We’ve written several articles about the importance of protecting your chemical storage tank from aggressive chemicals. Mechanical stress can be just as significant, causing polyethylene chemical storage tanks to crack. Mechanical stress occurs when the tank’s structure is compromised after a physical impact with another object, over pressurization, or rigid plumbing that doesn’t allow natural expansion and contraction of the tank.
In cross-linked polyethylene (XLPE) tanks, tank cracking can produce small leaks. In linear polyethylene tanks, mechanical stress can lead to catastrophic failure. If a crack develops due to this stress in an XLPE tank, the cross-linked polymer chains provide a safeguard to help prevent further splitting—unlike potential “unzipping” that can occur in linear polyethylene tanks.
In this article, we’ll look at factors that cause stress cracking in polyethylene storage tanks, and offer solutions to reduce the impact of mechanical stress.
Mechanical Stress in Polyethylene Storage Tanks
Anytime you put a hole in a tank, it can provide an opportunity for stress cracking. We recommend putting sidewall fittings in a tank only when necessary. With cross-linked polyethylene, the polymer chains are bonded around the hole to minimize the risks.
The most common cause of mechanical stress in a polyethylene chemical storage tank comes from cutting a hole in the storage tank sidewall to accommodate fittings. When a hole is created and a fitting is attached, a binding effect is created, causing a cantilevering action. As a result, the polyethylene tank wall expands and contracts under stress. Rather than allowing the tank to expand and contract naturally, cracks can develop near the fitting holes. This makes flexible connections vital to a successful storage system.
Fittings should always be positioned above the knuckle radius where the wall thickness is most even. This allows the tank to expand and contract without causing the fitting bolts to bind in the area of the cut or drilled holes. Poly Processing's IMFO® Tank can eliminate the need for sidewall penetration altogether.
It's also important to use flexible connections or expansion joints with the correct performance specifications to allow for tank expansion and contraction. Poly Processing's team of chemical storage tank experts can assist in getting you the right fittings, proper plumbing, and accessories needed to provide the longevity you expect with a cross-linked polyethylene tank.
Manufacturing Robust Storage Tanks
The American Society for Testing and Materials (ASTM) has established four tests for cross-linked polyethylene tanks to help reduce the possibility of stress cracking. These tests ensure that the tank’s materials and composition meet the safety standards necessary for chemical storage tanks.
One of those tests, the ASTM Gel Test, provides a precise measurement of the concentration of cross-linked resin in a polyethylene tank. More cross-linked polymers provide a stronger molecular bond and greater resistance to mechanical stress. Poly Processing periodically tests a sampling of our tanks to ensure that we’re consistently manufacturing XLPE tanks that meet or exceed the ASTM standards and will provide the stress resistance you need.
Check out this video to see our rigorous testing procedures:
The Gel Test is performed after a tank is molded. We cut a small 1/8” sample out of the interior tank wall and weigh the sample. We use a piece of the interior tank wall because that’s where the polyethylene will interact with the chemical. After weighing, the sample is boiled in a solution of Cyanox 2246 and xylene, which strips away all of the resin that isn’t cross-linked.
The remaining sample is weighed and compared to the original weight. By doing a simple percentage calculation of the remaining weight of the tank sample, we get a reading on the tank’s structural integrity.
The ASTM Gel Test requires that the percentage of cross-linked resin in a polyethylene tank must be 60% or greater at the innermost 1/8” of the tank wall. Anything less is unacceptable and would result in a structurally unsound tank.