When you think about a chemical storage tank system, you have to consider the factors that impact the tank’s service life. Not only do you have to make sure that the tank material and fitting materials are compatible for the chemical you are storing, it is also important to consider the functionality of the tank. For example, how it responds to pressure.
Pressurization in Polyethylene Tanks
ASTM D 1998 standards allow for some working pressure in a polyethylene tank. A pressure rating of up to 10 inches of water column (0.361 psi) is considered acceptable, though limiting the water column height to 6 inches (0.216 psi) increases the margin of safety. Polyethylene tanks including Poly Processing's Crosslinked tanks are designed and rated for zero vacuum. Cross-linked polyethylene (XLPE) tanks are robust; the molecular bond gives the tank better mechanical properties over high density polyethylene. In a study conducted on pressurization, the XLPE tank could withstand more pressure than HDPE, and when the tank did eventually fail, it did not experience catastrophic failure as the HDPE did. But it’s still necessary to avoid tank pressurization, even with XLPE tanks.
Pressurization and Vacuum Affect Service Life
Pressure and vacuum are leading reasons why a tank will experience less than expected service life. Vacuum is the opposite of positive pressure, but it plays a role in the wear and tear on a polyethylene tank. If a tank is sealed and chemical is discharged, the tank could collapse if proper venting is not in place. Conversely, if too much air builds up in the tank, especially during pneumatic fill, the plastic experiences stress and can fail. Making sure there is only atmospheric pressure, without vacuum or positive pressure, is the best way to store chemical safety and to maximize the tank’s service life.
Chemicals and Tank Pressurization
Many static chemicals will build pressure given time. There are certain chemicals, however, that fume more than others, and that fuming creates more pressure. Hydrochloric acid, acetic acid, and ammonias are highly fuming chemicals that should be stored with a scrubber system that scrubs and discharges the treated chemical fumes. It is important to make sure the scrubber system does not create more back pressure in the tank than the ASTM D 1998 standard of 10 inches of water column. A scrubber system with proper pipe sizes and adequate plenum holes or slots should be used to avoid over pressurization of the tank. Poly Processing can match a specific scrubber system to a specific tank.
There are other ways to ensure that your chemical storage tank does not become over pressurized. Choosing the right manway cover, making sure the materials in your tank system are compatible with the chemical application, and making sure that all fittings and accessories are properly connected are all best practices that should be followed whenever you’re storing chemicals.
Talk to a chemical storage expert for further design recommendations.