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					Oil Movements Encyclopedia
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Tanks (design aspects, roof types)
05-03-97

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Tanks
1. Introduction
This section gives some further background on tank design. Note that all tank design issues are ORTEC/4's responsbility. The information given here is only to support the Oil Movements Technologist. The standardisation of storage tanks and fittings has gone to a great extent especially with regards to cone roof tank types for which standard drawings and calculations are available for tanks with a diameter range of 3 m up to 60 m.

After the selection of the tank type and pressure range, the tank size must be determined, which is normally based on the standard dimensions. Then follows the design and engineering, which normally is an ORTEC/4 or Opco responsibility. A logical engineering sequence is outlined hereunder:   A detailed soil investigation study is carried out resulting in a foundation engineering report highlighting expected settlement behaviour of the tank. Requisitions for the tank(s) are prepared. For fittings and accessories reference is made to applicable standard drawings. The foundation engineering report is included in the "invitation to bid" package. Various contractors prepare bid proposals, which provide details about the design, engineering fabrication welding and erection method, testing and (pre-) commissioning of the tank. Following bid evaluation and contract award, the tank manufacturer/contractor starts the detailed design, comprising static calculations for the various tank components, design and workshop drawings, erection diagrams, etc.

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2. Further background to tank design
2.1. Tank foundation
The tank foundations generally take the form of a tankpad, constructed from durable, inert, granular materials such as sand, covered by an erosion protection layer. Under the tank bottom a High Density Polyethylene layer of 1.3 - 2.0 mm thickness is installed to prevent leakage of hydrocarbons into the soil and groundwater.

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Tanks are rarely founded on piled concrete rafts unless all the available options such as relocation, pre-loading, soil replacement, future re-levelling or jack-up and foundation reinstatement have been considered in depth and proven to be unfavourable from an economic viewpoint. The main functions of a tank pad are the following:       To transfer and distribute the loading exerted by the tank, its content and external forces to the foundation receiving soils. To raise the tank bottom to such a level that it will be well clear of the highest ground water and capillary water levels. To provide a durable and smooth surface for tank construction.

A tank can de divided into three main parts: the bottom the shell the roof

2.2. Tank bottom
Bottom plates function as a membrane and transfer the liquid pressure directly on to the tank foundation. Therefore, liquid tightness is the governing criterion for the tank bottom. They are made of 6 mm thick rectangular steel plates, which are interconnected by overlaps, welded by a full filled weld. Load transfer by the rather thin bottom plate is not possible. An exception forms the annular plates which act as the transition section between the tank shell and the tank bottom. These annular plates placed under the tank shell are highly stressed due to bending by the horizontal liquid pressure exerted on the lowest part of the tank shell. Annular plates (applied on thanks > 12.5 meter diameter) are thicker (10-15 mm) and made of the same material as the shell plates. They are butt-welded on backing strips to obtain full weld penetration.

2.3. Tank shell
From a design point of view the shell is the most important part as it must withstand the liquid pressure. The tank shell is made of a number of courses, normally 1.5 to 2.5 m wide. The plate thicknesses gradually decrease upwards due to the reduction in horizontal loading of the liquid pressure. In the design calculations the liquid density is assumed to be 1.00 g/ml to allow for a multi-purpose hydrocarbon storage application, including water. As per DEP, the calculated plate thickness at the top is restricted to a minimum varying from 6 to 10 mm depending on the tank diameter. The tank shell is provided with a number of mountings and accessories such as stairways, nozzles for pipe and instrument connections, manholes etc.. For nozzles over 2", reinforcement plates are required to compensate the reduction in strength caused by the hole in the shell . Depending on their size and shell plate thickness the larger nozzles are to be prefabricated at shop and to be stress-relieved as a sub-assembly. Floating roof tanks are provided with a primary wind girder to maintain roundness when the tank is subjected to windloads. This primary wind girder is located at or near the top of the tank. Secondary wind girder are sometimes required for both floating and fixed roof tanks, to prevent buckling of the tank shell under wind and/or vacuum conditions.

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2.4. Fixed roof
The roof plates are 5 mm thick and are interconnected by lap-weld on the top side only. They are normally not fixed to the roof supporting structure. Above the shell the plates are welded to the top curb angle of the tank with a continuous seal weld, with the intention that the roof will rupture at the shell roof connection in case of an internal tank explosion. No damage to the shell bottom connection will then occur with the risk of loss of product. Mountings required for access, gauging, dipping and vents are placed on top of the roof. For safety reasons a railing is installed at the roof periphery.

