product and must be able to withstand considerable pressure. The drain Valve, which should be incorporated into the outer side of the containment basin, must be closed to prevent possible contamination to the environment.
We currently used tank designs, the fixed-roof tank is the least expensive to construct and is generally considered the minimum acceptable equipment for storing liquids. A typical fixed-roof tank consists of a cylindrical steel shell with a cone- or dome-shaped roof that is permanently affixed to the tank shell. Storage tanks are usually fully welded and designed for both liquid and vapor tight, while older tanks are often have a riveted or bolted construction and are not vapor tight.
Lifting and handling of a stainless steel storage tank:
A breather Valve (pressure-vacuum Valve), which is commonly installed on many fixed-roof tanks, allows the tank to operate at a slight internal pressure or vacuum. This Valve prevents the release of vapors during only very small changes in temperature, barometric pressure, or liquid level, the emissions from a fixed-roof tank can be appreciable.
Additionally, gauge hatches / sample wells, float gauges, and roof manholes provide accessibility to these tanks and also serve as potential sources of volatile emissions.
External Fixed Roof Tank
A typical external fixed roof tank consists of an open-topped cylindrical steel shell equipped with a roof that floats on the surface of the stored liquid, rising and falling with the liquid level. The fixed roof is comprised of a deck, fittings, and rim seal system. Fixed roof decks are constructed of welded steel plates and are of three general types: pan, pontoon, and double deck.
Although numerous pan-type decks are currently in use, the present trend is toward pontoon and double-deck type fixed roofs.
Manufacturer supply various versions of these basic types of fixed decks, which are tailored to emphasize particular features, such as full liquid contact, load-carrying capacity, roof stability, or pontoon arrangement. The liquid surface is covered by the fixed deck, except in the small annular space between the deck and the shell; the deck may contact the liquid or float directly above the surface on pontoons.
External fixed roof tanks are equipped with a rim seal system, which is attached to the roof perimeter and contacts the tank wall. The rim seal system slides against the tank wall as the roof is raised and lowered. The fixed deck is also equipped with fittings that penetrate the deck and serve operational functions. The external fixed roof design is such that evaporative losses from the stored liquid are limited to losses from the rim seal system and deck fittings (standing storage loss) and any exposed liquid on the tank walls (withdrawal loss).
Internal Fixed Roof Tank
That tank has both, a permanent fixed roof and a fixed roof inside. There are two basic types of internal fixed roof tanks:
Tanks in which the fixed roof is supported by vertical columns within the tank
Tanks with a self-supporting fixed roof and no internal support columns
The fixed roof is not necessarily free of openings but does span the entire open plan area of the vessel. Fixed roof tanks that have been retrofitted to employ an internal fixed roof are typically of the first type, while external fixed roof tanks that have been converted to an internal fixed roof tank typically have a self-supporting roof.
Tanks initially constructed with both a fixed roof and an internal fixed roof may be of either type. An internal fixed roof tank has both a permanently affixed roof and a roof that floats inside the tank on the liquid surface (contact deck) or is supported on pontoons several inches above the liquid surface (noncontact deck). The internal fixed roof rises and falls with the liquid level.
Domed External Fixed Roof Tank
Domed external fixed roof tanks have the heavier type of deck used in external fixed roof tanks as well as a fixed roof at the top of the shell like internal fixed roof tanks. Domed external fixed roof tanks usually result from retrofitting an external fixed roof tank with a fixed roof.
As with the internal fixed roof tanks, the function of the fixed roof is not to act as a vapor barrier, but to block the wind. The type of fixed roof most commonly used is a self supporting aluminum dome roof, which is of bolted construction. Like the internal fixed roof tanks, these tanks are freely vented by circulation vents at the top of the fixed roof. The deck fittings and rim seals, however, are basically identical to those on external fixed roof tanks.
Horizontal tanks are constructed for both above-ground and underground service. Horizontal tanks are usually constructed of steel, steel with a fiberglass overlay, or fiberglass-reinforced polyester. Horizontal tanks are generally small storage tanks.
