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Storage Tank Construction

To order

Storage tank construction should be performed in accordance with detailed metal framework’s design plan and Work Execution Plan. Work Execution Plan is the basic technological document in the course of oil tank installation.

The area of construction site must be arranged in conformity with the general layout and should include zones for operating and shifting the materials-handling machines, stockpiling zones, pioneer (temporary) roads, necessary premises and utility systems (electricity, water, communications facilities), firefighting means.

Prior to construction of a storage tank all works connected with basement and foundation arrangement should be completed. Acceptance of basement and foundation is done by the ordering customer with the representatives of construction company and installer, participating as well. Acceptance of the finished basement and foundation is fixed in the corresponding document (acceptance certificate/act).

1. Work execution plan

Welding and assembly works are performed based on the Work Execution Plan for the oil storage tank construction. It is the basic technological document, worked out by a specialized engineering design organization on the basis of metal frameworks project.

Work Execution Plan should include and provide:

  • Master plan of the construction site stipulating the materials-handling equipment and its positioning;
  • Description of procedures, aimed at providing the necessary assembling accuracy and the tank elements’ space stability in the course of top assembly and installing to the project position;
  • Measures, providing the load-bearing capacity of the construction elements in the course of storage tank erection;
  • Requirements for the quality of welding and assembly works for each procedure of oil storage tank construction;
  • Types and the scope of control measures for the storage tank erection;
  • The order of tank testing procedures;
  • Safety regulations and labor protection norms;
  • Environmental protection requirements.

Work Execution Plan sets the order of oil tank elements’ installation, including the use of special facilities and equipment. The project also stipulates measures, aimed at providing the needed geometrical accuracy of the tank frameworks and reducing deformation process resulting from the shrinkage loss of the welding seams.

Technological standards for welding, included in the Work Execution Plan, should stipulate:

  • Requirements for preparation of edges for welding;
  • Requirements for joints’ assembly for welding;
  • Welding methods and regimes;
  • Welding materials;
  • The order of procedures;
  • The order of welding passes and welding of joints;
  • Requirements for heating the junctions depending on the air temperature and the tempo of their cooling;
  • The need for protection cover in the welding zone;
  • The necessity of after-welding heat treatment of the junction;
  • Necessary equipment and technological facilities for the oil storage tank construction;
  • Methods and the scope of control of the welding seams.

The Register of operational control is to be seen as the essential part of Work Execution Plan, as it sets the requirements for quality control of the welding and assembly procedures.

2. Welding and Other Procedures of Storage Tank Construction

2.1. General requirements for welding

In the course of construction of a storage tank, as well as in fabrication, electric arc welding methods are used in either of the following ways:

  • Mechanized arc welding by consumable electrode in shielding gas;
  • Automatic submerged arc welding;
  • Mechanized arc welding with self-clinching flux-cored wire;
  • Mechanized arc welding with self-clinching flux-cored wire in shielding gas;
  • Manual arc welding.

The welding is done in the course of installation in accordance with the Work Execution Plan, stipulating the following:

  • The most effective methods of arranging assembly joints;
  • Welding materials;
  • The form of welded elements’ preparation;
  • Technological welding regimes;
  • Necessary technological equipment and facilities;
  • Climate conditions for performing of the work.

The shape coefficient of the built-up seam (pass) should fall into the limits between 1.3 and 2.0. It is allowed to make dashed welding seams in one pass in the joints of the tank’s elements that do not affect the leak tightness.

Temporary technological elements, welded to the tank in the course of production and assembly, should be removed without any impact force to the tank’s elements. The residues of the welding seams are scraped bright flush with the parent metal.

2.2. The order of construction of storage tanks

Correct organization of the work and the order of welding and assembly operations is essential because of big tanks’ dimensions and long welding joints. Correctly performed tank installation enables to reduce the residual stress, arising from the welds’ shrinkage, to its minimum and to prevent distortion of the framework sheets.

The order of installation procedures is shown in scheme 1.

