The debate rolls on: stainless steel versus plastic tanks
Stainless may be more glamorous, but when it comes to process suitability and costs, which really is best for your unique application?
By Rob Jewett
Good managers always look for ways to streamline business processes, be more efficient and reduce expenditures. This applies also, of course, to the business of production processing and managing fluids. Managers who oversee acquisition of the required tanks and associated equipment must understand the role tanks play in their specific applications and the range of available tank options, along with procurement and maintenance costs.
Tanks used in processing need to meet specific standards and criteria based on fluid types involved and the particular process or stage of its management. This is especially true in high-purity water, biopharmaceutical, food-related and laboratory applications. Here, tanks are often custom built and expensive. But costs in this area can be reduced by choosing tanks made from materials other than the old standby, stainless steel.
”Stainless” has long been the automatic choice in industries where high purity, easy cleanability, and entirely repeatable processes are critical. It’s shiny and attractive, it’s reliable and it just feels like the right choice — or even the only choice — to lots of companies needing tanks for processing and storage. Many in industry continue to recite the “stainless mantra” when it comes to tanks and tank materials. However, for a great number of applications, today’s plastic tanks provide the same or better performance — often at a lower cost and with significantly shorter lead times.
Alternatives to stainless
Application specifics themselves go a long way in answering the question as to which tank material is more appropriate. Most important is understanding the properties of the materials handled and full range of process or storage requirements. Quite simply, if a tank is open to the atmosphere and extremely high temperatures aren’t involved, you don’t need steel. Answers to a few other key questions about the fluid and application requirements help determine whether a stainless or plastic tank is the right choice.
First, what is the process pressure inside the tank? If it will be filled or drained faster than the tank’s vents allow air to move, stainless steel may be needed. This is so because of steel’s tensile strength under pressure. It also can more readily be fabricated to withstand vacuum situations.
Nevertheless, while a plastic tank molded from polyethylene (PE) or polypropylene (PP) has less ability to resist differential pressure, it can be fitted with special hardware to enable it to balance atmospheric pressure and maintain the tank’s integrity.
Next, define dimensional tolerances. A molded plastic tank may have dimensional tolerances as large as +/- 3%, and, with temperature change, it expands and contracts more than steel. This fluctuation and variability can be accommodated with simple measures, such as flexible piping or hose used for plumbing connections. In those rare applications that require all plumbing to be rigid, stainless steel is the better choice.
Hot to the touch
Another important consideration is temperature. What is the temperature of the fluid to be stored? If it falls between -40 C and 95 C, polyethylene or polypropylene can accommodate it, whereas stainless steel — or a specialized fluorinated polymer such as polyvinylidene fluoride (PVDF) — will be needed for heated liquids.
Polypropylene tanks work well in heat-sanitized ultrapure water systems, water-for-injection applications and other processes that call for handling fluids up to 100 C.
Finally, what are the physical and chemical properties of the substance that will fill the container? Abrasive substances, solvents, petroleum products and even some detergents with particularly strong surfactants should be stored and processed in high-grade stainless steel because they can cause plastic to degrade. It should be noted, however, that lower grades of stainless steel, which have minimal concentrations of molybdenum (the element that resists corrosion), are far more apt to corrode, meaning any cost-saving opportunity over higher-grade steel is lost because the cheaper steel will likely fail.
The likelihood of corrosion is higher as the concentration of acids, chlorides or oxides in the fluid increases; corrosion also occurs more readily in high-temperature processes. Again, before making the final selection of tank and tank material, the process manager must understand the properties of the fluid at hand.
Everyone understands the simple adage that a penny saved is a penny earned, and plastic can save a pretty penny over stainless steel and other tank material choices. For processing a given fluid that is compatible with stainless or plastic, plastic can deliver significant savings. That’s the case regardless of whether the comparison is to lower-cost 304 steel or higher-grade 316L.
