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Summary 1. Types of storage tanks 1.1. Atmospheric storage tanks 2.2. Low pressure storage tanks
2. API standards 3. API 650: welded steel tank for oil storage 4. Causes of catastrophic Tank Failure 5. API 653: Tank Inspection and Maintenance
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1. TYPES OF STORAGE TANKS
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1. TYPES OF STORAGE TANKS Storage tanks Atmospheric storage tanks
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Low pressure storage tanks
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1.1. ATMOSPHERIC STORAGE TANKS
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1.1. ATMOSPHERIC STORAGE TANKS The most common type of atmospheric storage tank is the fixed cone roof tank, These roofs are normally supported by internal structural but can be fully selfsupporting in smaller diameters (typically, 60 ft [3 m] diameter or less). up to 300 ft (91.5 m) in diameter & 64 ft (19.5 m) in height (larger diameter tanks have been built, mostly outside the U.S.)
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1.1. ATMOSPHERIC STORAGE TANKS The umbrella roof has radially-arched segmental plates with integral framing support members (usually without internal support columns).
In the steel dome roof tank, the roof plates are usually formed with curved segments joined to be self-supporting.
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Types of Storage Tanks
1.1. ATMOSPHERIC STORAGE TANKS The floating-roof tank is designed to minimize filling and breathing losses by eliminating or minimizing the vapor space above the stored liquid. The shell and bottom of this type of tank are similar to those of the fixed roof tanks, but in this case, the roof is designed to float on the surface of the stored liquid. Older styles of floating roofs include single steel deck details without annular pontoons. Such roofs have no reserve buoyancy and are susceptible to sinking in service.
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Types of Storage Tanks
1.1. ATMOSPHERIC STORAGE TANKS
Annular-pontoon and double-deck Some floating-roof tanks have fixed aluminium geodesic dome roofs installed on top of the tank shell to reduce product vapor loss or to eliminate the need to drain rainwater from the roof. These are considered internal floating roofs is a tank with a fixed steel cone roof over a floating roof.
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1.2. LOW PRESSURE STORAGE TANKS
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Types of Storage Tanks
1.2. LOW PRESSURE STORAGE TANKS Low-pressure storage tanks are those designed to operate with pressures in their gas or vapor spaces exceeding the 2.5 lbf/in.2 (18 kPa) gauge permissible in API Std 650, but not exceeding the 15 lbf/in.2 (103 kPa) gauge maximum limitation of API Std 620. 1.2.1 Types of Low Pressure Tank Roofs Tanks that have cylindrical shells and cone or dome roofs are typically used for pressures less than about 5 lbf/in.2 (34.5 kPa) gauge. Tank bottoms may be flat or have a shape similar to the roof. Hold-down anchorage of the shell is generally required.
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1.2. LOW PRESSURE STORAGE TANKS For pressures above about 5 lbf/in.2 Gauge (34.5 kPa), hemispheroid, spheroid, and noded spheroid tank types are commonly used. Tanks for this application are now typically constructed as spheres. Such tanks are designed to withstand the vapor pressure that may be developed within a tank having no devices or means to change or relieve the internal volume.
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Plain Hemispheroids
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1.2. LOW PRESSURE STORAGE TANKS
Hemispheroid
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Plain Spheroid
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2. API STANDARDS
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2. API standards: API RP 653: Lining of above ground storage tank (AST) API RP 575: Inspection of atmospheric and low pressure storage tanks API RP 571: damage mechanisms affecting fixed equipment in the refining industry API RP 651: Cathodic protection of above ground storage tank (atmospheric storge tanks) API 650: Welded steel tank for oil storage API 653: Tank inspection, repair, alteration and reconstruction
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3. API 650 : Welded steel tank for oil storage
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WHAT IS THE API 650 SPECIFICATION Worldwide Standard for Above Ground Storage Tank Design and Construction: This Standard establishes minimum requirements for material, design, fabrication, erection, and testing for vertical, cylindrical, aboveground, closed- and open-top, welded carbon or stainless steel storage tanks in various sizes and capacities REFERING TO ASME Section V, IX
Provides requirements for calculations of shell plate thickness, man-way and nozzle design Provides procedures for shell, roof and floor construction Specifies material requirements and minimum thickness requirements Specifies weld construction requirements, weld spacing, and X-ray requirements
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3.1. IMPROVING INDUSTRY NEEDS • Global improvement on industry is going on with increasing population and technology. • To solve raw material needs for serial production increasing number of storage tank is unavoidable. • Increasing number of vehicle in traffic causes construction of new storage tanks. • Old storage tanks, generally built up before 1970s need to be replaced with new ones due corrosion on material and wrong manufacturing/erection techniques in past.
