Mechanical seals are containment device that keeps product from leaking out and keeps contaminates from entering into the product being pumped.
Mechanical seals are used in rotary shaft equipment which move solids, liquids or gases, such as pumps, mixers, blenders, agitators and compressors, as well as spas and swimming pools.
Why do most seals fail prematurely?
- One of the seal components becomes damaged.
- The seal faces open.
If the pump manufacturers information is unavailable, the following are critical for providing the right part. Shaft Size, Head Type, Seat Type, Seat O.D., Seat Thickness. If possible bring the seal into Acumen Seals for proper selection.
What is considered good life for a mechanical seal?
- 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 always correctable.
- 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 seal life should be the lubricating quality of the product you are sealing. Hot water, many gases and most solvents are typical non-lubricants.
- With all of that said, the fact is that in excess of eighty-five percent 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 the exception.
- Corrosion of one of the seal components.
- Physical damage that includes the affects of 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. It:
- Became viscous.
- Built a film on the sliding components and the lapped faces
- The product vaporized across the lapped faces blowing them open.
- Most carbons and hard faces can tolerate a lot of heat. The elastomers (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 face.
- 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 as 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 them selves flat in operation.
- 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.
- 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 can 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 its self square to the shaft. Some seal companies offer some type of a self aligning design to solve this problem.
Why do most original equipment seal designs frett and damage the shaft under the dynamic elastomer or spring loaded Teflon?
- Corrosion resistant shafts and sleeves protect themselves from corrosion by forming a protective oxide (ceramic) layer on the metal surface. The dynamic elastomer in the seal polishes this layer away as the shaft slides through the elastomer because of shaft vibration, pipe strain, misalignment etc.
- The ceramic protective oxide that is removed by the polishing action imbeds its self into the elastomer causing it to act as a grinding wheel that increases the sleeve or shaft damage.
- There are two forces closing the seal faces.
- A spring force caused by the spring, springs, or bellows pushing on the seal face.
- A hydraulic force caused by the pressure of the fluid acting on the closing area of the seal faces.
- There are three forces opening the mechanical seal:
- A hydraulic force caused by fluid or vapor trapped between the lapped faces.
- Centrifugal force that is causing the rotating portion of the seal to try and become perpendicular to the rotating shaft.
- Hydrodynamic forces generated between the seal faces because for all practical purposes liquids are not compressible.
We balance these forces by reducing the closing area of the seal faces and thereby reduce the closing force. This is usually done by a small sleeve inserted into the seal or as step machined into the shaft. Metal bellows seals have an effective diameter measured through the bellows to accomplish the same thing.
- It is supposed to vulcanize its self to the shaft so that it can drive the rotating face. If you can remove it easily something is wrong. You probably used the wrong lubricant on the rubber during installation. This is a case where the lubricant we use is supposed to attack the rubber and make it swell.
Not at all. Touching seal faces seldom causes problems. We are trying to keep solids from penetrating between the lapped faces, so the less you handle them the less likely solids will be deposited on the faces.
The rotating portion of the seal is mounted on a cartridge sleeve and this assembly is connected to the stationary portion of the seal along with the seal gland to form a cartridge assembly. Cartridge seals simplify the installation process and allow you to make impeller adjustments without upsetting the seal face loading.