High-density polyethylene chemical storage tanks can be made of linear polyethylene or cross-linked polyethylene. They are both made with resins that are heated to create a hollow plastic part, yet the differences in their development create very different polyethylenes and structural strength.
Linear polyethylene parts are created when thermoplastic resin pellets are ground and then heated to create the fluid plastic that will harden and cure into a linear, high-density polyethylene surface. Envision a rope where the individual threads of fiber are twisted together, but not tied. The result is a linear resin that has polymer chains “tangled” together, rather than tied.
Linear polyethylene is cost-efficient, and is safe for storage of benign and non-corrosive chemicals.
Cross-linked polyethylene (XLPE) is high-density polyethylene that is manufactured by adding a catalyst to the thermoplastic resin. This turns it into a thermoset. The catalyst causes a covalent bond that links the molecules together. Picture a chain linked fence where the metal is actually linked together. The result is a plastic that possesses impact, tensile strength and resistance to fracture.
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This makes it an excellent choice for chemical storage applications when tank integrity is of utmost importance.
Because cross-linked polyethylene offers longer tank life and more protection in the long run, there is a little more upfront investment. Cross-linked polyethylene offers 20 times the environmental stress crack resistance of high-density linear polyethylene, has 10 times the molecular weight, and greater impact and tensile strength as well.
Another difference comes in testing the two plastics. When linear polyethylene fails, it fails catastrophically, because the linear polymer chains “unzip.” Cross-linked polyethylene might develop a small tear or pinhole but cannot catastrophically fail, because of the covalent bonds that connect the polymers.
Oxidation resistant resins
If you’re storing an oxidizing chemical like sulfuric acid or sodium hypochlorite in a polyethylene tank, an antioxidant barrier will give you much greater tank life.
Antioxidant resins contain saturated amounts of antioxidants. Most polyethylene resin already contains some antioxidant. Resin producers intend to protect the plastic resin itself from heat created in the molding process. It is spent during the heating process and non-existent at the completion of the molded part. Having performed the task it was charged with (protecting the plastic during production) it is no longer available for any post-production purpose.
The presence of antioxidant resin in a completed storage tank, however, addresses the chemical resistance of oxidizing chemicals for 15 to 20 years after the tank is shipped.
Innovative tank configurations
There are several different polyethylene tank configurations available in the marketplace today. Two of the innovative tank configurations are full drain tanks and double wall tanks.
Full drain tank systems
One of the biggest challenges in vertical tank design is maintaining the tank’s integrity while still allowing for it to be fully drained, cleaned, and properly maintained. Some companies create tanks out of one substance and insert or mold a flange, post-production or during production, for drainage. This works, but there are other alternatives such as an integrally molded flanged outlet (IMFO).
An inserted flange is common in the chemical tank industry, but there are decided benefits to a molded full drain. The main differences lie in impact on the structure and structural integrity, net positive suction possible, and draining capabilities of the tanks.
One-piece construction enhances long-term integrity, both for the tank as a whole and in the knuckle radius, which is the point where the floor of the tank meets its sidewall. The knuckle radius is the thickest part of the tank, so, adding an insert post-production or during construction can compromise the tank. An insert can impact the tank’s hoop integrity as well, which puts the entire tank at risk of structural stress from volatile chemicals.
When the insert is made of metals or alloys, chemical reactions where the tank material meets the insert can create a stress point for the plastic itself. This can cause leaks or cracks over time.
With an IMFO® system, the drainage flange is molded while the tank is processing, making it a one-piece, stress-free integrated part of the tank. The flange is also created from cross-linked high density polyethylene.
An integrally molded flanged outlet has several key advantages:
A flange located at the bottom of the tank permits full tank drainage since the outlet is below the tank knuckle radius. This eliminates the need to enter the tank for cleaning.
- One-piece construction enhances long-term performance of the tank, since it doesn’t compromise the tanks hoop integrity or structural design.
- In aggressive oxidation environments, the antioxidant resin system can protect the drainage components from deterioration.
- Highest amount of static head pressure.
Poly Processing has used its IMFO system on tanks from 750 L – 52,000 L.
Double wall tanks
Safe operation and minimizing the risk and hazards associated with spills and leaks is vital in chemical storage. An effective way to contain the spill or leak is a secondary containment system called a double wall tank, or “tank-within-a-tank.”
A properly designed “tank-within-a-tank” system keeps contaminants, such as debris and rainwater, from entering the interstitial area. These tanks provide secondary containment to avoid damaging equipment or property, loss of chemical, or injury to employees in the event of a spill.
Advantages of a properly engineered double wall tank system include:
- Provides 110% secondary containment.
- Will equalize the liquid and allow the chemical to be used until it is convenient to inspect and repair the tank.
- Is ideal for chemicals like sulfuric acid that can have dangerous exothermic reactions to water.
- The expense, cost and maintenance of secondary concrete containment is eliminated.
- The footprint of the containment system is minimized. Secondary containment is achieved in a more compact way.
- Addition of a transition fitting maximizes the double wall tank system’s performance.
Selecting the right fittings for a polyethylene chemical storage tank is crucial to ensuring optimum function, personnel safety, and maximum life of the polyethylene tank. Proper sidewall fittings prevent tank leaks and spills. Selecting sidewall fittings purely based on cost can lead to an expensive mistake.
Double wall fittings
The outlet on a double wall tank becomes the most vulnerable and most commonly used component that wears. A cost saving technique uses a rubberized doughnut between the inner and outer tank, which bridges the two tanks. The challenge with this is that it reduces flex which the tank needs, but greatly compromises true secondary containment. The pressure that the bottom of the tank experiences in combination with an inflexible fitting creates a hinge point. This commonly fails due to wear and leads to complete siphoning of the tank outside of the containment to the ground.
To minimize these problems, Poly Processing offers a bellows transition fitting. It features a flexible expansion joint that protects one tank wall from the other against hinge and flex lock. A fitting is attached to the inner tank, while a flexible expansion joint allows the inner tank to expand and contract. The piping is fully contained by this fitting, meaning that there is no hinge point, and the fitting is able to fully contain any leaks that may take place.
Adding the enhanced bellows transition fitting will maximize a double wall tank system’s performance by allowing drainage of the primary tank through single or double wall piping.
Plastic encapsulated bulkhead fittings
A bolted one piece sure seal fitting or B.O.S.S.® style fitting is a streamlined one-piece fitting without separate bolts. The fitting connects to the tank with a single gasket to greatly minimize the potential for leaks. A ring with welded studs, available in stainless steel, titanium or alloy C-276, is injection molded with polyethylene around it. This provides a one-piece fitting with a single sealing point. This is the fitting of choice for high profile applications like sodium hypochlorite and sulfuric acid.
Poly Processing has developed a B.O.S.S. fitting and can install it on almost any tank in any location where needed.
This article appears in ES&E Magazine’s June 2017 issue.