SUBJECT: Maintenance practices
that cause high seal and bearing maintenance problems
Maintenance departments seldom return savings to
the company management. They fear that if they do not spend this
year's budget, next year's will be reduced. Management views
maintenance savings as bottom line money and works at reducing
maintenance man power and inventory costs. Here are some of the
maintenance practices that increase the pump failure rate:
- Failure to align the pump and driver.
Misalignment will cause the mechanical seal to move excessively,
increasing the chance for the seal faces to open and fail the
- Pipe strain is another cause of misalignment
between the seal's stationary and rotating faces. Wear ring damage
is common if pipe strain is present.
- Failure to dynamically balance the rotating
assembly can result in "whip, wobble, and run-out
- Damage to the shaft and bearings during the
sleeve removal process. Banging on the sleeve with a large hammer
or heating the shaft with a torch are common methods used to
remove sleeves. Needless to say the seal and bearings stand a good
chance of being destroyed in the process, along with the shaft
that will be bent or warped.
- Using the coupling to compensate for
misalignment. A coupling is used to transmit torque and compensate
for axial growth of the shaft, nothing else! It cannot compensate
for misalignment between the pump and its' driver.
- Trimming the impeller without dynamically
re-balancing it. The impeller casting is not homogeneous, it must
be re-balanced after any machining operation has taken
- Throttling the pump discharge to stop a
cavitation problem. The more you pump the more
N.P.S.H. you need, so throttling does
work, but you may be now operating off the pumps' bep. resulting
in shaft deflection.
- Failure to machine the stuffing box square to
the shaft will result in excessive seal movement.
- Repairing the cutwater to the wrong length can
cause a cavitation problem known as the "Vane
Passing Syndrome" that will damage the
tips of the impeller blades and damage the volute just beyond the
- Failure to properly adjust the open impeller
clearance or letting the closed impeller wear ring clearance
become excessive can make the pump run inefficiently and
- Turning down a shaft and repairing it with a
polymer material will weaken the shaft making it more sensitive to
deflection forces. That practice was common with packed pumps, but
should be avoided when mechanical seals are being
- Substituting a globe valve for a gate valve
will throw the pump off of its bep. causing shaft
- Any alteration in the piping system or failure
to prevent solids "build up" in the lines will have the same
- Mounting the pump and motor on too light a
foundation. The foundation should be at least five times the mass
of the equipment sitting on it or vibration will become a problem.
Proper grouting is also necessary to mate the base of the pump to
handling practices can also lead to premature
- Installation problems:
- The seal is installed at the wrong length.
There are a lot of ways to do this from reading the print wrong
to the sleeve moving after the seal was attached to the
- The wrong lubricant was used on the dynamic
rubber part causing it to be chemically attacked. Petroleum
grease on Ethylene Propylene O-rings is a good example of this
problem. In salt water applications zinc oxide should be used
on all rubber parts and metal components that clamp
- The seal was installed before the impeller
setting was made or an impeller adjustment was made without
resetting the mechanical seal. In most cases this will cause
the seal faces to open prematurely.
- The shaft or sleeve is out of tolerance.
This can cause serious problems with those seal designs that
have a dynamic elastomer sliding on the shaft (most original
equipment seals fit into this category).
- The sleeve was hardened to resist packing
wear causing the seal set screws to slip and the faces to
- The elastomer (rubber part) exceeded its'
shelf life. This is a real problem with the Buna "N" material
found in most rubber bellows seals.
- Installing a stationary seal on a cartridge
will cause the rotating face to "cock" when the set screws are
attached to the shaft.
- An environmental control was lost while the
seal was installed in the pump. Typical environmental controls
- Clean flushing liquid to keep solids away
from the moving seal parts.
- Controlling stuffing box temperature with a
cooling or heating jacket. If the circulating water is "hard"
condensate may have to be substituted to prevent the cooling
jacket from becoming coated with calcium and other solids that
will interfere with the heat transfer.
- Barrier fluid is used to circulate between
two mechanical seals. Sometimes the circulation is done by
simple convection, but pumping rings and forced circulation are
common also. Check to see if your convection tank has to be
pressurized. This is a common problem with many original
equipment seals. Feel the convection lines to make sure the
convection is taking place in the right direction.
- A steam quench is often used to remove
dangerous vapors and to keep the seal area warm when the pump
is shut down. Metal bellows applications use the steam quench
to cool down hot oil to prevent unwanted "coking".
- A stuffing box vent should be connected
from the area of the seal faces to the suction side of the
pump, or some other low pressure area to prevent air from being
trapped at the seal faces.
- A discharge recirculation line, and a
bushing in the bottom of the stuffing box are often used to
pressurize the stuffing box to prevent the product from
vaporizing at or between the lapped seal faces.
- Is there enough clearance between the seal
outside diameter and the inside of the stuffing box? Solids build
up in the stuffing box can interfere with the free movement of the
- The seal was installed with unidentified
materials, making troubleshooting almost impossible.
- Which carbon is being used? There are a
hundred available and they are not all alike.
- Which elastomer are yoiu using? Do you know
both the material and the grade?
- What material are the metal components
manufactured from? Not all stainless steel grades are alike, and
stainless steel springs or metal bellows should never be used
because of potential problems with chloride stress
- There are many hard seal faces in use. All
ceramics, silicone carbides and tungsten carbides are not
- The outside springs were painted on a double
seal when the pump area was refurbished.
- The pump discharge recirculation line is
handling abrasive solids. They are being directed at the lapped
seal faces or at the thin metal bellows.
- If the open impeller is adjusted backwards
(this is a common problem if a facility has both Flowserve
(Duriron) and Goulds pumps), it can create a vacuum in the
stuffing box as the "pump out vanes" are running too close to the
- Do not shut off the stuffing box cooling
jacket when a metal
bellows seal is installed. The stuffing
box is cooling down the shaft as well a the seal area. Shaft
cooling is necessary to prevent heat conduction to the
bearing maintenance practices are a major
cause of premature bearing failure.
- If the oil level is too high or the bearings
are over greased, the low specific heat of the lubrication and its
poor conductivity will cause the bearing area to over
- The inside of the bearing case must be
protected against rust when it is stored as a spare. The bearings
should be coated with an appropriate grease because they can rust
- During storage, or while in a standby
condition, nearby equipment that is vibrating can induce vibration
into the static bearings causing "false brinneling" or hardening
of the bearing balls and races.
- If the oil becomes contaminated with water
you'll experience a very rapid bearing failure. The water can
enter through the grease or lip seals from several sources:
- leakage through the packing or mechanical
- From the water hose that is used to wash
down the base plates and pump area.
- From moisture in the air as the moisture
enters the bearing casing through "aspiration".
- From the quench gland on an
- The bearing was installed improperly:
- The shaft outside diameter was the wrong
tolerance. Remember that the tolerance is given in tenths of
thousands of an inch or thousands of a millimeter.
- Too much pressure was put on the arbor
press during the assembly sequence.
- The bearing was heated in contaminated oil
that has deposited the contaminates in the races
- The oil was over heated and varnish
particles are now in the race ways.
- The bearing was pushed too far up a tapered
shaft increasing the radial load.
- The thrust bearing is being retained by a
simple snap ring. During operation the shaft thrust is usually
toward the volute and against this thin ring.
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