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• Natural reasons such as eartquake, wind, snow also cause new storage tanks or reinforcement on in-service tanks • Change in service or operation cause new storage tanks or change in tank structure or geometry. API 650 & API 653 Training
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OTHERWISE,
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2. Design - Basic design approach and verification 2.2 Materials used in construction
Section 4 of API 650 define materials can be used in construction of a storage tank. Accoding to this section mainly below materials can be used; Also design metal temperature should be taken into account according to location of storage tank. Material type of shapes and pipe is also deifned in section 4. Welding electrode for; Material has Ut550 MPa, E80 series
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2. Design - Basic design approach and verification 2.3 Basic Design Approach and Verification 2.3.1 Correct thickness? Shell thickness easliy can be verified by 1-foot method; td: design thickness tt: hydrotest thickness D:nominal tank diameter H:design liquid level G:specific gravity of product Sd:allowable stress for design St:allowable strss for hydrotest CA:corrosion allowance
In any case min thicknesses as follows;
Bottom plates shall be minimum 6 mm thick exclusive of CA unless othervise specified. API 650 offers 1800 mm width plates.
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2. Design - Basic design approach and verification 2.3 Basic Design Approach and Verification Annular plate thickness shall be decided acc. to below table;
The ring of annular plates shall have a circular outside circumference, but may have a regular polygonal shape inside the tank shell, with the number of sides equal to the number of annular plates.
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2. Design - Basic design approach and verification 2.3 Basic Design Approach and Verification 2.3.2 With or Without Annular plate? Bottom shell course in Group IV, IVA, V, or VI, butt-welded annular bottom plates shall be used unless maximum product stress for the first shell course is less than or equal to 160 MPa or the maximum hydrostatic test stress for the first shell course is less than or equal to 2.4MPa. Opennings correct bottom location? 171 In this caseinlap-welded plate maybe used.
Location shell or roof opennigs may be verified with below tables;
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2. Design - Basic design approach and verification 2.4 Opennings in correct location?
Tank design review can prevent many imperfection during fabrication or erection. API 650 & API 653 Training
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3. Fabrication - Important points for fabrication at workshops or site • During fabrication of tank material in workshop or site maximum • care shall be taken not to cause failure during erection or service. Edges of plates may be sheared, machined, chipped, or machine gas cut. Shearing shall be limited to plates less than or equal to 10 mm thick used for buttwelded joints and to plates less than or equal to 16 mm thick used for lap-welded Joints • Shaping of shell plates before erection should be done according to below figure;
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3. Fabrication - Important points for fabrication at workshops or site
• Good engineering design can accelerate erection by pre-drilled openning. Drilling of telltale holes shall be done before erection • Radius and weld bevel control during fabrication can prevent any time/effort loss during erection • Proper marking of shell/bottom plates with a prestudied drawing can save time and effort during erection. • Plates and tank material shall be loaded in a manner that ensures delivery without damage • Shop primer on plates and protective oil on flange faces can prevent material from corrosion till final process API 650 & API 653 Training
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4. Erection - Methods and their advantages/disadvantages 4.1 After foundation works and laying bottom/annular plates, there are mainly two different erection method for storage tanks; 1. Conventional method: Starting from bottom course and raising tank putting upper courses on top of bottom one via crane.
