WARNING-PRICES SHOWN ARE 2006 CURRENTLY BEING UPDATED

List No.130
TANK COATINGS AND LININGS

INTRODUCTION
Carbon steel tanks and vessels can be surface coated to protect against atmospheric corrosion, or lined to prevent attack from tank contents. A wide variety of paint systems are available for external applications but in recent years the paint industry has developed many finishes which can withstand prolonged immersion in oils, chemical products and solvents. Alternatively a variety of rubber, synthetic rubber, composite and plastic linings are available from specialist applicators.

This summary selects some of the proven protection systems, indicating the applications, approximate costs and possible manufacturers/suppliers

Paint manufacturers' or specialist applicators' advice should ALWAYS BE SOUGHT before selecting a lining for any particular duty.

STANDARDS APPLICABLE
The current British standard code of practice for protective coating of iron and steel structures is BS 5493 : 1977 although this document does NOT include specific recommendations for INTERNAL tank linings. BS 5493 partially superseded by BSEN ISO 12944

BS 6374 : 1984 covers the lining of equipment with polymeric materials including thermoplastic, thermosetting resins and rubbers

SURFACE PREPARATION
The less efficient the method of preparation in removing millscale, rust, any surface contaminants, then the lower is the comparative performance achieved by the paint system and the more frequently will the surface have to be maintained, particularly under immersion conditions. A typical common standard for steelwork to be prepared by abrasive blast cleaning is to BSEN ISO 8504-1 and materials applied within the appropriate time. Grade SA2.5 shall be used for visual checks of blast standard. The European Standard for grit blast cleaning is ISO 8501-01, the common grade of surface preparation being ISO Sa2.5 (near white finish) with ISO Sa3 being specified for very aggressive materials.

TYPICAL COATINGS/LININGS

Epoxies
These paints are resistant to alkalis, oils and solvents but should not be used unless the highest quality of surface preparation and application can be assured. Cured films are hard and solvent resistant. Coal Tar Epoxies are generally cheaper, and may be easier to apply, but are restricted to darker colours and have lower solvent resistance.

There are a number of systems available under the general heading of 'epoxy'. Most systems consist of two or more components which are mixed immediately before application and the cure proceeds at ambient temperature. The base resin may contain solvents, however with the new environmental requirements to reduce solvent emission most paint manufacturers are increasing the solids content to increase the number of solventless and solvent-free epoxies available.

Epoxy resins have good resistance to many organic chemicals but not phenols and methanol and are unsuitable for solvents such as ketones and esters. All epoxies have good resistance to water and alkalis and oxidising acids up to about 10% strength.

Typical epoxy formulations include:-

AMINE CURED SOLVENT SYSTEM - applied as a multi-coat system, based on liquid resin, the hardener being amine based.

PHENOLIC SOLVENT SYSTEM - better chemical resistance than above and also has good resistance to hot water.

HIGH SOLID EPOXY SYSTEM - very low solvent content based on liquid epoxy. Hardener is either an amine, polyamine or a polyamide

SOLVENT FREE EPOXY SYSTEM - developed for food and drink products to give a non-toxic and taint free lining, based on liquid epoxy with no solvent present. These coatings can be reinforced with glass flake or glass fibre.

COAL TAR EPOXY SYSTEM - a further 2-pack system, usually contains some solvent. Applied in 2 or 3 coats up to 400 microns dry film thickness. Suitable for water up to 50°C and heavy fuel oils.

Drying Oil Type Paints and Alkyds
These cover a wide range of products, which, in the main are relatively slow drying but more tolerant of less than perfect surface preparation. Phenolic variations will dry at lower temperature. Recoating usually presents no problems, chemical resistance is poor but weather resistance is good.

Bituminous Coatings
Generally these are low-cost coatings whose protective properties depend on film thickness. Bituminous coatings have good resistance in water immersion conditions, but are not resistant to hydrocarbons and solvents. They may become brittle in cold weather and soften in hot weather.