Horizontal tanks are constructed such that the length of the tank is not greater than six times the diameter to ensure structural integrity. Horizontal tanks are usually equipped with pressure-vacuum vents, gauge hatches and sample wells, and manholes to provide accessibility to these tanks. In addition, underground tanks may be cathodically protected to prevent corrosion of the tank shell. Cathodic protection is accomplished by placing sacrificial anodes in the tank that are connected to an impressed current system or by using galvanic anodes in the tank. However, internal cathodic protection is no longer widely used in the petroleum industry, due to corrosion inhibitors that are now found in most refined petroleum products.
Variable Vapor Space Tank
Variable vapor space tanks are equipped with expandable vapor reservoirs to accommodate vapor volume fluctuations attributable to temperature and barometric pressure changes. Although variable vapor space tanks are sometimes used independently, they are normally connected to the vapor spaces of one or more fixed roof tanks. The two most common types of variable vapor space tanks are lifter roof tanks and flexible diaphragm tanks. Lifter roof tanks have a telescoping roof that fits loosely around the outside of the main tank wall. The space between the roof and the wall is closed by either a wet seal, which is a trough filled with liquid, or a dry seal, which uses a flexible coated fabric.
Flexible diaphragm tanks use flexible membranes to provide expandable volume. They may be either separate gasholder units or integral units mounted atop fixed roof tanks. Variable vapor space tank losses occur during tank filling when vapor is displaced by liquid. Loss of vapor occurs only when the tank's vapor storage capacity is exceeded.
Advantages of using of internal fixed roofs:
Level of Evaporation of Storage Product Is Decreased;
Risk of Fire Is Lower
Internal Fixed Roof Could Be Aluminum. It Has Low Height and Storage Capacity Is Increased;
The Internal Fixed Roof Is Protected From Ambient Climatic Conditions and They Could Be Used In Various Earth Regions;
It Is Not Necessary Mounting Of Roof Drain.
Dome roof with internal fixed roof
Tanks with cone or dome roofs and internal fixed roofs have reduction loss of evaporation of the product. When the internal pressure is increased the loss is less.
Internal fixed roofs could be: made of steel - they are similar to external fixed roof, but roof drain is not necessary;
According to static scheme of the roof:
Self-supporting roofs - cone and dome roofs;
Self-supporting cone roof
Self-supporting roofs, especially dome roof are very suitable when internal pressure is increased. Latterly internal fixed roofs can be mounted when fixed roofs are supported and there are columns inside. When the roof is self-supporting internal space of the tank is free and it is more convenient to install internal fixed roof. Conditions of exploitation of internal fixed roof are improved.
Self-supporting dome roof
It is the most light and economical type of fixed roof.
It can be used when:
The spans are very big;
The internal pressure is increased;
Aluminum is used as material for construction;
Roof plates usually are welded to supporting structure. According Bulgarian experience cone and dome roofs can be ensured with frangible joints. It can be achieved when part of the welds between cover plates and structures has decreased height.
Roof cover plates work only on tension condition and they could be used sterling. Plates work in two directions;
This type of roof is easy to prepare;
Hanging roof could cover big spans;
They are very light.
The tanks are made in various capacities
Volume: Ranging from 200m3 to 15 million liters and above
Diameter: Ranging from 5 mtrs to 60 meters and up
Height: Ranging from 5 mtrs to 20 meters and up
Design code: API 650 and others
Past Project for Fixed roof tanks:
• Gasoline storage tank - 4 x 8000 m3
• Jet fuel storage tanks - 3 x 2 million gallons
• Petrol storage tank - 6 x 8500 cum.
• Kerosene storage tank - 5 x 4 million gallons
• Crude oil storage tank - 5 x 10000 m3
• Ethanol storage tank - 5 x 8500 cum.
• Molasses storage tank - 2 x 2500 cum