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Tanks for use in the chemical storage, processing and water and waste treatment industries must not be taken as a commodity, according to Forbes Group.

They must be safe, as the liability and risk to personnel and the environment is extremely high.

To the engineer without experience in the selection of materials and design, tanks seem to be simple structures, almost purchased from a product catalogue.

This approach can lead to failures within the lifetime of the tank.

It is essential that the design, materials of construction and type of manufacture are carefully assessed to meet the application and design life required, at the same time producing the most economic solution.

An inadequate tank is regularly selected by the purchaser purely on economic grounds to meet a demanding budget.

Virtually all tanks are regarded as long-term capital investments.

In particular, the water industries often demand at least a 20-year design life.

The selection of the design standards and materials of construction must follow suit.

No matter what the application is, it is essential that the purchaser fully engages with the manufacturer to ensure that a safe, correctly designed tank is selected.

Part of this initial selection process involves the critical factor relating to the materials of construction.

According to Forbes, customers should not be misled by manufacturers that can only offer very limited choice of materials.

Customers must consider the potential long-term effect of the contents on the materials of construction.

This is particularly important when handling liquids in a thermoplastic tank where the thermoplastic is not only the chemical resistor but is also the load bearing structure.

All plastics suffer from a small degree of permeation and many engineers discount this long-term effect on the polymeric structure, which can lead to environmental stress cracking or softened material.

Customers should insist on a certified design standard with full design calculation, thus ensuring that the tank will meet requirements.

Materials can have a relatively short life because of degradation when the tank shell is under load.

Long-term exposure to a number of chemicals when under stress can induce environmental stress cracking.

Material selection is always a difficult area for the purchaser to consider, with various claims being made in relation to the expected service life by different manufacturers.

The safest approach is to fully engage with an experienced manufacturer who can demonstrate successful long-term applications.

The next safeguard is the requirement for detailed design.

The product must be fully compliant to a recognised British or European Union (EU) standard without exceptions.

The suggested selection process includes material selection.

Thermoplastic can be selected as the chemical resistor and load bearing structure.

Alternatively, glass-fibre-reinforced vinylester and polyester (GRP or FRP) laminate with a composite inner corrosion barrier can be chosen.

The thermoplastic inner shell corrosion barrier can be the chemical resistor, with the external GRP laminate being the load bearing structure – this being fully protected from the effects of the contents.

Examples of such tanks, in continual service for more than 35 years, can be demonstrated by Forbes.

All materials have a degree of degradation over time, particularly in chemical environments.

There are reduction factors published in EN 1771 for a number of chemicals in contact with thermoplastics, where permeation does have an effect on the physical properties of the material.

For GRP tanks and pressure vessels, long-term experience to be demonstrated by the manufacturer or guidance notes can be applied in BS 4994-1987, which will give the appropriate safety factor.

There are a various typical examples of points to raise with the manufacturer.

First, the long-term chemical resistance must be confirmed, with examples.

While the corrosion resistance charts indicate satisfactory resistance, if the thermoplastic is also acting as the load bearing structure, customers must ensure that the design gives a low level of strain as environmental stress cracking can occur long term.

Many chemical resistance charts are based on short-term testing only.

Second, customers should ensure that the grade of thermoplastic is correct for the application.

Third, customers must confirm that there is no risk of temperature excursions beyond the design temperature.

The mechanical strength of thermoplastics rapidly reduces with relatively small increases in the temperature of the contents.

Customers also need to ensure that the chemical resistance for GRP is compatible for the resin selected and that a low level of strain is maintained, thus avoiding the risk of strain corrosion.

To achieve a long-term safe working product, it is imperative that the tank is designed to a recognised standard.

This will reduce the purchaser’s health-and-safety liability in the event of anything going wrong during the working life of the tank.

It is important that all aspects of the chosen standard are met.

Prior to specifying and manufacturing items to the relevant standard, the following information must be agreed and documented between the purchaser and manufacturer and confirmed by the manufacturer prior to design and manufacture: details of the liquids to be handled including any trace contaminants; the confirmation of chemical resistance of the thermoplastic and the agreed reduction factor for the long-term effect of the contents on the material of construction as detailed in EN 1771; the maximum operating temperature; the design temperature; and the safety factor.

For thermoplastics, customers should use BS EN 12573 or, for guidance, see EN 1771.

In terms of the type of loading and application, typical safety factors adopted are as follows: static load at room temperature and constant conditions equals no possible danger to persons, objects and the environment in the event of failure (safety factor 1.3); and loading under alternating conditions (such as temperature or filling levels) equals possible danger to persons, objects and environment in the event of a failure (safety factor 2.0).

The factors of safety quoted are for two loading cases depending on the potential hazard posed by the tank.

In each individual case, the designer or engineer decides which classification is appropriate for the tank being designed and what level of safety factor is to be used.

For GRP and GRP with inner corrosion barrier of thermoplastic, the safety factor is calculated within BS 4994-1987 and EN 13121-2/3.

The operating requirements specified by the tank owner and the calculations by the supplier to determine the design life and critical features of the tank should form the basis of the documentation retained for the life of the tank.

For thermoplastics for demanding duties, customers should use the DVS 2205 German standard.

EN12573, a less demanding standard that many engineers consider is lacking in a number of areas, will provide basic design parameters but will not address many areas covered in DVS2205.

For GRP-/FRP- and thermoplastic-/GRP-reinforced products, users should apply the BS 4994:1987.

The recent EN 13121-3 standard is primarily a pressure vessel code for GRP vessels with up to a 10bar pressure rating to meet the European Pressure Equipment Directive (PED).

As a result of the complicated nature of the EN 13121-3 code, it has been suggested that the standard be withdrawn and that the BS 4994-1987 be the mainstay for the design and manufacture of safe and reliable tanks and vessels that can exceed 200m3 in capacity.

The most important message is to fully engage with the manufacturer to build a working relationship.

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