The order of assembly operations

 

 

3. Tank Bottom Assembly

3.1. Plate-by-plate method of bottom installation

Storage tank bottom installation (plate-by-plate)If the bottom is supplied by the producer in plates, it is assembled in the following way:

There are 1 meter-long logs with rectangular or semi-circular cross-section, arranged in squares on the installed and accepted foundation (picture 14), the cross-section area is 0.1×0.1 m. The top row of the squares is better to make of 1.2-1.3 meters’ logs. The height of the squares is equal to 0.8 m, to ensure the possibility to weld the flow-joints and tar the bottom. The centerlines of the squares should be located at not more than 3 meters’ distance from each other, while the distance between the centerlines of the squares’ rows should be twice as big as the width of the sheets minus two widths of the bottom seams’ shaping. Boards (planks) are put along the squares, the bottom is installed on them.

The two elements of the bottom – segmental ring with the welded first ring of the shell and the central part are assembled and welded independently. The welding joint, connecting them together, — the so-called “expansion” seam – is welded only after the complete installation of each separate element.

Assembly of the central part of the bottom starts with the line (zone) leading through the basement center of the tank. Then all the lower sheets of the bottom are assembled in the direction from the center to the edges. Butt-welds of the sheets are tacked in 6-7 positions, the end tacks are located at 50 mm distance from the edges and are made flush. The butt joints are welded after assembling the whole band. The ends of the joints of 50 mm are welded flush so as to ensure the tight contact between the upper and lower bands. After the lower bands are welded, the upper bands are assembled and welded in the same way. The overlap between the bands should not be less than 50 mm.

Assembly of the central part of the bottom is started from the central bands. They are overlapped and assembled on tack-welds, located at the same time at the top and at the bottom on both sides of the workpiece every 250-300 mm in the direction from the center to the edges of the bands. To ensure the adjustment of the bands of the central part of the bottom, the edges of the end-sheets are left without tacks at the length of 750-800 mm in the course of joining the bottom with the segmental ring.

Welding of the bands is done with the overlap seam from the middle of the band to its edges, using the backstep procedure with the step length of 200-250 mm. At first all top overlap seams are welded, then the lower, overhead joints are welded afterwards. After this the butt-joints of the bands are backwelded by overhead weld.

Segmental sheets of the edges are assembled on 10-12 supporting stands, placed along the perimeter of the basement. Segmental ring is assembled in a way to put the two butt-joints on the centerline of the central band and to ensure that the gap clearance between the elements of the ring will not exceed 3-4 mm. After thorough check of the horizontal parameters of the segmental ring the butt-welds are tacked at the ends of the joints; the inside is left without tacks so as to have the possibility to level the segmental ring in the strictly horizontal position in case of distortion in the course of welding procedures.

Prior to assembly of the lower angle joint it is important to weld the parts of the butt-joints of the segments, to which the angle joint is placed. The welding is done in two layers, scrapping the slag and backwelding the overhead seams. The beads are cut down with chipping chisel flush with the surface of the sheets of the segmental ring.

After two circular guidelines are drawn on the segmental ring, which correspond to the inside and outside diameters of the circumference angle, the first angle section is installed and tack-welded. Tacking is done along the outside circle perimeter from the angle center to the ends every 500-600 mm in areas 30-40 mm long. The ends of the angle section are left without tacks at the length of 600-700 mm to ensure the possibility of easy adjustment of the other parts. The other angle sections are assembled on both sides of the first one. They are installed with the overlap of 3 mm, after which they are butt-welded. Then the attached sections are adjusted by the guidelines with tacks to the segmental ring from the butt-joints to the free ends. The final section, which is not less than 1 m long, is adjusted and cut on-site. The vertical panel of the angle should be strictly perpendicular to the segmental ring. The first sheet of the first ring is installed on segmental ring in strictly vertical position after chipping the edges in the lower corners at the height of the corner block and at 1 mm depth to let the butt-joint be afterwards welded to the vertical panel of the corner block. The first sheet is tack-welded at the same time both to the segmental ring and to the angle in checkerboard order from the middle of the sheet to the edges every 400-600 mm in areas 40-50 mm long. The ends of the first sheet are left without tacks at the length of 600-700 mm to ensure the possibility of easy adjustment of the other sheets. The other sheets of the first ring are installed on both sides of the first sheet with a gap clearance of 2-3 mm between the sheets and connection of the edges. The tack-welding is started from the butt-joint with the first sheet, the tacks are made in 4-6 positions each is 60-75 mm long. Then the tack welding is done along the lower edge of the sheets from the tack-welded butt-joints to the free ends. The final sheet of the first ring is adjusted and cut on-site.

