Viton seal

EPDM and FKM barely overlap, and that is why both exist. ERIKS puts FKM's strengths as mineral oils and greases, aliphatic, aromatic and special chlorinated hydrocarbons, petrol, diesel fuels, silicone oils and greases. Every one of those swells EPDM.

Grundfos demonstrates the inversion inside a single table. Its list of pumped liquids for the CR names a preferred shaft seal per medium, and the elastomer flips with the chemistry.

Same pump, same hydraulics, four liquids, and the rubber changes twice. Nothing about the casing material decides that. PTFE is the third option, and it covers a wider range than either elastomer at the cost of resilience.

Grundfos attaches a warning to that list which is worth repeating. The table is intended as a general guide only and cannot replace actual testing of the pumped liquids and pump materials under specific working conditions, and Grundfos names concentration, liquid temperature and pressure as the factors that move the answer.

Viton is a registered trademark of The Chemours Company, applied to fluorocarbon rubber polymers. Specifying it as though it were a material grade is how the wrong compound arrives on site.

Three names describe the same family. FKM is the ASTM D1418 short-form designation. FPM is the DIN/ISO 1629 abbreviation for the same thing, which is why Alfa Laval writes FPM on a hygienic pump leaflet while Grundfos codes the identical rubber as V in a CR type designation. TYM Seals & Gaskets, which sells both branded and generic material, says plainly that the naming causes problems when ordering.

The family is also wide. DuPont, which developed Viton before the brand passed to Chemours, splits its own polymers by fluorine content: A-type at 66%, B-type at 68%, F-type at 70%, with the flat statement that an increased fluorine level means better fluid resistance. Specialty grades run the other way. GLT and GFLT are formulated for low-temperature flexibility, and two further grades are singled out for outstanding resistance to strong bases and amines.

"Viton" on a quotation therefore narrows the polymer family and tells you nothing else. Ask which type, and check that against the medium.

Ketones, esters, amines and strong bases. Those are the media that attack FKM, and EPDM handles every one of them.

• Ketones and low molecular weight esters swell it. DuPont makes the point vividly in its own fluid resistance guide: methyl ethyl ketone and ethyl acetate are used as solvents for uncured Viton. The more polar the medium, the more likely it is to swell the cured rubber.
• Amines embrittle it. They react with the polymer backbone rather than swelling it, so elongation drops off sharply and hardness rises. An FKM O-ring that has gone hard and split has usually met a base, not a temperature.
• Strong bases degrade it. DuPont names sodium hydroxide at relatively high concentrations. Grundfos agrees at the pump end and puts an EPDM seal on 20% caustic at 50°C.

None of that makes FKM a poor material. It makes the medium the specification, which is the conclusion the corrosive duty page reaches from the casing end. Where a medium sits outside both the EPDM and the FKM window, the answer is PTFE or a build with no elastomer in the wetted path at all.

The temperature figure on an FKM datasheet is not the temperature figure for an FKM seal on a water duty, and the gap is wide enough to matter.

ERIKS gives the polymer a working range of roughly -26°C to +205/230°C. Then look at what a pump manufacturer will stand behind. Grundfos publishes its CR cartridge seals as a face pair plus an elastomer, and two of them differ only in the rubber.

Same pump, same faces, and EPDM has the wider window at both ends. Ebara puts a harder number on the same point. On the 3 Series the H, HS, HW and HSW builds run to +110°C, and Ebara still recommends a maximum fluid temperature of 75°C where the pumped fluid is water in combination with FPM elastomers, to guarantee a longer life of the sealing parts. That is 35°C of rating you do not get to use.

One caveat travels with the Grundfos figures and it matters more than the numbers. Grundfos states that the range of standard shaft seals applies to clean water and water with anti-freeze liquids. It is a rating for a clean duty, not a promise about what is actually in your line.

If the duty is hot water, steam or a cleaning-in-place cycle, FKM is a downgrade carrying the price of an upgrade. Specify EPDM.

Oils and fuels are most of the FKM we specify. Two other cases get missed, and both cost somebody a seal.

Oils, fuels and hydrocarbons. Mineral oils, greases, petrol, diesel and silicone oils all sit inside the FKM window, and no other common elastomer covers them.

A medium that is both hot and oily. ERIKS puts sulphur-cured EPDM at a maximum of +120°C and peroxide-cured EPDM at +150°C, against roughly +205/230°C for the FKM polymer. On a hot oil there is no EPDM answer at all. The assembly still bounds the duty, and on a pump the shaft seal normally gets there first.

Fats and edible oils on a hygienic line. EPDM is the standard product-wetted elastomer on hygienic pumps because a cleaning cycle is hot caustic, and Alfa Laval lists EPDM as the product-wetted elastomer on the DuraCirc with FPM as the other option. A line carrying fats or edible oils is what that option is for. Check the compound's food-contact declaration separately from its chemical rating. They are two different documents and one does not imply the other.

Where it is not the answer: water, hot water and cleaning chemistry, where it costs you temperature and buys nothing; ketones, esters and amines, where it fails outright; and any medium outside both windows, where you move to PTFE or design the seal out.

The medium, its concentration and its temperature. Those three fix the elastomer, and they are the three an enquiry usually arrives without.

Then two warnings from the people who make the material, both worth more than a compatibility chart. The first is about additives. DuPont's guide notes that commercial and proprietary fluids may contain additives to improve handling or end-use conditions, that these can vary between manufacturing locations under identical product labels, and that it is the additives rather than the major constituent which may have an adverse effect on the Viton. A hydraulic fluid, a coolant or a detergent is a formulation, not a chemical.

The second is about testing. DuPont describes its own fluid resistance data as a starting point, based on laboratory work done under controlled conditions, and recommends testing under actual or closely simulated field conditions wherever possible. Grundfos says the same about its list of pumped liquids.

Where a seal choice is genuinely difficult, the seal manufacturer's advice is the one to take, and we will go and get it. And if your medium turns out to want EPDM, we will say so, even where somebody has already quoted you the FKM build as the better one.