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4.
Erection - Methods and their advantages/disadvantages
2. Jacking method: Starting from top courseand raising bottom courses on bottom of top one via hydrojacking.
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4. Erection - Methods and their advantages/disadvantages Avantages/ Disadvantages of Each Method Conventional method needs crane which costs huge amount if erection works delay ( generally occurs) Jacking method is safer especially against site accidents Conventional method provide less temporary welds and eventually less cleaning and NDT works. Jacking method provides generally working on ground level and high weld quality Conventional method is faster if no repair works retrospectively. Jacking method helps worker for inside work during raising of tank. Conventional method complicate final inspections Jacking method needs NDT results and repairs(if exist) before putting bottom one Conventional method complicate NDT works Jacking method needs more workmanship during raising operation. Convetional method is more dependent on climatic conditions
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4. Erection - Methods and their advantages/disadvantages 4.3 Important points during erection Welding procedures and welder certificates shall be prepared according to ASME Sec.9. For welds on non-pressure parts AWS D1.1 can be used. The welding of the shell to the bottom shall be practically completed before the welding of bottom joints that may have been left open to compensate for shrinkage of any welds previously made is completed. Upper plate at a horizontal butt joint shall have a 4:1 taper when its thickness is more than 3 mm greater than the lower plate
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4. Erection - Methods and their advantages/disadvantages 4.3
Important points during erection
Misalignment during shell welding, vertical or horizontal, shall be controlled. Limits around 1,5 and 3 mm, respectively. After first pass of shell-to-bottom weld from outside weld should be checked against leakage with suitable manner. Shell butt welds shall be examined with either RT or UT. Plumbness and roundness control shall be taken into account before hydrotest. Foundation requirements shall be provided to achieve other dimensional tolerances
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5. Testing – NDT, Pneumatic and Hydrostatic Testing. 5.1 Pneumatic and Hydrostatic Testing Welds of bottom shall be checked with vacuum box with air pressure between 20kPa and 70 kPa. Before hydrotest pads shall be tested pneumatically with air up to 100 kPa During hydrotest water level shall be increased at least 50 mm above the weld of roof-to-shell(top angle) connection For carbon steel equipment where water contact exceeds 14 days, including filling and draining owner may consider adding an oxygen scavenger and a biocide, and raise the pH by the addition of caustic. Aim of hydrotest is checking welds against leaking as well settlement control. Roof plates shall be tested by vacuum box or bubble test. If bubble test is used, before draining water after hydrotest volume above water level shall pressurized with air not exceeding weight of roof plates. API 650 & API 653 Training
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5. Testing – NDT, Pneumatic and Hydrostatic Testing. 5.2 NDT(Non-Destructive Test)
Number of RT films should be determined based on the figure below
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5. Testing – NDT, Pneumatic and Hydrostatic Testing.
5.2 NDT(Non-Destructive Test) We offer shortly; one spot from each first vertical, each “T” joint, one spot per 20 m vertical weld and one spot per 40 m horizontal weld . Annular plate butt weld shall be examined via RT RT shall be applied according to ASME Sec. 5 and evaluated according to ASME Sec 8. Penalty films shall be taken into account carefully. Magnetic and Liquid penetrant testing shall be applied according to ASME Sec 5. and evaluated according to ASME Sec.8. Moreover operator shall have eye – vision capability test report not older than 1-year. Reinforcement of welds and undercut shall be xamined carefully according to limits given in Sec. 8 of API 650. Arc strikes are generally evaluated as harmless or ignored. However they can initiate big failure. Those shall be cleaned carefully.