The welding of the bottom, assembled in this way, and the first tank ring is done in the following order:

  1. All joints of the first ring are welded at the height of 200-300 mm from the segmental ring and at 50 mm from the edge in the upper part, flush with the surface of the sheets to ensure the tight contact of the second ring’s sheets in the course of the successive installation.
  2. All circular seams are welded: the first ring is welded to the segmental ring by double seam, after that the circumference angle is welded by single seam: first to the segmental ring, then to the first ring of the tank.
  3. The butt-joints of the segmental ring elements are checked and undercut (if necessary) to eliminate waviness and to install 3-4 mm gaps, after which the butt-joints are welded with overhead back-welding and reinforcing with steel straps of sheet steel 8-10 mm thick. At the same time the butt-joints of the circumference angle are reinforced by welding pin of corner steel.

Prior to welding the central part of the bottom with the segmental edge, the joints’ edges of the lower bands are planned, cut with a 2-3 mm gap, tacked and then welded with overhead back-welding. Then the ends of the upper bands are planned, cut with an overlap not less than 30 mm, tack-welded at the long parallel edges at first and then to the segmental ring. The welding is done in the same order as tacking. The welding operations at the points of seams’ crossing should be done only by highly-professional welders.

3.2. Assembling coiled bottoms

Storage tank coiled bottom installationBottoms of tanks of loading capacity up to 2000 m³ and up to 12 m in diameter, as a rule, are completely welded at production site and coiled. The coil is then rolled to the basement in a way to ensure that the middle of the coil is positioned along the centerline of the basement. The bottoms of larger tanks, having more than 12 m in diameter, cannot be fully loaded to the open wagon, which is 13.66 m long. They are made of several parts, placed on each other while coiling.

The coil with the bottom, consisting of two parts, is placed on the basement in a way to provide that the first half of the bottom, which is the outside cover of the coil, will be installed in the planned position after uncoiling. In this case the second half of the bottom will be put on the first one.

Panels, fixing the coil, are cut with oxygen, after which the coil is unfolded when the loop of the cable rope is eased off. In case the coil failed to unfold spontaneously (under the influence of elastic forces) the further uncoiling is done with the help of truck tractor or hauling winch. When the coil is fully unrolled the center of the circular edge of the top half-bottom has a brace welded on it, meant for fixing the end of the cable rope for moving the second part of the bottom to its project position with truck tractor or hauling winch. Then the butt-joint of the two halves of the bottom is assembled for welding. It is always done with an overlap. It is fixed with tack welds from the center of the bottom to its edges, tightly pressing both panels to each other.

Storage tank coiled bottom installationIf the bottom is assembled from three strip panels successively coiled, the first coil is unfolded to its project position, then it is loaded to the sledge together with the other two and they are moved by truck tractor in a way, enabling to uncoil the second strip panel to the project position. After this the last coil is again loaded to the sledge and transported to the other side of basement to unroll the third one.

Pictures 7-13 show the order of assembly operations for coiled bottom of a tank of 400 m³ loading capacity.

3.3. Extreme deviations of dimensions and shapes of the installed bottom

Regardless the method of the bottom installation, the deviations of its size and shape should not exceed the following rates:

  • Maximum allowable height of local bulges and buckles in the central part of the bottom is determined by the formula: f ≤ 0,1R ≤ 80 mm, where stands for maximum pointer – сarrow dents of a bulk or a bulge on the bottom, mm; R stands for the radius of inscribed circle at any area of a bulk or a bulge, mm. Cutting (sharp) bends and wrinkles are not allowed.
  • Local deviations from the planned shape in the zones of radial assembly welds of the edges’ circle (angularity):±3 mm (the measurements are done with the pattern with 200 mm base).
  • The rise of edges in the connection zone with the central part of the bottom is determined by the formula:
  • fa ≤0,03L for bottom’s diameter equal to 12-25 m;
  • fa ≤0,04L for bottom’s diameter more than 25 m,

where fa – the height of the edge’s rise, mm, L — the width of the edge, mm.

