Tank Bottom Maintenance

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TANK BOTTOM MAINTENANCE

Klaas van der Mije

Denmark December 2012


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The existing stock of tanks in many areas is now ageing.

In Europe for example, most tanks are over 20 years old.

These vary from lightly to severely corroded. Tanks are due for major maintenance

Systems should give additional life-time

AGE AND CONDITION OF TANKS

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In the past the externals of tanks were almost always painted.

Interior linings were not so often applied. (Only when the

contents had to be kept free of contamination or were knownto be corrosive).

Storage tanks are built to standard such as API 650 or BS

2645 . These standards provide a corrosion allowance (extra

steel thickness). This only works when corrosion is slow andeven, but in practice metal loss is often concentrated in small

areas (pitting).

CORROSION ALLOWANCE

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Heavy fuel oils or crude oil were once thought to be non-

corrosive.

This turned out not to be the case, particularly when the

products were contaminated with water and acids.

Presently there are more than 440 types of crude oil.

Experience has shown that the bottoms and the first one to

two meters of the vertical walls are the most heavily

attacked when the cargo is (sour) crude oil, heavy fuels

contaminated with water and other similar oil related

products

CORROSION

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Floating roof

Crude oil

Water

Tank shell

CORROSION


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The main types of corrosion seen in originally uncoated

tanks are:

Concentration cell corrosion

Galvanic cell corrosion

Sulphate reducing bacteria (SRB)

Product corrosion

Generally more than one form of corrosion can be

identified in any one tank.

CORROSION

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CORROSION

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A corroded tank bottom has a surface

which is far from ideal for coating.

Cleaning + dry abrasive grit blastingleaves a surface which is a mixture of

swallow, pitting and omega () pitting

alternating with areas where the

surface is more even.

SURFACE CONDITION

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Flat Bottom

Vertical Tank Shell


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CORROSION AND PITTING OF BOTTOM PLATE

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Corroded cleaned from contamination After blasting to SA 2


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Corrosion penetration


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Omega

Open

Pitting Types


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Historical and for economic reasons, multi-coat tar

epoxy coatings were used for tank bottom maintenance

for crude petroleum products. These performed quite

well, but there were some problems.

TRADITIONAL COATING SYSTEMS

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With solvent containing coatings such as epoxy and tar epoxy

Very thick areas:==>solvent retention

==>shrinkage

==>build-up of residual stress levels

==>lack of adhesion in pits and cracking

Areas of retained solvent:

==> osmotic blistering if water in tank

TRADITIONAL COATING SYSTEMS

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Application of Solvent Based Coating to Pitted Steel

TANK BOTTOM REPAIR SYSTEMS

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Low DFT weak spots

Void

Tanktop / bottom plating

Void

Excessive

film thickness(leading to solvent

retention, cracking and

breakdown)


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Filling of Omega Pit using Epoxy Pit Filler

TANK BOTTOM REPAIR SYSTEMS

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Weak spots

Tanktop / bottom plating

Pit filler

Void(suitable site for starting

corrosion cell)

Solvent based epoxy coating


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Solvent free coating systems which can be applied by

cold airless spray provide much better performance.

The ideal products are so formulated that they are free

of solvent and do not shrink. Because of this they can

be applied in high dry film thicknesses, from 300

microns (12 mils) to several millimeters (on horizontal

areas).

NEWER TYPES OF COATING SYSTEM

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FREE FILM FLEXIBILITY

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LIFTING OF TANK

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SigmaGuard CSF 650

NovaGuard 840

NovaGuard 890

Solvent free coating making entire coating operation safer

Easily applied by cold airless spray

Light colour & glossy for good visibility

Easy over-coating enables stripe coating of critical areas to be used

for extra security

has a wider spectrum of use than coal tar epoxies e.g. it is resistant

to the bulk of the worlds tank stored petroleum liquids


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SOLVENT RELEASE

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65m diameter tank, coated 1.5m up walls

Medium Solids

0 liters

thinner

Solvent Free CSF

500 liters

thinner


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TANK BOTTOM LIGHT REFLECTION

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Advantages of Cold Sprayable Solvent Free Epoxies

Sprayable pit filler

Flows to give optimal edge coverage, no shrinking away fromsharp edges

Excellent adhesion

Does not shrink

Sufficient flexibility enabling it to move with steel


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Sigmaguard CSF 610

Sigmaguard CSF 650

Novaguard 840

Novaguard 810

Novaguard 890

The requirements for surface cleanliness, abrasive blasting

ISO Sa 2, must be at least as good as for solvent

containing systems.

System dry film range

1 x 300-500 m

or

2 x 300 m


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SIGMAGUARD CSF APPLICATION

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SPRAYING CSF

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INSPECTION FREQUENCIES STORAGE TANKS

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Classification Service Conditions External Internal

Ultrasonic Thickness Sample or Single Tank Max. for Group

Group 1

Slops, corrosive or

aggressive chemicals, raw

water, brine. No coating

1 year 3 years 10 years

Group 1A With proveninternal coating 5 years 7 years 15 years

Group 3Crude oil, intermediate

products5 years 8 years 18 years

Group 4 Fuel oil, gas oil& lub oil 8 years 16 years 20 years

Group 5 JET A 1 coated 10 years 15 years 30 years

Group 6Light products, kerosine,

JET A 1 uncoated5 years 10 years 20 years


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Experience has shown:

Internal linings prevent leakage through small underside

(soil side) corrosion holes in bottom plates

TANK BOTTOM CORROSION

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The problem of leakage through underside corrosion holes can

be a nightmare for tankfarm management.

