Material Selection for Sealing Products: A Guide to Carbon, SiC, Elastomers and PTFE

Material selection for sealing products means choosing, for every wetted part of a seal, the material that simultaneously meets chemical compatibility, temperature and pressure limits and wear behavior. In a mechanical seal the decision covers three groups: the face pair, the elastomer or PTFE secondary sealing elements and the metal parts; for gland packings and gaskets it is the braid or sheet material that is selected.

These groups are not independent. A correctly chosen silicon carbide face can start leaking within weeks because of an o-ring that is incompatible with the fluid, and a durable elastomer cannot save a spring that cracks in chloride-bearing water. Seal life is often set by the weakest material. For the role each component plays inside the seal, see what is a mechanical seal.

This guide first lists the factors that drive selection, then treats face pairs, elastomers, PTFE, metal parts and gland packing and gasket materials separately. It closes with a selection table by medium and the typical failures caused by material incompatibility. The ratings in the tables are a qualitative pre-screening tool; the final choice is made with a fluid analysis and the manufacturer's material data.

The material decision is made on the full set of operating conditions, not on the name of the fluid. The same pump may see process fluid, flush water and cleaning chemicals in a single day; selection is based on the most demanding item on that list.

The factors constrain each other: an elastomer that is compatible with a fluid at room temperature can swell in the same fluid at high temperature, and a carbon grade that runs for a long time in clean water can wear out quickly once sand enters the same water. Compatibility charts are therefore always read together with temperature and particle content. The factors in this section's list are usually assessed in this order.

• Medium (fluid): chemical composition, pH, solvent content, oxidizing behavior and lubricity. This is the first screening criterion for the elastomer and the carbon impregnation.
• Temperature: not only the continuous operating temperature but also the minimum temperature and the peak reached during cleaning cycles, steam sterilization or short dry running. The elastomer class is checked against this peak rather than the continuous temperature.
• Pressure: raises the load on the faces and therefore the frictional heat; in the elastomer it increases the risk of extrusion into the gap. High pressure may call for a harder elastomer grade or a backup ring.
• Speed: peripheral speed, together with pressure, forms the PV value. As PV rises, face materials with high thermal conductivity come to the fore.
• Particles and abrasive content: as the solids load rises, a soft carbon face wears quickly; a hard/hard face pair and a protected mechanism are needed. For details see the guide to mechanical seals for abrasive fluids.
• Food, pharmaceutical and drinking water requirements: FDA sets the rules governing the formulation of materials in contact with food in the United States; WRAS is an approval scheme in the United Kingdom for products and materials in contact with drinking water. These requirements do not select the material class but the suitable grade within that class.
• Balance of cost and service life: FFKM or a hard/hard face pair where none is needed adds cost without extending life, while a cheap material running at its limit gives the saving back through early failure.

The primary seal is formed by two faces, one rotating and one stationary, and this is the most heavily loaded group among seal materials. Established practice is for one face to be softer than the other: the soft face conforms to its counterpart and wear progresses in a controlled way. Pairs in which two hard faces run together are generally reserved for abrasive duties, high face loads or fluids in which carbon is chemically unsuitable.

On the soft-face side the choice is almost always carbon graphite. What matters for selection is the impregnation the carbon is ordered with: resin-impregnated grades are preferred for water and general chemicals, and antimony-impregnated grades for higher pressures and temperatures and against the blistering seen in high-viscosity oils; in caustic and strong acid service the impregnation must also resist that medium. Carbon's self-lubrication protects the seal during brief moments of dry running; in solids-laden fluids, however, its softness is its greatest weakness and it wears quickly.

On the hard-face side the choice usually starts with silicon carbide (SiC); its hardness, thermal conductivity and chemical resistance make it the most common hard face. What decides the choice by medium is which of the two SiC grades is specified. Sintered SiC (SSiC), which contains no free silicon, is the safe choice for caustic, strong acid and high-purity services; the more economical reaction-bonded SiC (RBSiC) is sufficient in neutral fluids and clean water, but its free silicon can dissolve in strong alkalis and hydrofluoric acid. Writing only SiC, without the grade, into a specification for a caustic line is therefore an incomplete definition. Both grades are brittle.

Tungsten carbide (WC) is the alternative to SiC where toughness matters more than hardness: in pumps with vibration, cavitation and impact loads, the face edge chips less. The detail to watch in material selection is the binder; in acidic or seawater-bearing media a nickel-bound grade is specified instead of cobalt. Alumina ceramic is the hard face for clean water applications where cost is decisive; its low thermal conductivity makes it sensitive to thermal shock, and it is generally not chosen for lines at risk of dry running.

In the product data, MS-7 is listed with SiC, tungsten carbide and carbon face options. For a detailed comparison of hardness, thermal conductivity and material codes, see the guide to silicon carbide vs tungsten carbide.

The table in this section compares the most common face pairs against the three criteria that most often decide material selection: dry running and thermal shock behavior, suitability for abrasive service, and limits in acids and alkalis. Whether a pair is suitable depends not only on the material but also on the seal design, the flush arrangement and the operating point.