  • The point of the outer profile of the bottom:
Empty tank: Tank diameter
  up to 12 m 12-25 m 25-40 m 40 m
The difference in the marks of neighboring points at a distance of 6 m around the perimeter 10 mm 15 mm 15 mm 20 mm
Difference of marks of any other points 20 mm 25 mm 30 mm 40 mm

 

Water-filled tank: Tank diameter
  up to 12 m 12-25 m 25-40 m 40 m
The difference in the marks of neighboring points at a distance of 6 m around the perimeter 10 mm 15 mm 15 mm 20 mm
Difference of marks of any other points 20 mm 25 mm 30 mm 40 mm

3.4. Corrosion preventive treatment of the bottom:

After thorough cleaning of the lower surface of the bottom with wire brushes a cool prime coat is applied – a thin layer of primer (a solution of stearine pitch in benzol or bitumen in petrol).

After the primer becomes dry the bottom is covered with two layers of hot bitumen added with filling material, as it is done in the course of pipelines’ insulation.

To ensure applying the coat to the whole bottom’s surface the squares are replaced from one position to another.

4. Assembly and Welding of a Storage Tank Shell

4.1. Assembly of the tank shell by plate-by-plate method:

Assembly of the tank shell (plate-by-plate)Assembly of the tank shell (plate-by-plate)Assembly of the tank shell (plate-by-plate)Assembly of the tank shell (plate-by-plate)

This method implies assembly of the shell starting from the 1st ring with subsequent installation of the shell plates in their planned positions in the upward direction by the rings.

While using this method of assembly it is important:

  • To assemble the plates of the 1st ring considering the extreme deviations, stipulated in the Work Execution Plan;
  • To fix together the shell plates with each other and with the bottom plates with the help of assembly tools;
  • To assemble the vertical and horizontal butt-joints of the shell with planned gaps for welding.

Resistance to wind stress is provided by installing bracing and temporary stiffening rings.

Prior to tack-welding the attached elements should be tightly pressed to each other with different push tools. Pressing-through (as in case of assembly bolts) is not allowed.

4.2. Installation of a coiled wall:

Installation of a coiled wallInstallation of a coiled wallInstallation of a coiled wallInstallation of a coiled wall

Installation of a coiled shell is done in four stages:

  • lifting the coiled shell in a vertical position;
  • uncoiling the shell panel;
  • shaping of the end areas of the shell panel;
  • assembling the butt-joint of the shell.

Assembly and welding of a vertical tank wallIn case a jib crane of necessary carrying capacity is available at the construction site (either crawler caterpillar or pneumatic-tyred) the coil of the shell is unloaded to the bottom by this crane. In case the crane cannot be provided, the coil is rolled by a truck tractor or hauling winch over the cross ties or logs, attached by construction cramps.

The joint movement of the coil and the pallet in the course of uncoiling is provided by angle-bars – restrictors, that are welded to the pallet along its circular profile in a way to ensure that these angle bars remain inside the coil after it is lifted up. Installing the coil from horizontal to vertical position is made by turning method, similar to that of lifting towers. Special rotary joint, welded to the bottom and fixed to the coil with locking band, provides turning of the coil and prevents its lower edge from damage. To avoid bumps of the coil and the bottom after the “dead point” is passed, it is possible to attach a brake guy line of cable rope to the top edge of the coil, the other end of the guy line is fixed to the winch drum or to the truck tractor. The “dead point” is the position, where the coil’s center of gravity and the centerline of the supporting rotary joint match together along the vertical line. When the coil reaches the position near the “dead point”, the guy line is held tight. After the coil passes the “dead point” it is lifted down to the pallet by the brake guy line. There is also the possibility to lift the coil by a crane. Continuity of the bottom in the course of crane operations is kept with the help of arranging the cross tie decking. However, when the net weight of a coil equals to 30 t and the height is about 12 m it will demand high carrying capacity cranes, which is not always available at the construction site. 

