carbon vs silicon carbide mechanical seal in stock

You must consider the “environment” the seal will be exposed to when selecting the design, and importantly, the material of your mechanical seal. The saying “pay me now, or pay me later” very much applies to seals as not selecting the right material will cost more in the long run.

For all environments the material used for the seal face must be stable, be able to conduct heat, be chemically resistant and deliver good wear resistance. However, certain environments will need these properties to be stronger than in others.

Abrasive and harsh environments mean that the material selected must be able to withstand this, which can be more expensive. However the cost will be returned to you over time as poor material grade selection will only result in costly shut downs, repairs, refurbishments or replacements of the seals once again.

Various materials can be used for seals depending on the requirements and environment they will be used for. By looking at material properties such as hardness, stiffness, thermal expansion, wear and chemical resistance, you are able to find the ideal material for your seal.

When mechanical seals first arrived, seal faces were often made from metals such as hardened steels, copper and bronze. Over the years, more exotic materials have been utilised for their property advantages, including ceramics and various grades of mechanical carbons.

Selection of the proper seal face materials is essential for the successful operation of the mechanical seal. In fact, it could be argued that selection of materials is the most important decision to be made by the seal designer.

In evaluating materials for seal faces both the properties of the individual materials and the combination of the tribological pair must be considered. In general, dissimilar materials are used for seal faces. These materials are frequently thought of as the “soft face” and the “hard face” although sometimes two “hard faces” are used.

Mechanical seal design would be considerably simplified if the “perfect” seal face material could be found. With such a material, the designer would not be concerned about balance ratio, face widths, heat generation, flushing, corrosion, etc. Therefore there is a tremendous incentive to develop improved seal face materials.

Even though a perfect seal face material is not likely, the ideal face seal material can be described based on our experiences and problems with existing materials. This ideal material would have the following characteristics:

Leakage is probably more a result of the seal design rather than a property of the material but good face materials can certainly promote low leakage seal designs. In most seals, the actual face separation is strongly related to the surface finish of the materials. Therefore, materials which have and maintain smooth surfaces generally leak less than those with rough surfaces.

Leakage is also related to the compliance, or ability of the seal faces to conform to each other. Compliance is generally thought of as a function of the seal shape; however, it is strongly influenced by the modulus of elasticity. Materials with a low modulus, such as carbon, are more easily made into compliant shapes than materials such as tungsten carbide.

Mechanical seal calculations are considerably simplified through the use of a coefficient of friction. Unfortunately, this coefficient of friction is not a constant and ranges from around .03 to .3. Naturally, the coefficient of friction is a function of the tribological material pair but it also depends on the fluid being sealed. To make matters worse, it turns out that the coefficient of friction also depends on the seal face load and is reduced when the seal leaks.

In spite of these limitations, the coefficient of friction is a useful means of comparing seal face materials, especially when tests are done under similar conditions. Table II shows coefficients of friction for various face combinations.

As shown in Table II, there is a considerable variation in coefficient of friction for various materials. Even when specific material formulations are tested, the coefficient of friction depends on the fluid being sealed, the seal load and aspects of the seal design such as face distortion.

A good mechanical seal material must not only be strong enough to resist the stresses of normal operation, it must also be strong enough to survive the manufacturing process, storage and the rigors of installation.

The strength, hardness and rigidity of carbon graphite based materials is generally an order of magnitude less than that of metals and ceramics such as steel, tungsten carbide or silicon carbide. This means that more design effort is normally directed toward the component which is manufactured from carbon graphite. The primary reason for the use of carbon graphite in mechanical seals is it self lubricating qualities — not its strength.

Tungsten carbide is at the other extreme from carbon graphite. Tungsten carbide has a very high compressive and tensile strength, is very hard and has a high modulus of elasticity.

Silicon carbides are even harder than tungsten carbides but are much more brittle and greater care must be taken during installation and removal. These difficulties in handling have caused many users to prefer tungsten carbide in spite of the low frictional characteristics of silicon carbide.

The thermal aspects of mechanical seals are a major factor in seal performance and reliability. Two of the major material properties are thermal conductivity and thermal expansion.

The thermal shock characteristics of materials have already been discussed. Although thermal conductivity enters into the thermal shock parameters R2 and R3 directly, its effect on seal face temperature is probably more important.

Carbon graphite materials generally have a thermal conductivity of around 5 to 8 Btu/hr ft F; metal filled carbons are somewhat higher. In contrast, tungsten carbides and silicon carbides have thermal conductivities ranging from 40 to 100 Btu/hr ft F. This means that, in a typical seal with carbon versus tungsten carbide or silicon carbide faces, the major of the heat transfer takes place through the non-carbon element.

Stainless steels, Stellite and alumina have much lower thermal conductivities than tungsten carbide and seals using these materials will run considerably hotter than one using tungsten carbide or silicon carbide.

