Tank Bottom Maintenance

TANK BOTTOM MAINTENANCE

Klaas van der Mije Denmark December 2012

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AGE AND CONDITION OF TANKS

• 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

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CORROSION ALLOWANCE

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 known to 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 and even, but in practice metal loss is often concentrated in small areas (pitting).

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CORROSION

Heavy fuel oils or crude oil were once thought to be noncorrosive. 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 5

CORROSION

Floating roof

Tank shell

Crude oil

Water

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CORROSION

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.

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CORROSION

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SURFACE CONDITION

• A corroded tank bottom has a surface which is far from ideal for coating. Vertical Tank Shell • Cleaning + dry abrasive grit blasting leaves a surface which is a mixture of swallow, pitting and omega (Ω) pitting alternating with areas where the surface is more even. Flat Bottom

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CORROSION AND PITTING OF BOTTOM PLATE

Corroded cleaned from contamination

After blasting to SA 2½

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Corrosion penetration

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Pitting Types

Omega

Open

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TRADITIONAL COATING SYSTEMS

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.

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TRADITIONAL COATING SYSTEMS

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

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TANK BOTTOM REPAIR SYSTEMS

Application of Solvent Based Coating to Pitted Steel Low DFT weak spots

Excessive film thickness

Void

(leading to solvent retention, cracking and breakdown)

Void Tanktop / bottom plating

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TANK BOTTOM REPAIR SYSTEMS

Filling of Omega Pit using Epoxy Pit Filler Weak spots

Solvent based epoxy coating

Void Pit filler

(suitable site for starting corrosion cell)

Tanktop / bottom plating

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NEWER TYPES OF COATING SYSTEM

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).

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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 world’s tank stored petroleum liquids

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SOLVENT RELEASE 65m diameter tank, coated 1.5m up walls

500 liters thinner

0 liters thinner

Medium Solids

Solvent Free CSF

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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 from sharp edges • Excellent adhesion • Does not shrink • Sufficient flexibility enabling it to move with steel

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NEWER TYPES OF COATING SYSTEM Sigmaguard CSF 610 Sigmaguard CSF 650 Novaguard 840 Novaguard 810 Novaguard 890 • The requirements for surface cleanliness, abrasive blasting ISO Sa 2½, must be at least as good as for solvent containing systems. •

System dry film range 1 x 300-500 µm or 2 x 300 µm

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SIGMAGUARD CSF APPLICATION

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SPRAYING CSF

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INSPECTION FREQUENCIES – STORAGE TANKS 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 proven internal coating

5 years

7 years

15 years

Group 3

Crude oil, intermediate products

5 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 6

Light products, kerosine, JET A 1 uncoated

5 years

10 years

20 years

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TANK BOTTOM CORROSION

Experience has shown: Internal linings prevent leakage through “small” underside (soil side) corrosion holes in bottom plates

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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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INTERNAL REINFORCED LININGS

Sigmaguard CSF 650 or Novaguard 840 or 890 • Cold Solvent Free Epoxy very versatile with respect to reinforcement a) glass-flake b) chopped fibres c) mat - non - woven d) mat - woven

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HOLE BRIDGING CAPACITY OF EPOXY LINING

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.

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Hole Bridging Capacity - 18m high tank

Hole bridging radius Note: a 50% safeguard has been included

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CONCLUSION

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.

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EPILOGUE

Glass-fibre reinforced systems are used to prevent tank leakage caused by perforations from underneath and for the protection of the inside from corrosion.

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TANK CLEANING BEFORE STEEL CONDITION INSPECTION

• • • • •

Sludge removal Degreasing Inspection blasting-areas to be coated Sweepblasting of areas not scheduled for coating Grit removal

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SURFACE PREPARATION FOR COATING

• • • •

Blasting abrasive Blasting equipment (dry) Dehumidification/ventilation Removal/cleaning of blasting abrasive • Inspection of blasting

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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

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

Omega

Open

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CORROSION PITS FILLED WITH CSF

Omega

Open

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GLASS FIBRE FOR LAMINATE

• Glass fibre type: E glass (finish - suitable for epoxy resin) • PPG data sheet • Cutting size of chopper gun: 1 inch

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GLASS FIBRE FOR LAMINATE

Coating

Air

Glass fibre Chopper gun

Coating nozzle

Coating nozzle

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GLASS FIBRE CHOPPER GUN

Omega

Open

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CLOSE-UP CSF & FIBRE

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APPLICATION CHOPPED GLASS FIBRE

Omega

Open

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TIME FOR MAINTENANCE, 2750M

Hole Bridging Capacity - 18m high tank Preparation Blast & prime First coat Final coat Pit filling Caulking Hand lay-up or chopped fibre Steel Renewal

Note: Holding primer can be omitted if tank is small or RH kept below 40% during blasting and first coat

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COST COMPARISON TANK BOTTOMS 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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SUMMARY

• • • • •

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

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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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60


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EXPECTED LIFE TIME TANK-LININGS

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.

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Visual inspection

Omega

Open Blistering

Cracking 64


Physical inspection

Omega

Open Adhesion 65


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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Corrosion Protection of Organic Coatings Increasing Corrosion Protection

4

Omega Excellent

Good

Poor

6

8

10

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

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EIS

Omega

Laboratory

Open

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 • 17 year old Cold Solvent Free epoxy • 17 year old Cold Solvent Free epoxy recoated with new layer Cold Solvent Free epoxy

= 9,6 Z = 8,9 Z

= 9.5 Z

• Adhesion value on top roughened old > 20 Mpa Cold Solvent Free epoxy

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QUESTIONS

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Tank Bottom Maintenance

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