Designs for life

Tanks for use in the chemical storage and processing and the water and wastewater treatment industries must not be taken as a commodity. Tanks must be safe, as the liability and risk to personnel as well as the environment is extremely high.

To the engineer without experience in the selection of materials and design, tanks seem to be simple structures. But this approach can easily lead to failures within the lifetime of the tank.

It is essential that the design, materials of construction and type of manufacture are 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.

Virtually all tanks are regarded as a long-term capital investment. In particular the water industries often demand at least 20 years design life. Therefore, selection of the design standards and construction materials must follow suit.

No matter what the application, 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 that 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 tanks where the thermoplastic is not only the chemical resistor, but also the load bearing structure. All plastics suffer from 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 a certified design standard with full design calculation, thus ensuring the tank will meet requirements. Materials can have a short life due to degradation when the tank shell is under load. Long-term exposure to chemicals when under stress can induce environmental stress cracking.

With material selection the best approach is to engage with an experienced manufacturer that can show successful long-term applications. The next safeguard is detailed design. The product must be fully compliant to a recognised British or EU standard. The suggested selection process includes material selection:

• Thermoplastic as the chemical resistor and load bearing structure
• Glassfibre reinforced vinyl-ester and polyester (GRP or FRP) laminate with a composite inner corrosion barrier
• Thermoplastic inner shell corrosion barrier as the chemical resistor with the external GRP laminate being load bearing structure, and being fully protected from the effects of the contents

The area not always addressed by the manufacturer, is proven expected long-term life. 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, the appropriate safety factor can be applied through long-term experience demonstrated by the manufacturer or applying the guidance notes in BS 4994 – 1987.

Points to raise with the manufacturer include:

• The long-term chemical resistance is confirmed, with examples. Whilst 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 in the long term.
• Ensure the grade of thermoplastic is correct for the application
• Confirm there is no risk of temperature excursions beyond the design temperature
• 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 a long-term safe working product, it is imperative that the tank is designed to a recognised standard. This will reduce the buyer’s health and safety liability in the event of anything going wrong during the working life of the tank.

Prior to specifying and manufacturing items to the relevant standard, the following information must be agreed between the buyer and manufacturer, and confirmed prior to design and manufacture:

• Details of the liquids to be handled including any trace contaminants
• 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 temperature
• Design temperature
• Safety factor. Guidance for thermoplastics – BS EN 12573 – can be seen in EN 1771 (see Table). 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

• Operating pressures
• Design pressures
• Site conditions
• Hazardous area requirements
• Imposed loads
• Operating conditions
• Method of support and holding down design
• Nozzle connections, size, standard and layout
• Seismic loadings
• Site access and layout
• Transportation, installation and handling

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 will give you basic design parameters but does not address many areas covered in DVS2205.

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

Due to the complex nature of EN 13121-3 code, coupled with an estimate of 40-plus within the document, there are suggestions that EN 13121-3 is withdrawn and BS 4994-1987 remains the mainstay for the design and manufacture of safe and reliable tanks and vessels that can exceed 200m3 capacity.

Lee Forbes is co-founder of Forbes Group, chairman of BS 4994-1987 and a member of BSI committee for EN 13121. T: 01366 389625