Storage Tanks : Design, Construction, Maintenance, operation & Trouble shooting.
A Typical Storage Tanks.
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Standards & Codes : Design, Construction & Testing
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API Standard 650 : Welded Steel tanks for Oil Storage API Standard 620 : Design and Construction of Large, Welded Low pressure Storage tanks API Standard 653 : Tank Inspection, Repair & Alteration & Reconstruction Indian Standard, IS 803 : Welded Steel tanks for Oil Storage British Standard Spec. BS 2564 : Vertical Steel Welded Storage Tanks with Butt Welded shell. DIN 4119 Part 1,2 : Above Ground Cylindrical Flat Bottom tank installation of metallic materials.
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Design Criteria of a Code
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The Design Code Criteria are the basic rules upon which the design is based. They Cover the design formulas or methods, the design loads and allowable stresses.
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Design Criteria of API Std 650
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API Std 650 is based on maximum Principal stress theory. Thickness of tank shell is calculated by membrane stress due to hydrostatic pressure + internal vapour pressure. Allowable Stress : Design Condition (Sd) – Min of 2/3 of Yield Stress and 2/5 of UTS Allowable Stress : Hydrostatic Condition (St) – Min of 3/4 of Yield Stress and 3/7 of UTS
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API Std 650 : Scope
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The standard is meant for design, selection of materials, fabrication, erection & testing Vertical, cylindrical, above ground, closed and open top, welded steelstorage tanks. Internal Pressure approximating to atmospheric pressure. However, tanks can be designed for pressure up to 0.175 Kg/cm2. The tanks entire bottom is uniformly supported. The tanks are in non refrigerated service and maximum temp of 200 deg F.
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API Std 620 : Scope
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The standard is meant for design and Construction of large, welded low pressure, CS above ground storage tanks. Vertical, cylindrical, above ground, closed, welded steel storage tanks. Pressure in the vapour space not more than 15 lbs/sq inch( 1.055 kg/cm2). The tanks are for metal temp not greater than 250 deg F.
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Factors to be specified for design
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Metal Temp Specific gravity Corrosion allowance for each component Wind velocity Special consideration to foundation, corrosion allowance, hardness testing, etc.
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Design Consideration
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Dead Load : Weight of the tank or tank component External Pressure : shall not be less than 1 inch of water pressure Hydrostatic load: load due to filling the tank with water to the design liquid level
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Tank Capacity
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Tank Components
• Bottom Plates : (i)
All Bottom Plates Min Thkness 6 mm exclusive of any CA. (ii) Bottom plate shall project outside shell by 25 mm.
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Annular Bottom Plate : 6-9 mm. if 1st Shell course is less than 19 mm. 6-11 mm. in case shell course is 19-25 mm 6-14 if 25-32 8-17 if 32-38 Variation is due to 1st shell course stress.
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Tank Components
•Shell Thickness:
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Nominal dia of Tank
Nominal Plate thickness
<15 M 15 M to 36 M 36 M to 60 M >60 M
5 mm 6 mm 8 mm 10 mm
Tank shell to be checked for stability against buckling from the design wind speed. If required for stability, intermediate girders, increased shell plate thickness or both shall be used. 3/8/2013
Tank Components
Shell Thickness: •Calculation of thickness by 1-Foot Method- calculates the thickness required at design points (0.3 m above of the bottom of each shell course) It is used up to tank dia of 60 M.
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Tank Components
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Tank Components : Shell Openings
A Tank shell opening that is larger than 2’’ to be suitably reinforced. Typical reinforcement details:
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Tank Components : Shell Manhole Cover Plate & Bolting Flange
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Tank Components : Shell Manhole Neck Thickness
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Tank Components: Top & Intermediate Wind Girder
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•
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Open Top Tanks to be provided with stiffening rings to maintain Roundness at or near the top of the course preferably at outside. When Wind girders are located more than 0.6M below the top of the shell, the shell to be provided with a top curb angle : size 64 x 64 x 4.8 mm for shell 5 mm thick and angle size of 76 x 76 x 6.4 mm for shell more than 5 mm thk Stiffening Ring or wind girder can be used as walkway. Width of walkway not to be less than 24 inch. 3/8/2013
Tank Components: Stiffening Ring Sections
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Tank Components: Wind Girder with walkway Separate Top curb angle
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Tank Components: Roof
Different types of Roof : A. Fixed Roof : Most widely used and integrated with tank structure. (a) Cone Roof (used for atmospheric tanks) (b) Dome Roof ( Used for pressurised tanks) Cone Roof : (i) Self Supported – used for small tank dia 4- 6 m. (ii) Supported by Rafter – used for tank dia 8-12 m (iii) Trussed – used for tank dia up to 60 m (iv) Column Supported – economical for large dia tanks but not recommended because diff settlement can cause overstress in shell and roof. Dome Roof : (i) Self Supported – used for tank dia of 4-6 m (ii) Rafter Supported – used for large dia tanks.
