Plastic tanks for hazardous chemical storage are becoming more commonplace as a cost effective alternative to stainless steel or fibreglass and generally come in two varieties; fabricated or rotationally moulded. Although both are made from plastic they differ in construction method, material, longevity, cost, overall long-term safety and standards compliance—they couldn’t be more different.
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Fusion Fabricated Tanks
Fusion Fabricated tanks are made from flat sheets of material in a technique similar to manufacturing stainless steel tanks. Most commonly, Fusion uses sheets of HDPE (high density polyethylene / PE100) or PP (polypropylene).
Sheet Material Benefits
High Density Polyethylene (PE100)
- 100% weld integrity with PE100 pipe
- Superior, long term material strength
- Exceptional weld strength
- Construction of higher tanks
- Superior acid resistance
- Shorter welding times
- Easier cold bending of thinner sheets
- Weatherproof (exceptional UV stability)
Polypropylene Homopolymer (PP-H)
- Very high chemical resistance
- Excellent high temperature resistance
- Excellent impact resistance
- Higher scratch resistance than HDPE
- Excellent moisture resistance
- Food grade
Guaranteed Consistent Wall Thickness
Flat plastic sheet has very little variance in overall wall thickness and is manufactured to within a ±0.1mm tolerance; thereby making it a consistently strong material.
To build a fabricated tank, flat plastic sheet is bent into a open-ended cylinder and the two parallel, touching edges are welded together. The base and roof are cut from sheet plastic and welded to the open ends of the cylinder to form a complete, enclosed tank.
Greater Design Flexibility
Customisability is a considerable benefit in the fabricated tank manufacturing process. Sizes can be scaled up and down from 300L to 60,000L and vary in shape / diameter as this process doesn’t require expensive moulds to be setup.
Fusion fabricated tanks are designed and manufactured to DVS2205 & BS/EN12573 which are accepted globally as the comprehensive standard for the calculation of tanks and apparatus made of thermoplastics. These standards include design procedures that take into account load stresses, environmental effects and safety features, along with strict rules on welding and construction methods. Adherence to these standards allows Fusion to manufacture long lasting tanks in a large variety of sizes with varying nozzle arrangements (shown in Fig. 1); making them perfect for chemical processing and storage.
Figure 1. A range of Fusion fabricated chemical tanks being transported to site.
Fusion fabricated tanks can also be custom manufactured to include a variety of different modifications which have been used in the water treatment, mining, oil and gas, manufacturing, food and beverage, pharmaceutical markets, industrial and various other chemical processing sectors.
Fusion fabricated tanks can also be manufactured with structural and nozzle variations which meet a range of Australian and German Engineering Standards. These can meet:
- Chemical storage and processing safety requirements (vents and overflows);
- Working at heights platforms, including balustrades and ladders in chemical resistant FRP;
- Seismic hold down clamps and stays;
- Roof mounted and sideways mounted manways for confined space access; and
- Rated lifting lugs for trouble-free maneuverability and install of tanks.
Left: Figure 3. A fabricated thermoplastic storage tank with various accessories to comply with Australian Standards.
Fusion fabricated tanks can also be manufactured with a range of conical or sloped bases for ease of draining. Mixing baffles can also be included for optimal chemical blending and powder dissolving processes with the addition of both plastic and/or steel structural support frames which are designed using harmonic analysis techniques to minimise the effect of vibrational fatigue on the vessel. An example of a conical mixing vessel is shown in Figure 4 below:
Left: Figure 4. A fabricated thermoplastic mixing vessel with a conical base, mixing baffles and a harmonically balanced agitator structural support.
Due to the design flexibility of the manufacturing process, Fusion fabricated tanks can be engineered to meet a multitude of site and space requirements. Examples of this include:
- Rectangular tanks to meet restrictive space requirements, and
- Dual wall tanks to meet AS3780-2008 The storage and handling of corrosive substances requirements.
Left: Figure 5. A fabricated thermoplastic rectangular vessel with a galvanised steel frame (frames can also be manufactured from thermoplastic) designed to DVS2205-5 standards.
Material Density & Structure
The molecular arrangement in HDPE is in the form of long chains which allow them to pack closely and tightly together in a crystalline structure. This gives HDPE its high density, and also provides high strength, stiffness and chemical resistance.
