SUBJECT: Operation practices that cause
frequent seal and bearing maintenance problems 6-7
Wouldn't it be wonderful if the plant operation and maintenance
departments could work independently? The fact of the matter is that
there are three types of problems we encounter with centrifugal pumps
and poor operation is only one of them. If you're curious, the other
two are design problems and poor
Seals and bearings account for over eighty-five percent (85%) of
premature centrifugal pump failure. In the following paragraphs we
will be looking at only those operation practices that can, and will
cause premature seal and bearing failure. Design and maintenance
practices will be discussed in other papers in this series.
When pumps were supplied with jam packing, the soft packing
stabilized the shaft to prevent too much deflection. In an effort to
save flushing water and to conserve power, many of these same pumps
have since been converted to a mechanical seal and the radial
stabilization the packing provided has been lost.
The bad operating practices
Running the pump dry will cause over-heating and excessive
vibration problems that will shorten seal life. Here are some of the
common reasons why a pump is run dry:
- Failing to vent the pump prior to start-up.
- Running the tank dry at the end of the operation cycle.
- Emptying the tank for steaming or introduction of the next
- Running on the steam that is being used to flush the
- Starting the standby pump without venting it. Venting a
hazardous product can cause a lot of problems with the liquid
disposal. Many operators have stopped venting for that
- Tank vents sometimes freeze during cold weather. This will
cause a vacuum in the suction tank, and in some cases could
collapse the tank.
- Sump fluids are often dirty, corrosive or both. The control
rods for the float switch will often "gum up" or corrode and give
a false reading to the operator. He may think that there is an
adequate level, when in fact, the tank is empty.
Dead heading the pump can cause severe shaft deflection as the
pump moves off of its best efficiency point (bep.). This translates
to excessive heat that will affect both the seal and the bearings as
well as causing the seal faces to open, and the possibility of the
impeller contacting the volute when the shaft deflects.
- Starting the centrifugal pump with a shut discharge valve is
standard practice with many operation departments. The concern is
to save power without realizing the damage that is being done to
the mechanical seal, impeller, wear rings and bearings.
- Some pumps are equipped with a recirculation valve that must
be opened to lessen the problem, but many times the valve is not
opened, or the bypass line is clogged or not of the correct
diameter to prevent the excessive head. Another point to remember
is that if the bypass line is discharged to the suction side of
the pump the increased temperature can cause cavitation.
- After a system has been blocked out the pump is started with
one or more valves not opened.
- Discharge valves are shut before the pump has been
Operating off of the best efficiency point (bep.) causes shaft
deflection that can fail the mechanical seal and over-load the
- Starting the pump with the discharge valve closed to save
- The level in the suction tank is changing. Remember that the
pump pumps the difference between the discharge and suction heads.
If the suction head varies, the pump moves to a different point on
- Any upset in the system such as closing, throttling or opening
a valve will cause the pump to move to a new point on the curve as
the tank fills.
- Pumping to the bottom of a storage tank will cause the pump to
move to a different point on the curve as the tank fills. Some
systems were designed for a low capacity positive displacement
pump and have since been converted to a centrifugal design because
of a need for higher capacity. Centrifugal pumps must discharge to
the top of the tank to prevent this problem.
- If the discharge piping is restricted because of product build
up on the inside walls, the pump will run throttled. This is one
of the reasons that it is important to take periodic flow and
- Increasing the flow will often cause cavitation problems.
Seal environmental controls are necessary to insure long
mechanical seal life. It is important that operations understand
their function and need because many times we find the controls
installed, but not functioning.
- Cooling-heating jackets should show a differential temperature
between the inlet and outlet lines. If the jacket clogs up, this
differential will be lost and seal failure will shortly
- Barrier fluid is circulated between two mechanical seals.
There may or may not be a differential temperature depending upon
the flow rate. If a convection tank is installed, there should be
a temperature differential between the inlet and outlet lines. The
line coming out of the top of the seal to the side of the tank
should be warmer than the line from the bottom of the tank to the
bottom of the seals, otherwise the system is running backwards and
may fail completely. The level in the tank is also critical. It
should be above the tank inlet line or no convection will occur.
Some convection tanks are pressurized with a gas of some type.
Many original equipment (O.E.M.) seal designs will fail if this
differential pressure is lost.
- Some seal glands (A.P.I. type) are equipped with a quench
connection that looks like the seal is leaking water or steam. If
there is too much steam pressure on this quench connection, the
excessive leakage will get into the bearings causing premature
failure. The steam is often used to keep the product warm to
prevent it from solidifying, crystallizing, getting too viscous,
building a film on the faces etc. Operating people frequently shut
off the quench to stop the condensate from leaking.
