Most asked questions, seals
SUBJECT: The most asked questions about mechanical seals 10-2
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’re 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.
Why do most seals fail prematurely?
- One of the seal components becomes damaged.
- The seal faces open.
What are the most common causes of component damage?
- Corrosion of one of the seal components.
- Physical damage that includes the affects of high heat or excessive pressure
What are the most common causes for the lapped seal faces to open?
- 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.
Do seal faces have to be lubricated? Can they run dry?
- 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.
Do seal faces have to be kept cool?
- 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.
When should you use two hard faces?
- 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.
Why not standardize on 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.
Do seals have to leak.?
- 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.
Do you have to flush most slurry applications?
- It depends upon the percentage of solids. Most fluid with entrained solids can run without flush if you have met the following conditions:
- The packing stuffing box has been replaced with a larger inside diameter version. Centrifugal force will throw the solids away from the lapped seal faces.
- You are using a hydraulically balanced seal that generates low heat.
- The seal springs are not located in the fluid.
- The fluid is at the seal outside diameter.
- The dynamic elastomer moves to a clean surface as the carbon wears.
- You are using suction recirculation to get flow in the stuffing box.
I am looking for a simple solution to a difficult problem. Do discharge recirculation filters or cyclone separators installed between the pump discharge and the stuffing box make sense in slurry applications?
- I wish they did! Filters clog and then there is no circulation in the stuffing box.
- Cyclone separators were never intended to be a single pass devise. The also require a substantial difference in pressure between the discharge and the clean liquid connections. In a pump application these pressures are too close together.
If I put a higher fluid pressure barrier fluid between dual seals, shouldn’t that keep the faces clean?
- No, the clean fluid always takes the path of least resistance. That is the same reason that higher pressure air does not keep dry solids from penetrating the lapped faces.
- Centrifugal force will pack solids in front of the inboard seal face and restrict its movement.
Do you need a higher pressure barrier fluid between dual seals?
- Higher pressure is called barrier fluid; lower pressure is called buffer fluid.
- The only dual seals that require a barrier fluid are the “back to back” rotating, unbalanced versions, and you shouldn’t use them any way.
- Balanced tandem seals (one behind the other) use a buffer fluid that will not dilute your product if the inner seal fails. They also put the pumping fluid at the inner seal outside diameter where it belongs.
- Dual seals should be hydraulically balanced in both directions so that they will stay shut regardless of the direction of the fluid pressure.
How does seal hydraulic balance work?
- 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.
Is it O.K. to have a third party rebuild my mechanical seals?
- Not really. If you’re happy with your seal have the manufacturer, or the company that sold it to you do the rebuilding. Here are a couple of reasons why:
- Carbon/graphite has to be molded in a sintering process and the third party doesn’t own the molds for your carbon/graphite face. Machined carbons don’t have the density required for good seal faces.
- There are many grades of elastomers. How do you insure you have the right grade. You can’t tell by looking at the part.
- Lapping is a real art. The temperature has to be closely controlled to get the right flatness.
Should I be using split mechanical seals?
- There are places where they are the only logical solution:
- Double ended pumps. If one seal is leaking why take the pump apart and change both? Change only the one that is leaking.
- Large vertical pumps. Sometimes you have to take the roof off the building to remove the solid mechanical seal.
- Large size shafts are a natural for split seals.
- Changing a seal means doing a re-alignment. Why go through that again?
- If you have to remove a lot of pump insulation to get to the seal.
- If the pump is in an awkward location, split seals make sense.
- Many split seal designs can run with no visible leakage, but they seldom can pass a fugitive emission test that calls for leak rates in the order of parts per million.
If I touch the lapped faces, are they ruined?
- 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.
Why should you not use stainless steel springs or stainless steel bellows in mechanical seals?
- Chloride stress corrosion is the problem and chlorides are every where. Use hastelloy “C” springs and metal bellows and you’ll never have this problem.
Why not standardize on Teflon as the preferred rubber part in a mechanical seal?
- Teflon® is not an elastomer, it doesn’t have a memory and has to be spring-loaded to the sleeve or shaft. This spring loading interferes with the flexibility of the seal and prevents the elastomer part from flexing and rolling to compensate for minor shaft movements.
Why not mount the seal outside the stuffing box and then dirt and solids will not get into the springs and sliding parts of the mechanical seal?
- The sealing fluid will be at the inside diameter of the lapped faces where centrifugal force will throw solids into the faces.
- Solids will pile up in front of the seal preventing the faces from moving forward when the sacrifical carbon wears.
What is a cartridge seal?
- 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.
Do I need the new gas seals if I want to seal fugitive emissions?
- Not really. Rotating seals do not pass fugitive emission tests because of their sensitivity to misalignment. Stationary seals usually do not have this limitation.
- The difficulty arises when you try to install a stationary seal on a cartridge sleeve. When you tighten the sleeve set screws to the pump shaft you introduce misalignment between the rotating seal face and the rotating shaft. Hysteresis (delay or lag) problems take over and the result is the stationary seal design fails to pass the fugitive emission test. Any good cartridge mounted self aligning seal can resolve this problem.
- Although a single seal can pass the test, a dual seal is recommended with a low pressure buffer fluid between the seals to act as a back up when the first seal wears out or fails. The buffer fluid will prevent unwanted product dilution and simplify the installation because there is no need for a compatible high pressure barrier fluid that is often hard to find.
Why does my outside mounted seal make a whistling sound?
- The seal faces are running dry. The product you are trying to seal is not a lubricant.
Every time I remove a rubber bellows seal from my pump it is stuck to the shaft. Why?
- 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.
When my metal bellows seal fails because of breakage at the plates, the break is always near the end fittings and never in the middle of the bellows. How is that explained?
- This is the common mode of failure for excessive vibration. Metal bellows seals need some type of vibration damping to stop harmonic and “slip-stick” vibration problems.
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