Custom Plastic Tank Fabrications vs Roto-moulded Tanks: Asset or Liability?

An asset is an item you own that can provide you with future economic benefits in years to come – for example, a custom plastic tank fabrication that has been designed in accordance with the international standard DVS2205.

A liability is an item that over time will eventually start to cost you money – for example, a roto-moulded tank that deteriorates over time because it’s not suited to the desired application.

In short, assets put money in your pocket, and liabilities take money out.

Here are our thoughts on custom plastic tank fabrications vs roto-moulded tanks and what makes our custom plastic tank fabrications assets for your business;

Ability to design to a Known Life

All plastics that are subject to stress will age with time, just like us humans, stressful situations have an impact on our bodies.

Tanks storing harmful media are manufactured from materials whose aging behaviour has already been accurately determined by long-term industry testing. Thermoplastics such as high-density polyethylene (HDPE) and polypropylene are the only materials for which long-term (30+ years) pipe pressure tests have been performed. These material results (referred to in DVS 2205 and BSEN 12573) allow us to design our custom plastic tank fabrications to a known working life with confidence.

In comparison, there is no such long-term test data available for the roto-resins that are used in the manufacture of roto-moulded tanks. Critical information such as creep strength, long-term elastic modulus, and allowable hoop stresses are simply unavailable. Without this key information, it is almost impossible for a roto-moulded tank to be “designed” to a known life.

Tank fittings and their safety

Virtually all polyethylene pipe and pipe fittings types in Australia are manufactured from High-Density Polyethylene. This creates a huge problem when trying to connect HDPE fittings to a roto-moulded PE tank which is manufactured from roto-resins.

A common practice is to weld HDPE fittings to the tank wall. However, in roto-moulded tanks, the two materials are effectively different molecules meaning the welds are highly susceptible to cracking, especially when exposed to oxidising chemicals. This leads to inevitable and well-documented failures within the industry.

A custom plastic tank fabrication has no dissimilarities problems when welding. Using extruded HDPE and HDPE fittings give clarity ensuring optimum weld strength and a life equivalent to the tank.

Material Properties

The primary difference between the High Density Polyethylene used for custom plastic tank fabrications, and the Polyethylene used for roto-moulding, is in the length of the ethylene molecules. HDPE has an approximate molecular weight of over 300,000 whilst the Polyethylene utilised in the rotomoulding process, typically MWPE or LLDPE, has an approximate molecular weight between 50,000 – 100,000.

With the HDPE molecules being three times longer than that of the MWPE or LLDPE means increased toughness, creep resistance, and stress-cracking resistance over time.

Design Codes

Design codes for custom plastic tank fabrications generally follow DVS2205 and the BS EN12573. Both codes are internationally accepted as state of the art for thermoplastic storage tank 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. The temperature of operation for the tank is limited to 24 deg C and the standards do not support design to a known life.

Cost

One of the main driving factors for customers is the cost of a custom plastic tank fabrication vs a roto-moulded tank. Roto-moulded tanks will always be cheaper compared to fabricated.

However, at Industrial Plastics we are trailing new innovative technologies to reduce the costs of custom plastic tank fabrications –not to compete directly with roto-moulded, but to offer some middle ground to smaller businesses and for more cost-sensitive applications.

For certain applications, a custom plastic tank fabrication would be a superior asset to a small business but the CAPEX v OPEX tradeoff does not equate resulting in the purchase of a liability.