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Tank installation

Assembly of the tank frameworks should be performed in accordance with detailed metal framework’s design plan, Work Execution Plan, requirements of State Standard GOST 52910-2008. Work Execution Plan is the basic technological document in the course of 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 tank installation 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. 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 assembly;
  • Requirements for the quality of welding and assembly works for each procedure of installation;
  • Types and the scope of control measures;
  • The order of tank testing procedures;
  • Safety regulations and labour protection norms;
  • Environmental protection requirements.

Assembly and welding technology provided by the Work Execution Plan should ensure conformity of the installed unit to the requirements of the metal frameworks project and State Standard GOST 52910-2008.

Work Execution Plan sets the order of 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;
  • 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. Transporting, unloading and acceptance of the rolled workpieces of a vertical steel tank

The rolled frameworks are usually transported by eight-wheel rail open wagons with the loading capacity of 60 t. The coils are loaded off from the wagon depending on net weight and the height of each item and on availability of load lifting equipment. Either of the following methods can be used: by weight carrying crane (considering the center of gravity marked by the producer) or by truck tractors. While being unloaded by truck tractors, the open wagon is fixed by brake shoe holders. Two unloading beams are installed and special supporting stay braces are put under the edge of the wagon. The coil is taped around along the center of gravity by several turns of brake cable, fixed to the holding truck tractor. At 500-800 mm’s distance from the edge on the thick sheets’ side the coil is taped with several turns of the other cable rope, fixed to the pulling truck tractor, located aside the way of the coil’s rolling down. After the fixing elements of the coil to the open wagon are removed, the pulling truck tractor smoothly moves the coil to the unloading beams, while the holding truck tractor prevents it from spontaneous rolling on the beams.

Panel roof workpieces are transported in special packing.

The coils are transported from the place of unloading to the construction site by trailers (in case of road availability) or by sledge in winter and in case of lack of proper roads. In case the distance is not big and the surface is flat it is possible to roll the coils by truck tractors. The twisting direction of the coil while rolling should be backhanded (inverse) to that of its rolling in the course of production. This is important because only this can ensure safety of the panels, preventing the coil from elastic unwinding.

Acceptance of the metal frameworks for assembly should be done by the ordering customer’s representative and the installing company with signing an act (certificate) in the approved form. The acceptance certificate should also have the following supplements:

  • Detailed metal frameworks design project of the producer
  • Packing (delivery) lists
  • Results of measurements and testing in metal roll’s initial check at production plant and certificates forwelding materials
  • Physical control charts for welding joints.

The quality of supplied elements and substructures should meet the requirements of the technological documents of the installing company, the metal frameworks design documents, detailed metal frameworks plans and State Standard GOST 52910-2008.

3. Welding and assembly procedures

3.1. General requirements

In the course of assembly of tanks, as well as in production, 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 design documents determine the level of mechanic properties and the cold resistance of the welding joints. Theirdefect level is set by the State Standard GOST.

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.

3.2. Requirements for mechanical properties of the welding joints

Mechanical properties (hardness excluding) of the metal of corner, overlap and T-joints are determined on samples, cut out from the butt-welded joints – test-pieces. The type of steel, welding materials and equipment, used in test-pieces’ production, are the same as those meant for welding of the above-mentioned types of junctions.

3.2.1. Requirements for strength characteristics:

The metal of welding joints should be equal in strength to the parent metal. Tests are made on three samples of types XII or XIII according to State Standard GOST 6996. The metal of corner weld joint rim weld (the joint of the wall with the bottom) is subject to additional demand of strength uniformity with the parent metal at the standard rate of yield point.

3.2.2. Requirements for impact resistance (impact bending) of the welding joints

Impact resistance at the set testing temperature should be not less than the limits, stipulated by the State Standard GOST 52910-2008 for planning the metal frameworks of a tank.

The testing temperature is set in accordance with State Standard GOST 52910-2008.

The test of impact bending (impact resistance) is made for the metal of welding joints and the heat affected area of the butt-welded joints of elements in groups A and B. The impact resistance in this case is determined for the seam’s metal and the heat affected area on three transverse specimen (three for the seam and three for the heat affected area) with a sharp cut: type IX for the parent metal’s thickness of 11 mm and more, type X for the parent metal’s thickness of 6-10 mm according to State Standard GOST 6996.

