Proper chemical storage is an important aspect of operating a plant or facility. In order to ensure that chemicals are stored safely in polyethylene tanks, you have to choose or custom-build a tank with the correct wall thickness. But what goes into determining the correct wall thickness for your chemical storage tank?
Typical Polyethylene Tank Walls
Tank walls come in varying thicknesses depending on what they are designed to store and its basic geometry. Starting with the chemical and operating temperature, typical tank thicknesses vary based on the Specific Gravity Rating (SPG) of the tank. We manufacture tanks with spg’s of 1.35, 1.65, 1.9 and 2.2. The tanks can range from .20 inches thick to 1.5 inches thick, depending on its geometry and where the measurements are taken. A 1.65 spg tank that is very short and flat will have a thinner wall than a 1.65 spg tank that is tall and narrow. This is because as the diameter of a tank base decreases and the height increases, the pressure on the bottom sidewall of the tank increases based on the weight of the fluid. In order to offset this weight, a thicker wall is needed to accommodate for the extra pressure that is exerted on the sidewall and floor of a tall, narrow tank.
A fluid's spg and operating temperature is directly proportional to the weight and pressure a chemical exerts at the bottom of a tank. People often ask how thick a chemical storage tank wall needs to be in a 1.65 spg tank. It will vary based on the geometry of the tank.
Factors in Determining Wall Thickness
The formula for this geometry comes down to the allowable hoop stress, basically the force applied perpendicular to the radius of an object, of the construction material, which refers to the strength of the material at any given area of the tank sidewall. Hoop stress will be high if the tank has a small diameter, because it is measured in pounds per square inch of pressure in that tank.
A six-foot diameter tank holding the same chemical and gallonage as a twelve-foot diameter tank will have less surface area on the bottom sidewall which means it will require thicker walls to compensate for the increase in hoop stress calculated for that tank design.
Testing to Ensure Correctness in Wall Thickness
Once the necessary wall thickness is determined and the tank has been manufactured, we perform impact tests that will tell us the impact strength of that particular cutout of plastic and let us know that the tank was properly cured. We then can run ultrasonic thickness tests on different degree locations and elevations of the tank sidewall in order to determine wall thickness in specific areas. These tests ensure that the wall thickness of our tanks conform to ASTM standards and that our tanks can handle the pressure exerted from the chemical being stored.