2.5. Floating roof
A floating roof is a roof structure designed to remain floating on the surface of the liquid in an open tank. The roof is in complete contact with that surface, so that no vapour space exists in the tank and vapour losses are minimised. Their buoyancy and loading conditions have to fulfil the requirements specified in [3]. Floating roofs are not designed to operate above atmospheric pressure. Consequently they can become unstable if raised out of the liquid and supported on vapour. Therefore the volatility of the product and the storage temperature must be controlled, particularly for very volatile products in hot climates, in order to know whether the products can be safely stored in a floating roof tank. Spiking of the oil stored with butane or propane should be avoided as much as possible. If spiking is applied, it should be done in a controlled manner to ensure that no free gas will be trapped under the floating roof, as this would cause an unstable condition for the roof. When free butane or propane gas escapes via the rim space it will be a serious handicap to extinguish a rim fire, if this were to occur. Three types are in use:   A pontoon type roof has a continuous angular pontoon divided by bulkheads into liquid tight pontoon compartments. The central area is covered by a single deck diaphragm. A double-deck roof has both an upper and lower deck extending over the full area of the liquid surface. The lower deck in contact with the liquid surface is separated from the upper deck by rim plates and bulk head plates to form liquid-tight pontoon compartments. A SIOP type roof is a pontoon type roof with a central circular pontoon compartment which is radially connected to the angular pontoon by a number of rectangular hollow steel stiffeners.

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The floating roof is provided with a number of mountings and accessories. The most important are:   A centre rainwater-drain consisting of an articulated piping or hose drain system, which is connected to an outlet nozzle in the bottom course of the tank shell. An access ladder from the top of the tank shell to the floating roof is equipped with self levelling stair treads. The rail track is placed at such a height above the centre deck to ensure free movement of the ladder rollers at times of snow or rain water on the deck. Furthermore, the ladder bottom shall be equipped with an anti-derailing device to prevent uplift of the ladder during heavy wind. Earthing facilities, which are vital to the safety of the floating roof tank. Special steel strips (shunts) are therefore applied for the earthing of the floating roof across the seal mechanism. Automatic bleeder vents are installed to vent the air from under the floating roof at times of initial filling. They shall also open automatically just before the roof lands on its supports thus preventing the development of a vacuum under the roof.

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A guide and gauge pole to maintain the roof in its position (to prevent rotation) and to allow level reading of the liquid.

Pontoon type roof

The pontoon type roof is normally used for tanks up to 60 m diameter. For larger diameters of tanks the double deck and the SIOP type are used. These types are less vulnerable to strong winds (no fatigue cracks due to waving of liquid and plates).
Double deck roof

The double deck roof is always used for small diameter tanks (i.e. up to 15 m diameter) because with these small diameters the centre deck of the pontoon would be too small to produce a diaphragm effect. Furthermore, they are many times used for large diameter tanks, e.g. over 50-60 metres in areas with frequent strong winds. Strong winds may cause fatigue cracks in the single, centre deck of large pontoon roofs, resulting in oil seepage onto the centre deck. The lower deck rests on the liquid and some distance above this, the upper deck rests on the lower deck, supported by vertical bulkheads and supporting concentric rings. The air space between the two decks provides an effective insulation against solar radiation. Double deck roofs have a weight of about 105 kg/m2. The upper deck has a slight incline towards the centre of the roof. For very large diameter roofs even a double incline may be used.
SIOP type roof

The SIOP type roof was developed to avoid fatigue cracks in the centre decks of the floating roof with a diameter over 50 metres. It is a pontoon type floating roof, the centre deck of which is reinforced on the upper side by sturdy radial stiffeners (usually 60 cm wide and 80 cm high). The roof is erected with a downward slope to the centre.
Operation floating roof

Each floating roof is a moving structure and requires attention in order to maintain maximum efficiency and to prevent accidents. The following guidelines are recommended:

After construction and water testing of the tank, the roof will be standing in its high (i.e. maintenance) position. Before the floating roof is taken into service the following actions are required:        Check that the valve of the roof drain at the tank shell is fully open. Check that the roof drain and drain holes in the centre sump of the roof are not plugged by dirt or any foreign matter. Close the drain plug in the centre of the roof. Check that the first 2-3 metres of filling are done at a reasonably low filling rate, to prevent damage to roof drain or roof. Check that the side-entry mixers are not switched on before the roof is at least a few metres above the impellers of the mixers. Adjust all roof supports and automatic bleeder vents into their low (i. e. service) position when the roof is floating. Observe the roof when being floated into service.

