SUBJECT : Common seal
installation mistakes that cause premature mechanical seal failure.
problems with non cartridge designs.
- The seal is operating with the wrong
compression causing it to either overheat or open up prematurely
as the faces wear.
- The pump open impeller was adjusted after
the seal was installed.
- The pump shaft sleeve moved when the
impeller was tightened to the shaft shoulder.
- The installation dimension was measured
incorrectly by the mechanic that installed the
- The mechanic did not use the print, or
could not read the print.
- The pump has been modified and the print
was not revised.
- The mechanic took his measurement from the
wrong location. He measured from the impeller locating shoulder
instead of the stuffing box face.
- The gasket thickness was
- The stuffing box face has been machined to
provide a gasket surface.
- The wrong measuring tool was
- Temperature growth has changed the initial
- The pump is fitted with sleeve type
bearings, and the axial movement is excessive.
- The seal was rebuilt to a different axial
- The mechanic was not wearing his glasses
and he could not read the correct measurement on his
- The seal was installed using the old
set&endash;screw mark as the reference. The reference was
- The sleeve or gasket surface is damaged. This
is causing leakage between the elastomer and the shaft sleeve, or
between the sleeve and the shaft.
- This is a common problem in all packing
conversions. There are many products available that you can use
to repair worn or damaged metal parts to insure a good gasket
- The seal faces are not flat.
- The seal was accidentally dropped and no
one dares tell the boss.
- Poor packaging. The seal should be able to
survive a 39 inch (one meter) drop.
- Over tightening of the stationary face
against a rough stuffing box in a rotating seal application can
distort the lapped face.
- The stationary face clamping areas are not
equal and opposite, causing the faces to go out of
- The seal was lapped flat at ambient
temperature and then put into cryogenic service where it went
out of flat at the colder temperature.
- A recirculation line is aimed at the seal
faces. The high velocity recirculating fluid will interfere with
the ability of the seal to follow whip, wobble, or run out. Be
careful of this configuration when using metal bellows designs.
The recirculated fluid may contain abrasives that will wear the
thin metal plates.
- A gasket or some type of fitting is protruding
into the stuffing box and touching a flexible rotating seal
- The stationary face is not centered to the
- Shaft radial movement can cause the seal
faces to separate if the hard face is not wide enough. In some
cases the stationary face was broken when the deflecting pump
shaft contacted it.
- The stationary seal face has been installed
backwards and you are running on a non lapped surface.
- Only one side of a hard face is usually
lapped flat. Many manufacturers do a poor job of identifying
the un lapped face.
- The seal was set screwed to a hardened
- Seal set screws are manufactured from
corrosion resistant materials. They are, therefore, softer than
conventional set screws and can vibrate loose.
- Many packing sleeves were hardened to
resist packing wear.
- The elastomer (rubber part) was placed on a
damaged portion of the shaft or sleeve.
- The wrong lubricant was used on the elastomer
and it is chemically attacking the elastomer.
- Any petroleum lubricant will damage an
ethylene propylene (EPR) O- ring causing it to
"swell&endash;up". This failure usually occurs within five
days. The flexible seal components will "lock up" and prevent
the lapped faces from staying together
- The seal face was lubricated and then put into
cryogenic service. The cold temperature will freeze the
- The application needs some type of
environmental control and the connection has not been made. If the
connection has been made, see if it is hooked up incorrectly or
the supply has failed.
- Environmental controls are used to control
the temperature, pressure and cleanliness of the product in the
stuffing box. These controls include flushing, quenching,
heating, cooling, recirculating, venting etc. Make sure these
controls are connected to the correct stuffing box, or seal
gland port. Flushing, as an example, should come into the
bottom of the stuffing box or seal gland.
- The shaft/ sleeve tolerance or finish is not
- Unbalanced seals are sensitive to the
shaft&endash;sleeve diameter and surface finish. Most balanced
seals have an internal stepped sleeve, so they are not
sensitive to small variances in shaft sleeve
- In vertical pump applications the stuffing box
has to be vented. Horizontal pump stuffing boxes can be drilled to
insure good venting also.
- The shaft sleeve was removed to make
additional room in the stuffing box.
- The sleeve was positioning the
- The sleeve was providing corrosion
- The stationary face is not perpendicular to
the shaft in a rotating seal application. This will cause
excessive axial movement of the rotating components.
- The use of a stuffing box facing tool can
just about eliminate this problem.
- Stationary seal designs and self aligning
designs can compensate for this problem.
- The rotating face is not perpendicular to the
shaft in a stationary seal application. This will create the same
problem as mentioned above, except that the stationary face will
do the moving. This is also the main reason you should not
cartridge mount stationary seals.
- The stationary face was not centered to the
- Many cartridge seals have centering clips
to solve this problem. Seal designs that utilize slotted glands
should use some type of centering shims.
- If you have not provided centering, the
seal faces can separate if you are using "out of balance" or
- This can cause a pumping action in
cartridge seals, that could cause a convection tank to run
backwards and the barrier fluid to overheat.
- The API.gland
connections are hooked up incorrectly. Be sure that you understand
the difference between flushing, quenching, drain and
- No recirculation line was hooked from the
bottom of the stuffing box to the pump suction. Use a discharge
recirculation line if you are pumping a fluid close to its vapor
- In isolated cases the set screws have loosened
when the sleeve was made of too soft a material.
