SUBJECT: Is there a reliable
method of introducing a centrifugal pump predictive maintenance
Probably not! But if you want to try you are first
going to have to define what you mean by predictive maintenance. If
you mean that you're going to inspect the pump and based on your
observation, you're going to accurately predict future life, you're
going to have a problem.
The relationship between life to date and future
life is generally accepted as valid. As an example:
- Measure the depth of the tread on your
automobile tires, record the distance driven on the tires, and if
you do not change your driving habits, you can accurately predict
the life remaining.
- Do the same thing with the shoes you are
wearing and you'll come up with a similar result.
These are items that tend to "wear out" so life to
date is a valid measurement. The problem with centrifugal pumps is
that seals and bearings account for over 90% of premature pump
failures and neither of these items ever "wears out". Seals should
run until the sacrificial carbon face has worn away, but a close look
at used seals will demonstrate that wear is actually a minor problem.
In excess of 85% of mechanical seals leak with plenty of wearable
face still visible.
Bearings do not "wear out" like mechanical seals.
They have a predictive fatigue life that is based on load and cycles.
Properly loaded they could last a hundred years, but like seals, they
experience a very high premature failure rate. All of this means that
the measurements you are taking today are no indication of what is
going to happen tomorrow. It's like trying to predict an automobile
accident. There are precautions you can take, but accidents still
Most companies base their predictive maintenance
programs on vibration analysis or interval timed, visual inspection.
and that is why we find "reactive maintenance" the norm in most
plants. How many times have we heard the expression "I did not have
time to do the job correctly (realignment, dynamic balancing, etc.)
because I had to get the pump back on stream".
A more sensible approach to predictive maintenance
is to monitor the equipment for changes that could be destructive in
the future, but allow you to correct them before the destruction
starts. I spent my formative years in nuclear power. If, as an
operator, you did something wrong that would be harmful to the atomic
reactor it would
and shut down immediately. But if you took an action that could be
potentially dangerous, the reactor would start an
that would start to slowly shut down the reactor and give you time to
correct what ever it was you did.
Medical people use a predictive maintenance
program when they:
- Monitor your cholesterol level. If it exceeds
some preset number (two hundred in the U.S.) it means that your
arteries are in danger of clogging, so you should change your diet
before it becomes serious. (insertion)
- If your blood pressure is too high you could
get a stroke. (insertion)
- A high fever indicates a need to get medical
attention before destruction starts. (insertion)
- Some types of pains initiate an immediate
- You do the same thing with your
- A high engine water temperature is a sign
of engine failure in the future. You better check the fan belt
and look for water leaks. Nothing is serious yet, but you
should react to the warning signs. (insertion)
- High fuel consumption indicates a need for
an engine tune-up. (insertion)
- A loss of oil pressure means shut off the
engine and react immediately. (scram)
Pumps also "scram" and give "insertion" signals."
Unfortunately vibration analysis indicates that destruction has
already started (scram). Let's look at some of the "insertion"
The stuffing box temperature is increasing. If it
gets too hot you're going to have a problem. You had better correct
the condition if you do not want to experience a premature seal
failure. What can happen if the stuffing box temperature gets too
- The product can change state. It can stop
being a lubricant and quickly become a destructive solid or
- It can vaporize, expand and blow the seal
faces open, leaving destructive solids between the
- It can become viscous, interfering with the
free movement of the springs and bellows.
- It can solidify, gluing the faces together
or making the moveable components inoperable.
- It can crystallize and interfere with the
moving parts of the seal.
- It can cause the product to build a film on
the faces (hot oil as an example) and sliding components,
making them inoperable.
- Corrosion increases with increasing
- Temperature causes materials to expand. Seal
faces can go out of flat, and pressed in carbon faces can loosen
in their holder.
- Metal bellows
vibration dampers can stick to the
shaft sleeve, opening the faces.
- Some seal faces can be damaged by high heat.
Plated materials and filled carbons are two such examples. Voids
in some carbon faces can expand causing pits in the lapped
- Elastomers can experience "compression set"
problems, causing them to leak or in some cases fail completely at
higher heat levels.
What could be
causing this high heat? If you take no corrective action one of the
above will occur.
