Fluorinated plastics can be used as internal linings in process-equipment components for highly corrosive applications in chemical processing, and material development has allowed conductivity and compliance with food-contact applications
The industrial production of chemicals forms the foundation for a wide range of downstream processes and applications that are indispensable in our modern world. When highly corrosive media, such as hydrochloric acid (HCl) or caustic soda (NaOH), are produced and transported, the selection of suitable wetted materials for pipelines, containers and equipment components becomes especially important. These materials must reliably withstand not only mechanical stresses, but also chemical and thermal loads. Common metallic materials, such as stainless steels, are often inadequate in these cases, and special alloys (including nickel-based alloys, zirconium, titanium or tantalum) either have limited application ranges, or are ruled out for economic reasons.
Therefore, in highly corrosive applications, containers, pipelines and equipment components, such as pumps and valves, are primarily equipped with internal fluoropolymer linings. In this design, the metallic shell bears the mechanical load, while the inner lining ensures chemical resistance. Fully fluorinated plastics, such as polytetrafluoroethylene (PTFE) and perfluoroalkoxy co-polymer (PFA), are particularly used as lining materials due to their universal chemical resistance.
The sectional illustrations (Figures 1A and 1B) highlight the design principle of a highly corrosion-resistant pump and valve lined with fluoropolymer.
FIGURE 1A. This cutaway shows a fluoropolymer lined pump
FIGURE 1B. This cutaway shows a fluoropolymer-lined valve
Application areas for corrosion-resistant process equipment are found in basic chemical industries, as well as in specialty chemicals. Typical processes for such systems include bromine production, chemical waste treatment, chlor-alkali electrolysis, methylene diphenyl diisocynate (MDI) and toluene diisocyante (TDI) production, acid production and treatment, titanium dioxide production and fertilizer manufacturing.
In the market segments for fine and specialty chemicals, as well as in the pharmaceutical and food industries, processes include the production of crop-protection agents (insecticides/herbicides), dye and pigment manufacturing, biopharmaceutical technologies, and active pharmaceutical ingredient (API) production.
Additional market fields for lined components include ultrapure water for semiconductor manufacturing, water treatment, clean-in-place (CIP) systems in food-and-beverage production, mining, metal recovery and the transport of hazardous materials by road, rail and ship.
PTFE versus PFA
Polytetrafluoroethylene (PTFE) was discovered by American chemist Roy J. Plunkett and later became globally known under DuPont’s tradename Teflon. PTFE is a fully fluorinated plastic. Its carbon backbone is completely shielded by fluorine atoms, which gives it universal chemical resistance and the highest thermal stability among plastics.
Due to its high molecular weight and melt viscosity, PTFE is processed by pressing and sintering. In the manufacturing process, it is either hydraulically pressed into molds, or a rubber bladder is inserted into the hollow body to be lined — a valve housing, for example — and the gap between the bladder and the metal wall is filled with PTFE powder. The powder is then compressed under high static pressure in a process known as isostatic pressing. The PTFE lining is subsequently sintered in an oven.
Perfluoroalkoxy copolymer (PFA) offers chemical resistance comparable to PTFE, but unlike PTFE, it is thermoplastically processable and transparent. This allows PFA to be processed using methods such as transfer molding, injection molding, extrusion or rotomolding. When melted, PFA forms a homogeneous melt that, in combination with metal-centered molding tools, ensures a uniform lining and consistent wall thickness. Variations in lining thickness — common in isostatic PTFE pressing using a rubber bladder — are eliminated with PFA. This offers a significant quality advantage to users. The transparency of the PFA lining also simplifies quality inspections by allowing visual evaluation of the lining results.
Conductive fluoropolymer
When examining the physical properties of PFA and PTFE, one finds a high specific volume resistivity of approximately 1018 Ω·cm. These two fluoropolymers are therefore not electrically conductive. When a PFA-lined pump or valve is used with a chargeable liquid (conductivity <10–8 S/m), electrostatic charges can accumulate on the surface of the lining. This can lead to discharges, both inside and outside the component.
Conductive fluoropolymer contains a small amount — about 2% — of conductive pigment in the form of graphite, carbon black, or graphitized carbon. The filler is not chemically bonded to the fluoropolymer’s molecular chain, but rather is embedded within the material matrix.
As a result, dangerous electrostatic charges cannot build up, since they are safely dissipated through the lining and the metallic housing (surface resistance <109 Ω and discharge resistance <106 Ω). The conductive version of PFA enhances safety by dissipating static electricity, thereby preventing the risk of sparking or explosions caused by electrostatic discharge.
Another significant advancement is the FDA compliance of the conductive PFA lining. This ensures that the materials and products meet the strict requirements of the U.S. Food and Drug Administration (FDA; Washington, D.C.; www.fda.gov). This is especially important for applications in the pharmaceutical and biotechnology industries, where the highest standards of safety and purity are required. FDA-compliant materials are characterized by their durability and their ability to avoid releasing harmful substances into the products that come into contact with those materials.
