The Differences between Fabricated and Roto-moulded Tanks

There are five important areas where Chemical Vessels fabricated from extruded HDPE differ from Roto-moulded vessels manufactured from LLDPE, LDPE or LMDPE.

Ability to Design a TANK that suits its PURPOSE

All plastics materials that are subject to stress will age with time. This requires that vessels holding dangerous chemicals are manufactured from materials whose aging behaviour has been accurately determined from long-term testing. High Density Polyethylene and Polypropylene are the only materials for which long-term (+30 years) pipe pressure tests have been performed. The results (referred to in DVS 2205 and BSEN 12573) allow engineers to design to a known working life with confidence.

In comparison, there is no such long-term test data available for the roto-resins resins used in the manufacture of roto-moulded vessels. Such critical information such as creep strength, long-term elastic modulus and allowable hoop stresses are simply unavailable to the design engineer. Therefore, it is not possible for a roto-moulded tank to be designed to a known life since the basic behaviour of the material with time is not accurately known.

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Material Properties

The primary difference between the high density polyethylene used for fabricated vessels, and the Polyethylene used for Roto-moulding, is in the length of the ethylene molecules that make up each material. HDPE has a molecular weight of over 300,000 whilst the Polyethylene utilized in the roto-moulding process, typically MWPE or LLDPE, have a molecular weight from 50,000 – 100,000. That is, the HDPE molecules are up to three times longer! This extra length allows the HDPE molecules to create more ‘knots and eyes’ between them, increasing the toughness, creep resistance, and stress cracking resistance whilst retaining an optimum level of crystalinity to also provide a high tensile strength and stiffness.

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Environmental Stress Cracking Resistance (ESCR)

Over time, corrosive chemicals attack at the micro-cracks in any polymer surface. The degree to which this is resisted by the polymer depends on several factors, however, it has been shown via independent tests that HDPE resins have far superior ESCR resistance compared to those resins used for roto-moulding. This is reflected in the different standards that are employed to measure ESCR for pipe grade HDPE and roto-moulded materials.

For pipe grade HDPE resins the requirement is for a specimen immersed in surfactant at a temperature of 80deg C to sustain a full load of 4MPa for 100 hours. The standard for a roto-moulded resin requires that a specimen be subjected to a temperature of 60deg. C for only 20 hours. The different requirements reflect the difference in capabilities of the materials.

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Design Codes

The Pre-eminent Design codes for fabricated thermoplastic Chemical vessels are the DVS2205 (a German guideline available in English) and the BS EN12573. Both codes are internationally accepted as state of the art for thermoplastic chemical vessel design. They provide detailed design procedures taking into account long-term hoop stress, wind-loading, nozzles, manholes, temperature effects up to 60deg, and stress concentration factors.

In comparison, the AS/NZS (Int.) 4766 –a preliminary roto-moulding design standard – provides only one calculation in the whole document to calculate hoop stress. Temperature of operation for the vessels is limited to 24 deg C and the standards do not support design to a known life because, as stated before, there isn’t the data to support it.

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Safety of Connections

Virtually all PE pipe and pipe fittings in Australia are manufactured from High density Polyethylene. This creates a problem when connecting to a Roto-moulded PE vessel made from Roto-resins. The most common practice is to weld the HDPE pipe fitting to the roto-moulded tank wall. This leads to inevitable and well documented failures. The two materials are effectively different molecules and the welds are highly susceptible to cracking, especially when exposed to oxidizing chemicals.

There is no similar problem when welding HDPE pipe to an HDPE tank as they are the same material providing optimum weld strength and a life equivalent to the tank.