3.2.3 Requirements for technological tests on bending of the welding joints

While testing the welding joints for static bending the arithmetic mean value of the bending angle of six transverse test specimen (type XXVII according to GOST 6996) should not be less than 120°, while the minimum bending angle of one specimen – not less than 100°.

In case of metal thickness up to 12 mm (inclusive) the tests are made by bending the sample with the seam’s root to the inside (on 3 samples) and the root to the outside (on 3 samples). When the metal is more than 12 mm thick the tests are made by bending the samplesedgeway (on 6 samples).

3.3. Technical requirements for welding joints

The construction scheme of welding joints of the tank’s elements should correspond to the requirements of metal frameworks design plan and Work Execution Plan.

The view of the welds must meet the following standards:

- the metal of the seam should have smooth fit with the basic metal;

- the seams should not have cracks of all types and sizes, ripples, gas pockets and their chains, burnthrough and blowholes.

Parameters of undercutting of the basic metal should not exceed the ones, stipulated in the Table.

name of welded connection permissible value between when the level of responsibility of the tank
  IV III I; II
vertical belt joints and connection with the bottom wall 5% of the thickness, but not more than 0,5 mm not more than 0,5 mm not more than 0,3 mm
horizontal connections wall 5% of the thickness, but not more than 0,8 mm 5% of the thickness, but not more than 0,6 mm 5% of the thickness, but not more than 0,5 mm
other connections 5% of the thickness, but not more than 0,8 mm 5% of the thickness, but not more than 0,6 mm 5% of the thickness, but not more than 0,6 mm

The length of the undercutting should not be more than 10 % of the joint’s length within the limits of the sheet.

Crown out of the butt-end joints of the tank’s elements should not exceed the ones, stipulated in the Table.

 

the thickness of the sheet, mm the maximum value of convexity, mm
  vertical connections wall other compounds
up to 12 inclusive 1,5 2,0
over12 2,0 3,0

For butt-welded joints of the tank’s elements of the same thickness the following shift of the welded edges is allowed:

- 1.0 mm for the elements not thicker than 10 mm;

- 10 % of thickness, but not more than 3.0 mm, for elements thicker than 10 mm.

Maximum weld legs should not exceed the rate, equal to 1.2 of the thickness of the thinner element in the junction.

For elements of 4-5 mm’s thickness the weld leg must be equal to 4 mm. For elements of bigger thickness it is determined by calculation of design, but anyway it should not be less than 5 mm. This is not applied for the size of the welding joint, fixing the decking of the easy-to-remove roof to the top ring of the wall.

Crown out and inward bulging of the corner joint should not exceed the weld leg by more than 20 %.

The weld leg may be decreased not more than by 1 mm. It can be increased by not more than:

1.0 mm – for weld legs up to 5 mm;

2.0 mm – for weld legs more than 5 mm.

Overlap joint, welded by continuous seam on one side, is allowed only for joining the bottom and the decking of the fixed framed roof. The size of the overlap should not be less than 60 mm for joining the strip panels of the bottom and not less than 30 mm for joining the sheets of the roof and the bottom. It should anyway be not less than 5 times as thick as the thinnest sheet of the joint part. 

3.4. Development of assembly methods for vertical steel tanks.

Formerly vertical tanks were installed only by per-sheet method, i.e. by assembling the wall from separate sheets. Per-sheet method is applied with the help of mobile cranes, equipped with jibs of necessary length. Installation of the wall and its welding are carried out after the bottom is installed. Nowadays the per-sheet method is the basic one for construction of tanks with high loading capacity.

Development of methods of tank construction was closely connected with working out the method of rearing, aimed at partial transfer of the labour intensive operations to construction and building enterprises.

In 1929 specialized production plants began welding and coiling separate rings of the walls for tanks, not exceeding 300 m³ loading capacity. They were afterwards installed by method of rearing.

There is a certain drawback of this method, apart from its labour intensity and complicated equipment, It is to be seen in the necessity to provide rigidity of the tank in the course of its installation. For the sake of protecting against the wind stress the walls should be thoroughly braced with the set of anchors.

