FFKM and Kalrez®: Choosing the Elastomer in a Mechanical Seal

FFKM is a perfluoroelastomer with a fully fluorinated backbone, used as the last-resort material for the secondary seals of a mechanical seal. It has the widest chemical resistance and the highest temperature limit of any elastomer class, and it keeps a seal running in aggressive or mixed media where NBR, EPDM and FKM swell, harden or crack.

A significant share of early seal failures in the field starts not at the face pair but in the secondary sealing element — the o-ring, bellows or wedge. Even with the right face material, an o-ring that is incompatible with the medium swells and hardens within weeks and stops the seal.

This guide covers where FFKM departs chemically from FKM, and how brand names such as Kalrez® relate to the generic material. For seal types see mechanical seal types; for face and metal components see material selection for sealing products.

FKM, the fluorocarbon rubber family, is built on vinylidene fluoride, so its chain still carries carbon-hydrogen bonds. Those bonds give good resistance to oils and hydrocarbons, but ketones, esters, amines and hot steam attack the polymer at exactly those sites.

In FFKM the backbone is fully fluorinated. Tetrafluoroethylene and perfluoromethyl vinyl ether are polymerized with a third, cure-site monomer that supplies the cross-linking points, and no vulnerable carbon-hydrogen bond remains. Chemical resistance then approaches that of PTFE while the material keeps the flexibility and the recovery of an elastomer.

The trade-offs fall under three headings: unit price well above the other elastomers; limited low-temperature flexibility on standard grades, so cold duties need dedicated grades; and, in pressurized gas service, explosive decompression after a rapid pressure drop, which calls for decompression-resistant grades.

Kalrez® is a registered trademark of DuPont and names one of that company's products in the FFKM class; it is not the generic name. The generic name is FFKM, or perfluoroelastomer. An engineer who types "kalrez" into a search box is usually looking for an FFKM-class secondary seal, not for one brand.

The same distinction applies on the FKM side: Viton® is a registered trademark of The Chemours Company for FKM elastomers, and the query «viton salmastra», common in Turkish-language searches, in practice means a mechanical seal fitted with FKM elastomers.

Writing a brand name into a specification narrows the supply chain for no technical gain. State the generic class and the requirement instead: elastomer class FFKM, operating temperature, the media it contacts, and where relevant a note on food contact or decompression resistance.

In a mechanical seal the face pair forms the primary seal. The elastomer is the secondary sealing element: it seals the rotating ring against the shaft or sleeve and the stationary ring against the housing. It appears as an o-ring, an elastomeric bellows or a wedge; IS-1 Istanbul lists o-ring and wedge mounting in its data.

This element looks static but is not: the rotating ring must move a few millimeters axially so the faces can follow wear and shaft movement. A hardened or swollen elastomer locks that travel, the faces lose contact, and the leak starts at the elastomer.

Elastomer failures take five forms: swelling as the medium penetrates the polymer; hardening and cracking from temperature or oxidation; extrusion under high pressure through a wide gap; explosive decompression after a rapid vent in gas service; and compression set, the loss of recovery under sustained squeeze.

The figures below are approximate, grade-dependent ranges for a qualitative first screen; exact limits must be confirmed against the manufacturer's material data. DIN 24960 and EN 12756 identify elastomer classes by letter code: P = NBR, E = EPDM, V = FKM, K = FFKM and T = PTFE.

The FFKM decision follows from the whole duty envelope, not one chemical. Duties that combine an aggressive medium with high temperature come first: a chemical process that approaches the temperature limit of FKM while also carrying amines, ketones or caustic will not run on standard elastomers.

The second group is mixed and changing media. Where one pump handles acid, then solvent, then wash water, a single elastomer must suit the whole list, and FFKM is the class that covers it alone.

The third group is cleaning and emission duty. Hot caustic CIP and steam SIP cycles expose the elastomer to hot alkali and steam every batch; FDA-compliant food-contact grades are available. Where fugitive emissions are classified under ISO 15848-1, compression-set resistance is one of the elastomer properties that governs whether the required tightness class is held.

Using FFKM where it is not needed spends budget without buying service life. In clean water, boiler feed water and neutral industrial media, NBR or EPDM run well inside their limits. In food and beverage duties dominated by hot water, steam and caustic cleaning, EPDM is already the correct class and behaves better than FKM.

In mineral oil, hydraulic fluid, fuel and general hydrocarbon service, FKM is sufficient and economical; below the approximate upper limit of FKM there is no technical case for FFKM. What sets FFKM apart is a broad media list combined with rising temperature.

Once the seven questions below are answered, usually one elastomer class remains. Where two items point to different classes, the more restrictive one wins.

• Has the full media list been written down? Cleaning, flushing and commissioning fluids touch the elastomer too, and the most aggressive entry decides it.
• What is the peak temperature? The peak during a CIP/SIP cycle or a short dry run governs the choice, not the continuous operating temperature.
• What is the lowest temperature? Standard FFKM grades have limited cold flexibility, so a low-temperature grade must be requested.
• Hydrocarbons or polar solvents? FKM where hydrocarbons dominate, EPDM where hot water and polar solvents dominate, FFKM where both appear on the same duty.
• Is there food, beverage or pharmaceutical contact? FDA-compliant grades exist in both EPDM and FFKM, so medium and temperature still decide it.
• Is there pressurized gas or a rapid pressure drop? If so, a decompression-resistant grade and a suitable groove geometry are required, or the elastomer cracks from the inside.
• What if the items conflict? When temperature points to FKM while cleaning chemistry points to EPDM, neither suffices alone and FFKM is the single common solution.

Meccanotecnica Umbra Turkey is the Turkish company of the Italian mechanical seal manufacturer Meccanotecnica Umbra S.p.A. From Ikitelli OSB, Istanbul, it manufactures and supplies mechanical seals, gland packings, gaskets and rotary joints for automotive, home appliance and industrial pump applications.

In the product data, the FFKM option is listed on the Carrara-branded Valgasket O-Ring, while the mechanical seal ranges list NBR, EPDM and FKM. For the face-pair side, see silicon carbide and tungsten carbide.

For elastomer and face options by range and the technical PDFs, see the product catalog; once the media list, peak temperature and pressure are known, the class and grade are settled together.