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Tank Components: Floating Roof Tank
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Tank Components: Roof - Cone Roof & Dome Roof
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Tank Components: Roof - Cone Roof & Dome Roof
Different types of Roof : Floating Roof : used for storage of volatile products – reduce costly evaporation loss and increase protection from fire. (a) Open Top Tank Floating Roof (a) Pan type (b) Ring Pontoon type © Ring Pontoon with deck pontoon (d) Double deck type (b) Internal Floating Roof (a) Open pan type (b) Ring pontoon type © Double deck type
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Tank Components: Roof - Cone Roof & Dome Roof
Different types of Roof : Floating Roof : Light Weight Internal Floating Roof (a) Aluminium IFR – most commonly used (b) Fibre glass roof – used in chemical applications where metal materials not compatible with stored product.
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Tank Components: Roof – Floating Roof
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Tank Components: Roof – Floating Roof
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Tank Components: Roof – Floating Roof
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Tank Components: Roof – Floating Roof
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Tank Components: Roof – Internal Floating Roof
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Tank Components: Roof – Internal Floating Roof
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Tank Components: Roof – Internal Floating Roof
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Tank Components: Roof Design
• Fixed Roof : • Dead Load : (a) Self weight of members including the weight of joints & connections. (b) Weight of covering plates (C) Weight of bracing members (d) Weight of nozzles and attachments etc. • Live Load : Roofs and all supporting members to carry a uniformly distributed live load of 125 kg/m2. This includes wind load and internal vacuum load also.
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• Seismic Load : Basic seismic coefficient are given in IS 1893-1984. These loads are to be considered in conjunction with 3/8/2013 the self and live load.
Tank Components: Roof Design
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Fixed Roof
(1) Roof plate shall have a minimum nominal thickness of 5 mm. (2) Roof plates shall be attached to the top angle of the tank with continuous fillet weld on top side only. (3) A roof is considered frangible if shell to roof joint fail prior to the shell to bottom joint in the event of excessive internal pressure. (4) Slope of the roof at the top angle attachment does not exceed 2 in 12. (5) The roof is attached to the top angle with a single continuous fillet weld that does not exceed 5 mm.
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Tank Components: Roof Support
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Supported Fixed Roof
(1) Minimum thickness of any structural member, including any CA on the exposed side or sides shall not be less than 6 mm for columns, knee braces and beams. (2) It shall be 4 mm minimum for any other members. (3) For columns the value of L/rc (maximum slenderness ratio) shall not exceed 180.
(4) For all other members except tie rods, the L/rc shall not exceed 300. where, L = unbraced length, rc = least radius of gyration of column and r = governing radius of gyration.
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Tank Components: Roof Support ( typical column base details)
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Tank Components: External Floating Roof
• This is covered in Appendix-C of API Std 650. (1) Deck : Roof in corrosive sour services such as Crude Oil should be contact type designed to eliminate the presence of any air-vapour mixture under the deck. (2)All deck plates shall have minimum nominal thickness of 5 mm. (3) Pontton : Pontoon Design shall be based on a liquid with a specific gravity of 0.7 and with primary drain inoperative for the condition as under: 250 mm of rainfall in 24 hours period . Single deck and any two adjacent pontoon compartments punctured and flooded.
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Double deck and any two adjacent compartment punctured and flooded without any water or live load. 3/8/2013
Tank Components: External Floating Roof
• Ladders : A ladder to be provided for access to the roof. Ladder shall be automatic adjusting type. All deck plates shall have minimum nominal thickness of 5 mm. • Roof Drains : Shall be not less than 3 inch for tank dia up to 36 M and for tank dia more than 32 M, the roof drain shall be 4 inch NB. Drain shall be provided with an NRV. • Emergency Roof drain : to be provided in double deck design to take care of severe rainfall.
• Vents : To prevent overstressing of the roof deck or seal membrane, automatic bleeder vent is furnished. • Supporting Legs : Removable or non removable. Size >= 2 inch NPS
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Tank Components: External Floating Roof
• Roof Manholes : Minimum diameter of 24 inch. Number of Manholes in the roof shall be as follows:
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Tank Components: External Floating Roof
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Peripheral Seal: Also referred as Rim Seal.
