In virtually all sterile or sanitary applications the first choice of materials for hardware construction is stainless steel. Use of this material throughout the pharmaceutical and food and beverage industries is extensive and includes storage and transfer vessels, piping, valves, pumps, etc. It was included as a requirement in the proposed CGMP for Large Volume Parenteralsunder “Water and other liquid handling systems.” There are numerous classifications of stainless steel alloys designed for a wide range of applications. First stage blading for combustion gas turbines, for example, are exposed to corrosive gases at temperatures of 1400°F or higher and must withstand these conditions for periods of months or even years. Special alloys have been developed to meet these applications.
In the sterile and sanitary process industries these extremes are not encountered, although high purity water is known to be corrosive and, as seen earlier, it is frequently stored and recirculated at 80°C. In addition, they are subjected to steam sterilization at temperatures of 121°C or higher. The stainless steel grades most often selected are 304 and 316. Grade 316 contains approximately 17% chromium, 13% nickel and 2.5% molybdenum. Addition of the molybdenum distinguishes 316 from 304 and provides a higher degree of corrosion resistance.
Low carbon content 316 stainless steel, designated at 316L, wherein the carbon content is no greater than .03%, is the primary choice for product contact surfaces, including WFI. The low carbon content limits chromium carbide precipitation at grain boundaries, which will lead to intergranular corrosion, particularly at weld areas.
When employed in sanitary systems these materials are provided with a surface finish designed to impart the characteristics of cleanability and avoidance of contamination hold-up along with resistance to corrosion. A number of surface finishes are available, such as that referred to as Number 4, a mechanical finish achieved by finish grinding with 140 to 150 grit. Final polishing is generally done to a minimum 180-grit finish. The term grit relates to the mesh size of the abrasive particles used in polishing.
Electropolishing is an anodic dissolution process that provides an extremely smooth surface free of pitting or crevices that can result in contamination entrapment. Since the process is one of removing the peaks of surface grooves the resulting finish contains the least surface area and is the least susceptible to corrosion. The process is applied to virtually all product and water contact surfaces and sometimes to non-contact surfaces as well for similar reasons as well as aesthetics.
ErtelAlsop designs proprietary plate and frame and lenticular filter cartridge equipment for the pharmaceutical, biological, biotech, food and beverage and cosmetics industries. You may choose from a variety of materials of construction to meet your needs, including carbon steel and polypropylene. But for sanitary applications, our first recommendation is one of the 300 series mentioned above with appropriate surface finish. ErtelAlsop also manufactures highly effective, validated pharmaceutical depth filter pads and lenticular format filter cartridges.
Please download our publication PH-2000 for additional information.