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4. CAUSES OF CATASTROPHIC TANK FAILURE
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4. TANK FAILURES COMMON CAUSES Tank shell plates not thick enough for specific gravity of product stored Over-filling of tank – over pressure Cut-down and re-erected tanks using improper cutting and re-welding procedures Improper weld procedures • Lack of full weld penetrations (lack of weld fusion) • Improper weld seam spacing • Lack of radiograph (x-ray) of newly constructed tanks or on repaired tanks
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4. TANK FAILURES COMMON CAUSES
Lack of weld fusion - #1 common cause Welders not certified and tested to weld procedure Weld deterioration/corrosion – especially in lower horizontal and vertical weld seams Brittle fracture of steel Lack of proper certified inspections Tank erectors who know little about API Specifications and procedures – these companies attract customers with a “cheap” price. Make certain your contractor can verify that your tank meets API Specifications.
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4. TANK FAILURES INCORRECT CONSTRUCTION
Lack of weld fusion - #1 common cause Welders not certified and tested to weld procedure Weld deterioration/corrosion – especially in lower horizontal and vertical weld seams Brittle fracture of steel Lack of proper certified inspections Tank erectors who know little about API Specifications and procedures – these companies attract customers with a “cheap” price. Make certain your contractor can verify that your tank meets API Specifications.
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Stairs welded directly to tank shell
Improper patch plates on Bottom plates
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Incorrect reinforced plate application
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Deep corrosion
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Weld deterioration
Plate corrosion
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Interior piping corrosion
Shell corrosion
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Weld deterioration and four-way junction – NON API
Four-corner insert with reinforcing backup – NON API
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Evidence of interior shell corrosion
Foundation evaluation
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Floor plate corrosion
Lap-welded seam leak
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Floor coupon with bottom side corrosion
Floor top side corrosion
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Severe roof corrosion
Star light . . . Star bright!
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5. API 653 :TANK INSPECTION AND MAINTENANCE
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5. API 653 :TANK INSPECTION AND MAINTENANCE This standard covers Above ground steel storage tanks built to API Standard 650 and its predecessorAPI 12C.
It provides minimum requirements for maintaining the integrity of such tanks after they have been placed in-service and addresses inspection, repair, alteration, relocation, and reconstruction.
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5. API 653 Tank Inspections Why Inspect Your Tanks? Prevent leaks into your secondary containment or to groundwater (if you do not have a secondary containment system) Establish a baseline of tank condition and corrosion rates Identify problems to perform repairs before you have a significant leak or release - Maintain your capital asset Minimize chance of catastrophic tank failure
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PROPER INSPECTION PROTOCOL INSPECTOR CREDENTIALS Certified API 653 Inspector
Four years minimum experience with storage tanks
Must pass test conducted by American Petroleum Institute (API)
o
Inspectors receive an individual inspector number
o
Inspector testing required every three years
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PROPER INSPECTION PROTOCOL INSPECTOR CREDENTIALS Visual inspection of welds, plates, and appurtenances UT (Ultra-sonic Thickness) testing of shell courses, floor, and roof Vacuum testing of all floor weld seams – unless epoxy coated Identify bottom side corrosion on floors Settlement Survey
Checking for planar tilt
Check for floor bulges or depressions
Provide calculations for safe or maximum fill height
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TANK MAINTENANCE COMMON SENSE APPROACH Conduct monthly/weekly walk-around of your tank(s)
Look for stains on steel where leak may be occurring Check valve function and nozzle welds Check associated piping Check foundation for wash-out/deterioration
Keep good records of product in and out API 650 & API 653 Training
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TANK MAINTENANCE COMMON SENSE APPROACH Open up your tank a minimum of every two years and conduct your own visual inspection inside
Check for weld deterioration and corrosion
If tank is coated, visually check coating for blisters or cracks
Keep a record of inspections and results If tank has an internal containment liner, check leak monitor weekly Conduct an API-653 inspection of your tank every five years as API 650 & API 653 Training recommended by TFI (The Fertilizer Institute)
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THANK YOU FOR YOUR ATTENTION Contrôle & Inspection Moderne Tel (LG) : (216) 71 770 137
Tel (LD) : (216) 71 770 142 Fax: (216) 71 770 136 E-mail:
[email protected] Address: 10, Street n:° 8161 Olympic city 1003 Tunis
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