SUBJECT: Pump and seal preventative
maintenance - what is it all about? 2-6
Smart shop maintenance can be approached several different
- Reactionary Maintenance - The equipment has failed and you
have to fix it right now! If you have an installed spare it helps,
but you must fix it immediately because you can't afford to run
without a spare. This is the "norm" in most plants.
- Preventative Maintenance - You'll take appropriate actions and
thereby prevent the unit from failing . Most companies are still
trying to figure out what those appropriate actions are.
- Predictive Maintenance - By taking selected readings we hope
to be able to predict an impending problem and calculate how much
longer the unit will run before failure. A lot of information is
being collected, but the concerned parties are still trying to
figure out how to use it. Most predictive maintenance calls for
shutting down the equipment when some arbitrary time limit has
been reached and this puts you back to reactive maintenance
- Continuous Diagnostic Maintenance - You'll take constant
readings and note any significant change in these readings.
Hopefully you'll then be able to predict impending failure. This
is very similar to reading the instruments on the dashboard of
- Machinery History - By keeping good records we hope to predict
the life of the unit or its individual components. This system
assumes that the life of the previous unit somehow relates to the
life of the present one.
The problem with most of these systems is that we collect more
data than the operator or any one else can deal with. The result is
that Reactionary Maintenance is a "reality" in most plants today.
Since the taking of readings is part of most of these programs let
us take a look at the type of information we can gather for analysis.
can monitor :
- Heat - Especially in the seal chamber and bearing case. A
changing reading at the pump suction would be helpful in
predicting cavitation. Volute casing readings could indicate
internal recirculation and minimum flow problems as well as an
indication of impeller rubbing.
- Pressure - You can take readings at the pump discharge,
suction and stuffing box to determine where you are on the pump
curve and see if you're within the operating range of your
- Speed - To see how it affects pump curve data. The pump curves
were generated with a variable frequency motor at a speed
different than your induction motor.
- Noise - To indicate cavitation, rubbing, location on the pump
curve, bad bearings, or some other abnormal condition.
- Flow - To check the status of wear rings, impeller adjustment
and the discharge recirculation system.
- Strain - To anticipate rubbing and stress corrosion
- Liquid level - To anticipate npsh,bep and air ingestion
- Leakage and Fugitive Emissions - To check the seal performance
in both the stuffing box and bearing case locations.
- Product contamination- To monitor the performance of dual
seals and flushing controls.
- Functioning of stuffing box environmental controls - To
anticipate seal failure.
- Power Consumption - To check pump efficiency and to anticipate
- Vibration - At multiple locations in the system to indicate
that a failure has already started.
monitoring of vibration is confusing to many people. We hear
about frequency, amplitude, velocity, acceleration, I.P.S. and all
sorts of technical jargon. Probably the system verbalized the most,
is the reading of acceleration ( in./sec2 or mm/ sec2).
The problem with this system is that it is dependent upon the
frequency of the vibration. Other companies use decibels as a method
of measurement with a decibel defined as:
20 log10 input /reference
In this system everyone uses a different reference except the
people measuring sound who have agreed upon background noise as their
reference. Since this is a logarithmic scale it allows you a big
range to compute change in levels. In fact each 6 db is equivalent to
a two times increase in vibration level.
The bottom line is, regardless of the method you are using, only a
relative number. Most people agree that a two times increase in
reading is cause for concern and the equipment should be shut down
for a visual inspection.
The transducers that pick up this vibration can be either
permanently mounted or portable, with permanent being the preferred
method. Be sure to install the transducers on a flat, clean surface
and be careful how you screw them down. To insure good contact it
helps to place a small amount of silicone grease under the transducer
to fill in irregularities that might trap air and give a false
If you are going to use the portable type of vibration analyzer
you should drill a small recess at the location you wish to monitor
and lubricate it with silicone grease to prevent corrosion. This
recess should match the curvature of the probe. Be sure the area is
clean before placing the probe in the recess and be sure to hold the
probe in a vertical or horizontal position, never upside down. If it
must be at an angle you must try to duplicate the same angle each
time you take a reading. Your readings will be relative readings so
they will have no meaning outside of your own organization and this
particular piece of equipment.
Many problems become visible when we look at the disassembled
inspection of individual components is still one of the best
methods of troubleshooting. You can see :
- Evidence of rubbing.
- Product attaching to the hardware.
- The presence of foreign objects.
- Missing parts.
- A wrong part.
Be sure to note the order in which the parts came out to determine
an improper assembly.
