Mechanical Seals - Face Materials
Mechanical Seals come in a wide range of designs however the most important factor when determining how long a seal is going to last, is deciding on the right seal face materials. The old adage "pay me now or pay me later" applies to seals. You must consider the environment the seal will be exposed to. Abrasive environments require seal faces that will hold up to the wear and tear of abrasive environments. It makes little sense to save money on the seal only to have to spend more money to replace the seal later. On the other hand if you are pumping very clean fluid that has a lubricating quality such as machine oil or Ethylene Glycol, save your money for better quality bearings.
The most common types of seal faces are: Carbon, Ceramic, Ni-resist, 17-4, Silicon Carbide, Tungsten Carbide, GFPTFE (glass filled PTFE.... often called Teflon (R)). Another group of seal faces would be those of coated seal faces. The coatings are "plasma coatings" and are generally a form or silicon or tungsten carbide sprayed onto a stainless steel seal head. They have been sold by various trade names by the major mechanical seal companies but in our opinion are not worth the money. We have found the coating will always eventually wear and once it has will need to be completely recoated. In contrast, if a Silicon Carbide or Tungsten Carbide seal face is worn, it can generally be re-lapped and polished , bringing it back to "like new" condition and allowing a second, third or even fourth use of the same seal head.
Ceramic - Generally a 99.5% aluminum oxide offering excellent wear characteristics due to it's hardness. It is chemically inert and can be applied to nearly any product. Ceramic cannot however handle the thermal shocks that Ni-resist or 17-4 seats can. They will also shatter like a plate if dropped on concrete.
Silicon Carbide - Is a bluish-black material created by fusing silica and coke. It is in the same family as Ceramic (due to the Silica), but has much better lubrication qualities and is harder. The most common is reaction-bonded silicon carbide. In Chemical applications however, Alpha Sintered Silicon Carbide may be recommended. Alpha Sintered Silicon carbide does however sacrifice the Pressure Velocity ratios due to the lack of un-reacted free carbon. Either grade of SIC can usually be re-lapped and polished to be reused.
Tungsten Carbide - A very versatile seal face like Silicon Carbide. It is very hard like silicon carbide however it is very heavy to the touch, making it easy to distinguish from silicon carbide. Ideal for high pressure applications due to a high modulus of elasticity which helps prevent face distortion. Tungsten carbide also can be re-lapped and polished to be re-used.
Ni-resist - Is a form of Stainless steel which has had nickel added to it to lower the friction generated by the rotating face. it is an inexpensive seal face and ideal for fresh water applications.
GFPTFE - Say that one ten times.... Glass Filled PTFE (most people just say Teflon (R)). Gives the chemical resistance of PTFE however glass must be added to give the face hardness and to prevent cold flow issues associated with PTFE.
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