Environmental Science & Engineering - www.esemag.com - November 2003
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Double wall polyethylene tanks can reduce costs and expedite compliance

By Michael Spurrier, MBA,
Snyder Industries Inc.

A recent installation of an indoor tank farm of 1100 USG Captor Containment Tanks which are storing various industrial chemicals.

While US federal containment regulations do not offer much detail in secondary containment system designs and plumbing apparatus requirements, state and local governments are becoming increasingly stringent in the formulation and enforcement of chemical containment legislation.

Containment systems thought to be modern in recent years are now found to be inadequate and not compliant with forthcoming regulatory deadlines. New technologies in double wall tank designs for above ground storage are providing more options in addressing the performance and cost concerns related to the prevention of hazardous waste or hazardous constituents released into the environment. Double wall tanks are being increasingly utilized as an alternative or supplement to secondary containment requirements.

A variety of material constructions are now available in double wall tank systems to ensure chemical compatibility with most hazardous substances being stored. Specifically, new advancements in rotationally moulded polyethylene containment systems are offering timely, cost-effective solutions to the ever-changing regulations. While the use of basic polyethylene tanks in non-critical water and agricultural applications is widely known, the enhancements in resins, processes, and designs are increasing the use of engineered polyethylene tank and containment systems often in demanding hazardous chemical and waste applications.

Resin selection and processing make a difference
Polyethylene materials (PE) used in double wall tank systems provide broad chemical resistance at a comparably low cost, especially in corrosive and oxidizing chemical services. PE containment suppliers should be able to offer and support chemical specific recommendations that PE resin is best suited for a particular application. All PE tank and containment systems are constructed of either linear or crosslinkable PE. Linear PE is capable of being recycled and welded, and it also meets the requirements of US Food and Drug Administration (FDA) Regulation 177.1520. Crosslinkable PE is a tough material that cannot be recycled or welded, and should not be used for a food or potable water application.

Unfortunately, some refer to all PE tank products as being of HDPE, or high-density polyethylene, material construction. Consequently, decisionmakers are not educated on the variations in resin density, and the different performance attributes inherent in the variety of resin choices. Different materials and process methods can cause variations in the density and other physical characteristics of a tank’s construction. Therefore, it is important that purchasers and specifying engineers know and compare the density of the resin used in their tank and containment systems, because generally speaking, as the molecular weight of polyethylene increases, the resistance to chemical permeation and environmental stress cracking improves.

The process utilized to manufacture most PE double wall tanks, rotational moulding, has evolved to the point that modern design advancements can provide specific structural integrity features to handle a variety of mechanical and climatic stresses absorbed in daily operations throughout a tank’s useful life. Rotational moulding is a unique plastic process in that the raw material utilized to form the finished part is never exposed to any pressure during the moulding process, which eliminates built-in stresses inherent in most other plastic processes. As a result, tanks manufactured from the rotational moulding process have superior impact strength, which can be demonstrated through a manufacturer’s quality test data.

An example of demanding impact test criteria is a 400 ft./lb. impact at minus 40 degrees Fahrenheit. Impact test statistics are important because the greater the PE’s impact strength the better the tank’s overall physical properties. The ideal tank cure will maximize impact strength at the highest possible resin density. When comparing polyethylene tank suppliers, purchasers and specifying engineers should inquire about the tank manufacturer’s documented impact test criteria for the tank sizes under consideration.

Design features can increase safety and value
One of the most misunderstood elements of tank design is the measurement of specific gravity, which relates to the amount of weight a particular tank is designed to safely store. The American Society for Testing and Materials (ASTM) committee formulated design standards for polyethylene storage tanks, which applies to the individual tanks within a double wall containment system. When followed, these standards help ensure consistency and safety in polyethylene tank system design and manufacturing. Unfortunately, there are a number of PE tanks used in hazardous chemical applications that may not meet ASTM standards. Consequently, purchasers and specifying engineers should require containment system suppliers to provide wall thickness audits and corresponding calculations to verify ASTM standards have been met before a containment system is accepted.

The rotational moulding process allows much flexibility in the moulding of different tank features without incurring additional labour or parts costs. Tank features that can be easily incorporated within the rotational moulding process, like seismic tie down lugs, top lifting eyes, and bottom drain outlets, are extremely labour intensive when supplied on a fabricated tank, which increases the total procurement costs for a containment system.

More and more specifications are calling for seismic restraints on their containment systems, so it is becoming increasingly important to invest in containment tanks that have the ability to be retrofitted with seismic restraining systems in preparation for any possible regulatory changes in this regard.

Abridged

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