When the roof is floating on oil, it shall be checked that no compartments are leaking. Owing to the deeper immersion of the roof in oil than in water and the easier penetration of oil (compared

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with water during testing) through pinholes, it may be possible that small leaks are detected which were not found during water testing. During operation of the roof it shall be ensured that:    The roof is always kept in a floating condition. Side-entry mixers are not switched on when the roof is less than a few metres above the impeller of the mixers. Filling and emptying rates are not increased above their original values without the approval of the design department.

By means of inspection at frequent intervals it shall be checked that:          The valve of the roof drain at the tank shell is fully open. No oil is leaking out of the roof drain. The roof drain and the drain holes in the centre sump of the roof are not plugged by dirt or foreign matter. The centre deck is not leaking. The pontoon compartments are not leaking. The earthing shunts at the periphery are in a good condition (not broken). The wheels of the rolling ladder are running smoothly over the rail track. The roof supports and automatic bleeder vents are all in their low (service) position. The seal is operating within its tolerances for inward and outward movement (the seal may pull away from the shell or press too tightly if the tank is too much out of roundness due to soil settlement).

Before a floating roof is landed on the supports, the following actions are required:        Check that the valve of the roof drain at the tank shell is fully open. Check that the roof drain and drain holes in the centre sump of the roof are not plugged by dirt or foreign matter. Check that the side-entry mixers are switched off when the roof is less than a few metres above the impellers of the mixers. Check that the centre deck is not leaking. Check that the roof compartments are not leaking. Adjust the roof supports and the automatic bleeder vents into their high (maintenance) position. Observe the roof when being landed.

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During standing the following actions are required:      Check that the valve of the roof drain at the tank shell is kept fully open (if necessary, by means of locks). Check that the roof drain and drain holes in the centre sump of the roof are not plugged by dirt or foreign matter. Instruct maintenance personnel and contractors of the importance and necessity of adhering to above points. Ensure that side-entry mixers are not switched on, even for desludging purposes. Open the drain plug in the centre of the roof.

NOTES  In order to facilitate the adjustment of roof supports and automatic bleeder vents, it is recommended that the openings be marked by colour bands, e.g. red for maintenance and green for service positions. It should be understood that the roof is in its most vulnerable position when standing on its supports with respect to vertical loading by rain water. During periods of heavy rainfall (e. g. in tropical areas) special attention is therefore required.  For SIOP-type roofs the installation is recommended of an additional opening for drainage into the tank. This shall be opened when the roof has landed on its supports. Instructions for the proper use of this opening shall be fixed on top. This additional opening is to be considered as an equivalent to the drain plug provided in pontoon-type roofs and is allowed to be open only if the roof is standing on its supports. It must be closed before the roof is floated.

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2.6. Roof drain (Floating roof)
Floating roof tanks should be equipped with means of draining rainwater from the upper surface of the roof. Any rain falling on the roof is collected in a sump at the lowest point of the roof and discharged via an articulated pipe drain installed between the sump in the roof and the nozzle in the lowest shell course. In the past some tanks were equipped with flexible rubber hose drains, but these drains are no longer recommended, as fatigue cracks and kinking have caused damage or collapse of such drains in several cases. Where these hoses are still being used it is essential to ensure that the hose cannot be trapped between the roof support legs and the tank bottom. For pontoon type roofs with internal articulated pipe drains a non-return valve should be provided near the roof to prevent back flow of stored product on to the roof in case of leakage in the jointed pipe. For double deck type roofs this non-return valve is not necessary owing to the extra height of the double roof, but an emergency roof drain should be fitted. For pontoon type roofs the pipe drains in use have diameters of 3, 4 and 6 inches, the actual size and number applied depending on the size of the tank and the rainfall intensity in the area where the tank is to be built. In areas with normal rainfall (e. g. Europe) the following is recommended:

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     

one 3-inch diameter pipe drain for tanks up to 20 m in diameter, one 4-inch diameter pipe drain for tanks from 20 m to 54 m in diameter, one 6-inch diameter pipe drain for tanks over 54 m in diameter.

