- By Mark Wysong
- Jul 01, 1999
While 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, are now found to be inadequate and not compliant with forthcoming regulatory deadlines.
New technologies in double-wall tank designs for aboveground storage are addressing the performance and cost concerns related to preventing the release of hazardous waste or constituents into the environment. Double-wall tanks are being increasingly utilized as a supplement or alternative to secondary containment requirements.
A variety of construction materials is now available in double-wall tank systems to ensure chemical compatibility with most hazardous substances. Specifically, new advancements in rotationally molded polyethylene (PE) containment systems are offering timely, cost-effective solutions to ever-changing regulations. While the use of basic PE tanks in non-critical water and agricultural applications is widely known, enhancements in resins, processes and designs are increasing the use of engineered PE tank and containment systems for demanding hazardous chemical and waste applications.
Resin selection and processing make a difference
PE materials used in double-wall tank systems provide broad chemical resistance at a lower cost than comparable traditional steel aboveground storage tank (AST) systems, especially in corrosive and oxidizing chemical services. PE containment suppliers should be able to offer and support chemical-specific recommendations on which 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 meets the requirements of U.S. 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, the majority of the industry often refers to all PE tank products as being of HDPE, or high-density PE, material construction. Consequently, decision-makers 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 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 as the molecular weight of PE increases, the resistance to chemical permeation and environmental stress cracking generally improves.
Rotational molding, the process utilized to manufacture most PE double-wall ASTs, has evolved to the point that modern design advancements can provide specific structural integrity features to handle a variety of the mechanical and climatic stresses absorbed in daily operations throughout a tank's useful life. Rotational molding is a unique plastic process, in that the raw material utilized to form the finished part is never exposed to any pressure during the molding process, which eliminates built-in stresses inherent in most other plastic processes. As a result, tanks manufactured from the rotational molding 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 foot-per-pound (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 PE 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 & Materials (ASTM) committee formulated design standards for PE storage tanks, which apply to the individual tanks within a double-wall containment system. These standards can help ensure consistency and safety in PE tank system design and manufacturing.
Unfortunately, there are a number of PE tanks used in hazardous chemical applications that do not meet ASTM standards because their tank operators either knowingly or unknowingly fail to follow these guidelines. The misplacement of a containment system with tanks that do not meet ASTM D-1998-96 into a hazardous chemical application can lead to catastrophic results. 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 molding process allows a great deal of flexibility in the molding of different tank features, without incurring additional labor or parts costs. Tank features that can be easily incorporated within the rotational molding process, like seismic tie down lugs, top lifting eyes and bottom drain outlets, are extremely labor-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. Molded-in top lifting eyes allow larger empty tanks to be accessed and maneuvered more easily by crane and hoist material handling equipment, which reduces the cost of tank handling and installation and increases safety for workers involved in tank installations. Rotational molding produces seamless one-piece tanks, so the entire tank surface and molded-in outlets are free of weld seams, which overcomes the potential leak paths and structural weaknesses that can be attributed to substandard or compromised welding procedures. Another molded-in tank feature that reduces leak paths is the integration of large flat surfaces on the tank for more secure fitting attachments. Flat areas provide a stronger and mor
e uniform sealing surface for fitting installations than traditional rounded tank surfaces.
Rotationally molded, double-wall tanks are now available with an enclosed design, where the inner and outer tank are interlocked, and the outer containment tank is protected from collecting any debris, rain or ice. The enclosed design helps address ever-changing overflow capacity requirements, because there is no possibility of rainwater causing an overflow situation in the case of an inner tank failure or leak. Open top containment systems are becoming obsolete in outdoor applications because of the complications caused by the possibility of rainwater coming in contact with residual or spilled hazardous material found in the outer containment tank. Furthermore, the sight of any liquid in the outer containment tank can create concern, even if it is just water.
The lightweight, durable PE construction of a double-wall tank makes shipping, handling and installation easier and more cost-effective than other tank material construction. The durability of a properly molded and designed double-wall tank addresses the damage expense that can occur with the handling and shipping of the heavier steel or more brittle fiberglass tank systems. Moreover, PE tanks' light shipping weight can reduce comparable freight costs. Existing concrete dike systems that have grown out of compliance often can be remedied through the addition of double-wall PE tanks, which increase the safety factor at a cost far less than a new dike system investment.
Most industry officials agree that a true double-wall tank should be plumbed from the top, rather than through the secondary tank sidewall, which decreases the chance of a leak with the omission of any fitting connections in the liquid phase of the inner tank's sidewall. Of course, the pumping equipment required to effectively discharge material from the top of a double-wall tank could be complex in design and very expensive, especially in corrosive and oxidizing chemical services.
There is, however, new technology available that enables gravity discharge through the means of a transition fitting that effectively seals the inner and outer tanks within the fitting outlet. The transition fitting's gravity discharge provides easier use and greater cost-effectiveness in tank unloading. The decision of top or bottom unloading is an argument of cost and ease of use versus safety considerations.
Applications warrant discussion
Purchasers and specifying engineers should place great importance on the data collection process in each secondary containment application, and discuss details with the manufacturer's representative. Type of chemical, concentration, specific gravity, temperature range, dimensions, mechanical loading and physical location that should all be discussed before making a decision on a double-wall tank or containment design. The resulting data will put the tank manufacturer in a better position to recommend a material or process technology that maximizes the physical properties of the PE containment system.
Before making a final decision on a PE tank or containment supplier, decision-makers should verify competitive quotations are meeting comparable specification requirements. Here are some points to consider:
- Can the tank manufacturer provide wall thickness audits to verify ASTM D-1998-96 standards have been met?
- What is the standard impact test criteria used by the manufacturer for the tanks under consideration?
- What is the processed density of the PE tank's finished wall?
- Does the tank manufacturer's warranty cover my specific chemical application?
- What chemical compatibility testing has the manufacturer completed to support their material recommendations?
- What is the containment system's seismic rating or capability?
- Can the tank or containment be shipped and installed as a unit?
- What is the track record and viability of the double-wall fitting configuration?
Once it has been determined that an application is correctly specified within a tank manufacturer's capabilities, double-wall PE tanks can be a cost-effective solution to a variety of hazardous chemical and waste services.
This article originally appeared in the 07/01/1999 issue of Environmental Protection.