TROUBLESHOOTING MECHANICAL SEALS TUTORIAL S015
Of all the seal related activities, analyzing mechanical seal failure continues to be the single greatest problem for both the consumer and the seal company representative. If you’ll take a little bit of time to familiarize yourself with the following outline you should feel a lot more comfortable the next time you’re called upon to do some seal troubleshooting.
When you begin to troubleshoot any type of a seal failure, consider the following:
- Rebuilt seals show evidence of damage and failure that might not be related to the problem you’re analyzing. Unless the metal case has been sand blasted and all of the other components replaced, the rub marks and signs of corrosion may not be relevant to the current problem. Before you do any thing else, find out if you are looking at a rebuilt seal and then find out which parts were replaced.
- Inquire to learn if this same design seal is running successfully in another pump with the same operating conditions. You’ll often find this is the case, allowing you to concentrate on what is different about this particular pump, or set of operating conditions.
- Do not attempt to analyze the failure from some one else’s description of the symptoms, or failure. If you cannot see the parts yourself it’s almost impossible to come to any sensible conclusion.
- People are reluctant to admit they dropped the seal, put it in wrong, or anything else that might reflect on their skill or dedication. You’re not always being told the truth. Let what you see, speak for its self.
- Intermittent service pumps experience more seal problems than continuous running pumps. Frequent stopping and starting is a major cause of seal failure because of “breakaway friction.”
- If you had a hundred years of troubleshooting experience you could see only three things when you look at a failed mechanical seal:
- You can see evidence of parts rubbing.
- You can see damage to a seal component.
- You can see something restricting a sliding or moving seal component.
- Mechanical seals should run leak free until the sacrificial face (carbon) is worn away. More than ninety percent of mechanical seals begin to leak long before this happens. They fail for only two reasons:
- .One of the seal components becomes damaged.
- The seal faces open
Rubber bellows seals are often supplied by pump companies as original equipment seals.for water and oil applications. They are a different kind of seal. Be aware of their problems.
We will begin with damage to one of the seal components. This damage is almost always visible, so look for two types; corrosion and physical damage.
Corrosion damage, The elastomer swells or the other seal parts become “sponge like” or pitted. We do not think of elastomers corroding we tend to call it chemical attack or chemical incompatibility, but regardless of the words, the affect is the same.
- The product you are sealing is attacking one of the seal components.
- The attack is coming from the cleaner or solvent used to clean the lines between batches or at the end of a “run”.
- The attack is coming from lubricants put on the elastomers or seal faces. Petroleum grease on ethylene propylene (epr) O-rings will cause them to “swell up”.
- Galvanic corrosion – happens with dissimilar materials located close to each other and connected by an electrolyte. As an example: stainless steel can attack the nickel binder in a tungsten carbide face.
- Oxidizers and halogens attack all forms of carbon including black O-rings.
- Wear or rubbing of a flexible or moveable component.
- Thermal shock of coated or plated seal face materials. Some ceramics also have this problem.
- Thermal expansion of the shaft/sleeve can break a stationary seal face or interfere with the free movement of a dynamic elastomer.
- The rotating seal hits something because of shaft deflection.
- Temperature extremes (both high and cryogenic) will destroy elastomers and some seal face materials.
- Erosion from solids in the product you are pumping.
- Fretting caused by the O-ring, V-rings, or Teflon wedge removing the passivated layer from the corrosion resistant shaft or sleeve.
- Fluid abrasion that can weaken materials and destroy critical tolerances.
- A discharge recirculation line can break a metal bellows and injure lapped seal faces, as well as interfere with the free movement of the seal.
- The elastomer can swell and breaks the face.
- Problems at installation. These include mishandling, setting at the wrong compression, putting the wrong lubricant on the elastomer etc.
- Fatigue of the springs caused by severe misalignment.
The seal faces opening is the second cause of premature seal failure. Scoring or wear of the hard face is the most common symptom of this failure. The scoring occurs because the solids imbed into the softer carbon face after they open.
The seal faces must stay in contact, but there are all kinds of conditions that are trying to force or pull them open.
Physical causes of the faces opening
- Axial shaft movement (end play or thrust). This is normal at start up.
- Radial shaft movement (run out or misalignment)
- Operating off of the pump best efficiency point.
- Hysteresis caused by a viscous (thick) product.
- Centrifugal force tries to separate the faces in a rotating seal application.
- Hydrodynamic forces generated between the lapped faces.
- Pressure distortion caused during pressure peaks such as water hammer and cavitation.
- Thermal distortion that can cause the seal face to separate from its holder or “go out of flat”.
- Thermal expansion of the shaft that can cause the shaft to engage the vibration damper used in most rotating metal bellows seals. Once engaged the vibration damper can pull the lapped faces open.
- A failure to provide equal and opposite clamping across the stationary seal face will cause distortion.
- A hardened sleeve can cause the seal set screws to slip.
- A wrong initial setting of the face load.
- Springs clog if they are located in the product.
- Loose set screws. If the sleeve is too soft they can vibrate out.
- Shaft tolerance and finish are out of specifications.
- The rotating shaft or seal hits something.
- A discharge recirculation line can force open the faces.
- Outside seal springs painted by maintenance people.
- A cartridge seal installation method can compress one set of faces and open the other.
- Fretting hang up.
- Cartridge mounted stationary seals move excessively unless they have some type of “built in” self-aligning feature.
Product problems are another cause of the lapped seal faces opening. With the loss of an environmental control. The fluid can:
- Vaporize between the faces forcing them open and causing a “chipping” of the carbon outside diameter as well as leaving solids between the lapped faces.
- Become viscous preventing the faces from following normal “run out”.
- Solidify between the lapped faces, or around the faces.
- Crystallize between the faces or around the dynamic portions of the seal.
- Build a film on the sliding components or between the faces causing them to separate.
- Be a slurry and/ or abrasive
- Operate in a vacuum causing the ingestion of air between the faces of some unbalanced seal designs.
- Swell up the dynamic elastomer, locking up the seal.
- Cause slip-stick between the faces if the sealed fluid is a non or poor lubricant
- A combination of some of the above
Here are some of the common causes of shaft displacement.
- Operating off the pump’s best efficiency point (BEP).
- Misalignment between the pump and its driver.
- The rotating assembly is out of balance.
- A bent shaft.
- A non-concentric sleeve or seal.
- Using sleeve bearings rather than precision bearings
- Passing through, or operating at a critical speed.
- Water hammer in the lines.
- The stuffing box face is not square to the shaft, causing misalignment problems.
- Pipe strain.
- An impeller adjustment is made to compensate for normal impeller wear.
- Thermal growth of the shaft in both a radial and axial direction.
- Bad bearings or a poor bearing fit.
- Reversing axial thrust at start up is normal.
- The motor is finding its magnetic center.
- Cavitation – there are four separate types of damage that can be observed .
- The sleeve moved when the impeller was tightened.
- The unit is pulley driven causing side thrust
- The impeller is positioned too far from the bearings. This is a severe problem in mixer or agitator applications.