Assembly and welding of a vertical tank wallWhen roping the coil from below the crane’s carrying capacity is all the time bigger than the effort, charged to the hook, which is the basic condition of safety of lifting operations. When roping the coil at the top the crane’s carrying capacity at final stages becomes less than the effort charged to the crane, i.e. it leads to overload and thus it must not be allowed. The coil, installed to the pallet, is bound with a loop of cable rope and moved by a truck tractor to the edge of the bottom to a position, ensuring that the closing edge with the fixed stiffening stay brace and staircase is placed in the planned position. For the sake of this the center of the bottom is marked after the welding, then the circle is drawn from the center, the radius of which is equal to the external radius of the lower ring of the tank shell. Angle bars are welded evenly along the drawn circle at a distance 1 m from each other. These angle bars are meant to serve as rest arms while the coil is unrolled. Afterwards the panels, preventing the coil from unrolling, are cut with oxygen, using the staircase on the stiffening stay brace, the loops of cable rope remain tight. The top of the stay brace is initially unfixed in radial direction with two guy lines. While the loop is smoothly eased off, the coil is unrolled with elastic forces that appeared in the course of its winding. The free outside edge of the coil is tightly pressed to the angle stop and tack-welded to the bottom.

Prior to installation of the closing board it is necessary to remove the shaft staircase that serves as a frame for the last coil of the shell. The angle-bars – restrictors are cut off the pallet and it is pulled out. The lower closing (free) edge of the coil is temporarily tack-welded to the bottom, after which the welding joints, fixing the vertical edge to the stands of the staircase’s frame, are cut off. The free staircase is pulled out by a crane through a hatch in the coverage. The assembly butt-joint of the shell is usually welded with overlap. To ensure this, the lower edge of the shell is released from the tack to the bottom, after which it is pulled to the starting edge of the shell, tightly pressed along the full height with tightening devices, and then the closing board of the roof is installed. Then the coverage (only spheric) is uncentered, the temporary supporting stand is removed through the crown, the central board of the roof is placed and welded. In the course of unrolling the coiled shell and the coverage boards it is necessary to check the deviation of the shell from the vertical mark, which may not exceed 90 mm throughout its full height. 

4.3. Extreme deviations of dimensions and shapes of the installed shell:

Regardless the method of the wall production and installation, the deviations of its size and shape should not exceed the following rates, stipulated in the table:

Name of the parameter and notes

Limit deviation, mm, if the diameter of the tank

 

12 m

12-25 m

25-40 m

40 m

Inner diameter of 300 mm from the bottom (measured in four sizes under an angle 45)

0,005r

0,003r

0,002r

0,0015r

The height of the wall:

— up to 12 meters, inclusive:

— from 12 to 18 m:

— more than 18 m:

(measured in four sizes under an angle 45)

±20

±30

±40

Yaw forming at a height of each zone (N is the distance from the bottom to the point of measurement). Measurement is carried out not less often than once every 6m around the perimeter of the walls in the range of 50 mm below the horizontal seams.

±1/200 h

Local deviation from the project form. Measurements are vertical rack or horizontal pattern, made on the project to the radius of the wall.

±15

Local deviations from the project forms in the areas of Assembly welded seams (angular). The measurements are carried out template, made on the project to the radius of the wall.

In accordance with project requirements KM

 

5. Installation of tank roof and pontoon

5.1. Installation of fixed roofs.

Installation of fixed roofs of storage tanksInstallation of fixed roofs of storage tanksInstallation of fixed roofs of storage tanksInstallation of fixed roofs of storage tanks

Depending on the construction scheme of a fixed roof either of the following procedures is performed:

  • Installation of framed conical and spheric roofs – with the central supporting stand;
  • Installation from the top, without central supporting stand: this is applied for frameless conical and spheric roofs and for framed conical and spheric roofs with separated elements of framework and decking;
  • Installation from inside the tank, without the central supporting stand; this is applied for roofs with separated elements of framework and decking;
  • Installation of framed spheric roofs inside the tank with their subsequent lifting to the planned position.

In the course of working out the technology for installation of roofs it is essential to consider the assembly load on the roof as a whole and on its construction elements. It may be needed to install temporary bracing beams, ties and other devices, meant to prevent deformation.