- A loss of flushing fluid. There are multiple
reasons why this could happen and I'm confident you can think of
many of them.
- Loss of barrier or buffer fluid between two
mechanical seals, or the convection of the barrier fluid has
stopped for some reason. Keep in mind that petroleum products need
forced lubrication or a pumping ring because of the petroleum low
specific heat and poor conductivity.
- Loss of the quench in an A.P.I.
- Loss of the discharge recirculation line
because of a clogged filter, cyclone separator or heat
- Loss of suction recirculation because of
solids in the fluid.
- Loss of cooling in the stuffing box cooling
jacket because the circulating water was "hard" and has deposited
an insulating layer of calcium on the inside of the cooling
- The seal is running dry because the stuffing
box was not vented in a vertical application.
- The seal was installed incorrectly. There is
too much spring load on the faces.
- You need a hydraulic
balanced seal. The unbalanced design
cannot compensate for the high stuffing box pressure.
- Thermal shaft expansion is over compressing an
outside seal design, or one of the seals in a dual seal
- The open impeller adjusting technique can over
compress some seal designs.
- The stuffing box is running in a vacuum
because the supply tank is not vented properly or cold weather is
freezing the tank vent.
- Water hammer, pressure surges and cavitation
will all alter seal face loading.
A change in the stuffing box pressure can
- The product to vaporize, opening the lapped
- O-rings and other elastomer designs to extrude
and jam the sliding components.
- Lapped seal faces to distort and go out of
- A stuffing box vacuum can blow open unbalanced
- A differential pressure across the elastomer
can cause ethylene oxide to penetrate into the elastomer and
destroy it as it expands in the lower pressure side.
If you are monitoring temperature and pressure in
the stuffing box area you will note the changes mentioned and
depending upon your knowledge of the above, you will have time to
react before seal failure occurs.
An increase in the bearing case oil temperature is
significant because the life of bearing oil is directly related to
the oil temperature. Lubricating oil has a useful life of thirty
years at thirty degrees centigrade (86°F) and its life is cut in
half for every ten degree centigrade (18°F) increase in
temperature. You can figure the temperature in the bearing is at
least ten degrees centigrade (18°F) higher than the oil sump
temperature. At elevated temperatures the oil will carbonize by first
forming a "varnish like" film that will turn into a hard black coke
at these higher temperatures. It is these formed solids that will
destroy the bearing.
What is causing these elevated temperatures? There
are a number of possibilities:
- Loss of circulation in the stuffing box
- Loss of cooling in the bearing case cooling
- Some one is cooling the outside of the bearing
casing causing the outside diameter of the bearing to shrink,
increasing the load.
- The bearing was installed
- The bearing is over lubricated. The oil level
is too high or there is too much grease in the
- The lubricating oil is contaminated with
- The shaft is overloaded because the pump is
operating off of the B.E.P., misalignment, unbalance,
- There is too much axial thrust.
Oil sampling is always a good idea. It can tell
- If water is getting into the oil.
- If the oil additives are still present and
- If the oil is carbonizing due to high
- If there are solids due to corrosion, bearing
cage destruction, or some other reason.
If you monitor pump
suction and discharge pressure and coordinate this information with
flow and motor amperage readings you can come up with a lot of useful
information such as:
- You can tell if you have the right size
- You can estimate where you are in respect to
the bep. and know if the shaft is deflecting, or is about to
- You can tell if the motor is close to an
- You will know when the impeller needs
adjusting, or the wear rings need replacement.
- You can spot poor operating practices if you
have a chart recorder installed, instead of pressure and
- You can tell if the tank you are pumping from
is losing the proper level or if the suction lines are
- You can tell if you are getting close to
It goes without saying that constant monitoring is
the most sensible answer to predictive maintenance. It is the same
logic you use with your automobile. You believe that the extra
expense of installed gauges is a cheap investment for longer engine
There is nothing wrong with vibration analysis (an
E.K.G. is still part of taking a physical) but do not substitute it
for sensible monitoring. The "scram " is too expensive in this very
competitive world of ours.
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with over 600 Seal & Pump Subjects
to Mc Nally home page