SUBJECT: Preventing premature seal
Here is a three question quiz:
Question: How do you prevent
premature mechanical seal failure?
Answer: Find out what is causing the seals to fail and prevent it
Question: How long should a
mechanical seal last?
Answer: Until the sacrificial carbon seal face wears away.
Question: How often do seals wear
Answer : Less than ten percent of the time.
If you understand that simple logic you'll be happy to lean that
seals fail for only two reasons:
- The lapped faces opened.
- One of the seal components becomes damaged.
Don't make it too complicated. If you inspect a failed seal and
see no evidence of a component damage, then the seal faces must have
opened, because seals fail for only two reasons.
the most common reasons that lapped faces open
- The set screws slipped on a hardened shaft sleeve.
- The dynamic rubber part (the rubber part that moves) stuck to
the shaft or sleeve.
- The 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.
- 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 tolerance for
- The 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..
- The O-ring groove was out of tolerance. There is too much
interference on the shaft preventing the O-ring from flexing and
- The 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.
- Too much axial movement of the shaft:
- The seal was installed on the shaft and then the semi-open
impeller was adjusted to the pump volute for the inital clearance
setting, or to compensate for normal impeller wear.
- Thermal growth of the shaft. Every inch of shaft will grow
0.001" for each 100°°F (0.001 mm/mm/ °C of
- Sleeve bearings allow excessive axial movement. Some
centrifugal pump designs utilize this type of bearing as a thrust
- Too much 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.
- Harmonic vibration is a major source. The seal is vibrating in
harmony with another piece of equipment
- Cavitation is very common in many applications. Remember that
there are five types of cavitation and all of them can cause
- "Slip stick" vibration can occur between seal faces if the
pumping fluid is not a lubricant. Hot water is a good example of a
non lubricating liquid. Most gases and dry solids are obvious
- Hitting a critical speed is another cause of vibration. Pumps
that have this problem are said to have flexible rather than rigid
- Installation errors:
- Not enough load on the lapped seal faces. Do not be tempted
to increase the designed load because the additional heat
generated can be a cause of seal face damage.
- The seal was installed at a wrong inital setting.
- The sleeve moved when the impeller was tightened.
Measurements should be taken after the sleeve to shaft gasket
- The mechanic misread the installation print.
- The mechanic used the old set screw marks as a guide and
their location was not correct No print was available at the
- The 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
- 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
- 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.
- The wrong lubricant was used on the dynamic o-ring causing
it to swell up and lock the seal to the shaft.
- Reversing stuffing box pressure can cause most unbalanced seal
designs to open.
- 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) is just about the limit for rotating seal
- The shaft is fretted (grooved from the dynamic elastomer)
causing the moveable face to hang up as it tries to compensate for
- The lapped faces are not flat.
- The faces never were lapped flat
- The lapped face was installed backwards. You are running on
the non-lapped side.
- High stuffing box pressure can distort a lapped face.
- Thermal distortion can distort a seal face.
- Seal used in cryogenic service (very cold) must be lapped
at cryogenic temperatures.
- The product changed state and is restricting the movement of
- The product can crystallize if you change the temperature
in the stuffing box.
- A temperature change or agitation can cause a product to
- Products can solidify with a change in temperature,
pressure or agitation.
- A change in temperature or pressure can cause a product to
build a film on the seal sliding surfaces.
- The product vaporized between the faces and blew them open.
This can happen with an increase in temperature or a decrease
in stuffing box pressure.
- Solids clogged the springs or some other part of the seal,
restricting seal movement.
- Solids outboard the seal can restrict axial movement as the
seal moves to compensate for carbon wear.
- Ice can form when some products vaporize, or cold
weather can freeze moisture in the air.
- Crystalls and solids can form outboard because of seal
leakage or dirty quench fluid.
- If you are using a gland quench connection, the
quenching fluid must be clean or it will deposit
contaminants outboard the seal.
- Discharge recirculation lines aimed at the moving seal parts
can restrict their movement.
- The seal face hung up in the fretted groove that we find so
common in most original equipment seal applications.
The easiest way to tell that 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 faces open, it will allow solids to penetrate between the
lapped faces and then these solids will 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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