A moderately sized (up to 1,500 gallons) stainless steel tank with a bottom-entry industrial mixer, for instance, can cost twice as much as the same mix tank made of plastic. With larger tanks, savings are realized more in terms of time than cost; large stainless tanks generally take from 12 to 20 weeks to fabricate and deliver, compared to 6 to 10 weeks for poly tanks.
When cost is a consideration, alloys such as Hastelloy and specialty polymers such as PVDF are immediately out of the running. And while Teflon has a high-temperature capacity and is impervious to nearly everything, it, too, is expensive — as well as making fabrication complex and dangerous.
Plastics clearly lead the way for ease of fabrication — translating immediately into lower costs and shorter lead times. Polymers can be molded and fabricated into a wide variety of sizes, shapes and styles, including flat bottom, cone bottom, open top and dome top (a necessity for ultrapure water applications) — all with less difficulty and expense than when working with stainless steel.
Plastic’s other advantages
If pigments are omitted, plastic tanks can be — and often are — made to be translucent. Fluid levels can be determined at a glance, whereas tanks made from stainless have mechanical gauges, viewing ports, electronic sensors or other devices to monitor changing volumes.
Interestingly enough, translucence can actually be a requirement for certain specialized applications. One example is a specific type of biofuel production that uses photosynthetic bacteria to produce ethanol. Here, sunlight is a necessity, so the opaque stainless tank is not an option for this process.
Other specialized processes that are better suited to the use of poly tanks are those with high-purity requirements, such as biopharmaceutical applications. As long as the fluid at hand is not highly abrasive or acidic, as discussed earlier, it is better suited to a plastic tank because of the material’s resistance to pitting. Stainless steel, especially the lower grades, is often subject to adverse chemical reactions between the tank contents and the tank, causing pits to form on the inner surface. These areas can become nucleation sites for bacteria, quickly contaminating the contents. This is especially detrimental in the pharmaceutical and food industries, where companies must demonstrate to the FDA a 100% repeatable cleaning process — something that cannot be achieved with a pit-damaged tank.
Plastic tanks also offer superior performance in ultrapure water (UPW) applications. Stainless steel — even a higher grade such as 316L — often fails in such applications, as the ionized water draws ions out of the steel, creating ferric oxide, hydroxide or carbonate as the tank’s inner surface degrades. Because this rusty residue is generally red (although it can range from orange to black), the phenomenon is called rouging. The rouging residue covers the tank and is distributed and deposited throughout the entire processing system — obviously an unacceptable situation. Plastic used in tank fabrication, though, is chemically inert, so it is immune to corrosion and rouging.
The safe choice?
Stainless steel has a large following in industry. This is perhaps more because of human nature than good science. Sometimes it’s just force of habit; using steel is just “what we do,” they say. Others succumb to the over-engineering syndrome — it’s a safer bet, and safer is better. And then there are some who simply aren’t aware of their other choices.
However, as the number of engineers who exclaim, “I didn’t know you could weld plastic!” falls, the number of thermoplastic tanks put into use is rising. Ultimately, when companies start to look hard at the options — all the options — they’re likely to embrace plastic tanks in even greater numbers.
Plastic has become ubiquitous in industry — and in everyday life — because of its great versatility as a problem solver. This is certainly so when it comes to fluid management. In many applications, today’s plastic tanks outperform stainless steel tanks — at a lower cost and with shorter lead times. Managers who make informed decisions and employ plastic tanks where they are well suited can save considerable time and money while improving their manufacturing and processing capabilities. The overall result will be a business that moves ahead — and stays ahead — of the competition.
Rob Jewett is the president of Terracon Corp. He has 30 years’ experience in water treatment and fluid monitoring, control and containment. Terracon Corp. began manufacturing non-metallic fluid-management tanks, vessels and mixers in 1976 as a cost-saving alternative to stainless steel. Since then, Terracon has established a reputation for solving complex fluid management challenges with creative and cost-effective custom solutions.