Not all liquids are the same,

 

Not all processing requirements are the same,

 

And storage tanks are definitely not all the same!

 

Cheaper, mass-produced tanks are manufactured by a process known as roto-moulding. To enable the moulding process, the plastic that is typically used is LDPE (low density polyethylene) or LLDPE (linear low density polyethylene).

 

Custom fabricated tanks are designed and built specifically to the customer’s requirements by machining and welding rigid plastic sheets and pipe. This enables the tanks to be made from a wide range of plastics suited to the particular chemicals and conditions inside the tank.

 

Material

 

HDPE (high density polyethylene) is most often used for fabricated tanks and is a close relative of the LDPE and LLDPE used in roto-moulded tanks as they are all made up of ethylene molecules. However, the molecules in HDPE are in the form of long chains which allow them to pack closely and tightly together. This gives it it’s high density, but also provides higher strength, stiffness and chemical resistance.

 

The molecules in LDPE and LLDPE are made up of many irregularly spaced branches. These branches stop the molecules from packing together so tightly which, you guessed it, is what makes it low density. This also means that it has lower strength and stiffness.

 

In addition to HDPE, fabricated tanks can also be made from a variety of other plastics which all have particular characteristics suited to specific performance requirements such as different chemicals, temperatures, flexibility or rigidity, impact resistance, UV stability, fire retardancy, etc. Polypropylene (PP), polyvinylchloride (PVC), polyvinylidene difluoride (PVDF) are some examples of other plastics that can be fabricated due to their unique performance characteristic. Combinations of different plastics can also be incorporated into a tank design to really leverage the benefits of different materials.

 

Flanges and Fittings

 

Virtually all polyethylene pipe and pipe fittings in Australia are manufactured from HDPE. This can create a problem when connecting to a roto-moulded tank made from LDPE or LLDPE. The most common practice is to weld the HDPE pipe fitting to the roto-moulded tank wall. This often leads to inevitable and well documented failures due to the materials different molecular structures. The welds are highly susceptible to cracking, especially when exposed to oxidizing chemicals. There is no such problem when welding HDPE pipe and fittings to a HDPE tank as they are the same material providing optimum weld strength.

 

Pipe and fittings are also available in materials such as PP and PVC which enable fully welded connections to tanks fabricated from those same materials.

 

Design Flexibility

 

Roto-moulded tanks are formed inside moulds built to a predetermined size and shape. These moulds are intended to produce many tanks of exactly the same size and shape. This mass production process is one reason roto-moulded tanks are a cheaper option but limits the customer to what is available “off-the-shelf”.

 

Because custom fabricated tanks are designed and built specifically to the client’s requirements, different sizes, shapes and operating conditions can be considered. Additional features such as baffles, manholes, sloping floors, inspection hatches, ladders and guard rails can also be incorporated into the fabricated tank design.

 

Design Codes and Standards

 

The pre-eminent design codes for fabricated plastic tanks are the European BS/EN 12573 and German DVS 2205. Both codes are internationally accepted as state of the art for thermoplastic chemical vessel design. They provide detailed and comprehensive design procedures taking into account long-term stress, wind-loading and seismic effects, nozzles, manholes, elevated temperature effects, stress concentration and safety factors. Furthermore, the German DVS 2207 codes provide strict guidelines on the welding and construction of fabricated tanks.

 

In comparison, roto-moulded tanks are governed by AS/NZS 4766 which provides only one calculation. This calculation determines the wall thickness due to hydrostatic pressure but does not calculate the influence of higher temperatures, chemical degradation, long-term stress or other forces on the tank.

 

Summary

Roto-moulded tanks can provide a suitable, up front cost-effective solution for many storage applications. However the demands of certain chemicals, required extended lifespan, operating parameters, or even just size and shape constraints may warrant a custom fabricated tank. Contact the design team at Industrial Plastics to discuss the best options for your particular storage requirements. We have an in-house engineer as well as an experienced technical team ready to provide you with the right solution.

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