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Primary Seal : Can be either a mechanical shoe type or a Foam and nitrile rubber casing arrangement. The foam and nitrile rubber casing is protected from weather with a metallic weather protector/ compression plate.
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Secondary Seal : It shall be wiper type
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Gauging Devices : (1) Manual Gauging Port with cover. – slotted guide pole gauge wells. – it must be provided with a gauging platform. (2) Automatic Level Gauge : tank need to have ground level reading automatic float level gauge.
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Tank Layout
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Tankages installation as per OISD 118.
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Aggregate Cap of tanks in one dyked enclosure shall not exceed
(a) 60000 Cu M for a group of fixed roof tanks (b) 120000 Cu M for a group of floating roof tanks •
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Dyked enclosure shall be able to contain complete contents of the largest dia tank.
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Tank Layout
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Dyke volume shall be calculated deducting other tanks volume and tank pads volume.
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Height of tank enclosure dyke shall be between 1 M & 2 M.
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Process equipments not to be installed inside dyke.
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Separation distance between nearest tanks in separate enclosures shall not be less than dia of the larger of the two tanks or 30 m, whichever is more.
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Dyke volume shall be calculated deducting other tanks volume and tank pads volume. 3/8/2013
Tank Layout
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Tanks shall be arranged in maximum of two rows.
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Tanks having 50000 CuM cap & above shall be laid in single row.
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Fire wall of minimum height 600 mm to be provided where more than one tank is located.
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A group of small tanks each not exceeding 9 m dia and in all not exceeding 5000 CuM in capacity to be treated as one tank for fire wall provision.
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Tank Components: Tank Fabrication & Erection
Typical Joints :
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Tank Construction
Typical Joints :
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Tank Construction
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Tank Construction : Major Jobs
A. Foundation • Stone column Foundation – Less Costly. But chances of settlement
is there. Settlement occurs during hydrotest. • Concrete Pile Foundation – Costly. But No settlement. B. Mechanical Work
• Scaffolding work and round shell work as per height – concern to Safety. More time consuming. • Jacking Method – Safe, Easy work. – Fast working.
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Tank Construction : Concrete Foundation
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Tank Construction : Use of Jacking Method
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Tank Construction
• Connected piping Work – Nozzle, Manhole, Roof drain, Shell drain and clean out door. • Structural work • GI sprinkler & Foam Pourer system. • Goose Neck, Flame Arrester, Vent, Pressure Vacuum Relief Valve, Jet Mixer, Agitator Assembly. • Instrumentation- Gauging, Manual & Auto • Painting • Insulation – wherever necessary.
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Tank Construction : Flame Arrester in Fixed Roof Tank
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Tank Construction : Goose Neck in Fixed Roof Tank
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Dimensional Tolerances
• Plumpness : Top of shell relative to bottom not to exceed 1/200 of total tank height. • Roundness : Radii measured at 1 ft above the bottom corner weld shall not exceed the following :
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Tank Dia
Radius Tolerance
< 40 Ft
+/-1/2’’
40 to 150
+/- ¾’’
150 Ft to 250 Ft
+/- 1’’
>=250 Ft
+/- 1.25’’
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Testing
• Bottom Plate welding Testing by Vacuum Box method • Reinforcement plate welding test by applying 15 PSI gauge pressure. • Roof plate welding test by vacuum box test method. • DP test for Shell to bottom/ annular plate joints. • Radiography at random at Butt Joints of shell. • Hydrotest by filling water up to maximum liquid level or up to curb angle. Tank shell to inspect frequently during hydrotest.
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Check Points During Construction
A) Tank Foundation
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1. Soil Test Report 2. Reference Level 3. Pile Details ie Quantity and Size 4. Method of Piling being used (Manual or Automatic 5. Type of concrete Mix ( M15, M20 ,M25 or M30 6. Cube test Result for 7, 14 and 28 days. 7.Settlement reading during load test 8. Pile cut off level 9.Pile cap ( Single or Combined footing) 10. Concrete Foundation including ring wall 11. Layer wise Compactness checking during Sand filling. 3/8/2013
. Method of Piling being used ( 12. Premix Carpeting Ratio ( Sand and Bitumen ) 13.Strength test report of Premix Carpeting. 14. Slope of Tank pad and Level w.r.t Reference Level
B) Bottom Plate Laying
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1) Annular plate cutting layout checking w.r.t Approved Drawing. 2) Sketch plate cutting layout checking w.r.t approved drawing 3) Checking of bottom underneath paint thickness. 4) Annular plate fitment checking including baking strip. 5) Welding electrode quality and no. of run. 6) DP and Radiography as per approved QAP/ITP. 7) Bottom sketch plate fitment and lapping dim.