Ecological problems Economical problems

Fines can be given


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Important factors which have an impact on underside

corrosion:

Type of raised foundation Seal between tank and foundation

Drainage of the foundation

Availability of soluble salts

Climate

Temperature


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TANK FARM SINGAPORE

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LIFTING OF TANK

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LIFTING OF TANK CLOSE-UP

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Sigmaguard CSF 650 or

Novaguard 840 or 890

Cold Solvent Free Epoxy very versatile with respect toreinforcement

a) glass-flake

b) chopped fibres

c) mat - non - woven

d) mat - woven

INTERNAL REINFORCED LININGS

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Based on:

Ultimate tensile strength

E-modulus

Thickness of the coating Radius of the hole

Deflection

Maximum load/pressure

Tank load density

Gravitational constant of load

The hole bridging capacity is determined based on experimental

data and calculation in analogy with polyester technology.

HOLE BRIDGING CAPACITY OF EPOXY LINING

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Hole Bridging Capacity - 18m high tank


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Hole bridging radius

Note: a 50% safeguard has

been included


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Glass-fiber reinforced systems have structural properties with the

loading configuration found over holes in the bottom plates of land

storage tanks.

The structural properties depend strongly on the type of

reinforcement.

Chopped fibre systems offer a practical and economical solution.

CONCLUSION

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Glass-fibre reinforced systems are used to prevent tank leakage

caused by perforations from underneath and for the protection of

the inside from corrosion.

EPILOGUE

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Sludge removal

Degreasing

Inspection blasting-areas to be coated

Sweepblasting of areas not scheduled for coating Grit removal

TANK CLEANING BEFORE STEEL CONDITION INSPECTION

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Blasting abrasive

Blasting equipment (dry)

Dehumidification/ventilation

Removal/cleaning of blastingabrasive

Inspection of blasting

SURFACE PREPARATION FOR COATING

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EFFECT OF HUMIDITY ON CORROSION RATE

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VACUUM GRIT REMOVAL

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FINAL VACUUM CLEANING

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PIT-FILLING

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Omega

Open

Pit-filling of corroded tank-bottom for glass fibre reinforced system


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CORROSION PITS FILLED WITH CSF

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Omega

Open


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Glass fibre type: E glass

(finish - suitable for epoxy resin)

PPG data sheet

Cutting size of chopper gun: 1 inch

GLASS FIBRE FOR LAMINATE

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GLASS FIBRE FOR LAMINATE

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Coating

Glass fibre

Air

Coating

nozzle

Chopper gun

Coating

nozzle


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GLASS FIBRE CHOPPER GUN

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Omega

Open


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CLOSE-UP CSF & FIBRE

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APPLICATION CHOPPED GLASS FIBRE

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Omega

Open


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Hole Bridging Capacity - 18m high tank

TIME FOR MAINTENANCE, 2750M

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Note: Holding primer can be

omitted if tank is small or RH

kept below 40% during blasting

and first coat

Preparation

Blast & prime

First coat

Final coat

Pit filling

Caulking

Hand lay-up or chopped fibre

Steel Renewal


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COST COMPARISON TANK BOTTOMS

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Price Sq.m Index Time in Days Remarks

New steel bottom 100 60

Additional:

Hydrostatic testing

Reinstalation of out fittings

Chopped glass fibre system 40 30 Extended service life up to 25 years

Single coat 25 21 Extended service life up to 20 years


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Time saving due to less coats

Safety-net when reinforced system is applied

Extended inspection interval

Easy to clean for inspections Extended service life of the tank

SUMMARY

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FINISHED TANK

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TANK OPENED AFTER 5 YEAR SERVICE WITH BLACK

PRODUCTS AND IN EXCELLENT CONDITION

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SIGMAGUARD CSF 850 AFTER 17 YRS

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Visually

Blistering, break down due to end of service life

Cracking, not sufficient flexibility for following the movement of the substrate

Corrosion breakthrough

Physically

Adhesion when down to 70 % of the original value, coating system in good

condition, when lower system degradation on the way. Interface at steel

coating should be clean no corrosion formation

EIS

Electrochemical Impedance Spectroscopy non-destructive resistance

measurement of the coating indicates performance of the tank-lining system.

EXPECTED LIFE TIME TANK-LININGS

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Omega

Open

Visual inspection

Blistering Cracking


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Omega

Open

Physical inspection

Adhesion


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EIS

Method is under development since 1980

ISO 16773 1,2,3,4 is laying down the standard for measurement Originally developed for life time establishment of external pipe coatings

which require long life time 50 years

Present data bank is mainly for atmospheric coatings

Immersion linings, ballast tanks, drinking water pipes building up of data

over the last years


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Omega

Open


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OmegaPoor

Corrosion Protection of Organic Coatings

4 6

Coating Impedance, Log Z (Z in ohms cm2 @ 0.1 Hz)

8 10

Good Excellent

Increasing Corrosion Protection


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Omega

Open

EIS

Laboratory Field


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EIS

Experiment upgrading old tank-lining which is in good

condition, after17 year in service, for extending service life.

Cleaned and sweep-blasted the panel

Applied a new Coat of Cold Solvent Free epoxy

Eis measurement up to level of new system.


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Values for tank-linings

New Cold Solvent Free epoxy = 9,6 Z

17 year old Cold Solvent Free epoxy = 8,9 Z 17 year old Cold Solvent Free epoxy

recoated with new layer

Cold Solvent Free epoxy = 9.5 Z

Adhesion value on top roughened old > 20 MpaCold Solvent Free epoxy


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QUESTIONS

Tank Bottom Maintenance

Documents

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