SURFACE PREPARATION
The key to effective lining/coating is the preparation of the steel surfaces. Hand and/or mechanical surface preparation is not sufficient to remove millscale or corrosion. Heavy duty coatings should be applied over a blastcleaned substrate. In the case of large surfaces a holding primer may be applied to preserve the surface in its blast cleaned state preventing re-rusting until it is possible to apply the main paint specification. Some tank coatings do not permit the use of pre-fab primers, this should always be confirmed with the paint manufacturer.

PREPARATION OF WELDS
The life time till first maintenance of any protective system is also determined by the dry thickness of the coating system, particularly on weld seams, sharp edges, nuts and bolts. All these critical areas should be given extra stripe coats to ensure the specified dry film thickness is achieved. By giving more attention to these difficult areas, the life time of the system will be greatly extended. Hand laid welds which have a bead with a surface irregularity exceeding 3mm or with a sharp crest having a radius of less than 2mm should be ground mechanically. Sharp edges should be rounded off with a grinder of 2mm or more. Pittings in excess of 2mm in depth under 5mm in diameter should be filled by welding or using a suitable filer.
INTERNAL TANK LININGS (SHOP FABRICATED TANKS)

EXTERNAL TANK COATINGS

The above tables refer generally to paints and coatings by the generic descriptions. It is advisable to consult manufacturers for advice on particular applications, but for information only a selection of manufacturers and products or series names are listed below.

Rubber/Ebonite Linings
Applied by specialist applicators, e.g. Dexine Ltd. and BTR Vitaline Ltd. at their works. The size of work is limited by the size of the autoclave at these companies, i.e. 4.4M dia x 9.1M length and 3.6M dia 6.8M length respectively.

Rubber is adhesive bonded and the whole is vulcanised and spark tested. The development of cold vulcanising materials is important especially for reducing costs and problems with site application.

Butyl Rubber 'Bag' Liners
Available in 0.75, 1.0 and 1.5mm thickness, material is elastomeric synthetic rubber polymer and is an ideal waterproof membrane for site bolted tank applications, having been developed early in 1940 in the USA. Lining costs are highly competitive.

Stoved Linings
Developed mainly for the brewery and drinks industries because of their solvent free properties. Typical examples are Prodorglas 'S' lining, Prodorfilm 'E' lining and stove phenolics such as Lithglow 'Saekaphen' and 'Calvinac' stoved and cold cured systems which can be shop or site applied by expert applicators.

Flake Glass Lining
An important class of heavy duty linings in which very small glass plate or needle shaped particles are dispersed in an epoxy, vinyl ester, polyester or furane medium, giving a wide range of chemical resistance to acids, alkalis or solvents. Well proven systems available from main suppliers include ARCHCO, RIGIDON, CORROCOAT, GLASS SHIELD and CEILCOTE, for which specific high performance quotations will be submitted on request.

Zinc Silicate Lining
In use for many years for lining storage tanks, probably the cheapest lining available to preserve purity of many organic chemicals. Not suitable for acidic conditions.

Polyurethane Lining
Suitable for abrasive chemicals with moderate chemical resistance applied with trowel or spray gun.

Galvanising
Hot dip galvanising is widely used for domestic and small industrial sized water storage tanks. BS 417:1987 refers to domestic applications while BS729:1986 covers more general galvanising standards. The main limitations are size and the need to avoid heat distortion, which is inevitable when immersing certain types of tankage (especially rectangular) into hot molten zinc.

Cost is generally based on tank weight but to obtain approximate costs for comparison with alternative lining, the following prices are indicative:- 3mm plate plate £86/M2, 4mm £65/M2, 5mm £52/M2, 6mm £44/M2.

COMPARISON OF COSTS OF CONSTRUCTION/LINING PER UNIT CAPACITY
Costs below are based on vertical cylindrical, flat based vessel 10,000 gall capacity with standard fittings.