There are things you can measure as well as things that can be
monitored or observed:
- Clearances - At the wear rings and bearing fits.
- Dynamic balance - of the entire rotating assembly or the
- Alignment - Between the pump and the driver as well as the
piping and the pump flanges.
- Settings - For the seal face loading and impeller
- Shaft deflection - To insure that rotating parts will not
contact stationary parts.
- Shaft axial movement - Especially equipment with sleeve or
babbitt bearings. Both impellers and mechanical seals are
sensitive to this movement.
- Oil analysis - To learn if we are experiencing excessive wear
or if our lubrication is breaking down. An 18 degree Fahrenheit
(10 C.) increase in oil temperature will cut the service life of
the oil in half.
- X-Ray - To detect cracks in metal, especially at the welds or
to indicate evidence of Stress Corrosion cracking.
- Thermal imaging - To detect rubbing and heat losses.
- Magnetism - Especially in the bearing area. Magnetized
bearings or seals attract the metal particles found in worn
Lists like the one above could keep a maintenance staff busy
forever, and no one could deny that the information would be
valuable. The real question, however, is how practical would it be to
do those things? A human being could be wired to give constant
readings of his blood pressure, pulse, E.K.G., cholesterol etc.. but
no one would think of doing it unless he were in terrible health and
in intensive care.
Most maintenance programs start with the false assumption that the
life to date is some how related to how much service life is left in
the equipment. In other words; if half of the seal wearable face is
still left then the seal can be logically expected to run the same
amount of time as before. The problem with this logic is that it only
works if the components are wearing out. In the case of seals and
bearings, failure is the most common condition with "wearing out"
taking place less than fifteen percent of the time.
You only have to look at the mechanical seals that have been
removed from your pumps to verify this statement. The only
sacrificial part of any mechanical seal is the carbon face and an
inspection of used seals will show that in better than 85% of the
cases, the used seals have substantial face material left. Normally
fatigued bearings are even more rare than worn out seals.
Some years ago the U.S. Navy contracted for the building of K
(Killer) Class submarines. They were super SONAR (listening) ships
with the capability of detecting enemy submarines from a long
distance. They did an excellent job of detecting enemy submarines,
but were too slow to catch and destroy them. The result was that they
recorded only the passing of ships and were eventually scrapped. I
see this as the same problem with most of these maintenance programs.
We are recording the data, but the seals and bearings are still
failing at the same rate.
I have no problem with people who want to monitor equipment, but I
do have a problem with people who want to substitute monitoring for
good maintenance practices. Unfortunately these two groups are often
composed of different people operating under different budgets.
Lecturing to maintenance groups, I find very little concern with
sensible maintenance practices and a growing concern for monitoring.
The common complaint among maintenance people is that there's no time
to do the work correctly because of the pressures of production. I
also find a lack of training in the basics such as :
- How to read a pump curve
- How to make a system curve and how to relate it to the pump
- The causes of Cavitation and how to stop it.
- How to align the pump and motor.
- How to prevent pipe strain.
- Good piping practices to prevent liquid turbulence.
- Troubleshooting pumps and seals.
- How to set impellers.
- How to install a bearing.
- How to install a Mechanical Seal and still be able to adjust
open impellers for thermal growth and wear.
- How to install wear rings.
- And the list goes on......
Most experienced people, and almost all pump manufacturers agree
that the main cause of premature pump shutdown is seal and/or bearing
failure. What then would be minimum good maintenance practices for
seals and bearings?
shaft deflection. It'll cause problems with packing,
mechanical seals, bearings and will change critical dimensions such
as impeller clearances, wear ring clearances, seal settings etc.
- Use "C" ( Metric uses "D") frame adapters to simplify
- Use Centerline wet ends if the operating fluid temperature
exceeds 200° Fahrenheit (100° C)
- Balance all shaft assemblies and check they are straight.
- If you're using open impellers keep them adjusted to the
correct "hot" setting.
- Maintain the correct oil level and change bearing oil
frequently. If you're using grease lubrication it's more difficult
to change the grease, but it has to be done. Two thousand hours
(83 days) should be a maximum unless you can guarantee there was
no moisture ingestion or the lubricant was not overheated. Be
careful not to over lubricate the bearings.
- Use labyrinth, or positive face seals to keep moisture out of
the bearing lubrication and to prevent shaft fretting damage.
