19-07 SEAL FACES OPEN
Please note that in this paper I identify a rotating seal as one that has the springs rotating with the shaft and a stationary seal as one where the springs do not rotate, but instead remain stationary in the gland. Self - aligning seals have springs in both the stationary and rotating faces.
Most seal failures are associated with shaft displacement. See “Shaft displacement problems” for details.
Physical causes of the faces opening:
- Axial shaft movement (endplay or thrust). This is normal at start up.
- Back to back rotating seal designs can hang up the inner seal if there are solids in the pumpage. The solids pack up in front of the inner seal moveable face
- Cartridge mounted stationary seals move excessively unless they have some type of "built in" self-aligning feature.
- Discharge recirculation lines aimed at the moving seal parts can restrict their movement.
- Distortion of the face when subject to “water hammer,” cavitation, etc. …
- Dynamic elastomer stuck to the shaft
- Elastomer swelled up and seized the shaft because it was not compatible with the pumping fluid or a cleaner that was circulated through the lines between or after batches.
- Expansion of the rubber part sealing the seal face to its holder can force the lapped face out of its holder or break the hard face. (ID fit
- Failure to provide equal and opposite clamping across the stationary seal face will cause distortion of the lapped face.
- High shaft speed will cause centrifugal force to drive the rotating face square to the shaft, opening the lapped faces. 5000 fpm (25 meters/sec), measured at the center of the lapped faces is just about the limit for rotating seal designs.
- High stuffing box pressure can distort a lapped face.
- Hydrodynamic forces generated between the lapped faces.
- Hysteresis caused by a viscous (thick) product.
- Operating off of the pump best efficiency point bends the shaft.
- O-ring groove was out of tolerance. It is causing too much O-ring interference with the shaft preventing the O-ring from flexing and rolling.
- Outside seal springs painted by maintenance people.
- Pressure distortion caused during pressure peaks such as water hammer and cavitation.
- Radial shaft movement (run out or misalignment)
- Reversing stuffing box pressure can cause many unbalanced seal designs to open.
- Rotating shaft or seal hits something.
- Seal face hung up in the fretted groove that we find so common in most original equipment seal applications.
- Set-screws slipped on a hardened shaft sleeve causing the seal to lose its spring load.
- Shaft tolerance and finish are out of specifications.
- Shaft vibration and no vibration damping. This is a big problem with the metal bellows seals we find in high temperature applications. Vibration damping interferes with the frequency of the vibration. The O-ring found in many seal designs is a natural vibration damper.
- Sleeve finish was too rough. You want a finish of 32 rms (0,8 microns) or better, unless you are using rubber bellows designs that call for a finish of no better than 40 rms (1 micron).
- Spring load on the wedge, V-rings or U-cup used with most pusher type seals was too high. The Crane #9, Durametallic ROTT and Borg Warner "U" are typical of these designs. A high spring load causes too much interference fit on the shaft.
- Springs can clog if they are located in the product.
- The shaft or sleeve diameter was too large. + 0.000 inches to - 0.002 inches (+ 0.00 mm to - 0,05 mm) is a good shaft or sleeve tolerance for mechanical seals.
- 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.
- Wrong initial setting of the face load.
- Too high a load can cause a product to “flash” and open the seal face.
- Too low a face load can cause the faces to separate when the pump stops.
- Product problems are another cause of the lapped seal faces opening. With the loss of an environmental control the fluid can:
change state and restrict the movement of the seal.
- Change in temperature or pressure can cause a product to build a film on the seal sliding surfaces.
- Product can crystallize if you change the temperature in the stuffing box.
- Slip-stick between the faces if the sealed fluid is a non or poor lubricant
- Some liquids can solidify with a change in temperature, pressure or agitation.
- Temperature change or agitation can cause a product to become viscous and interfere with seal movement.
- Vaporize between the faces and blow them open. This can happen with an increase in temperature or a decrease in stuffing box pressure.
- Solids can clog the springs or some other part of the seal, restricting seal movement. This is a big problem with rotating, "back to back" dual seal designs. The solids collect between the rotating shaft and the inner seal face inside diameter causing the moveable face to "hang up".
- Solids outboard the seal can restrict axial movement as the seal moves to compensate for carbon wear.
- Ice can form when some fluids vaporize, or cold weather can freeze moisture in the air.
- Crystals and solids can form outboard because of seal leakage.
- If you are using a gland quench connection, the quenching fluid must be clean or it will deposit contaminants outboard the seal. If shop water is used in a quench gland to cool a hot fluid be aware that the calcium in the water will deposit outboard of the seal and restrict its movement.
- The seal can operate in a vacuum causing the ingestion of air between the faces of some unbalanced seal designs.
- The dynamic elastomer swelled up because it was not compatible with the pumping fluid or a cleaner that was circulated through the lines. In some designs this expansion of the rubber part can force the lapped face out of its holder or break the hard face.
- An oversized packing sleeve (no tolerance)
- Cartridge seal was installed by pushing on the gland, over compressing the faces in a single seal
- Cartridge dual seal push on with gland loading inside face unloading outside face
- Double ended pump sleeve not sealed to impeller
- Environmental control hooked up wrong
- Gasket protruding into the stuffing box
- Gland flushing ports off center
- Impeller was adjusted after the seal was attached to the shaft. Duriron is the exception because its impeller adjusts towards the back plate causing the seal to over compress.
- Lapped faces are not flat.
- High stuffing box pressure can distort a lapped face.
- Installed backwards. You are running on the non-lapped side.
- Never were lapped flat
- Seals used in cryogenic service (very cold) must be lapped at cryogenic temperatures.
- Thermal distortion can distort a seal face.
- Measured from a shoulder.
- Mechanic misread the installation print.
- Mechanic used the old setscrew marks as a guide and their location was not correct.
- Metal bellows free length changed when the bellows was over compressed.
- No print was available at the installation site. The boss has it locked in his filing cabinet so it will not get dirty or lost.
- Not enough load on the lapped seal faces. Do not be tempted to add springs to increase the designed load because the additional heat generated can be a cause of seal face damage.
- Rotating type mechanical seals need the stationary face installed square to the shaft to prevent excessive axial movement. Stationary designs need the rotating face to be installed square to the rotating shaft. This is much easier to do as long as the seal face is not set screwed to the shaft. It should be butted up against a square shoulder or some other type of "squareness" must be provided.
- Seal face in backwards or un-lapped side is being used
- Seal face was lubricated
- Solids were introduced into the lubricant
- The lubricant froze in a cryogenic application.
- Shaft or sleeve thermal growth. In most seal designs this will unload the seal faces. The seal must be attached to the shaft after all thermal growth has occurred. You are going to need a cartridge seal to do this. Outside mounted, non-metallic seals can be an exception. They will over compress with thermal growth.
- Sleeve moved when the impeller was tightened. Measurements should be taken after the sleeve to shaft gasket is compressed.
- Some people need glasses to read a print or scale. They can see the 1 and 2 on the scale, but not the funny little lines in between.
- Spring load on the wedge or V-ring too high
- Shaft or sleeve diameter too large. Interfering with movement of elastomer
- Stationary seals require the rotating face to be square to the shaft. Cartridge seals can cause a problem
- Wrong lubricant was used on the dynamic O-ring causing it to swell up and lock the seal to the shaft.
The easiest way to tell if you are having seal face opening problems is to inspect the hard face for evidence of wear. Common sense dictates that carbon cannot wear a hard seal face.
If the lapped faces open for any reason, they will allow solids to penetrate between the faces and embed into the softer carbon when the faces close. The contaminated carbon will then act as a grinding surface making wear marks in the harder face.
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