The height marks of the central board and the assembly stand on tanks with framed spheric roof should be determined considering the planned height and camber, stipulated by the design project.

Extreme deviations of the installed roof’s size and shape should not exceed the following rates, stipulated in the table:

Name of the parameter and notes Limit deviation, mm, if the diameter of the tank
  12 m 12-25 m 25-40 m 40 m
The mark of the top of conical and spherical roofs (measurements are carried out through the central branch pipe) ±30 ±50
The difference between the marks adjacent nodes top of radial and girders:
— in the interface zone with the shell:
— in the interface zone with the central shield:
— in the area of joining of radial beams of spherical roofs:
20
10
15
Deviation from the design radius of spherical roofs. Clearance between the template and the curved surface (measurements are carried out on each radial beam and truss). 5,0

 5.2. Installation of floating roofs and pontoons

A pontoon or a floating roof is installed at the bottom of the tank after it is assembled and controlled to be leak proof.

Extreme deviations of the installed roof’s size and shape should not exceed the following rates, stipulated in the table:

Name of the parameter and notes Limit deviation, mm, if the diameter of the tank
  12 m 12-25 m 25-40 m 40 m
Mark the upper edge of the outer ring sheet (Board):
— the difference between the marks of the neighboring pixels at a distance of 6m on the perimeter:
— the difference between the marks of the other points:
30
40
Deviation of the outer ring sheet from the vertical height of the sheet (the measurements are carried out at least every 6 m along the entire perimeter) ±10  
Deviation guides from the vertical to the entire height of the guides N, mm, in the radial and tangential directions 1/1000 Н
   
The gap between the top edge of the outer ring sheet and the vessel wall (dimension spend every 6m on the perimeter. Position — pontoon at the bottom) 10  
The gap between the sending and pipe in a pontoon or a box floating roof (position — pontoon at the bottom) 15
Deviation of supports from the vertical when resting on them pontoon or a floating roof 30

Installation of tank roof and pontoon

6. Installation of manway plates and fitting pipes

Installation of manway plates and fitting pipesWhile marking the planned positions for installing manway plates and fitting pipes in the tank shell it is essential to meet the requirements for allowed distances between the welding joints.

The distance between the outside edge of reinforcing plates to the central line of horizontal butt-joints of the shell should not be less than 100 mm. The distance to the central line of the vertical butt-joints of the wall or between the outside edges of the two neighbor reinforcing plates of the fitting pipes should not be less than 250 mm.

It is allowed to cross-cover the horizontal joint of the wall with a reinforcing sheet of the intake-dispensing fitting pipe or the access manway plate on the area not less than 150 mm form the profile of the reinforcing plate. The covered part of the joint should be controlled by radiographic method.

While installing fitting pipes and manway plates on the tank their positions on the wall and the roof should be controlled in accordance with the table:

name of parameter limit deviations
  hatches nozzles
Mark the height of installation ±10 mm ±6 mm
The distance from the outer surface of the flange to the vessel wall ±10 mm ±5 mm
Turn the main axes of the flange in the vertical plane ±5° ±5°

 

7. Quality control and testing

7.1. Methods of quality control of the welding joints in the course of assembly works

Quality control of the welding jointsQuality control of the welding jointsQuality control of the welding jointsQuality control of the welding joints

Quality control of the welding joints in the course of tank construction should imply:

  • The use of welding methods, ways and scope of control of the welding joints, applicable to the tank’s importance level;
  • The use of effective technological welding procedures and materials according to the stipulations of metal frameworks plan and Work Execution Plan;
  • Carrying out technical and construction design supervision.