- Do not use shafts with an L3/D4 ratio
above 60 (2,0 metric)
- Try to keep Suction Specific Speed numbers below 8500 (10,000
metric) and never above 15,000 (16,500 metric)
- Maintain the correct clearance between the impeller and the
pump cutwater or tongue. It should run between 4% and 6% of the
impeller diameter. Use 4% for impeller diameters up to 14" (355
mm) or less and 6% over 14" (355 mm).
- Use corrosion resistant solid shafts only. Sleeves do not add
strength to shafts. Sealed pumps shouldn't need sleeves, unless
you're using the type thet frett and groove shafts.
- Make sure you have enough Net Positive Suction Head Available
(npsha) to prevent cavitation.
- Do not let air enter into the system. Air comes in through
shaft packing, flanges, and valves above the water line.
Vortexing, aerating the incoming liquid, and pumping the suction
dry are some other common causes.
- Replace wear rings any time the original specified clearance
good practices :
- Pay attention to parts storage.
- As an example, Buna "N" rubber has a shelf life of only one
year because of ozone attack. Proper packaging can increase
this life considerably.
- Many pump power ends are already rusted internally at the
time of installation because of poor storage policies and lack
of internal corrosion protection.
- Lapped seals should be packaged in such a way that they can
survive a one meter (39 inch ) drop without injuring the lapped
- Use only hydraulically balanced seals for all of your sealing
applications. They'll be able to handle fluctuating system and
- Use only non-fretting seal
designs to prevent costly shaft damage. All real seal
companies have them available.
- If possible, bore out your present stuffing box, or install a
commercially available large bore sealing chamber to give the
mechanical seal room to move and centrifuge dirt and/or solids
from the faces.
- Use universal seal materials to prevent material mix up and
lower inventory costs.
- Grade 316 stainless steel seal metal components can usually
be used in any pump manufactured from iron, steel, stainless
steel or bronze. CAUTION do
not use stainless steel springs or metal bellows because of
Chloride Stress Corrosion. Hastelloy "C" would be the best
choice for these locations.
- Use only unfilled carbons. They'll be chemically compatible
with any chemical except an oxidizing agent. Oxidizing agents
combine with carbon to form carbon monoxide and carbon
- Silicone carbide is the best universal hard face material.
Two versions are available, reaction bonded and alpha sintered.
Alpha sintered is the preferred one
- Most of the chemical in this world can be sealed by either
Viton® or Ethylene Propylene elastomers. Dupont's Kalrez
®, Green Tweed's Chemraz or a similar product should take
care of the rest.
- Unless you're pumping a fluid at, or close to its vapor point,
connect a recirculation line from the bottom of the stuffing box
back to the suction side of the pump, in place of the line
installed from the discharge side of the pump to the stuffing
- To insure good seal life, be sure that the installed
environmental controls are working.
- Cooling/heating jackets can become clogged with calcium and
become inoperative. If your cooling water is too hard or dirty,
use condensate instead.
- Flushing pressures can vary, or flushing lines can become
clogged. You may have to install a separate system. A flow meter
will help yo to be sure that you're flushing the correct
- Quenching must be regulated to prevent water entering into the
bearings. (Another good reason to use labyrinth or face type
- Convection tanks can run backwards, make sure the piping is
installed correctly and the rotating components are centered in
the stationary gland.
- Install cathode protection where ever practical and
- Use o-ring seal designs whenever possible. O-rings seal iboth
vacuum and pressure, and can flex to compensate for minor shaft
vibration and movement. Remember also that the o-ring is a natural
- Avoid pipe strain problems by piping from the pump to the pipe
rack and use a "centerline" wet end any time the pumping fluid
temperature exceeds 200° Fahrenheit. (100 C.)
The most sensible thing you can do to prevent unexpected pump shut
down is to install a "back up" mechanical seal in each of your pumps.
Since the seal is the most likely component to fail. and you want to
maximize the seal life, the "back up" seal will allow you to run to
failure and will give you time to schedule a shut down at your
- You can use either tandem, face to face, or "two way balanced
seals", but never rotating, "back to back" designs. A convection
tank can be installed between the seals and the level / pressure
in the tank will indicate which seal has worn out or failed
The only other sensible solution to an unexpected costly shutdown
is a split mechanical seal that can get you back on line, usually in
less than an hour.
Once these maintenance practices have been initiated and back up
sealing provided, a well thought out monitoring
system can be of great value. If given a choice I would vote for
a constant monitoring type of system, but the fact of the matter is
that any of them would be of value.
® DuPont Dow elastomer
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