Make sure you choose the correct plastic tank for chemical storage

Paul Boughton

Virtually all tanks are regarded as a long-term capital investment. Industries often demand at least 20 years design life. Therefore selection of the design standards and materials of construction must follow suit. Lee Forbes reports

Tanks for use in the chemical storage and processing and the water and waste treatment industries must not be taken as a commodity. Tanks 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 bought from a product catalogue. This approach can easily lead to failures within the lifetime of the tank. It is essential 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. Often an inadequate tank is selected by the purchaser purely on economic grounds to meet a demanding budget.

No matter what the application is, it is essential the purchaser fully engages with the manufacturer to ensure a safe, correctly designed tank, is selected. Part of this initial selection process involves the critical factor relating to the materials of construction. Do not be misled by the manufacturer who can only offer very limited choice of materials. Look at the potential long-term effect of the contents on the materials of construction. This is particularly important when handling liquids in thermoplastic tank where the thermoplastic is not only the chemical resistor but 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 softening of the material. Always insist on certified design Standard with full design calculation thus ensuring the tank will meet requirements. Despite what some people think, materials can have a relatively short life due to 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 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 EU Standard without exceptions.

The suggested selection process includes:

Material selection: Thermoplastic as the chemical resistor and load bearing structure; Glassfibre reinforced vinylester and polyester (GRP or FRP) laminate with a composite inner corrosion barrier; Thermoplastic inner shell corrosion barrier being the chemical resistor with the external GRP laminate being load bearing structure, this being fully protected from the effects of the contents. Examples of such tanks, in continual service for over 35 years, can be demonstrated by Forbes.

The area, which is not always addressed by the manufacturer, is proven expected long-term life. 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 affect on the physical properties of the material. For GRP tanks and& pressure vessels, long term experience to be demonstrated by the manufacturer or apply the guidance notes in BS 4994-1987 which will give the appropriate safety factor.

Typical examples of points to raise with the manufacturer include: the long term chemical resistance is confirmed, with examples. While the corrosion resistance charts indicate satisfactory resistance, if the thermoplastic is also acting as the load bearing structure, ensure 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; ensure the grade of thermoplastic is correct for the application; confirm there is no risk of temperature excursions beyond the design temperature. The mechanical strength of thermoplastics rapidly reduces with relatively small increases in temperature of the contents; ensure the chemical resistance for GRP is compatible for the resin selected and a low level of strain is maintained, thus avoiding the risk of strain corrosion.

Design Selection: To achieve long term safe working product, it is imperative the tank is designed to a recognised Standard. This will reduce the purchaser's Healthy 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, with statements such as 'covered by the relevant parts of the standard' to be treated with a high degree of suspicion. 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.

- 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. For GRP structures where the inner corrosion barrier is a chemical resistant laminate such as vinylester, long-term test results for the particular application.

- Maximum operating and design temperature.

- Safety factor. For thermoplastics - BS EN 12573 - for guidance see EN 1771 (example below).

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

- Configuration and detailed dimensions (ie flat base, conical or dished base, sloping base, conical. Dished or flat top. The less preferred configuration of a horizontal or rectangular tank).

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, use the extensive German Standard DVS 2205. A less demanding Standard EN12573 (which many engineers consider is lacking in a number of areas) will give you basic design parameters but does not address many areas covered in DVS2205.

For GRP /FRP and thermoplastic/GRP reinforced products apply the very comprehensive BS 4994:1987. The recent issue of EN 13121-3 has caused much consternation within the industry. This standard is primarily a pressure vessel code for GRP vessels up to 10-bar pressure rating to meet the European Pressure Equipment Directive (PED).

Lee Forbes is chairman of BS 4994-1987 & a member of BSI committee for EN 13121 and Co-founder of Forbes Group, Denver, Downham Market, Norfolk, UK.