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Plastic or Stainless- What tank should I use?

September 16, 2013
Saint Gobain Polypropylene Process Vessels

Saint Gobain Polypropylene Process Vessels

The renaissance of plastic and single use products can be scary for stainless steel guys like us here at Holland Applied Technologies. However, as the industry looks to streamline and optimize processes, there isn’t any question that plastics will be an integral part of our future. While applications and uses of plastic tubing and flexible hose have been well documented, less focus has been places on the utility of plastic storage vessels. While stainless has historically been the automatic choice for the high purity industry, here are some things to keep in when purchasing your next tank.

First and foremost, the fluid and process specifics go a long way in selecting the appropriate tank material. For corrosive, abrasive, and high temperature products, plastics are out. However, if the process temperatures range from -40 C to 95 C, polypropylene and polyethylene will be able to handle it. Products with abrasives, solvents, and surfactants call for high grade stainless steel vessels as well. Custom PVDF and high density polyethylene’s can be used in these applications, but reduce commercial benefits of plastic bulk tanks. It’s worth noting, however, that lower grades of stainless, with low molybdenum contents, are far more likely to degrade and pit. Pitting creates a nucleation site where microscopic bacteria can proliferate. Plastic vessels are manufactured from virgin resins that make them easy to clean and effectively autoclaved.

The next thing to consider is process pressure and dimensional tolerances. As a general rule of thumb, if the product is not caustic or abrasive, between -40 C and 95 C, and open to atmosphere, a plastic tank is suitable. If it will be filled or drained quicker than the tank can vent, placed under vacuum, or jacketed, stainless will be the better choice. The tensile strength of steel gives it the ability to withstand high vacuum ratings, while polypropylene and polyethylene will need to be fitting with special vents and hardware to withstand high differential pressures. Plastic also expands and contracts much more than steel, with dimensional tolerances that can vary up to as much as +/- 3%.

Good applications for plastic tanks include high purity water storage, buffer mixing, and small scale production runs. Ionized water can draw ions out of steel, creating oxides that make a steel tank’s inner surface appear “rusty”. This chemical reaction, known as rouging, can be dispersed through the process, having negative effects when dispersed throughout the process.

Plastic tanks are versatile and can be more commercially viable than stainless. It is a common misconception that you cannot weld plastic. Not only can you weld plastic, but Holland provides all customers with blank tank drawings where they can pencil in which fittings they need and where they need them- it’s that easy. Turn around on custom plastic tanks- including fittings, mixer mounts, and custom stands can be as quick as 6-10 weeks. It often takes up to 6 weeks to get drawings on a steel tank and receipt of tank from date of order can often be in the 16-20 week range.

To conclude, while stainless steel has long been the staple of the industry, the “go-to” material, recent developments in materials and plastics engineering have made plastic storage tanks not only commercially viable, but a great alternative to the old standbys. For a great number of applications, plastic tanks offer the same or better performance at lower cost and a shorter lead time.

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