In areas with excessive rainfall over short periods, e.g. the tropics, the following is recommended: one 4-inch diameter pipe drain for tanks up to 20 m in diameter, two 4-inch diameter pipe drains for tanks from 20 m to 54 m in diameter, two 6-inch diameter pipe drains for tanks over 54 m in diameter.

If only one roof drain is installed, it is recommended to install the articulated double-type drain according to Standard Drawing S 51039. If two pipe drains are installed to one centre sump, it is recommended to install the articulated single-type drain according to Standard Drawing S 51038. These drains have been designed in such a manner that they have the required flexibility in any roof position. Previously many roof drains had insufficient flexibility with the roof in its lowest position. Water should be drained as necessary from floating roofs. There should be a clear policy as to whether roof drain valves are to be left open or closed. In determining this policy the following hazards must be recognised:  If the valve is left closed serious damage may be caused to the drainage system by, for instance, freezing of any water left in the drain. Additionally, flooding and possibly instability of the roof will result if the necessary drainage is forgotten. If the valve is left open drainage of hydrocarbon from the tank to the surrounding area will occur if the drain fails. Consideration should therefore be given to connecting the roof drains to an oily water system.

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Although lowering of the roof on to its supports during normal operations should be avoided, the roof drain valve must be left open if the roof has to be landed as the roof may not be able to withstand the weight of water collected during heavy rainfall. References: [4,5]

2.7. Roof supports (Floating roof)
All floating roofs are equipped with a system of adjustable legs on which the roof rests in its lowest position (0.90 m above the bottom) during operation and at 1.80 m above the bottom during inspection and maintenance periods. In operation it is designed to keep the roof above the inlet and outlet connections, the drainage system and any other accessories located near the tank bottom. Floating roofs are designed to operated in a floating position. After landing it is strongly recommended that openings be made in the lower spot of the deck to allow excessive rainwater to be discharged in the tank, to prevent overloading of the roof in this vulnerable position.

2.8. Tank heaters
The heating of tanks can be done in various ways via:    suction heater tank coils manhole heater

Depending on the storage temperature to be achieved/maintained generally low pressure (3 bar) or medium pressure (15 bar) steam is used, but hot oil and hot water are also used. Consideration should be given to providing facilities such as a low level alarm to ensure that the heating coils are always covered by liquid, to prevent overheating causing possible coking, frothover or product deterioration.
The suction heater

The suction heater contains a U-tube bundle and is used for tanks containing high viscosity hydrocarbons. By heating the liquid the viscosity is lowered and pumping is easier. In this manner the tank contents do not need heating to reduce pumping costs.
Tank coil heater

Tank coils are made in a variety of configurations. Helical and spiral coils are most commonly shop-fabricated, while the hairpin pattern is generally field-fabricated. The helical coils are mainly used in process tanks and pressure vessels when large areas for rapid heating are required. The coils may be sloped to facilitate drainage. Most coils are firmly clamped (but not welded) to supports. The supports should allow expansion but be rigid enough to prevent uncontrolled motion. Nuts and bolts should be securely fastened. Reinforcement of the inlet and outlet connections through the tank wall is recommended, since bending stresses due to thermal expansion are usually high at such points.
Manhole heater

The manhole heater is applied in cases where the tank is converted from non-heated to heated service.

2.9. Tank fire fighting facilities
ORTEO/14 is to be consulted on tank fire fighting facilities. The material below is intended as background information only.

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It is a well established fact that fires in storage tanks, if not quickly suppressed, can escalate into major fires which can form a threat to adjacent equipment and can affect the overall environment. Over the years storage tanks have increased in size and contain different ranges of hydrocarbons. Early detection, adequate contingency plans, proper fire fighting systems and the ability to protect adjacent facilities (e.g. by cooling) are therefore vital requirements. The industry can offer more advanced equipment which will save on the highly critical time between ignition of a fire and the start of the extinguishing action.