The table contains information on methods of quality control of the welding joints, applicable depending on the controlled element of the tank:

Control zone Control method
visual and measuring vacuum radio ultrasonic capillary (color) overpressure
BOTTOM
Bottom joints + + - - - -
Bottom joints at a distance of 250 mm from the outer edge + + + - - -
SHELL
Vertical joints of the 1st and 2nd zones + - + - -
Vertical joinsof  remaining zones + - + - -
Horizontal joints  + - + - -
Crosshairs of vertical and horizontal joints + - + - - -
Branch pipe-to-shell joints + + - + - -
Joint between a collar of a branch pipe (hatch) and 1m a belt of a shell + - - - + +
Joint between a collar of a branch pipe (hatch) and a wall (except for the 1st belt) + - - - - +
Radial joints of stiffening rings + - - - - +
Spots of removal of assembly devices, welded joints of structural elements after their heat treatment + - - - + -
Bottom-to-shell joint + + - - -
ROOF
Radial joints of the support ring + - - + - -
Joints of roofing, roofing shields + + - - - +
Branch pipe-to-roof joints + + - - - -
FLOATING ROOF
Joints of boxes (compartments) and plugs of racks
+ - - - - +
Joints of the central part + + - - - -
Branch pipes-to-roof joints + + - - - -
1 allowed the use of ultrasonic testing
2 allowed the use of radiography
3 control breakdown «chalk-kerosene» hold up welding joint from the inner side

The scope of physical control of welding joints (in percentage to the joint’s length) of the tank shell, depending on the tank’s class of hazard, should correspond to the requirements of the table: 

VOLUMES of the PHYSICAL CONTROL of  WELDED JOINTS of  TANK SHELL, as % of the length of the seam
Control zone hazard class
  IV III II I
    1 000 – 10000m³  10000 –20000m³     
vertical welded connections:
in zones 1-2 20% 25% 50% 100% 100%
in zones 3-4 5% 10% 25% 50% 100%
in zones 5-6 2% 5% 10% 25% 50%
the zones above the 6-th - - 5% 10% 25%
horizontal welds:
in zones 1-2 3% 5% 10% 15% 20%
in zones 3-4 1% 2% 5% 5% 10%
in zones 5-6 - - 2% 2% 5%
in other zones - - - 2% 2%

While choosing the areas of control the prevailing attention should be paid to the areas of joints’ crossing.

Assembly butt-joints of the tanks installed with coiled method with the loading capacity from 1000 m³ and more must be controlled on the full length of the joints.

The results of tests and quality control of the welding joints are fixed in certificates and acts and represent the essential supplements to the documents for the tank.

7.2. Final tests after the oil storage tank construction

The final stage of the tank testing is the hydraulic pressure test, meant to check the tightness of connections and the durability of construction on the whole.

Tanks with fixed roof without a pontoon are additionally subject to extra pressure and relative under pressure from the inside.

The table below stipulates the tests that need to be done for tanks of different types (1 - tanks with fixed roof without a pontoon, 2 - tanks with fixed roof and a pontoon, 3 - tanks with floating roofs).

type of test 1 2 3
1 Watertight test + + +
2 Strength test of tank body under hydrostatic load + + +
3 Air pressure tightness test of a fixed roof + - -
4 Test the stability of the tank body by creating a relatively rarefaction inside the tank + - -
5 Buoyancy test of floating roof - + +
6 Rolling ladder performance test - - +
7 Testing the stability of the base of the tank with the determination of absolute and uneven draft along the contour of the bottom, roll of the tank profile of the central part of the bottom + + +

Hydraulic pressure test of the tanks with floating roofs or a pontoon are done before installing the rim seals.

The tests of a tank of any type are carried out based on the testing program, included in the metal framework design project and Work Execution Plan.

The testing program should include:

  • Stages of testing, stipulating the level of loading/discharge of water and curing time;
  • Rates of excessive pressure and relative under pressure, test curing time;
  • Scheme of visual inspection;
  • Demands for measuring the necessary geometrical parameters of the construction elements of the tank and its foundation;
  • Testing results’ processing, checking calculations (if necessary), final report on usability and operating regime of the tank.

7.2.1. Hydraulic pressure tests of a tank

In the course of hydraulic pressure test the tank is gradually filled with water to the planned level. The loading is performed in stages with certain time gaps, meant for supervising its settlement and the state of welding joints, and for carrying out other measuring and inspection, stipulated by the testing program.

If leaking from under the edge of the bottom or in the first ring of the shell is revealed in the course of testing, the water is fully discharged (removed). If cracks are revealed in the shell seams the water is discharged to the level lower than the discovered defect. If the defect appears in the 2nd-6th ring, the water is removed to the level one ring lower the defect. If the defect appears in the 7th ring and higher – the water is removed to the 5th ring. After the defects are eliminated the testing goes on.

