There are five important factors for safety and longevity when deciding between rotomoulded tanks or a tank fabricated from engineering plastics.
Safety of Connections
Welding HDPE pipe fittings to rotomoulded PE tanks often fails due to the molecular differences between the materials, leading to cracks, especially with oxidising chemicals. This issue is avoided when welding HDPE pipe to an HDPE tank, ensuring stronger welds and longer lifespan.
Ability to design to a known life
Only engineered thermoplastics like High Density Polyethylene (HDPE) and Polypropylene (PP) have been tested for long-term durability, allowing engineers to design tanks with a known lifespan, whereas rotomoulded tanks lack such data, making it difficult to predict their lifespan due to the different materials used.
Material Properties
HDPE has a linear molecular structure with fewer branches compared to other forms of polyethylene, or the materials often used in rotomolding. This contributes to greater toughness, creep resistance, and most importantly, stress cracking resistance. HDPE properties also include high tensile strength and stiffness.
Environmental Stress Cracking Resistance (ESCR)
Over time, corrosive media attack at the microcracks in any polymer surface. HDPE resins have significantly superior ESCR resistance compared to rotomoulded resins. Under hydrostatic pressure standards, pipe grade HDPE resins must endure 80˚C and 4MPa for 100 hours, while rotomoulded resins are only tested to lower temperatures. This means pipe grade HDPE can withstand harsher conditions for enhanced durability.
Design Codes
The DVS2205 and BS EN12573 codes are internationally recognised standards for designing thermoplastic tanks, covering critical factors like hoop stress, wind-loading, and temperature effects up to 60°C. In contrast, the AS/NZS 4766 standard for rotomoulded is limited, providing only one calculation for hoop stress and restricting operation to 24°C.
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