Fire detection

Prompt detection of accidental hydrocarbon spills and early fire detection is of vital importance. For the detection of fires, human visual observation in combination with fire call points spread throughout the plot provides the main detection system. In addition, some automatic detection systems are used when response time is highly critical or when a fire would not be visible. In the case of floating-roof tanks, a fire starts at the rim which cannot therefore be observed from the ground. The detectors, therefore, are connected to an alarm system, which is presented on the fire supervisory panel as a fire alarm. The automatic detection systems generally consist of duplex runs of plastic tubing above the protected equipment. The tubing is pressurised with air, a pressure switch is connected to each tubing end and both switches are connected into an alarm system. The flame retardant polyethylene tubing (black coloured) will rupture at a temperature of approximately 95°C. If a single alarm is raised, this indicates a system fault and, if both detectors are in the alarm position, this raises the fire alarm. The tubing should be installed on a rack or along dummy piping, while the connection between the different racks shall be executed with galvanised piping or equivalent. This type of installation will keep the nuisance alarms to a minimum. If a fire is detected, the main fire hydrant water pumps will start automatically and activate the fire alarm systems.
Fire prevention

Water is the most commonly used agent for controlling and fighting a fire, by cooling adjacent equipment and for controlling and/or extinguishing the fire either by itself or combined as a foam. To protect a tank from possible radiation exposure in adjacent tanks, certain tanks are protected by a fixed spray system. A manually operated system for tank roof (two water rings) and tank wall (top water ring around the tank). The water rate for the roof shall be minimum 1.7 dm3/min/m2 surface area and that for the tank wall shall be 17 dm3/min/m2 tank circumference for installation where tank spacing conforms to the IP code. Only when installed within one bund with a fixed-roof tank, a manually operated spray system shall be provided for the exposed area of the tank wall (normally half of the tank circumference). The water rate shall be 17 dm3/min/m2 exposed tank circumference.

Fire-fighting equipment

On fixed roof tanks, fixed foam chambers used to be installed at the top course of the tank. Experience has indicated that these systems are not very reliable because of corrosion. This causes

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the rupture of the gas seal plate, allowing gas to pass to the inlet connection at the road and thus creating a hazardous situation. During an explosion preceding a fire the foam chambers/piping systems will generally be ruptured and be non-operational at the critical moment. Currently, the use of foam chambers is not recommended. A more effective method is to use the sub-surface method of foam injection into the bottom course of the tank via a separate connection. Foam is fed from a fire tender (or from a fixed system) via a mobile/portable high back-pressure foam generator to the tank inlet connection. Sometimes the inlet and/or outlet product lines of a tank are used to inject the foam. This, however, eliminates the possibility of emptying a tank during a fire. The types of foam which can be used for normal hydrocarbon fires in tanks are e.g. fluoro-protein type, AFFF type and alcohol resistant types. These systems are suitable for light products and also for products which form a hot zone when burning. In the case of a thick hot zone (heavy product burning), the sub-surface shall be used for injecting water and air bubbles for some ten minutes. During this period the hot zone will be broken up by mixing with colder bottom product. After water has been applied by the sub-surface method for some ten minutes, the fire will be readily extinguished by introducing the foam compound through the foam maker. In case the fire had a very long preburn time, the water injection method should be applied with great care because boilover could occur when a very thick hot zone has been formed. There are some products for which the currently available types of foam are not suitable for subsurface applications, because the foam will deteriorate when travelling through. Examples of such products are alcohol's, alcohol blends and slops and in such applications the semi sub-surface foam system shall be used (Alcohol resistant type foam can only be applied as semi sub-surface). The foam is fed from the fire tender via mobile/portable high back-pressure foam generators. The hose is pressed out of the container by the foam pressure and passed through the liquid to the burning surface, thus preventing the foam from coming in direct contact with the product. For the sub-surface and, preferably, for semi sub-surface systems on oil tanks fluoro-protein foams must be used.

Fire protection for floating roof tanks is based on rim fires only. There are two conditions which can lead to a rim fire:   An excessive gap between the roof seal and the tank wall as a result of poor maintenance. Excessive vapour pressure of the material stored in the tank.

