How to Determine Proper Venting for Polyethylene Tanks

Proper venting of chemical storage tank systems ensures safe operation and also extends the useful life of the tank. This article will examine how to determine proper venting for polyethylene chemical storage tanks.

Tank Pressurization Study

At Poly Processing we noticed a great deal of damage to tanks from over pressurization. So, several years ago, Poly Processing commissioned a third party engineering firm to study the impact of pressure on pneumatically filled chemical storage tanks due to different applications or settings. The prevailing industry standard regarding venting was to use a vent one inch larger than the fill line; for example, a two-inch fill line requires a three-inch vent. The study determined the standard to be adequate is pumped mechanically. However, bulk storage tanks are often pumped off a tanker truck using air. As the rate of pumping increases, so does the chance for a surge of pressurized air entering the tank, which could result in over pressurization with standard venting. As a result, the thinking in the market shifted. Now the standard for proper vent size in pneumatically loaded polyethylene tanks is double the size of the fill; for example, a two-inch fill line means using a four-inch vent. This allows the air to evacuate at a faster rate, reducing the risk for over pressurization. 

Choosing the Right Venting System

Polyethylene tank fittings and accessories such as vents can be expensive. While vents add to the initial cost of the tank system, proper venting could help avoid far more expensive occurrences like premature tank failure or even catastrophic failure. In addition to standard venting, Poly’s SAFE-Surge manway cover ensures that your tank maintains the proper ACFM at all times- even if the primary venting fails. For plastic chemical storage tanks located outdoors where fumes are not a concern, the F.S. 2650® Manway Cover is a less expensive option for safely handling any air pressure surges. This combines a vent and manway cover into one engineered lid system that can evacuate air volumes up to 2650 ACFM, which is well within the calculated maximum potential pressure surges resulting using both 2 and 3 inch fill lines (maximum ACFM from a 2 inch standard line is 910; and 1150 from a 3 inch line).

Poly Processing meets or exceeds ASTMD 1998 guidelines for tank pressurization. For your specific polyethylene chemical storage system’s proper venting requirements, it’s best to consult with our engineering department. For instance, scrubber applications like hydrochloric acid or acetic acid demand an elaborate design to prevent over pressurization.

Technical information and guidance regarding tank pressurization and proper venting for polyethylene tanks is available in the third party study. This tank venting matrix offers calculations for venting design. 

For additional information on polyethylene tank venting or to determine a venting system that meets your requirements, please contact a chemical storage tank expert.