Chemical storage comes with many parts and pieces that need to be understood. One aspect we are often questioned about are the terms that are new to customers as they’re designing a chemical storage tank system.
Having over 40 years of experience in safe and effective polyethylene chemical storage, we recognize how a misunderstanding of complicated terms can affect your purchase decision. Our goal is to provide clarity on these common, yet misunderstood terms, explained for your convenience.
Let’s take a look at five key terms you might hear in the chemical storage industry.
You’ll see the term specific gravity used in regards to the chemical tank wall as well as the chemical itself. What does it mean? Broadly defined, specific gravity is “the ratio of the density of a substance to the density of a another substance.” In relation to the chemical itself, specific gravity is the ratio of the density of that chemical to the density of pure water. In relation to your chemical storage tank, specific gravity is a number that indicates wall thickness. A chemical’s specific gravity (SPG) of 1.9 requires specific inches of wall thickness, according to ASTM standards. At Poly Processing, a tank’s SPG is calculated for you to match what you need for the chemical you are storing. A 1.9 tank for example, meets ASTM guidelines of a necessary wall thickness to hold a chemical with a specific gravity of 1.9 or less. If your chemical had a specific gravity of 2.0, however, you would ask for the 2.2 tank wall option.
There is a big difference between cross-linked polyethylene (XLPE) and linear polyethylene. Linear polyethylene is a lower-density polymer that is made up of large chain branches. The larger branching restricts the polymer chains from getting closer to each other (less density). Its chemical makeup is weaker than XLPE because the molecules are not covalent bonds. Linear polyethylene has lower tensile strength that XLPE, can be broken down by strong oxidizers and can swell when storing certain solvents. XLPE is high-density polyethylene that contains a cross-link or molecular bond, which is when one polymer chain is linked to another. In the chemical storage world, XLPE is ideal because the thermoset resin can structurally withstand more stress. HDXLPE (high-density cross-linked polyethylene) is another term for XPLE, as cross-linked polyethylene is always high-density. For many related terms to polymers see our technical resource polymer glossary.
High-density polyethylene (HDPE) is a thermoplastic substance that has a high strength-to-density ratio. It has a higher tensile strength than low-density or medium-density polyethylene and can withstand higher temperatures. It does not provide the same safety features in a chemical storage tank that XLPE does, but there are many practical applications for it, such as water tanks.
TDH and Static Head
TDH (total dynamic head) is a term used in fluid dynamics, mostly by those sizing the correct pump to move chemical out of the tank. The tank itself can play a significant part in this calculation. TDH is the total corresponding height that a liquid or fluid is to be pumped, taking into account friction losses in the unit. TDH, therefore, is simply the pressure head difference between the inlet and outlet of the pump. In this measurement, "Static Height" is the max height reached by the fluid after the pump (the discharge point). This is known as the “discharge head.” The suction side of the pump, however, or "before the pump" uses a similar term called "static pressure head" or simply "static head", measured in length - such as the fluid height in a tank. So, static head is often the difference in chemical height in a tank and the pump inlet. The higher the static head (or the taller the tank), the easier the job for the pump, or less of a pump is needed. If a pump sizer needs more static head, one can provide a taller tank rather than a wider diameter. Likewise, a benefit of an IMFO design being flush with the tank floor provides additional feet of static head for the pump, among the many additional benefits.
This is also known as a bulkhead fitting. The bulkhead is a simple compression by threading, low cost fitting that is commonly used on water tanks and tanks under 1,000 gallons. A better-engineered alternative, Poly Processing’s B.O.S.S. Fitting® (bolted one-piece sure seal) is a tank fitting that utilizes a full face gasket for a single sealing point and without any bolt holes on the flange piece inside the tank. This significantly reduces the chance of leakage by eliminating bolt hole paths and includes an embedded steel ring which increases strength. Poly Processing’s B.O.S.S. Fitting® has a unique backing ring that makes it much stronger than plastic fittings and reduces the stresses associated with a mechanically attached fitting.
These are just of the few terms that come up during a conversation about choosing your chemical storage tank configuration.