Mechanical Pump Seals
Carrying on with our series on sanitary mechanical seals, the following will provide a good overview of the mechanical seal face materials we see in sanitary pump applications.
Note that we are only discussing seals and seal material for sanitary pumps. Sanitary pumps have unique characteristics in that they have to be hygienic or cleanable. Because of that, many commercial mechanical seals and packing type seals that do an excellent job from a sealing standpoint, are not suitable for sanitary pumps. So, we are keeping our comparisons to sanitary mechanical seals only.
First, it’s important to think about what makes a good seal face material. Few materials, in fact, are suitable for use in seals. As discussed in our previous blog post, to keep leakage to a minimum, the seal gap must be very small. As a result, the lubricating film is very thin. Consequently, the seal faces need to be able to withstand rubbing together at high speeds. And what also comes when things rub together at high speeds? Friction. Heat. So we also need to select materials that are able to withstand this heat. In sum, the best seal face materials have low friction, high hardness, good corrosion resistance, and high heat conductivity.
Now that we know what makes a good seal material, let’s look at some of the ones we see all the time on sanitary pumps.
Carbon seals. The old standby. Carbon seals offer the greatest economy and lubricity for sealing non-abrasive products. It’s good for clean, abrasive free materials. It self-lubricates to reduce heat and extend service life. It works great with all other seal materials.
Ceramic is much more resistive to abrasive materials than carbon. It’s a great all around seal material. It has good hardness and stiffness. It’s wear resistant, corrosion resistant, and cheap. Despite the material’s stiffness, it struggles in applications with thermal shock. It is most commonly paired with carbon and is Waukesha’s standard seal material in its sanitary positive displacement pumps. It’s the “can do” seal material in the “can do” sanitary pump.
Silicon carbide, specifically reaction bonded and self-sintered silicon carbide, offers superior strength, abrasion resistance, and thermal conductivity to alumina ceramic.
Reaction bonded silicon carbide is made by infiltrating compacts made of mixtures of SiC and carbon with liquid silicon. The silicon reacts with the carbon forming silicon carbide. The reaction product bonds the silicon carbide particles. Any excess silicon fills the remaining pores in the body and produces a dense SiC-Si composite.
Sintered silicon carbide is a premium version of silicon carbide. It contains no free graphite or silicon. Sintered silicon carbide is Waukesha’s hardest seal face for sanitary pumps. It contains no silicon that can leach into the process, is excellent in an oxidizing environment, and has good thermal shock resistance due to low thermal expansion coefficient and high thermal conductivity.
In summary, silicon carbide’s combination of hardness, strength, and temperature resistance gives it excellent capabilities for services in a wide range of applications where high speeds, high pressures, and chemical and abrasion resistance are required.
Commonly known as Purebide, siliconized graphite is technically a silicon carbide. It is made from specially formulated graphite whose surface has been converted to silicon carbide. This provides surface characteristics similar to silicon carbide but is less costly to produce than solid silicon carbide. Free graphite in the silicon carbide layer serves a dry lubricant, reducing friction and improving performance. Purebide is used exclusively in the Waukesha 200 series sanitary pumps.
There is nothing pure about this material. It is a composite. In order to hold the tungsten carbide particles together, a binder is required. Nickel is common in food applications due to its corrosion resistance and compatibility with food products. The metallic binder provides “toughness” to the seal material, making it perfect for heavy-duty applications where high starting torques or shock loading is present. It’s ideal for products that are sticky or set up, high temperature applications, and high loads. It is not inexpensive and should only be specified when absolutely necessary. Tungsten carbide mates with carbon, silicon carbide or itself.
Chrome oxide is an interesting material because most of the seal is actually machined from stainless steel. The base part’s seal face is then coated with chrome oxide. The stainless steel provides a strong base that resists high starting torques, impact loading, and rough handling. The chrome oxide coating provides a hard, wear resistant face that provides performance similar to solid ceramic.
To close, we hope is that this gives you a good idea of what seal face materials are available for sanitary pumps and when to consider them. A future post will expound on seal face combinations, focusing on specific applications when they should be applied, but for now, hopefully this table serves as a good recap:
Seal Face Pairings
|Economy, the can do seal
|Economy in an abrasive application
|Acids and Alkalis, thermal shock, high temperatures.
|Products that set up or crystalize. High torque or shock.
|Similar to TC. High starting torques. Sticky, abrasive, sugary products. CIP chemicals.
Holland Applied Technologies