In clean fluids most applications are solved with a carbon graphite pair; hard/hard pairs come in when abrasive particles or high mechanical loads are present. Hard/hard pairs have low dry-running tolerance, so they are installed together with a guaranteed, uninterrupted supply of lubrication, flush or barrier fluid.

Elastomers are the secondary sealing elements of a mechanical seal: they seal the rotating face on the shaft or sleeve and the stationary face inside the housing, and they are used as o-rings or elastomeric bellows. In o-ring (pusher) designs the dynamic o-ring slides axially so the faces can follow wear; a swollen or hardened elastomer locks this movement; the faces open, so the leak appears at the faces even though its root cause is the elastomer.

NBR (nitrile rubber) is an economical, general-purpose elastomer that is well suited to mineral oils, fuels, water and cooling water; it is limited in ozone, weathering and hot water. Its hydrogenated derivative HNBR improves heat and ozone resistance. EPDM performs well in hot water, steam, glycol-based coolant, dilute acids and alkalis, and ketones; on the other hand it swells on contact with mineral oil and hydrocarbons. For this reason a compatible lubricant, not mineral-oil-based grease, is used when installing EPDM elements.

FKM (fluorocarbon rubber) offers very good resistance to hydrocarbons, oils, fuels and aromatic solvents and good resistance to most mineral acids, and its high temperature capability makes it common in process pumps. Its weak areas are ketones, esters, amines, strong alkalis and hot water and steam; the low-temperature flexibility of standard grades is also limited. FKM and EPDM have almost opposite compatibility profiles, so the choice between them is usually settled by whether the fluid is a hydrocarbon or water and polar solvents.

FFKM (perfluoroelastomer), thanks to its fully fluorinated backbone, offers the broadest chemical resistance and the highest temperature capability among elastomers; it is used in chemical processes where aggressive chemicals meet high temperatures or the fluid list is very broad. The price is a high unit cost. Kalrez® is a registered trademark of DuPont; the generic name of the material is FFKM. For details see the FFKM and Kalrez® guide. Silicone (VMQ) offers a wide temperature range, but its low tear and abrasion resistance limits its use in dynamic sealing.

PTFE (polytetrafluoroethylene) appears in sealing products in three places: as a secondary sealing element in mechanical seals, as braiding yarn or impregnant in gland packings, and as sheet material in flange gaskets. A mechanical seal with PTFE sealing rings is one that uses a PTFE wedge, V-ring or bellows instead of an elastomer o-ring; it is preferred for broad chemical lists where elastomers are not compatible.

Two properties make PTFE the choice. The first is chemical resistance: because its carbon chain is completely surrounded by fluorine atoms, it is inert to acids, alkalis and the great majority of solvents; the known exceptions are highly reactive substances such as molten alkali metals and elemental fluorine. The second is low friction: it has one of the lowest friction coefficients of any solid material and offers a non-stick surface. Its typical continuous service temperature reaches about 260 °C, and it retains its toughness at very low temperatures.

Its limits, however, directly shape the design. PTFE flows over time under constant load, even at room temperature; this cold flow (creep) increases with temperature and leads to loss of tightness in a wedge and loss of bolt load in a flange gasket. Its resilience is very low: once compressed, it does not spring back like an elastomer. Its coefficient of thermal expansion is many times higher than that of metals, so during heating and cooling cycles its dimensions change differently from the surrounding metal parts. Its thermal conductivity and its wear resistance in virgin form are also low.

For this reason PTFE elements in mechanical seals are usually spring-loaded: the wedge ring is pushed into its seat by spring force, and the relaxation caused by cold flow is compensated by the spring. Where elasticity is needed, fluoropolymer (FEP or PFA) encapsulated o-rings are used; the jacket provides chemical resistance, while the FKM or silicone core inside provides resilience. These o-rings are still less resilient than elastomers, and the jacket must not be scratched during installation. Glass-, carbon- or graphite-filled PTFE grades reduce cold flow and improve wear resistance. In the product data, IS-1 Istanbul is listed with o-ring and wedge mounting and a PTFE option.

In gland packings, rings braided from pure PTFE yarn are used in chemically aggressive fluids and food applications; however, because of their low thermal conductivity they heat up at high peripheral speeds. Graphite-filled PTFE yarns remove heat better, while in PTFE-impregnated aramid or carbon braids the PTFE acts as a lubricant and filler. In the product data, SQ5000 is described as a pure PTFE gland packing. In gaskets, pure PTFE sheet remains limited by cold flow; filled (modified) PTFE sheets reduce cold flow, and expanded PTFE (ePTFE) conforms well to uneven flange faces.

The table in this section gives the general suitability of five secondary sealing materials by media class. Temperature values are approximate, grade-dependent ranges; different formulations within the same class can differ markedly. Exact compatibility must be verified against the manufacturer's material data, together with the fluid's concentration and temperature.

In the DIN 24960 and EN 12756 material code, these classes are identified by letters in the secondary seal position (third position); the common equivalents are P = NBR, E = EPDM, V = FKM, K = FFKM and T = PTFE. The same letters denote other materials in the face positions; for example, V in a face position is alumina ceramic. The range given for PTFE is the limit of the material itself; the wedge or bellows design and the pressure can narrow it.

