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Plastic Tanks

Why choose a plastic tank

Characteristics of Plastic Tanks

If you need to store large amounts of liquids, you generally use tanks, either under or above ground.

In ancient times, the use of natural cavities or underground cisterns was much more widespread, given the relative ease of set-up.

Then urbanization, growing needs and increasing availability of materials and processing techniques led to the preference for above-ground tanks.

Liquids

First in cementitious material, then in metal and finally today’s development, thanks to thermoplastic materials.

In fact, thermoplastic has many advantages:

  • it can assume many shapes and volumes, from a few fractions of liters up to tens of thousands
  • it is light and resistant, easily workable, economical
  • lasts over time, remains unaltered in contact with water, resists the aggressiveness of the vast majority of chemical products, including concentrated ones

Today, for the multiple needs of homes (drinking water deposits, rainwater collection, wastewater treatment), the industry offers a wide range of solutions at reasonable costs and immediate supply.

Thermoplastic tanks are also widely used for community use. In fact, large quantities of liquid chemical products are used for the purification and treatment of urban waste, and consequently each plant must have adequate storage tanks.

Industry has very widespread needs to store liquids. Water for washing, for firefighting, for drains, for  chemical products to be stored as soon as they are produced or to be distributed for consumption, or to treat surfaces in metallurgy, to sanitize in food processing, to avoid scale in steam production, to make detergents for homes and communities. These are just some applications.

And thermoplastic plastic tanks are in most cases the best choice for functionality, economy and resistance.

 Some of the most commonly stored chemicals:

  • Hydrochloric acid
  • Nitric acid
  • Phosphoric acid
  • Diluted Sulfuric Acid
  • Concentrated Sulfuric Acid
  • Ammonia
  • Sodium bisulfite
  • Sodium chlorite
  • Ferric chloride
  • Sodium hypochlorite
  • Calcium nitrate
  • Hydrogen peroxide (hydrogen peroxide)
  • Aluminum polychloride
  • Sodium hydroxide (Soda)
  • Aluminum sulphate
  • Urea (AdBlue)

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Technology

Thermoplastic polymers for tanks (PE and PP) are generally used on the basis of 3 different technologies:

Rotational Molding (rotomoulding)

It uses only low density PE (LDPE) for its ductility. A hollow metal mold is loaded with granules of raw product and, by means of heat and rotation on the two axes, its liquefaction and consequent distribution on the walls are induced. Once cooled, the mold is opened and the tank, formed in a single block, is extracted. Then accessories are applied (through mechanical connections)

PROS

  • economy (because large production volumes are made possible by simple technology)
  • versatility (thanks to the various shapes and sizes that can be made)
  • ease of installation (the lightness and the single-mouth shape help portability and placement on site)

CONS

  • production standardization prevents particular uses, as: intermediate volumes, combinations of height and diameter not foreseen by the available molds, set-ups for specific uses
  • the ductility of linear PE, combined with the impossibility of differentiating the thicknesses between the various sections of the tank, limits its use when high volume values ​​are reached (above 10 / 15,000lt), static load (density of the liquid) and chemical aggression
  • the span life, although considerable, is affected by the degradation produced by solar radiation, by the static loads of the content (deformability) and by the corrosive action when used for chemical products

Rounded Sheet Tanks:

Extruded sheets (in PE or PP) are preformed (calendering) and joined with butt welding, by special equipment, to form the vertical cylinder, then joined to the bottom, the roof and the accessories by heat welding

PROS

  • duration over time
  • good chemical resistance to slightly concentrated substances
  • intermediate economic costs between rotational molding and rotational extrusion

CONS

  • the preformed sheets cannot have high thicknesses and do not allow a variation in the consistency of the walls, limiting the adaptability to high volumes (in vertical cylindrical form the weights weigh much more at the bottom than on top)
  • for the same reason (wall thickness), the resistance to corrosion cannot be the greatest (if there is not enough material behind the layer in direct contact with the product to compensate for the deterioration, the structure is at risk)
  • the presence of an extensive development of the joints (welding between sheets) cause an objective limit to the reliability and safety compared to monobloc structures

Rotational Extrusion ((helix (roll-extruded) tanks)):

A melted polymer ribbon (High Density PE or PP) is deposited on a horizontal cylindrical metal shape. By translating the extrusion mouth parallel to the rotating cylinder, a succession of partially overlapping coils is formed. By repeating the process, and each time differentiating the starting point for the formation of the coil, once the deposited plastic has cooled and the metal shape on which it is wound has been removed, a cylinder with different thicknesses is obtained. To this, by forming an extruded bead, a bottom (on the thicker side) and a roof are welded.

PROS

  • resistant and long-lasting even in the presence of large volumes (up to hundreds of thousands of liters) and highly corrosive products
  • flexibility in setting up and preparing for specific uses (mixing, heating / cooling, highly densities media, etc.)
  • reduced maintenance
  • guarantee of maximum safety allowed by plastic tanks: they can integrate the leak containment structure and failures to which they may be subjected do not produce structural collapses, but only leaks

CONS

  • significant initial cost

  • availability only on order with consistent delivery times and installation costs

Materials

The plastic materials used for the tanks are generally of two types

THERMOSETTING

THERMOPLASTICS

The former are a family of synthetic resins derived from petroleum, initially liquid, which harden (“curing”) by means of special catalysts, even when cold. And, since the process is irreversible, after they become solid they can no longer change shape. They are used in combination with glass fibers on which they are coated, so the final product is a composite that takes the name of Fiberglass (fiberglass, GRP etc.)

Thermoplastics, (“can change shape with heat”), are petroleum derivatives which, by aggregation between simple molecules, form complexes called polymers.

Not all of them are suitable for the production of plastic tanks. Some, like PVC, are too rigid, others, like PTFE (Teflon) too soft. PA (nylon) has little resistance to chemical attacks, PVDF has very high costs.

There are two polymers with the most favorable characteristics:

POLYETHYLENE

  • Belongs to the Polyolefin family. It is obtained from the polymerization (process that produces molecules consisting of many equal parts repeated in sequence) of ethylene and is one of the most processed and used polymers, constituting the largest fraction of the world consumption of plastics.
  • The main features:
    • resistance to corrosion and chemicals
    • impact resistance
    • easily weldable
  • It is distinguished by density (molecular weight):
    • Low Density Polyethylene (LDPE): it is much more branched than HDPE, therefore it is a more ductile and less rigid material
    • High Density Polyethylene: it is a little branched polyethylene, therefore it has high intermolecular forces and greater stiffness than low density polyethylene

POLYPROPYLENE

  • It also belongs to the polyolefin family. It is the second most used polymer in the world (after polyethylene, PE).
  • The main features:
    • heat-resistant
    • non-polar, remains chemically stable in contact with aggressive substances
    • wear resistant and easily workable

   

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Cisterne in Plastica

Maurizio Giulimondi, Via del Paleotto 9/4, 40141 Bologna (BO) C.F. GLMMRZ55R27H501Y
PEC: maurizio.giulimondi.@pec.it

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