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The UK's leading manufacturer of custom designed Plastic Tanks and chemical plant in plastics materials |
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Telephone: +44 (0) 1366 389600 or Use Main Contacts direct dial lines.
22/05/2013
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Main Section -
For the Technically Minded -
Advantages of HDPE Tanks
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Advantages of HDPE Tanks
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All plastics that are subject to stress will age with time. Stress is automatically produced when filled with liquid. This requires that tanks holding dangerous chemicals are manufactured from materials whose ageing behaviour has been accurately determined from long-term testing. For High Density Polyethylene 100 grades, long-term pipe pressure tests have been performed and extrapolated according to the international Standard ISO/TR 9080. The results (referred to in DVS 2205 and EN 12573) allow engineers to design to a known working life with confidence.
In comparison, there is virtually no such long-term test data available for the rota moulding polymers used in the manufacture of rota-moulded tanks; only single-spot hoop tests have been made. Based on that, rota-moulded polymers will never reach any MRS-strength similar to PE-100; for instance, at 40?C with a hoop stress of 6 MPa, rota-moulded materials may reach 13.000 hours ¹ , whilst PE-HD/ PE-100 has a design time of 200.000 hours at the same parameters. The higher hoop stress resistant grade, PE -100 is resistant to this stress due to chemical thermal ageing thus a design life of 50 years is possible.
Further critical information such as creep strength, long-term elastic modulus and the very important allowable hoop stresses are simply unavailable to the design engineer. Therefore, it is not possible for a rota-moulded tank to be designed to a known life since the basic behaviour of the material with time is not currently accurately known.
Material Properties
The primary difference between the high density polyethylene used for manufacture of tanks & pipes, and the Polyethylene used for Rota-moulding, is in the length of the ethylene molecules that make up each material. PE-100 used for tanks and/or pressure pipes has a molecular weight of about 300.000 g/mole whilst the Polyethylene utilized in the rota-moulding process, typically PE-LLD, have a molecular weight from 50,000 ? 100,000. That is, the PE-HD PE-100 - molecules are up to six times longer! This extra length allows the molecules to create more ?knots and links? 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.
Environmental Stress Cracking Resistance (ESCR)(see graph below)
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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 PE-HD polymers have far superior ESC resistance compared to those resins used for rota-moulding. This is reflected in the different standards that are employed to measure ESCR for pipe grade and rota-moulded Polyethylene materials. For PE 100 (pipe grade PE-HD) polymer the requirement is for a specimen immersed in surfactant at a temperature of 80deg C to sustain a full load of 4MPa for 1000 hours (Test standard: EN 12814-3). At present there is no requirement or standardisation currently in being for the ESCR of rota-moulded polymers, Initial discussions for a possible EN standard give a requirement of 15 hours at only 60?C this being considered to be at the top end. At 80?C, test results are obviously much lower (1 to 10 hours). The different requirements reflect the difference in capabilities of the materials.
Design Codes
The Pre-eminent Design codes for fabricated thermoplastic Chemical vessels are the DVS2205 (a German Merkblatt guideline available in English) and the less demanding EN12573. Both codes are internationally accepted as state of the art for thermoplastic chemical tank design. They provide detailed design procedures taking into account long-term hoop stress, wind-loading (only DVS, not EN 12537), nozzles, manholes, temperature effects up to 60deg, and stress concentration factors.
In comparison, today?s rota-moulding design standards provide only very limited calculations in the whole document to calculate the wall thickness. Effects such as temperature influence, long term creep, chemical ageing of the polymer etc are not considered at all because, as stated before, there isn?t the data to support it and the materials are known to have far inferior performance as outlined above.
Safety of Connections
Virtually all PE pipe and pipe fittings in Europe are manufactured from High Density Polyethylene, mostly PE 100. This creates a problem when connecting to a Rota-moulded PE tanks made from Rota-moulding polymers. The most common practice is to weld the PE-HD pipe fitting to the rota-moulded tank wall. This leads to inevitable and well documented failures. The two materials are effectively different molecular structure, the melt viscosity of rota-moulded polymer is much lower (i.e. more water like when molten). Thus, the welds are highly susceptible to cracking, especially when exposed to oxidizing chemicals.
There is no similar problem when welding PE-HD/PE 100 pipe to an PE-HD/PE 100 tank as they are the same material providing optimum weld strength and a life equivalent to the tank.
¹ based on the proposed hoop stress curve for rota-moulded resins; EN-Standardization. |
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