One of the versions of this approach is the corkscrew method, developed by Czech constructors. It implies that the first ring serves as a “pattern” and while the wall of the tank may be twisted by special adjustments with electrical drive, the welding of new rings can be done in one and the same specially organized and equipped place. Complexity of the used equipment and tools didn’t allow to spread this method to other countries.

The method of rearing was also developed in Sweden, implying the use of so-called “climbing” hosting jacks, installed on special stay braces, that enable to perform all assembly and welding operations on ground level.

In the 1950s the USSR has suggested and mastered an effective coiling method of large tanks’ construction, which met all modern requirements. This method implies welding of the bottom and wall elements at production plants, after which they are brought to the construction site in the form of rolled workpieces, where they are then uncoiled, joined, and then the roof is installed. 

4. The order of assembly operations

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 operationsThe order of assembly operations

5. Tank bottom installation

5.1. Installing coiled bottoms

Bottoms 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.

If 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.

5.2. Per-sheet method of bottom installation

If the bottom is supplied by the producer in sheets, 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 wall 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.

Picture 15 shows the push tools, often used in the bottom assembly.

5.3. 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:
With empty vessel: tank diameter
  up to 12 m 12-25 m 25-40 m 40 m
The difference between the marks of the neighboring pixels at a distance of 6 m along the perimeter 10 mm 15 mm 15 mm 20 mm
The difference between the marks of any other points of 20 mm 25 mm 30 mm 40 mm

 

When a water-filled tank: tank diameter
  up to 12 m 12-25 m 25-40 m 40 m
The difference between the marks of the neighboring pixels at a distance of 6 m along the perimeter 10 mm 15 mm 15 mm 20 mm
The difference between the marks of any other points of 20 mm 25 mm 30 mm 40 mm

5.4. 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.

The order of bottom assembly for a vertical steel tank 300 is shown in the picture

Tank bottom installation Tank bottom installation Tank bottom installation Tank bottom installation

6. Assembly and welding of a vertical tank wall

Assembly and welding of a vertical tank wall

6.1. Installation of a coiled wall:

Assembly and welding of a vertical tank wallAssembly and welding of a vertical tank wallAssembly and welding of a vertical tank wallAssembly and welding of a vertical tank wall

Installation of a coiled wall is done in four stages:

Assembly and welding of a vertical tank wall

In case a jib crane of necessary carrying capacity is available at the construction site (either crawler caterpillar or pneumatic-tyred) the coil of the wall 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 wall

 

When 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 wall. 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 wall. 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 wall is usually welded with overlap. To ensure this, the lower edge of the wall is released from the tack to the bottom, after which it is pulled to the starting edge of the wall, 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 wall and the coverage boards it is necessary to check the deviation of the wall from the vertical mark, which may not exceed 90 mm throughout its full height.

The order of uncoiling the wall is shown in the pictures:

Assembly and welding of a vertical tank wallAssembly and welding of a vertical tank wallAssembly and welding of a vertical tank wallAssembly and welding of a vertical tank wallAssembly and welding of a vertical tank wallAssembly and welding of a vertical tank wall

6.2. Assembly of the tank wall by per-sheet method:

Assembly and welding of a vertical tank wallAssembly and welding of a vertical tank wallAssembly and welding of a vertical tank wallAssembly and welding of a vertical tank wall

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

Assembly and welding of a vertical tank wallWhile using this method of assembly it is important:

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

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

Assembly of the wall’s elements is carried out by experienced specialists on tacks.

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.

 

6.3. Extreme deviations of dimensions and shapes of the installed wall:

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

 

7. Installation of tank roof and pontoon

7.1. Installation of fixed roofs.

Монтаж кровли с центральной стойкойDepending 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 conical and spherical roofs (measured through the Central nipple) ±30 ±50
The difference between the marks adjacent nodes top of radial and girders:
— in the zone of conjugation with the wall:
— in a zone of interface with the Central shield:
— in the bordering zone of the radial beams spherical roofs:
20
10
15
Deviation from the design of radius of spherical roofs. Clearance between the pattern and the bent surface is measured on each of the radial beam and the farm). 5,0

 

7.2. 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 pontoonInstallation of tank roof and pontoon

8. 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 wall 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 wall 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 Dy 800-900 mm 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°

 

