It’s hard to judge, with the naked eye, how a polyethylene tank is constructed. On the surface, a tank is just a tank. However, there is a big difference between a well-designed, properly constructed tank and a cheap one that doesn’t meet structural requirements.
Not all tank manufacturers follow ASTM D-1998 standards, which are set up by the American Society for Testing and Materials to ensure that tanks are safe by testing the materials that make up tank construction.
4 ASTM D-1998 Tests
ASTM standards require all polyethylene tank and tank materials go through 3 tests that measure the consistency in the tank’s walls, the amount of impact the tank can withstand, and the structural integrity of the tank as a whole. There is also an additional test for cross-linked polyethylene tanks to measure the chemical makeup of the tank materials.
The ultrasonic test is performed to ensure a consistent wall thickness around the entirety of the polyethylene chemical storage tank. This test incorporates a sonar instrument that sends out measureable sound waves. By measuring the time it takes for the waves to travel back, we can calculate the wall depth. We do this test at various points around the diameter of the chemical storage tank, all the way up the tank wall to ensure consistency.
We’ve talked about the ASTM Impact Test before, in our blog post ASTM D1998 Impact Test: Will My Storage Tank Meet or Exceed This Standard? Like the gel test, a sample of the tank’s material is taken, generally from the manway area, and tested. Unlike the gel test, the impact test is designed to test the tank’s structural integrity through impact, rather than a chemical process. The sample of the tank material is frozen in a -20 degrees Fahrenheit freezer overnight, then placed into a machine where a “dart” is dropped at a certain foot-pound to create the appropriate impact for testing. There are different calculations for the required impact. The thicker the tank material, the higher the impact. This test is administered to verify the tank material can withstand mechanical stress and other challenges associated with long-term chemical storage.
In this test, the tank is filled to the dome with water. ASTM requires that it sits for 30 minutes to verify that there are no leaks and the tank can withstand the hydrostatic weight of the water. Poly Processing’s internal standard is to let it sit for an hour, at least, but it is common for some of our customers to request a 4 hour, 24 hour, and even 48 hour water test. We can accommodate our client's hydrostatic test requests. This test is performed on the tank as a whole, not just a sample of the materials used, so it not only measures the tank’s ability to withstand hydrostatic pressure, but it measures the fittings and accessories as well.
Performed only on cross-linked polyethylene tanks, the gel test measures the percentage of cross-linking in the wall itself. A sample of the tank material is placed in boiling xylene, which breaks down the linear polyethylene, the base material of the resin itself. All that is left is the cross-linked materials. The weight of sample prior to this process is measured against the weight of the sample after the process. That is where the percent of cross-linking is determined, and to meet ASTM standards it has to be at least 60%. If the gel percentage is too high though, it creates a brittle material. Since some chemicals can only be stored in cross-linked polyethylene tanks, it is critical that the tank’s crosslink percentage hits that sweet spot of 60% to 70%.
At Poly Processing, we manufacture all of our tanks to meet ASTM D-1998 standards. In fact, we strive to exceed expectations to ensure that all of our tanks provide the best chemical storage applications in the industry.