Processes in the pharmaceutical, biotechnology, food and semiconductor industries must meet a high set of standards to ensure high product purity. Equipment criteria specific to high-purity processes are established to minimize contamination and maintain product integrity. In designing a pristine process, material and equipment style are of upmost importance. Bacteria is the main cause of contamination and is prone to growing in the dead cavities of equipment created by sharp corners, crevices, seams and rough surfaces. Another source of contamination is leaking, which allows undesirable chemicals to compromise the quality of the process ingredients, by causing contamination, rusting and particle generation.
Materials of Construction
Many factors must be taken into consideration when selecting materials of construction for use in pristine process applications where high-purity and sanitation are paramount. All surfaces should be constructed of a smooth material that will not corrode, generate particles or harbor dead cavities. These criteria can be met with three standard materials: 316L stainless steel (SS), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). The advantages and disadvantages of these materials are summarized below to facilitate the material selection process for a given application with consideration of chemical compatibility, cost, and temperature stability.
Equipment Style Selection
Critical factors in high-purity equipment selection include cleanability, cost, flow capabilities and product compatibility. With these considerations in mind, criteria useful for choosing pumps, valves, seals and piping are described in this section.
Pumps
A fundamental requirement of pristine processing pumps is the ability to clean a pump in place without disassembly. Pump seals, gaskets and internal surfaces should eliminate the buildup of material and should clean out easily during wash cycles. The most common pump styles for high-purity processes are centrifugal, lobe-style and peristaltic pumps, which are outlined below.
Valves
Valves should not harbor contaminants and must be easy to clean. By these criteria, diaphragm and pinch valves are excellent choices for ultrapure processes, as they have smooth, gently curved surfaces that will not harbor contaminants. Ball check, full-port plug and full-port ball valves are good choices as well, while butterfly, spring check, gate and swing check valves are all unacceptable, since contamination can collect in the corners that are essential to their design. Though several valves are appropriate for pristine processes, certain valves are better suited for particular applications.
Diaphragm valves are the most widely used in high-purity systems for their resistance to contamination and ability to be used as a control valve. Ball and plug valves, on the other hand, are less costly and are not limited by temperature and pressure. Also, in applications using sterile steam and freeze-drying, ball valves are preferred over diaphragm valves because they eliminate the risk of catastrophic seat failure.
Seals
As with all pristine processing equipment, high-purity seals should not have any cavities where contaminants can breed. By choosing a seal with gland rings that do not need to be threaded or ported, the areas where bacteria can breed are minimized. In choosing a seal material, it is important to find a compound that will not swell, crack, pit or flake, thus reducing seal failure and contamination. To ensure the success of seals, fluroelastomers are a top choice in pristine processing applications for their excellent thermal stability, chemical resistance and mechanical durability.
Piping
The surface of piping, as well as any wetted equipment parts, should have a very smooth surface. When 316 SS is being used, electropolishing is a good method for achieving an ultra-smooth finish. Joining methods should minimize crevices and dead cavities, and all materials should be free of biological degradable substances, leachable substances, and glues and solvents that may migrate into the product stream.
References
1. Smith, B., What Makes a Pump for High-Purity Fluids?, Chem. Eng., pp. 87–89, April 2002.
2. Schmidt, M., Selecting Clean Valves, Chem. Eng., pp. 107–111, June 2001.
3. Wulf, B., Pristine Processing: Designing Sanitary Systems, Chem. Eng., pp. 76–79, Nov. 1996.
4. Weeks, D. T. and Bennett, T., Specifying Equipment for High-Purity Process Flow, Chem. Eng., pp. 27–30, Aug. 2006.
In some cases, the cause of failure may be difficult to determine due to the complexity of the seal construction. These unique failure modes can result in flaking or peeling of the seal face, corrosion, flaking or pitting of the carbon faces, degradation of the elastomer energizer seals, and spring or bellows breakage. It is likely that these rapid degradations are a result of contamination, which can be avoided with careful installation or using pre-assembled, cartridge-type mechanical seals.
En la actualidad, en los modelos digitales aparece el valor del momento de torsión, el esfuerzo de cizalla y la viscosidad como un número en la pantalla digital. Los dispositivos electrónicos internos realizan los cálculos automáticamente. Los modelos más sofisticados tienen conexión con un ordenador y el software permite el almacenamiento de los datos y la representación en tiempo real de la curva de flujo, entre otras cosas.
Estas medidas con Brookfield en vasos de precipitados son medidas de viscosidad aparente, no absolutas (no reproducibles en otros viscosímetros diferentes), por lo que es necesario informar del cilindro y de la velocidad utilizada.