The thermal expansion of seal face materials is related to both the seal face temperature and the coefficient of expansion of the material. In order to minimize the effects of face temperature on distortion, a low coefficient of expansion is desired.

The coefficient of expansion of carbon graphites, tungsten carbides and silicon carbides is similar. This is fortunate and allows for some degree of substitution in seal face materials within the same design family. Alumina is higher and stainless steels still higher.

Any differences in coefficient of expansion become especially important when a seal is manufactured by shrink fitting components made from different materials. In this case, if the operating temperature is sufficiently different from the manufacturing temperature, the seal faces may become distorted. In an extreme case, the components may become loose.

Corrosion of carbon graphites is usually more related to the binder than the carbon graphite. Metal filled carbon are especially subject to corrosion but a suitable resin filled carbon can usually be found for most services. Carbon graphites are not recommended for aqua regia, oleum or perchloric acid. Resins in common use are attacked by lithium hydroxide, potassium hydroxide, sodium metophosphate, anhydrous ammonia, sodium diphosphate and sodium cyanide.

Alumina has good corrosion resistance and high purity alumina is very good. Before the introduction of silicon carbide, alumina was the preferred corrosion resistant material in many mechanical seal services.

The two most common variations of tungsten carbide are cobalt bound and nickel bound. Nickel bound tungsten carbide is the more corrosion resistant although the cobalt bound tungsten carbide is more than adequate for most services. Neither is as good as alumina.

The chemical resistance of silicon carbide is excellent. The two most common variations of silicon carbide are reaction bonded and alpha sintered. Of the two, the alpha sintered is the more corrosion resistant but even reaction bonded silicon carbide is very resistant to chemical attack. Both are generally better in corrosion resistance than nickel bonded tungsten carbide. The “free silicon” in reaction bonded silicon carbide can be attacked by strong oxidizing chemicals. Alpha sintered silicon carbide has no free silicon; it is considered to be the most corrosion resistant of all the seal face materials.

Many of the desirable material qualities for a seal face are not so desirable during the manufacturing process of that component. In particular, the hardness and high strength of many materials make manufacturing very difficult. A common approach is to mold the “green” material into a near finished shape before completing the manufacturing process.

Carbon graphites are typically molded to a rough shape before being impregnated with resin or metal binder. Some simple shapes with small cross sections may be machined from cylindrical stock. The final shape is machined. Faces are always lapped.

Seal components made of very hard materials such as tungsten carbide and silicon carbide are frequently repairable. The repair process consists of chemical and mechanical cleaning and relapping. Caution must be used to assure that dimensional tolerances are maintained.

Softer materials, such as carbon graphites, frequently are not reused, especially if they have been in service for an extended period of time. These softer components generally have more extensive face damage than the hard component and are also less expensive to replace. In the case of carbon graphites, there may also be a concern about chemical attack of the binder.

The cost of seal components is generally related to the hardness and chemical resistance of the material. This cost is normally considered to be a small fraction of the total cost of removing the pump from service and the labor involved in changing out the seal parts. For this reason, most seal users prefer to use the best available materials in their mechanical seals. Currently, the most popular material combination is a premium resin filled carbon graphite versus silicon carbide.

The additional cost of tungsten carbides and silicon carbides is somewhat offset by the fact that components made from them can frequently be repaired – meaning cleaned and re-lapped.

Carbon graphite seal rings or faces are typically molded to a rough shape before being impregnated with resin or metal binder Carbon fill with metal/antimony carbon seal faces or rings are especially subject to corrosion but a suitableresin filled carbon seal faces or rings can usually be found for most services. Carbon graphite seal faces or rings are not recommended for aqua regia, oleum or perchloric acid. Resin-impregnated carbon seal rings or faces in common use are attacked by lithium hydroxide, potassium hydroxide, sodium metophosphate, anhydrous ammonia, sodium diphosphate and sodium cyanide. A metal filled carbon graphite seal face or ring is generally better in strength, hardness and modulus of elasticity than resin filled carbon graphite seal faces or rings.

Silicon carbide faces in mechanical seal assemblies result in improved performance, increased seal life, lower maintenance costs, and lower running costs for rotating equipment such as turbines, compressors, and centrifugal pumps. CoorsTek seal faces help reduce the possibility of leakage and catastrophic failure to safeguard the environment from the risk of fugitive emissions. They also lower energy consumption with reduced friction on startup and shutdown, as well as reduced wear and erosion during operation.CoorsTekseal faces are exceptionally durable and help increase the mean time between failures,resulting in greater productivity and lower total cost of ownership for processing equipment.

Our NUTECH cartridge mechanical seals are designed to fit all ANSI pumps for ease of installation and are pressure tested at our Texas manufacturing facility prior to shipping. We can custom design any cartridge style seal to fit your most demanding applications. We use premium grade materials of construction throughout. All seals are made in the USA.