The metal parts of a mechanical seal are the springs, drive and retaining parts, sleeve, gland and, in bellows designs, the metal bellows. These parts do not provide the seal directly, but because they are wetted, corrosion resistance drives their selection. Most general industrial applications use AISI 316 austenitic stainless steel; low-carbon 316L is common for welded parts and hygienic applications. In less aggressive water applications grade 304 can also be used.

Chloride-bearing media are the weak point of stainless steel. Molybdenum gives 316 better pitting resistance than 304; however, in seawater, brine and hot chloride solutions 316 can also suffer pitting and stress corrosion cracking. In these media duplex stainless steels offer higher strength and chloride resistance; for more severe conditions super duplex, titanium or nickel-based alloys are considered.

Springs are the most sensitive parts in this respect: they work under constant stress and their wire sections are thin, so their corrosion allowance is close to zero. The small-section springs of multi-spring designs are more sensitive to corrosion and clogging than the heavy spring of single-spring designs. For strong acids and hot chloride media, springs can be made from nickel-molybdenum-chromium alloys such as C-276. In metal bellows, too, the thin wall means the material is often chosen to be more resistant than the housing; for where bellows designs fit, see the guide to mechanical seal types.

Two more details should not be overlooked. Contact between dissimilar metals in the same fluid can accelerate galvanic corrosion. In caustic solutions at high temperature and concentration, austenitic stainless steels carry a risk of stress corrosion cracking; nickel and nickel-based alloys come to the fore in these services.

Gland packings and gaskets work on a different principle from mechanical seals. In a gland packing, braided rings are compressed around the shaft by a gland and a controlled drip is left for cooling; in a gasket, the material is compressed between two stationary flanges to provide a static seal. The group's gland packing and gasket brand is Carrara. For a comparison of the two technologies, see mechanical seal vs gland packing.

In gland packings the material is selected by fluid, temperature, pressure and, above all, shaft peripheral speed. As speed rises, the ability to remove frictional heat becomes decisive, which is why graphite and carbon fiber come to the fore at high speeds and pure PTFE at lower speeds. In abrasive fluids, hard fibers such as aramid resist wear but can also wear the shaft or sleeve surface; in that case a hardened sleeve is preferred. In the product data, GR8800 is listed as a premium braid of PAN carbon yarn with graphite powder, and N1706 as an aramid fiber braid. The fugitive emission performance of valve stem packings, a separate application from pumps, is assessed with type-test standards such as API 622.

In gasket selection, flange type, bolt load and pressure class are decisive in addition to fluid and temperature. In compressed fiber sheets the temperature and chemical limits are set largely by the elastomer binder; graphite sheets are used at high temperatures and in steam and are often reinforced with metal; filled PTFE sheets are preferred for chemical flanges. In the product data, Planiflon B13 is described as a silica-filled modified PTFE gasket sheet.

The table in this section gives combinations that serve as a starting point in most applications for common media classes. The table is a pre-screening tool: selection changes as concentration, temperature, particle load and cleaning chemistry change; in mixed fluids the most restrictive component governs.

Even within the same media class, conditions can change the choice. FKM that is adequate for a dilute acid, for example, may fall short in the concentrated, hot form of the same acid; a carbon graphite / SiC pair that gives long life in clean water may have to give way to a SiC / SiC pair once sand enters the water.

Most material-related failures leave a readable trace on the removed seal. The symptoms in this section's list show which material was mismatched and point to what needs to change in the next selection. For the general method of failure analysis, see the guide to mechanical seal failure and maintenance.

• Elastomer swelling: the o-ring grows, softens and overflows its groove; the dynamic o-ring cannot slide and the faces lose contact. The cause is usually an incompatible fluid or an incompatible grease used during installation.
• Hardening, cracking and compression set: the elastomer has usually run above its temperature limit, overheated during dry running or suffered chemical attack (for example, amines on FKM); an o-ring with a flattened cross-section and a cracked surface no longer springs back.
• Blistering and flaking on the carbon face: seen in viscous oils and under frequent start-stop conditions; the impregnation type must be chosen to suit the application.
• Roughening of the SiC face: in strong alkali or hot caustic service the free silicon phase of reaction-bonded SiC dissolves; sintered SiC is chosen for these services.
• Grain pull-out on the tungsten carbide face: in acidic media the cobalt binder is selectively leached; a nickel-bound grade or SiC is considered.
• Thinning and load loss in a PTFE element: cold flow changes the shape of the wedge or gasket over time; spring loading, a filled grade or retightening is required.
• Spring breakage and pitting: in chloride media, pitting and stress corrosion cracking appear on stainless steel springs; a more resistant alloy or a spring arrangement kept out of the process fluid is needed.

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.

The product data for many industrial series lists SiC and carbon faces together with NBR, EPDM and FKM elastomers; DR2-S and HUHNSEAL are examples of these combinations. For a material recommendation, the fluid name and concentration, cleaning and flush fluids, temperature range and peak, seal chamber pressure, shaft diameter and speed, particle content and any food or drinking water requirement are assessed together. You can review the options by series in the product catalog or contact us with your application data.