Fusion fabricated tanks can also be made from a variety of other plastics including: Polypropylene (PP), polyvinylchloride (PVC) and polyvinylidene difluoride (PVDF). These can be chosen to match desired characteristics such as chemical resistance, operational temperatures and more. Designs can also include combinations of plastics to further optimise performance and obtain maximum longevity for your application.
Repairs & Maintenance
HDPE is used for both the tank, joins and fittings making a homogenous, optimal environment with no variances in thermal expansion (not susceptible to cracking), chemical resistance and temperature rating.
Maintenance is infrequent and repairs are less likely.
Initial costs for fabricated tanks are often more, however the return on investment is much greater when taking into account the lifetime of the tank, minimal maintenance costs and repairs. Subject to application, Fusion offers a 25 year design life on tanks. This statement can be made as a result of consistently accurate materials and specifically engineered per-application designs.
Rotationally Moulded Tanks
Rotationally moulded tanks are manufactured in a process whereby a charge of thermoplastic powder is heated and spun in a hollow metal mould. The heating and spinning turns the plastic molten; coating the mould, which is then cooled to set and form the tank.
This process has the advantage of fast and inexpensive mass manufacturing repeatability, making the end product cost of roto-moulded tanks desirable to consumers.
However, when compared to fabricated tanks, and especially in instances of chemical storage, there are a few key points which make this manufacturing technique less reliable; especially for longevity and safety.
Wall Thickness Variances
Wall thickness variances can be an issue and the process of heating the plastic powder must be very accurate. If over or under heated, the plastic can become brittle or may not completely adhere to the mould, making areas weak. Likewise, the cooling process must be monitored and undertaken slowly, otherwise warping can occur.
Material Density & Structure
Materials commonly used in roto-moulded tanks are LDPE, LLDPE or MDPE (Low Density Polyethylene, Linear Low Density Polyethylene or Medium Density Polyethylene) with other grades used for certain chemical storage applications. Generally these are lower in density and weight compared to common fabricated tank materials and further affect overall strength.
- Low specific gravity
- Chemical resistance
- Impact strength
- Very low strength/stiffness
- Poor heat resistance
- High coefficient of thermal expansion
Material Incompatibilities can be an issue for roto-moulded tanks. On a molecular level, the commonly used lower density plastics are non-uniform in structure. Inlets and outlets are generally manufactured from PE100 and welded directly to the LDPE tank wall. The molecular differences between the tank and these inlets / outlets can lead to failures and cause cracking; significantly reducing operational life.
Right: Figure 8. A roto-moulded tank with a leaking nozzle in the early stages of crack propagation.
Repairs & Maintenance
Roto-moulded tanks tend to fail at the weak points in the shell. This is often corresponds to joins connecting pipe fittings and additional fabricated thermoplastic components. If not repaired quickly, this cracking can propagate as per Figure 9 below, and initiate catastrophic failure which is a major OH&S Hazard.
Right: Figure 9. A roto-moulded tank with a in the late stages of crack propagation.
Repairs can be not only costly, but insufficient, as repairs involve welding virgin PE100 material to material which has been exposed to the environment. Figure 5 shows the variance between these materials. This can result in cross contamination, which cracks and leak points will often re-occur. When being used to store hazardous chemicals, cracks and leak points are not an option as they can result in damage to the environment, associated plant equipment, and operating personnel.
Right: Figure 10. Temporarily repaired roto-moulded tank.
More serious failures require costly intricate repairs which are inevitably only temporary.
Right: Figure 11. Intricate repairs of a base replacement for a roto-moulded tank.
While roto-moulded tanks have their place in the market, it is clear that the overall potential for variability in the manufacturing processes and issues of material incompatibilities, makes them a less reliable option for chemical storage tanks and process vessels. A Fusion Fabricated tank can provide exact specifications with little variance in wall thickness and no issues with material incompatibility. These features make Fusion Fabricated tanks a safe and reliable option for chemical storage and process applications.
By making the right choice and selecting a Fusion fabricated plastic tank in the project design stage, Fusion tanks will ensure years of reliable operation.