Product examples
For use in highly corrosive, safety-critical and purity-sensitive processes, a wide range of valves and pumps with conductive, FDA-compliant PFA lining (conductive PFA) is available. The lining is applied using the transfer molding process, ensuring a guaranteed minimum thickness and uniform wall structure. These products meet international standards, such as TA-Luft, DIN EN ISO 15848-1, FDA 21 CFR§177.1550, and the Pressure Equipment Directive 2014/68/EU.
Heavy-duty ball valves and control ball valves. These ball valves feature a one-piece, PFA-encapsulated ball/stem unit with a cavity-reduced design (Figure 2). The self-adjusting stuffing box is maintenance-free and gas-tight, in accordance with TA-Luft. The seat rings are made of pure PTFE. The design is suitable for both manual and automated operation.
- Temperature range: –60°C to +200°C
- Flange connections: ISO 7005-2 or ASME B16.5
FIGURE 2A. These ball valve housings have PFA lining
FIGURE 2B. The stuffing box of these ball valves is maintenance-free and gas-tight
Shut-off and control butterfly valves. These butterfly valves feature a one-piece disc-shaft assembly encapsulated with conductive PFA. The shaft sealing system is maintenance-free and self-adjusting. These types of valves are suitable for aggressive media, such as acids, bases and solvents, and offer high mechanical strength. Optional versions are available with pneumatic or electric actuators.
Bellows-sealed control valve. The bellows-sealed control valve features a dead-space-free design and a top-entry construction for easy maintenance (Figure 3). It is equipped with a thick-walled conductive PFA lining (≥5 mm), offering universal chemical resistance and FDA compliance. The valve enables precise control from as low as 0.01 m³/h and is suitable for both liquids and gases
FIGURE 3. Conductive-PFA-lined control butterfly valve are designed for aggressive media
Safety valves. The latest generation of safety valves is designed for maximum discharge capacity and complies with EN ISO 4126-1 and AD 2000 Code A2. The conductive PFA lining prevents static charging and is FDA-compliant (Figure 5). These valves are suitable for use in potentially explosive atmospheres.
FIGURE 4. A bellows-sealed control valve for high-purity processes is shown
FIGURE 5. Safety valves with conductive PFA lining can be used in hazardous areas
Sight glass. The sight glasses enable visual monitoring of processes under aggressive conditions (Figure 6). The housing is lined with conductive PFA, and the sight glass panes are made of borosilicate glass. The antistatic lining prevents electrostatic discharges. The design is suitable for CIP and sterilize-in-Place (SIP) processes.
FIGURE 6. Sight glass with conductive PFA lining can be used for visual process monitoring
Magnetically coupled centrifugal pumps. These pumps are fully lined with conductive PFA and feature media-wetted components made of PFA, silicon carbide (SiC), or zirconium oxide (Figure 7). They are hermetically sealed, leak-free and both ATEX- and FDA-compliant. The pumps are available in close-coupled (block) or frame-mounted (process) designs and are suitable for flowrates ranging from a few liters to several cubic meters per hour. Vertical installation is possible for automatic residual drainage
FIGURE 7. A magnetically coupled centrifugal pump with conductive PFA lining is shown here
Conductive linings
Market trends in lined pumps and valves show a steady increase in the use of conductive linings in recent years. With the availability of conductive and FDA-compliant materials, these linings are now preferred and increasingly used not only in large-scale chemical production but also in pharmaceutical and specialty chemical applications as a preventive safety measure.
Selection criteria
The decision to use a conductive PFA lining should not be made generically, but should be based on thorough application engineering.
Chemical resistance is limited to the resistance of the filler material. For oxidizing media, such as bromine or chlorine, or for substances that chemically attack carbon-based fillers, the material should not be used, as the carbon filler may be leached from the PFA matrix. If conductive PFA is used with media prone to high permeation, it must be evaluated whether the presence of filler could further increase permeation.
Despite the low filler content, the fluoropolymer is not transparent, but black. Therefore, visual inspection or high-voltage spark testing of the lining is not possible, unlike with transparent, unfilled PFA. As an alternative, surface inspection using dye penetrant testing can be applied. Today, food-grade dye penetrants are available for this purpose.
Concluding remarks
The application range of valves with thermoplastic PFA fluoropolymer linings has been significantly expanded through new material developments. Today, users have access to PFA materials that combine conductivity with FDA compliance. Successful implementation requires aligning material properties with process requirements to fully leverage the benefits of fluoropolymer linings.
Edited by Scott Jenkins
Acknowledgement
All photos appear courtesy of Richter Cheme-Technik GmbH.
Author
Manfred Sett is a Product Development Engineer at Richter Chemie-Technik GmbH (Otto-Schott-Strasse 2, D-47906 Kempen Germany); Phone 02152146149; E-Mail: msett@idexcorp.com). He has over 35 years of experience in design and construction of process equipment comprising control, safety and shut-off valves, materials used in corrosive applications as well as application engineering. He is a graduate engineer for mechanical engineering.