- The gland flushing ports are machined off
center. Shaft rotation will either help or hinder the flushing,
depending upon their location. The flush rate changes with shaft
- The Allen wrench, used to tighten the set
screws, has rounded corners and is not providing the proper
- The set screws were over tightened and
- The shaft was machined down to accommodate a
smaller diameter seal, or the shaft was machined for repair. In
either case the shaft has been weakened and is now more liable to
deflect with radial loading.
- A severely cocked stationary will break the
springs in the rotating unit because of the excessive back and
forth movement. Excessive lug wear will also be
- The stuffing box often has product attached to
its' inside surface. This product can interfere with the free
movement of the flexible seal components.
- In a double ended pump conversion from packing
to a mechanical seal it is common for the impeller to be
positioned by sleeves that are not sealed at the impeller or
sleeve inside diameter&emdash; causing leakage between the sleeve
and the shaft.
problems with cartridge seals
- The centering or installation clips were not
removed prior to starting the pump.
- The cartridge was installed by pushing on the
gland. The friction on the sleeve static elastomer is causing the
inner seal to over compress and the outboard seal to
- The cartridge was installed on the stuffing
box and then the stuffing box was placed over the shaft. This is
causing the seal to be non concentric with the shaft and could
cause the shaft to hit the close fitting bushing in an A.P.I.
(American Petroleum Institute) type gland.
- Check proper centering by using a spacer
(normally a centering clip) to insure that the gland is centered
to the shaft.
- In some double seal cartridge applications, a
pumping action will occur if the sleeve is not concentric to the
gland. This can cause a convection tank to run
- If you mount a stationary seal on a cartridge,
the rotating face will "cock" causing excessive moving problems.
You will need some type of a self aligning feature to solve this
problems with rubber bellows seals (Type #1)
- The wrong lubricant was used on the bellows
(Silicone grease is an example of this) and as a result, the
rubber bellows is not sticking to the shaft.
- The shaft or sleeve is too smooth . It should
be no better than 40 rms.
- The carbon has been installed backwards. It
will fit either way, so be sure you are running against the lapped
side of the face.
- The seal installation must be completed in
fifteen to thirty minutes, or the rubber bellows will vulcanize to
the shaft in the wrong location.
- Be sure that none of the pump, or seal
dimensions have been altered. The seal must be installed at a
fixed length that is not easily changed.
- Most of the rubber bellows designs use a "Buna
N" rubber boot that is sensitive to ozone attack. Buna N rubber
has a shelf life of only one year.
It should be noted that the normal mode of failure
for this type of seal is for the rubber boot to experience a rupture
or "blow-out" causing a massive seal failure. It is important
to use back up protection if you use this type of seal.
problems with metal bellows seals
- If you are using the rotating version of the
metal bellows seal, the close fitting anti-vibration lugs can hang
up on the sleeve. The tolerance of the sleeve outside diameter is
very critical with this type of seal.
- The face holder, in high temperature
applications, is made from a low expansion material that has
little to no corrosion resistance. If you lose cooling, the
resultant shaft thermal expansion can engage the "damping lugs" in
the low expansion steel face holder and "drag open" the seal
- The seal balance line shifts with
- A pump discharge recirculation line can wear
into the thin bellows section.
- Austenitic metals change their spring load
when the bellows is over compressed.
- At elevated temperatures the carbon face can
loosen in the metal holder. Watch out for glued in
- Good designs will rotate the fluid to prevent
a slurry from wearing the thin metal plates.
- Be careful of stationary seals in high
temperature applications. The flush connection will produce uneven
cooling causing face distortion.
- High temperature applications utilize heat
treated bellows materials. Be aware that metal can anneal and
loose its spring rate at temperatures as low as 300° F (150
with split mechanical seals
- Some designs use glued elastomers. This causes
a hard spot in the elastomer that will not seat properly causing
- Many designs cannot be used in an alternating
pressure/vacuum application. This can be a problem when you have a
mixer application to seal.
- Some elastomers are not available in split
o-rings, so watch out for availability.
- Many outside designs move towards the product
as the faces wear, making them ineffective in slurry
problems that can occur shortly after the seal is
- The cooling jacket on the pump builds up a
layer of calcium on the inside, interfering with the heat
- Condensate, steam, or heat transfer oil is
a good substitute for hard well water, or dirty shop
- Teflon® comes off of the metal
- The product is penetrating the porous
Teflon coating and attacking the base material. In some cases
you may have to remove the baked on coating with a fine emery
- A pressure drop at the seal faces, and across
the elastomer can cause some products to change state as the
solvent evaporates from the product. This can cause the fluid to
- Become viscous.
- Build a film on the faces.
- Blow out pieces of the
- Cryogenic temperatures can freeze the
elastomer or any lubricant placed on the seal faces.
- The wrong lubricant on the dynamic elastomer
can cause the O-ring to swell and lock up the seal. This usually
occurs within ten days.
- At elevated temperatures the differential
expansion between the metal holder and the seal face can cause the
seal face to become loose in its metal holder. Metal has an
expansion rate approximately three times that of carbon and most
other seal face materials.
- A convection tank installed between
seals is running backwards or is not
running at all.
- Check to see if the seal gland is
concentric with the shaft.
- Be sure that the piping is connected to the
correct gland ports.
- Check the recommended height and distance
- Be sure you have a proper liquid level in
- If there is a pumping ring, make sure it is
running with the correct rotation
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