Tanks

Design, Construction, Operation and Maintenance of Tanks

Shaji Hakeem February 5, 2011

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StorageTanks

February 5, 2011

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Tanks Applicable codes for Design

API: 650 Welded steel tanks for oil storage. API: 620 Design and construction pf large welded low pressure storage tanks. IS 803 IS: 875 (Part 3) Wind loads Code of particle for design loads (other than earthquake) for buildings and structures. IS: 1893 Criteria for Earthquake resistant design of structure. API: 2000 Venting atmospheric and low pressure storage tanks. IS: 2007 Method for calibration of Vertical Oil Storage tanks. IS: 2008 Method for Computation of capacity tables for Vertical Oil Storage Tanks. API: RP 2003 Protection against ignitions arising out of static, lightning and stray currents.

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February 5, 2011

Tanks - Design

Material of Construction Material to be used shall conform to : i) ASME boiler and Pressure vessel Code Section II. ii) Indian Standard Specification. 

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Gaskets for manholes and nozzles fitted with blind flanges shall conform to grafoil. Thickness of gasket shall be 1.5 mm for nozzle upto 450 mm NB and 3.0 mm for nozzle above 450 mm NB. Bolts and nuts for all nozzle fitted with blind flanges shall conform to ASTM A-307 Gr. B, A-193 Gr. B-7, and A-194 Gr. 2H. Bolts and nuts for all structurals shall conform to IS: 1363 of appropriate grade. Floating roof pipe supports and pipe sleeves shall be of carbon steel conforming to ASTM A-106, A-53, API.5L, IS: 1978 or equivalent

February 5, 2011

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Tanks - Design

Basic Design Loads: Dead Loads Varying Loads Wind Loads Seismic Loads Loads due to Connected Pipeline. -The Roof should be able to float for a liquid with minimum specific gravity of 0.7 -Should be designed to accept full range of roof movements during unattended operations from low operating level to maximum design liquid level

February 5, 2011

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Tanks Design - Floating Roof Tank

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February 5, 2011

Tanks Design –Floating Roof Legs

Long legs

Short legs

The lower position (minimum 1200 mm) shall permit the roof to go as low as possible without interface with any intermit accessories and roof seal mechanism. The upper position (for cleaning) shall provide a clearance of 1800 mm between the pontoon / double deck edges and bottom. 7

February 5, 2011

Tanks – Floating Roof Tanks

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February 5, 2011

Tanks - Floating Roof Tanks

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February 5, 2011

Tanks - Articulated Roof Drain Metallic

articulated pipe drains with swivel joints of approved make and type shall be provided to drain water collecting on the roof. Single pipe drain system shall be provided with suitable counter balancing weights. All swivel joints shall be tested to a minimum pressure of 3.5 kg / cm2 before putting into roof drain assembly. Swing check non-return valve with CI body and brass / bronze valve and seat shall be provided for the primary drain for single deck / double deck floating roofs The primary roof drain shall be designed for complete water drainage equivalent to maximumrecorded rainfall per hour for given site without allowing any accumulation of water February 5, 2011

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Tanks Emergency Roof Drains

Emergency drain shall be provided to take care of primary drain failure. These drains shall be equipped with water seal sump inside the roof to prevent back flow of the stored product. It should start functioning after accumulation of 200mm of rainfall on deck.

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February 5, 2011

Tanks – Floating Roof Seals

February 5, 2011

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Tanks - Welding Procedure Code: Welding procedure qualification - ASME Boiler and Pressure vessel Code Section IX.  Low hydrogen electrode shall be used for all welds of shell courses having a thickness of 12 mm or more and for attachment of shell course to bottom or annular plates. High cellulose/ rutile type electrodes a per the AWS classification SFA 5.1 for all other welding. Welding sequence shall be adopted so as to give minimum shrinkage and distortion.  All vertical and horizontal shell joints shall be full penetration and full fusion welds using any one of the edge preparation permitted by the code. Single side butt welds are not permitted. Top curb angle shall be lap or butt welded to the top course with flange fumed outside. Curb angle and stiffening ring joints shall not coincide with the vertical shell plate welds.  Vertical joints should preferably be offset by at least 1000 mm.  First shell course plates shall be arranged so that the vertical joints clear he annular ring weld or sketch plate weld by at least 300 mm. vertical joints of the shell courses shall also clear the nozzle attachment welds or reinforcement pad welds as per code and applicable appendices. February 5, 2011

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Tanks - Construction

Fixed Roof a) The roof plate shall be supported by structure as indicated in the drawings / standard. b) All supporting structure shall be designed to carry the minimum loads as specified in codes. c) Laps in roof plate shall be 30 mm minimum or 5 t: where t is the nominal thickness of roof with minimum 30 mm. laps shall be arranged with the lower edge of the upper plate underneath the upper edge of the lower plate. d Roof shall be joined to the shell by means of continuous fillet (5 mm max.) weld on the curb angle. Roof to shell be frangible type, otherwise suitable emergency venting shall be provided as per code. e) Roof plate shall not be welded to the supporting structure. The same shall rest on the structure.