- Flushing fluids are used for a variety of purposes, but most
of the time they are used to get rid of unwanted solids. The flush
can be closely controlled with a flow meter or throttling valve.
The amount of flush is determined by the seal design. As an
example, those designs that have springs in the product require
- It is important to check that the stuffing box has been vented
in vertical pumps. The vent should be coming out of the seal gland
and not the stuffing box lantern ring connection.
There are some additional things that all operators should know to
insure longer rotating equipment life. As an example :
- Mechanical seals have an 85% or more failure rate that is
normally correctable. This is causing unnecessary down time and
excessive operating expense. Seals should run until the
sacrificial carbon face is worn away, but in more that 85% of the
cases the seal fails before this happens.
- There are five different causes of cavitation.
- You should know where the best efficiency point (bep.) is on a
particular pump, and how far it is safe to operate off the bep.
with a mechanical seal installed.
- You should be aware that washing down the pump area with a
water hose will cause premature bearing failure when the water
penetrates the bearing case.
- Learn about the affect of shaft
L3/D4 on pump operation.
- Know how the pumped product affects the life of the mechanical
seal and why environmental controls are necessary.
- If you are not using cartridge seals, adjusting the open
impeller for efficiency will shorten the seal life. In most cases
the seal will open as the impeller is being adjusted to the
volute. Flowserve (Durco) pumps are the best example of the
exception to this rule. The popular Durco pumps adjust to the back
plate causing a compression of the seal faces that can create
mechanical seal "over heating" problems.
- Cycling pumps for testing will often cause a mechanical seal
failure unless an environmental control has been installed to
prevent the failure.
- Mechanical seals should be positioned after the impeller has
been adjusted for thermal growth. This is important on any pump
that is operated above 200°F (100°C) or you will
experience premature seal failure.
- Some elastomers will be affected by steaming the system. A
great deal of caution must be exercised if a flushing fluid such
as caustic is going to be circulated through the lines or used to
clean a tank. Both the elastomer and some seal faces (reaction
bonded silicone carbide is a good example) can be damaged. If the
elastomer is attacked, the failure usually occurs within one week
of the cleaning procedure.
- The stuffing box must be vented on all vertical centrifugal
pumps or otherwise air will be trapped at the seal faces that can
cause premature failure of many seal designs.
- Most original equipment seal designs cause shaft damage
(fretting) necessitating the use of shaft sleeves that weaken the
shaft and restrict pump operation to a narrow range at the
Here are a few common misconceptions that cause friction between
maintenance and operation departments
- Shutting the pump discharge valve suddenly, will blow the seal
- All ceramics cold shock.
- High head, low capacity consumes a lot of power.
- The pump must come into the shop to change a mechanical
- If you use two hard faces or dual mechanical seals in slurry
applications, you will not need flushing water with its
corresponding product dilution.
- If you use metal bellows
seals for hot oil applications, you will not need the stuffing
box cooling jacket operating.
- It's okay to use an oversized impeller because throttling back
will save power.
A few more thoughts on the subject
- Operators should receive proper schooling on the trouble
shooting and maintenance of pumps. In the military and many modern
plants, the operator and the maintenance mechanic are often the
same person. If the operator knows how the pump works he will have
no trouble figuring out the solution to his problem. Too often he
is told to keep the flow gage at a certain point, or between two
values without understanding what is actually happening with the
equipment. If the operator recognizes cavitation he can tell the
maintenance department and help them with their trouble
- As you wander around the plant, look out for painters that
paint the springs of outside and double mechanical seals. There is
a trend to putting two seals in a pump for environmental reasons
and the painting of springs is becoming a common problem.
- If someone is adjusting the impeller make sure he is resetting
the seal spring tension at the same time.
- If the pump is getting hot or making excessive noises, report
it immediately. After the failure, it does no good to tell
maintenance that it was making noise for two weeks.
- If you are the floor operator it is common knowledge that
taking temperature and pressure readings is very boring,
especially on those gages that are located in hot or awkward
locations. Avoid the temptation to "radio" these readings. From
hot to failure is a very short trip.
- Maintenance's favorite expression is "There is never time to
do it right, but there is always time to fix it." Try to keep this
in mind when the pressure is on to get the equipment running
- Do not let cleaning people direct their "wash down" hoses
directly at the pump. Water entering the bearings through the lip
or grease seals is a major cause of premature bearing failure.
Most water wash downs are used to dilute and wash away seal
leakage. Stop the leak and you have eliminated the reason for the
- A great many motor and electrical problems are caused by
these same wash down hoses.
- Cooling a bearing outside diameter will cause it to shrink,
and the bearing will get hotter as the radial load increases. Keep
the water hose and all other forms of cooling off of the bearing
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