9. Quality control, testing and acceptance of the tanks

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

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
seams bottom seams pads to the bottom + + - - - -
joints of the bottom at a distance of 250 mm from the outer edge of the + + + - - -
wall
vertical seams of 1-St and 2-nd zones + - + - -
vertical seams rest zones + - + - -
horizontal seams zones + - + - -
seams the crosshairs of vertical and horizontal seam + - + - - -
the seam between the pipe and the wall + + - + - -
the seam between the collar socket (Luke) and 1m belt wall + - - - + +
the seam between the collar socket (Luke) and the wall (except 1-St zone) + - - - - +
radial seams rings of rigidity + - - - - +
spot removal Assembly jigs, welded structural bonding after heat treatment + - - - + -
seam with the bottom wall + + - - -
roof
radial seams support ring + - - + - -
seams roof flooring, roofing shields + + - - - +
seam pipe with a roof + + - - - -
floating roof (steel pontoon)
seams Korobov (bays) and plugs racks + - - - - +
seams the Central part of the + + - - - -
joints of the pipes to the roof + + - - - -
1 allowed the use of ultrasonic testing
2 allowed the use of radiography
3 control breakdown «chalk-kerosene» hold up welding seam from the inner side

The standards for evaluation of the welds’ defect rates of the rates of allowed defects should be stipulated in the design documents.

Visual-measuring control is done for the full (100 %) length of the welding joints of the tank.

The quality, shape and sizes of the welding joints should meet the requirements of the State Standard GOST 52910-2008 and the standard documents in accordance with the Table 5 (see earlier), and the design documents as well.

Leak tightness control is carried out for welding joints, providing structural integrity of the body of the tank and the floating and leak-proof properties of a pontoon or a floating roof.

Dye penetrant inspection is done after the visual-measuring control.

Physical methods of control:

  • Welding joints of the wall and butt-joints of the edges in the wall-connection area are subject to radiographiccontrol.
  • Radiographic control is applied after acceptance of welding joints by visual control.
  • In the course of inspection of joints’ crossing, the X-ray films are put T-shaped or in a cross – two films on each joints’ crossing.
  • The length of the shot (picture) should not be less than 240 mm and the width is determined according to the State Standard GOST 7512.
  • Response characteristics of the shots should fall into the 3rd degree (class) in accordance with State Standard GOST 7512.

The types and sizes of defects allowed depending on the tank’s class are determined according to GOST 23055:

- for tanks of IV class of hazard – at the 6th class of joints;

- for tanks of III class of hazard — at the 5th class of joints;

- for tanks of I, I class of hazard — at the 4th class of joints.

Faulty fusion and lack of penetration are not allowed.

So as to reveal the inside and surface defects in the welding joints and the nearby area of the basic metal, ultra-audible defect detection is applied.

The scope of physical control of welding joints (in percentage to the joint’s length) of the tank wall, 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 vessel WALL, as % of the length of the seam
Control zone hazard class tank
  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.

9.2. Final tests of the installed tanks.

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, according to the State Standard GOST R-5291-2008, need to be done for tanks of different types (tanks with fixed roof without a pontoon, tanks with fixed roof and a pontoon, tanks with floating roofs).

 

type of test rvs rvsp rvspk
1 Test the tightness of the housing when the Gulf water + + +
2 Test the strength of the shell of a tank and hydrostatic load + + +
3 Tightness stationary roof RVS excessive air pressure + - -
4 Test the sustainability of the tank creation of a relatively vacuum inside the reservoir + - -
5 The test of buoyancy and health pontoons and floating roof - + +
6 Functional testing катучей stairs - - +
7 Test the sustainability of the bottom of the tank with the definition of absolute and differential settlement on a path bottom roll tank, profile 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.

9.2.1. 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 wall is revealed in the course of testing, the water is fully discharged (removed). If cracks are revealed in the wall 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 (provided this is authorized and agreed with the Rostehnadzor). Prior to such tests all welding joints of the wall, bottom, roof and manway plates/fitting pipes, and also connections of wall 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 walls.

9.2.2. 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 walls 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.

9.2.3. 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 shouldinclude:

  • 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;
  • Labour 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. The pipeline should be tested for pressure Р=1,25 Рраб.

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 wall 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.

Tank installation
To order