With regard of the second aspect SIOP currently specifies a maximum of 12.5 psia true vapour pressure for liquids in floating roof tanks. The criteria for setting this level is avoidance of damage to the roof. ASME states that above 11.5 psia true vapour pressure, vapour losses from floating roof tanks may become excessive. Hence it may be that, at such levels, flammable atmospheres capable of ignition by electrical discharges could exist above the tank roof. The effect of heat radiation from a rim fire to adjacent tanks may be ignored as long as the tanks are located in accordance with the IP Code. Therefore, water spray systems are not required except where a specific tank can be exposed to heat radiation from adjacent fires. As such, only a dry foam riser with foam dam must be installed. The dry riser must have two valved hose connections at the bottom and top levels of the tank and, as an extra feature, a single foam pourer with block valve above the top hose connections. A rim plate (foam dam) must be welded to the roof to retain the foam. Rim fires can also be extinguished by means of fixed foam

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pourers, installed for connection to mobile high back-pressure generators. A single dry riser, as above, must be installed as a back-up system. See also [6].

2.10. Tank cleaning/desludging
The build-up of sludge in refinery crude oil tankage, if not processed, must be removed by periodic cleaning. This is generally laborious, time consuming and expensive. The presence of water in crude oil enhances the deposition of sludge which is an oil-water emulsion stabilised with wax crystals, and sediments of sand and silt. The rate of accumulation varies with the quality of crude, particularly the wax content, viscosity and water content. Excessive accumulation may eventually require the tank to be taken out of service for a major sludge removal exercise. Some of the main reasons for sludge control being adopted are as follows:         Tankage is always available for storage and not out of commission for cleaning or repair. Accumulation of sludge can cause severe corrosion of the tank floor and lower shell plates. Hazardous and costly tank cleaning practically eliminated. No or reduced product loss as a significant proportion of the sludge is recoverable hydrocarbon. No or reduced environmental/sludge disposal problems. Avoidance of floating roof damage. Ability to drain water from tank. No pockets of water or plugged drains. No loss of dipping/stock measurement accuracy.

The preferred technique is to deal with sludge in-situ and hence avoid opening the tank. Most methods involve the use of heat and/or agitation, and sometimes the introduction of diluents. It has been argued that such a sludge control programme will cause unacceptable fouling in the downstream equipment. However, if the sludge is allowed to accumulate, the possibility of a sudden carry-over as slugs of entrapped water and/or deposits in the distiller feed will increase, with more serious consequences. With in-situ sludge control in the feed tanks, fouling of downstream equipment will be far less, and any progressive fouling can be detected and monitored in time. Routine tankage operations should aim to avoid sludge deposition by maintaining adequate circulation of the tank contents, although regular periods of stagnation are acceptable (for settling and draining). As such, side entry mixers are now applied almost as standard. If they are correctly installed and well maintained and if a mode of operation is established specifically for the service, then the tank should be kept in a clean state. The swivel-angle type mixer is recommended because fixed-angle mixers can leave dead spots in large diameter tanks where sludge may accumulate. Other means could include jet circulation, for instance, but this is less effective unless extra "bottom-sweeping" nozzles are permanently incorporated. In this system, crude oil is circulated for some time via a clean out nozzle which is inclined so as to create a stream some 5 degrees below the horizontal in order to sweep the floor of the tank. The clean out nozzle is either a specifically designed jet nozzle connected to the existing tank circulation or mixing system, or the existing mixing nozzle of the tank itself is designed to be used for this purpose. This method can be implemented during normal operation of the tank. Although it is probably not as versatile as the use of swivel-angle mixers which can be directed to different areas of the tank

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and can be operated independently of other systems, the capital cost will be far less than that for existing mixers if the circulation pipework and pumps already exist. Experience with such systems and details of particular designs can be found in [7], which also contains guidance on corrective cleaning methods should a substantial sludge build-up have taken place. Manual and hydromechanical cleaning, however, entail taking the tank out of service. Further information on tank cleaning can be found in [8].

References
[1] [2] [3] [4] [5] [6] [7] DEP 00.00.06.06-Gen. Group S-51. DEP 34.51.01.31-Gen. BS 2654. Refining Safety Code Part 3: IP Model Code of Safe Practice, October 1981 SIPM Memorandum MFE No. 175/87: Practical guidelines for Construction, maintenance and Operation of Conventional Storage Tanks Civil/Structural Engineering Aspects. DEP 80.47.10.31-Gen fire protection and fire-fighting system and equipment. Sludge Prevention and Removal methods for Crude oil Storage Tanks; Third Meeting on Water treating and pollution abatement techniques in Refineries, 1984. H.W.Schipper, SIPM(Paper S1) Gas Freeing and Cleaning of Oil Storage Tanks; Shell Safety and Health Committee. April 1989. MF report 89-0798.

[8]

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