Gene Vogel
EASA Pump & Vibration Specialist
The seal failed and you replaced it. But it failed again and your seal supplier explained that the replacement seal you bought was a lower cost seal, and they have more expensive seals that might hold up better. So, you bought a more expensive seal, but it failed too. What went wrong?
Your seal supplier is a catalog sales vendor, not a value-added seal supplier. They didn’t ask you the important questions: What are you pumping? What is the temperature and chemical content? Are there solids or abrasives in the pumpage? What is the pump rotating speed? What is the operating pressure (head) range?
The major factors in the cost of a mechanical seal are the contact face materials and the elastomer components materials. (See Figure 1.) For seal contact faces, carbon and ceramic are the standard materials. Typically, the rotating face is carbon, and the stationary face is ceramic. These materials are not expensive and can be polished to the smooth three helium light bands of flatness needed to develop the hydrodynamic coupling that forms the seal. Carbon has a very low coefficient of friction, which ensures excessive heat is not generated. For fluids with no abrasive content, this combination works very well.
The elastomer components form secondary seals between the rotating seal contact face ring and the shaft, and between the stationary ring and the housing. The common material for secondary seals is Buna rubber, a.k.a. Buna-N. It is a versatile and relatively inexpensive elastomer material and is compatible with most lubricating oils, water and some chemicals.
When a seal is ordered solely on dimensions, without any application data, that’s what will be supplied – carbon/ceramic seal face rings and Buna elastomers. These work well for common municipal water, HVAC circulation, pool and spa applications.
For more difficult applications, alternate, more expensive materials are needed. For wastewater and many dewatering applications, there are suspended abrasives in the pumpage that will erode the soft carbon rotating ring and can damage the stationary ceramic ring. For these and similar applications, harder materials such as silicon carbide and tungsten are recommended to resist the abrasive wear. Other hardened materials, even diamond facing, are available for very difficult applications.
When harsh, corrosive chemicals are included in the pumpage, the weak points of the seal are the elastomer components. High temperature applications also stress the properties of Buna rubber. The two most common chemical and temperature resistant elastomer materials are Viton and Kalraz (Teflon-(PTFE) is not commonly used for seal elastomers). There are numerous other material variants with extended chemical and temperature resistance properties. Many suppliers offer compatibility charts to help evaluate specific applications. (See Figure 2.)
The cost factor is significant when advanced materials are needed to meet an application’s requirements. Hard-face contact face materials may double or triple the cost of a mechanical seal. Viton elastomer components may be three to five times more expensive than Buna rubber, and Kalraz up to 10x the cost of Buna. The impact on the seal cost varies with the extent of elastomer material used.
When a seal vendor says they have a “better” seal that costs more, the difference is the materials used in the seal construction. But these more expensive materials are intended to address specific applications issues of abrasive, corrosive and high temperature applications. The need for these materials must be specifically addressed; that is, the seal must be engineered to fit the application. If a seal vendor doesn’t ask those critical questions about pumpage content, temperature, chemical content, pressure and shaft speed, they won’t have the information to properly engineer a mechanical seal. A value-added supplier is one that asks those questions up front to ensure the seal provided meets the needs of the application. Maintaining a relationship with a value-added supplier is an important strategy for successful pump repair service centers.
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