PVDF piping used for nuclear facility acid handling project

When a prominent nuclear fuel processer was in the design phase with a system processing combinations of hydroflu­oric acid, sulfuric acid, nitric acid and deionized (DI) water, it chose Kynar PVDF piping systems to secure the expected lifetime of its fluid handling process. Compared to commodity plastics and metals, the fluoropolymer, Kynar PVDF excels in withstanding highly acidic chemistries.

Industry experience indicates that sulfuric acid and hydrofluoric acid can be aggressive to high performance metals and typical polymers. There are countless stories of corrosive attack and stress cracking failures with those materials. Kynar PVDF, on the other hand, has many documented successful case histories handling these chemicals individually and as mixtures.

Major concerns in the aforementioned nuclear design were chemical attack, system pressure, wide temperature range and process safety. After evaluating a complete Kynar PVDF piping system including pipe, fittings and valves versus a similar system made from Hastelloy, the end user decided that the Kynar PVDF option would be the most cost effective for long life performance.

Chemical handling

There are a host of chemicals involved in the process and each of them on their own present difficult han­dling issues. In cases where they are combined, there are additional concerns related to potential tempera­ture excursions and by-products.

Simtech process systems has a complete line of plastic piping with various joining methods. Selec­tion of the proper joining method can save time and add safety to the construction of the fluid handling system. Simtech also provides systems fabricated offsite by their engineering team or onsite training for the maintenance professionals operating within a facility.

Along with the piping and fittings, Simtech offers a variety of valves and instrumentation made with Kynar PVDF to assure fluid contact surfaces can be the same material. The types of valves used in this process were horizontal check, pressure relief and manual and actuated ball. (See Figure 1 for photos of components.)

Figure 1: Close up of welded valves and piping. Courtesy: Simtech Process Systems.

The fluid handling systems involve outdoor transfer of chemicals to an indoor facility. Extra care was pre­ferred for this transfer so the user requested double containment for moving the acids from the outdoor unloading area to the indoor process area. Kynar PVDF piping was easily welded by socket fusion or butt fusion in the double containment system (see Figures 2 and 3).

Figure 2: Double containment using clear outer plastic containment. Courtesy: Simtech Process Systems.

Figure 3: Double containment assembly of Kynar PVDF piping. Courtesy: Simtech Process Systems.

The double containment systems handle 93% sulfu­ric acid and 49% hydrofluoric acid. The diameter of the inner pipe was 1 inch and 1½ inch and the outer pipe needs to be sized to easily fit the transfer pipe inside.

In the process system, the same sulfuric acid and hydrofluoric acid are processed through 1½ inch and above diameter dual laminate piping. The Armor Tech dual laminate piping combines an inner layer of Kynar fluoropolymer with an outside support layer of FRP. The outside support layer gives strength and protection to the more corrosion resistant thermoplastic liner. In pipe sizes 1 inch and below, for the same process system, Simtech provided solid Kynar PVDF piping that was joined by socket fusion for both acids. All systems on this project were successfully hydrotested before commissioning. See Table 1 for the piping specifications.

Table 1: Pipe specification for acid systems. Courtesy: Simtech Process Systems.

System design

For the solid piping system, butt fusion was chosen as the joining method because the thick lap joint provides a level of safety when containing very strong chemicals. In the dual laminate system, butt fusion was chosen because the overwrap of FRP provides the additional safety should a weld be somehow compromised (See Figure 4).

Figure 4: Simtech dual laminate piping. Courtesy: Simtech Process Systems.

In the process, the temperatures range from -18°C to 50°C and the pressures inside the pipe range from 20 psi to 50 psi. In this type of service, Kynar PVDF provides performance that other polymers could not achieve.

Recycle stream

To add to all of the design considerations, after the chemicals are used in the process, they are combined in a chemical waste line. The chemical waste line is made up of DI water, 67% nitric acid, 49% hydro­fluoric acid and 93% sulfuric acid. This process is expected to run at 50°C and 32 psi, however, it is designed for upset condition of 82°C and 40 psi. Again, the use of Kynar PVDF in this combination of chemicals and temperature gives a nice range of safety in the design. See Table 2 for actual conditions of the recycle stream.

Table 2: Recycle acid composition. Courtesy: Simtech Process Systems.

Final thoughts

By choosing Kynar PVDF to handle a combination of 93% sulfuric acid, 49% hydrofluoric acid, 67% nitric acid and DI water, the nuclear facility was considering long term operating costs with minimum expected maintenance over a long period of time. A combina­tion of solid Kynar PVDF socket fused pipe and butt fused dual laminate pipe were installed depending on the size of the pipe and the chemistry involved. In addition, by using double containment technology, the facility engineers were designing for the ultimate in safety.

References

Kim, John; “Selection of Materials Used in Power Plant Chemistry Equipment and Operation,” Ultrapure Water, December 2004, pp 20-26

“Materials of Construction Guidelines for Hydroflu­oric Acid Solution (Aqueous),” 2014 Hydrogen Fluoride Industry Practices Institute, Washington D.C, p. 7, p. 12.

“Kynar PVDF Helps Maintain Product Purity, Mini­mize Maintenance at Ashland Chemical,” Spotlight Kynar Volume 17 Number 2, A History of Performance, ELF Atochem, p.5.

Beach, Steve; “In-Situ Relining Saves Shutdown at Dow Corning,” Plant Services, Putman Publishing, October 2005.

Original content can be found at Plant Engineering.

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