Tanks for storing liquid materials with the mass density, exceeding that of water, as well as the tanks, installed at sites without water availability, can be tested with the product itself. Prior to such tests all welding joints of the shell, bottom, roof and manway plates/fitting pipes, and also connections of shell with the roof and the bottom should be tested for leak tightness.

The tank filled to the planned level must be kept under load within the following period:

  • Vertical steel tank V≤10000 m³ — 24 hours;
  • Vertical steel tank V=10000-20000 m³ — 48 hours;
  • Vertical steel tank V≥20000 m³ — 72 hours.

The tank is considered to pass the test if no leaking appears on its surface of at the edges of the bottom within the test period, if the level of test product does not decrease, if the settlement (sinking) of the foundation and basement becomes stable.

The hydraulic pressure test is recommended to be carried out at the temperature level not less than +5 °С. If the test is done in winter period the water should be heated, or its permanent circulation should be provided so as to prevent its freezing in the pipes and gate valves, and to prevent frosting of the tank shell.

7.2.2. Pressure/under pressure testing for the tank’s body and roof:

Fixed roof of a tank without a pontoon is tested for excessive pressure with the tank filled to the level, which is 10 % lower the planned level, with keeping the load for 30-minutes. The pressure is provided by the flow of water with all roof manway plates tightly closed. In the course of testing the full visual inspection of welding joints of the fixed roof is done.

The steadiness of the tank body is checked by relative under pressure inside the tank, filled with water to the level of 1.5 m, keeping the tank under the load within 30 min. The relative under pressure is provided by the discharge of the liquid product with all manway plates tightly closed. If no signs of unsteadiness are revealed (no canning, no buckles), the shells and roofs are considered to pass the test for relative under pressure.

The excessive pressure is applied at the rate of 25 % higher the planned level, the relative under pressure – 50 % higher the planned level, if the design documents do not contain other demands.

After the acceptance test is done, it is not allowed to weld any construction elements to the tank. It is possible to perform corrosion preventive procedures, heat insulation and installation of equipment, stipulated in the design documents.

After the hydraulic pressure testing is done, the actual technical condition of the tank’s metal framework, basement and foundation should be evaluated.

7.2.3. The basic demands for the arranging and performing the tests:

The final testing of a tank for durability, steadiness and leak tightness is done after all welding and assembly procedures are over, the quality of all construction elements, including welding seams, is checked and accepted by the technical supervision organization.

All tests are done according to the technological chart, included in Work Execution Plan. Technological chart should include:

  • The order and regime of the hydraulic pressure test;
  • The order and regime of testing for excessive pressure and vacuum;
  • Arrangement of temporary pipelines for water loading and discharging with safety and shutoff valves’ installation;
  • Control panel;
  • Labor safety requirements in the course of testing.

Temporary pipeline for water loading and discharging should be arranged outside the area of embanking. The scheme of water discharge is worked out for each particular case. In the course of testing the water is often pumped from one tank to another, and from the last one to the firefighting tank of temporary water body.

Diameter of the pipeline for loading and discharging water should correspond to the estimated level of loading-discharge operations’ efficiency.

Apart from the operation scheme of flow and discharge of water, an emergency unloading scheme should be foreseen in case cracks appear on the tank’s body. For emergency purposes it is recommended to use one of the intake-dispensing fitting pipelines and technological pipeline with a valve outside the area of embanking.

It is essential to set and specially mark with preventing and safety signs the borders of danger zone for the testing period. If an embanking or a protective shell is arranged round the tank, they are considered to be the border of the danger zone. In case the tank is installed without embanking, the limits of the danger zone are determined by the radius, drawn from the center of the tank for the distance equal to two diameters of the tank.

The testing is done by the assembly organization accompanied by representatives of technical supervision service and construction design supervisor. After the tests are finished, the installation specialists and the ordering customer sign a certificate (an act), stating the finish of the metal frameworks’ installation and acceptance of the tank for further corrosion preventive procedures, installing equipment and other works.

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