Mechanical Seals failures seem to fall into four broad categories:

The seal motion was restricted and the faces opened.
Heat caused the 0-rings to deteriorate.
The seal materials were attacked by the fluid sealed.
The seal was installed incorrectly.

Mechanical Seals motion restricted:

The spring-loaded (dynamic) seal face constantly moves to maintain full face contact with the stationary seal face. The main reasons for this movement are
1. The stationary face is not perpendicular to the pump shaft.
2. The pump has bearing end play. This means that the shaft moves back and forth a few thousandths of an inch at frequent but random intervals.
3. There is some impeller unbalance causing shaft whip.
4. The pump is operated away from its BEP, causing side loads on the shaft.
5. There is thermal shaft growth and Pump vibration that affects the seal.

Here are the major conditions that can restrict.. movement of the spring loaded mechanical seals face:
1. Solids have collected in the seal or around the dynamic seal ring.
2. The fluid sealed has caused the dynamic 0-ring to swell.
3. The temperature limit of the dynamic 0-ring has been exceeded and the 0-ring has lost its elasticity (compression set) or become hard.
4. Spring compression is inadequate because of incorrect installation.
5. Solids in the stuffing box, gasket protrusion or other foreign material restrict the motion of the dynamic seal ring.

Thermal degradation of Mechanical Seal 0-rings:

0-rings are the one part of a mechanical seal that are sensitive to heat because of the way they are manufactured. The ingredients are mixed together, put in a mold and cured at high temperature for a specific time. The compound will then assume the shape of the mold and its hardness, or durometer, will increase. When the 0-ring is placed in an 0-ring groove in a seal and heated to a temperature beyond its recommended limit, the curing process will continue and the 0-ring will take a compression set. This means that the 0-ring has lost some of its resilience and squeeze, and fluid may leak past the 0-ring. The higher the temperature, the shorter the time before the 0-ring takes a compression set. When an 0-ring is exposed to high temperature for a long period, it will become hard and brittle, causing mechanical seals failure.

Since heat is often a problem and seldom helps the mechanical seal application, what can be done about it?
1. Use a balanced seal to minimize the heat generated by the seal.
2. Use low-friction face materials. Carbon vs silicon carbide is the best choice.
3. Use a clean liquid flush or product recirculation to carry away heat.

Mechanical Seal materials attacked:

When the correct materials are not selected,

1. The 0-rings may swell locking up the mechanical seal,
2. The mechanical seal faces may deteriorate rapidly, and
3. The metal seal components may corrode.
All can cause the mechanical seals to fail.

Mechanical seals installed incorrectly:

Many mechanical seals fail at initial start-up or prematurely because they were not installed correctly. Cartridge seals eliminate all measurement, protect the seal faces from contamination and are easy to install. With these seals, installation problems are minimized. The Outside seal is preset and requires no installation measurement. Only in-line seals require careful measurement to insure correct installation. By following the mechanical seals installation instructions ,step-by-step correct seal installation is easily achieved.

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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.

Seal Faces:

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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