Tanks

Design, Construction, Operation and Maintenance of Tanks

February 5, 2011 1

Shaji Hakeem

February 5, 2011 2

StorageTanks

February 5, 20113

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.

February 5, 2011 4

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.

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

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 5

Tanks - Design

February 5, 20116

Tanks Design - Floating Roof Tank

February 5, 20117

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.

February 5, 20118

Tanks – Floating Roof Tanks

February 5, 20119

Tanks - Floating Roof Tanks

February 5, 2011 10

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 maximum-

recorded rainfall per hour for given site without

allowing any accumulation of water

February 5, 201111

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.

February 5, 2011 12

Tanks – Floating Roof Seals

February 5, 2011 13

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 14

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.

February 5, 2011 15

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 16

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)

February 5, 2011 17

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 18

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, 201119

Tanks – CorrosionWhat 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”

ANODIC AREACATHODIC AREA CATHODIC AREA

METAL SURFACE

Fe ++Fe (OH) 3

Fe (OH) 2

(CATHODE)

½ O2 + H2O + 2e = 2OH

(ANODE)

Fe = Fe + 2e++

(IN SOLUTION)

Fe (OH)2 + O2 = Fe (OH)3 + ½ O2

Fe ++

CORROSION

CURRENT

Fe + 2OH = Fe(OH)2

++

Fe = Fe2+

+ 2e-

E0

= +0.44 V

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.

February 5, 2011 20

Tanks – Corrosion

February 5, 201121

Tanks – Repairs

Code – API 653 Tank Inspection Repair, Alteration and Reconstruction

February 5, 2011 22

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.

API 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

February 5, 201123

Tanks - Repair

API 653

February 5, 2011 24

Thank you