For typical HVAC applications, carbon-ceramic have stainless steel metals, BUNA elastomers, a 99.5% pure aluminum oxide ceramic stationary seal face, and a carbon rotating face. These seals work well with the temperatures mentioned above and a pH neutral range of 7.0-9.0. They can handle up to 400 ppm of dissolved solids and 20 ppm of undissolved solids which satisfied most system requirements.

In most applications carbon-ceramic seals work fine but there are certain circumstances when a different type of material for the mechanical seal should be specified. These include:

Systems with high pH levels. Most HVAC applications maintain a pH from 7.0 to 9.0. Once in a while the pH is too high for the carbon-ceramic seal material. The main issue may be located in the chemical treatment portion of your specification. There are specifications that call for the pH to be maintained at levels in the 9.0-11.0 range. If your specification calls for this range, the pump seal material specification should be changed to EPR/Carbon/Tungsten Carbide (TC) or EPR/Silicon Carbide (SiC) /Silicon Carbide (SiC). We recommend the EPR/SiC/SiC material since that seal can handle pH up to 12.5 which gives some “wiggle room.”

Higher solids levels. Solids, otherwise known as dirt, are another area of concern for mechanical seals. If the system is dirty or has silica in the water, you may find, once again, that you need the EPR/SiC/SiC seal. The standard Buna/Carbon/Ceramic seal in HVAC systems cannot handle any silica and the solids handling capability was mentioned earlier in this article. The silicon carbide seal can handle 60 times the dissolved solids content and double the undissolved solids content with 20 ppm silica content thrown in for good measure.

So, why not just always specify EPR/SiC/SiC seals? There are two reasons: cost and lead-time. This seal will cost three times as much as the standard seal. In addition, since the carbon-ceramic seal is standard, there may be additional lead-time to get a pump with a special seal.

Improperly mixed glycols. The term glycol is a bit of a misnomer when it comes to heat transfer solutions in HVAC systems. Glycol is used in many applications from shaving lotions to whipping cream to automotive antifreeze. In HVAC systems you want a properly mixed glycol-based heat transfer fluid that has the correct inhibitors for the application. If you use automotive antifreeze in HVAC systems, the silica based inhibitors will create a gel in the coils that blocks heat transfer and flow. That is, if the seals don’t leak first!

We commonly see Dowtherm® SR-1 and Dowfrost™ HD heat transfer fluids used for these type of applications. These products are made for use in our industry but should be mixed according to the manufacturer’s instructions. Dow recommends they be pre-mixed with deionized water before filling the system. If you simply mix these product with city or well water the calcium and magnesium in water will mix with inhibitors and cause a particulate that exceeds the ppm of normal seals.

Occasionally contractors will put the glycol based fluid in the system, filled it with water and then turn on the pump to “mix” the solution in the piping system. This subjects the pump seals to shots of up to 100% ethylene or propylene glycol which is well beyond the maximum recommended amounts for even silicon carbide seals.

Our experience is that the standard carbon ceramic seal work fine with properly mixed glycol products designed for our industry. However, brands vary in their quality and make-up so we cannot say that carbon-ceramic seals are appropriate for all glycol mixtures. These fluids do carry a higher pH than water, so we recommend specifying a glycol seal if there is any question.

1.375" / 1.750" / 1.875" Shaft Diameter Type 98 Cartridge Seals are available for daily sales or exchanges. The WSC98 Cartridge Mechanical Seal is available with different Small Bore and Large Bore Seal Gland configurations to adapt to hundreds of different Centrifugal pumps.

Your WSC98 is available with silicon Carbide vs. Silicon/Tungsten Carbide Face combinations. Silicon Carbide vs. Carbon is standard, but the WSC98 with Carbide faces is particularly effective in abrasive slurries.

The WSC15 Single Cartridge Mechanical Seal, an exclusive WSC design, fits every Allis Chalmers/RO-FLO Compressor without any modifications needed, with a low face load, worry-free installation approach and low face wear, the WSC15 cartridge Mechanical Seal

From standard materials, to top of the line abrasive resistant seal faces, these seals have been proven to out perform any OEM supplied mechanical seal in any market.

Some knowledge of the different materials that are common within mechanical seals is important to determine the correct seal when it comes to seal faces combinations and the material of the o-rings.

With the mechanical seal we are dealing with two seals, primary and secondary. The primary seal is the seal created by the hydrodynamic film between the two (very flat) seal faces. The secondary seal is the seal between the stator part and the housing and the rotor part on the shaft. The secondary seal is (almost always) achieved by using o-rings or gaskets (elastomers). Below you will find a brief overview of the most commonly used materials and combinations. Contact Qseals directly for advice.