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Tanks - Construction a) Roof shall be provided with automatic bleeder vents designed to open before the roof reaches its lowest position and to class when the roof rises above this point. Also same operation should follow in the clean-out position. b) Rim vents of 150 NB about 30 meters peripheral spacing (minimum two nos.) shall be provided for venting the dead space between stored liquid and seal. They shall provide with pressure relief valve of approval make, set at 20 mm water column. Gauge Well / Anti-rotational Device Gauge hatch shall have a quick opening cover. Gauge hatch shall be spark proof type. This shall also act as anti rotational device. Earthing and Floating Roof a) Wire cables size shall be provided between ladder and track and also between ladder and shell. Two such independent connections should exist in parallel. b) Adequate number of earthing shunts shall be provided for the seal. February 5, 2011

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Tanks Construction Floating Roof a) Tanks having diameter greater than 12 m but less than or equal to 60 m, shall be of single deck annular pontoon type construction or without buoys. c) When floating roof is in the highest position, tank shall be capable of holding its design capacity. d) Floating roof shall be of low deck (minimum vapour space) type. a) Deck shall have level surface with a permanent slope towards the drain sump to avoid stagnation of rainwater. c) Pipes sleeves shall project sufficiently above the roof to take care of flooded condition of roof and extend sufficiently below the roof to prevent escape of vapour ( minimum 200 mm)

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Tanks – Inspection/Testing at Construction Stage Radiography test for weld Joints – As per Code. Liquid penetrant / magnetic Particle Examination / Ultrasonic Examination –as per code

Vacuum box testing - for detection of leaks in the tank bottom. Annular Plate weld Joint Test - Inner fillet weld of bottom to shell joints shall be inspected and tested prior to welding the outside fillet weld. Leak test shall be performed with penetrating oil after removal of slag. Oil shall be removed before, welding the outer fillet.

Hydrotest: The filling height, in case of fixed roof tanks, shall be upto the curb angle and in case of floating roof tanks shall be restricted to the maximum height so their so that weather shield does go beyond curb angle.

Fixed Roof Test: After filling the tank upto curb angle, all opening in the roof shall be closed and internal air pressure shall be applied equivalent to the weight of roof plates. All welded joints in roof shall be checked with soap studs for detection of leaks. February 5, 2011

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Tanks – Inspection/Testing at Construction Stage Floatation Test for Floating Roof Tanks : With primary drains closed, water equivalent to 100% of max rainfall over the tank area shall be poured on the deck and in the stabilised state submergence of outer rim shall not exceed 65% of its height at any point. Foam Seal shall be installed only after flotation test.

Pontoon Puncture test: For annular pontoon type floating roofs the condition of deck and two adjacent pontoon compartments punctured shall be simulated and the submergence of inner rim and outer rim at the punctured pontoon shall be compared with the calculated value.

Pontoon leak test: All compartments shall be tested for liquid tightness. Nozzle Reinforcing Plate Test. Nozzle reinforcing plates shall be pneumatically tested at 1.05 kg / cm2g with soap solution. This test shall be carried out before filing the tank for hydrostatic testing.

Roof Drain Test.Drain pipes in floating roof tanks shall be pressure tested with at 3.5 kg / cm2. During the flotation test, the roof drain valve shall kept open and observed for leakage of the tank contents into pipe drain. February 5, 2011

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Tanks – Corrosion What is Corrosion ? -

Any Metal that has been extracted from a naturally combined state, has a tendency to revert to that state under the action of oxygen and water. This action is called as Corrosion

The commonest example being the “ Rusting of steel”

CORROSION CURRENT

2+

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Fe = Fe + 2e E0 = +0.44 V

Fe ++

Fe ++

Fe (OH) 3 Fe (OH) 2

CATHODIC AREA

ANODIC AREA

CATHODIC AREA

METAL SURFACE

)

(CATHODE

½ O2 + H2O + 2e = 2OH

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(ANODE) ++ Fe = Fe + 2e

(IN SOLUTION) ++ Fe + 2OH = Fe(OH)2 Fe (OH)2 + O2 = Fe (OH)3 + ½ O2

February 5, 2011

Tanks – Corrosion Parts Prone to Corrosion • External parts of Annular Plates are prone to corrosion at the edges due to seepage/accumulation of water between the foundation and the plates • Bottom plates get corroded rapidly incase of sea water /sulphide content. • Underside of the roof and roof structural come under corrosive attack due to vapour corrosion • Atmospheric corrosion can occur on all external parts of the tank • Floating roof deck plates are prone to corrosion due to rain water accumulation on the deck. • Pontoon boxes on the floating roof are prone to corrosion at the fillet weld between the pontoon and deck plates • Rim plates at the outer periphery of the floating roof get corroded where liquid-vapour phase is maintained. • Roof legs get severely corroded at the liquid-vapour phase junction. • The roof leg sleeve gets corroded near fillet weld junction to the roof plates and at the bolt hole area. • Roof sleeve pad may get corroded at the underside if sealing run between roof plates and pad is not carried out. • Roof drain sump gets corroded due to water accumulation/ stagnation. • The water draw-off nozzles pipe get corroded due to water stagnation.

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Tanks – Repairs Code – API 653 Tank Inspection Repair, Alteration and Reconstruction

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February 5, 2011

Tanks – Repairs Code – API 653 Tank Inspection Repair, Alteration and Reconstruction Proper

Application of This standard ensures that tanks are adequately inspected, and necessary repairs are made in a manner that acheives quality and integrity API 653 thas guidelines for new materials, welding, design, and testing. Major

Repairs are subject to additional testing including hydrotest of complete tank before returning to service. Major

Repairs - Operations that require cutting, addition, removal and/or replacement of Annular plate ring, shell to bottom weld, or a sizable portion of the shell. Std. 653 Sec,10 – gives practices and procedures for exemption from full hydrotest after major Repairs. One method for exemption is Fitness for Service evaluation as per API RP 579 API

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Tanks - Repair API 653

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February 5, 2011

Thank you February 5, 2011

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