FAQ

The only part of a mechanical seal that is supposed to be sacrificial is the carbon face. The seal should run leak free until the carbon face is worn away. If the seal leaks for any other reason, we consider it a premature failure, and can be corrected.

Two hard faces are selected when carbon is not acceptable in the application and you have run out of options. You are then trying to get the longest life you can.

The only variable in a seal life should be the lubricating quality of the product you are sealing. Hot water, many gasses and most solvents are typical non-lubricants.

With all of that said, the fact is that in excess of 85% of mechanical seals fail prematurely. When seals are removed from the running pump, most of the carbon face is still intact. Little face wear is the rule not exception.

• One of the seal components becomes damaged.
• The seal faces open.

• Corrosion of one of the seal components.
• Physical damage due to high heat or excessive pressure.

• The seal was set screwed to a hardened shaft.
• Solids in the product you are sealing are clogging the moveable components.
• The product changed state and interfered with the free movement of the seal.
  • The product crystallized.
  • It became viscous.
  • It solidified.
  • It built a film on the sliding components and the lapped faces.
  • The product vaporized across the lapped faces, blowing the faces open.

The graphite in the carbon/graphite face is a natural lubricant. In operation the graphite separates from the mixture and transfers to the hard face. This means that the seal face combination you are normally running is carbon on graphite. The hard face is just some place to put the graphite.

Moisture must be present for the graphite to separate from the carbon/graphite mixture.

Running dry means higher heat at the faces. If you are using a good unfilled carbon/graphite (and you should be!), the faces are not going to be your problem. The elastomer and the product you are sealing can be very sensitive to a temperature change in the stuffing box, or an increase of temperature at the seal faces.

Most carbons and hard faces can tolerate a lot of heat. The elastomer (rubber parts) are the parts you have to watch. They are the most sensitive to a change in stuffing box temperature, especially if they are positioned in the seal faces.

Hydraulically-balanced seals generate very little heat between the faces.

Unbalanced seals usually require cooling because of the excessive heat they can generate.

Some face combinations generate more heat than others. Two hard faces are an example.

Some seal materials conduct heat better than others. Ceramic is a poor heat conductor and carbon is not much better. Tungsten carbide and silicone carbide are excellent conductors of heat.

• With any of the oxidizing agents.
• When sealing any of the halogens.
• If the product tends to stick the faces together.
• If you are sealing hot oil and you have to pass a fugitive emission test.
• Some de-ionized water will attack carbon in any form.
• When you are not allowed anything black in the system because of the possibility of color contamination.
• Any time carbon/graphite will not work for some reason.
• If the specifications call for two hard faces.

• They generate higher heat than the carbon/hard face combination.
• They are not very forgiving. If the faces are not dead flat at installation, they seldom lap themselves flat in operation.

• Any good quality mechanical seal should run without visible leakage
• Single, stationary (the springs do not rotate), hydraulically-balanced mechanical seals can pass a fugitive emission test as long as the rotating portion of the seal is designed to be located square to the shaft.
• Rotating seals (the springs rotate with the shaft) seldom pass a fugitive emission test. They are too sensitive to various forms of misalignment.
• Cartridge mounted stationary seals usually fail fugitive emission testing because the set screwing of the cartridge to the shaft prevents the rotating face from positioning itself square to the shaft. Some seal companies offer some types of a self-aligning design to solve this problem.