Code of Practice for the design and fabrication of thermoplastic vessels

Dotmar has been working with the plastic fabrication industry within Australia and New Zealand, Australian Standards and key suppliers to develop an Australian Code of Practice for the fabrication of thermoplastic vessels.

Thermoplastics such as the polyolefin group have different physical property and corrosion mechanisms compared with metals. It is essential that material selection, design calculations and fabrication control take account of the specific properties and behaviour under load of thermoplastics. The topics addressed below are intended to give those interested parties, without extensive plastics knowledge but involved in the use of chemical storage vessels, a code of practice in the absence of a current Australian Standard. The common objective is to provide a thermoplastic vessel engineered for the expected service life which provides paramount safety, performance and value for the end users.

Material selection

Unlike metals, media temperatures are limited to lower levels where thermoplastics are used as structurally self-supporting chemical storage tanks. This, of course, is subject to the chemical resistance limitation on temperature. The elastic modulus of thermoplastics is, unlike metals, highly temperature dependent.

Material grade selection based on the chemical media, concentrations and temperature must be based on empirically derived data published by experienced and reputed sheet manufacturers for use in vessels. This is because the chemical resistance of plastics is usually a go or no-go proposition, unlike metals where a corrosion allowance can be calculated from predetermined corrosion rates. Correctly selected, a service life of 20 years can be warranted as proven in Europe. Improper selection is likely to result in catastrophic failure at short notice.

Design calculations

The relatively low tensile strength and elastic modulus of thermoplastics is further reduced as temperature and service life is increased. Deflection limits must be carefully controlled, hence thermoplastic vessels used for chemical storage should only be designed using DVS 2025* as the standard and RITA^ as the calculation system. DVS 2025 is a widely accepted German standard for design calculations for containers and apparatus made from thermoplastics. RITA is a software package using the values of the DVS standard which incorporates material selection for the chemical media, design parameters and characteristic values for stress, strain, creep etc. RITA's output provides the statement of required material thicknesses, nozzle position limitations etc.

These integrate the media, temperature and hydrostatic pressure loads together with fabrication techniques and weld type to determine stress and deflection limitations for the required design life. In combination, DVS and RITA have delivered many cost-effective, correctly specified and fabricated vessels with proven safety and service life.

Fabrication techniques, equipment and quality control

Welding of thermoplastic chemical storage vessels should only be done by DVS-trained welders**. Ideally, vertical seam welds and joined sheets should be welded using automatic butt welding machines. Testing of welds as stated in the DVS must be incorporated in the fabricator's quality plan. Material certificates for the thermoplastic sheet should be obtained by the user to ensure the material grade is supplied in conformity with the design calculation. (HDPE for example is not an accurate descriptor as properties under the generic term HDPE vary significantly).

The correct material selection, design calculations using RITA, fabrication methods and quality control in accordance with DVS2025 standards will ensure the safety and lifetime effectiveness of thermoplastic vessels.

Notes:
*DVS: Deutscher Verband fur Schweisstechnik (German Welding Institute)
^RITA: Roechling Integrated Tank Assistant (view www.roechling.de)
**Trained welders: there are some 13 fabrication companies with 25 trained welders in Australia as at June 2009. A list can be viewed at www.dotmar.com.au/dvsfab.