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1) ayout checking w.r.t approved 8) Vacumn / Pneumatic test
C) Shell Erection 1) 2) 3) 4) 5) 6) 7)
Plate Cutting Layout Plate Bending Profile according to Jig. Vertical / Horizontal Joint fit-up Circularity / Verticality Welding Quality “ T” Joint DP / Radiography Test SR Report
D) Roof Erection
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1) Level of Roof Staging pipe 2) Pontoon / Deck Layout 3) Joint fit-ups 3/8/2013
4)Welding Quality. 5)Vacuum / Pneumatic Test E) Apparatunces 1) 2) 3) 4)
Compliance w.r.t Drawing Fitment Welding quality Heat treatment / testing
F) Testing
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1) 2) 3) 4) 5)
Floatation Test of Deck Puncturing test of Pontoon Roof stability test with 10” Water on roof. Roof Drain Hydraulic Test Swivel Rotation Test
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6) Hydraulic Test and Settlement Reading. G) Other Points 1) Dyke layout 2) 3) 4) 5) 6) 7) 8)
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9)
Valve Operability Sample point Operability Foam seal fitment Foam Pourer and Sprinkler fitment / operability etc. Earthing Continuity Level Gauge Operability Stair case / rolling ladder /wind girder / structural fitment & welding quality Surface Cleaning and Paint Application 3/8/2013
Tank Operational Checks 1) Simultaneous Operation of Valves during 2) 3) 4) 5)
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6) 7) 8)
change over Tank product Level to arrest overflow / maintain SFH. Condition of Roof Drain / operability Water draining at Regular Interval to Prevent corrosion Side mixer operation at regular interval to minimize sludge formation Condition of Spiral Ladder Condition of Roof Condition of Insulation 3/8/2013
09) Sampling at regular intervals for testing purpose. 10) Breather valve condition 11) Water seal of Emergency drain 12) Pumping Rate.
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Tank Maintenance : Major Jobs Involved
A. Foundation Related: • Sand Filling • Morum or PCC (1:2:4)
• Premix Bituminous Carpet –anticorrosive layer. • RCC of 100 mm thickness outside the tank Annular plate. B. Mechanical : • Bottom Plate Replacement. • Roof Plate Replacement. Nozzle work – if any.
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Tank Maintenance : Pontoon Replacement
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Tank Maintenance : Major Jobs Involved
B. Mechanical : • Floating Roof Tank – Deck, Pontoon repair/ replacement. • Structural repair/ replacement.
• Swivel joints repair for Roof drain. • Handrail and Roof walk way. • Repair/ replacement of Water sprinkler system/ Foam Pourer system. • Surface cleaning by Grid Blasting. • Painting and Insulation.
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Tank Maintenance : Major Jobs Involved-Swivel Joint
B. Mechanical : • Floating Roof Tank – Deck, Pontoon repair/ replacement. • Structural repair/ replacement.
• Swivel joints repair for Roof drain. • Handrail and Roof walk way. • Repair/ replacement of Water sprinkler system/ Foam Pourer system. • Surface cleaning by Grid Blasting. • Painting and Insulation.
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Tank Maintenance : Major Jobs Involved-Pontoon Replacement.
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Tank Maintenance : Major Jobs Bottom Plate Replacement
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Tank Maintenance : Major Jobs Seal replacement
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Some Typical Failure of Tanks
During Construction
1) Failure of Pile during Load testing 2) Failure due to material quality. 3) Uneven / excessive settlement of tank pad causing tilting of Tank during test. 4) Welding failure during Hyd. Test
5) Touching of Roof / deck during initial lifting 6) Collapsing of roof / deck due to excessive water accumulation and no emergency drain point.
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Some Typical Failure of Tanks 7.Improper lifting of Deck / Roof during Hydraulic Test.
During Operation and Maintenance 1)
2)
3)
4)
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Collapsing of roof & shell due to vacuum inside for Breather valve mal functioning. Connected equipment damage due to mal operation of valves during Tank change over. Roof collapse due to closing of roof drain valve / emergency drain valve. Tank fire due to lightening spark for poor earthing.
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Some Typical Failure of Tanks 5) Leakage of bottom plate during running. 6) Tank fire during maintenance due to improper evacuation of hydrocarbon. 7) Roof collapsing due to under insulation corrosion 8) Other failure due to corrosion / aging.
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Thank you
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