SUBJECT : Heat, how it affects the pump
and mechanical seal. 1-4
Every day salesmen call on customers and make claims that their
pump, or mechanical seal can take more heat than the other guys.
Before we rush out to purchase these wonder products, we should take
a closer look at the heat problem.
The heat comes from several sources:
- Generated at the seal faces or by packing rubbing against the
- Friction of the pump rotating parts, especially if the
discharge is throttled.
- Ambient conditions. The weather or atmosphere surrounding the
- The product contains a certain amount of heat
- Two parts rubbing together, that are not supposed to be
rubbing, can generate a lot of local heat.
- Grease,or lip seals rub against the shaft very close to the
- Running to the left of the best efficiency point (B.E.P.)
means that the discharge is restricted.
The heat will affect you in several ways. It can :
- Increase the corrosion rate of any corrosive liquid.
- Change critical tolerances.
- Destroy some seal faces
- Shorten the life of any elastomer (Rubber part) in the
- Change the state (ie. liquid to a gas) of the product you are
- Increase pipe strain.
- Waste valuable energy
- Change the viscosity of the bearing oil and eventually cause
- Heat on the suction side of the pump can cause
We'll look at each of these areas in detail, and at the end of
this paper make some recommendations to improve both your pump and
WHERE THE HEAT COMES
HEAT GENERATED AT
THE SEAL FACES :
The following numbers are typical of the conditions in a stuffing
box when you are sealing with a conventional original equipment,
Stuffing box pressure
Seal face diameter
Seal face area
Seal spring load
Face load from the spring
Stuffing box volume
1 pint of water
500 cc of water
Face coefficient of friction
We will make the first calculation in the inch size:
Hydraulic closing force = 100 lbs/in2 * 1 in2 = 100 lbs
Hydraulic Opening force = An average of 50 psi on the faces * 1
in2 = 50 lbs.
100 lbs closing + 30 lbs Spring force - 50 lbs opening = 80 lbs
80 lbs * 0.2 * 1885 F.P.M. = 30160 Ft lbs./ min
778 ft lbs. / min. = 1 Btu..
30160 / 778 = 38.8 Btu../min.
38.8 Btu../ min would raise 1 pint of water 38.8 degrees
Fahrenheit each minute, so we would have to flush in 38.8 pints (4.84
gallons per minute) of cooling water if we did not want the product
to get hot.
Metric looks like this :
A Newton Meter is a Joule so we have 690 Joules/ sec.
690 Joules/Sec.* 60 Sec./Min. = 41,400 Joules per minute.
41,400 * 0.239 joules per calorie = 9,895 calories (9,9 Kilo
Calories) per minute.
9.9 Kilo calories per minute would raise 9,9 liters of water one
degree Centigrade per minute.
Since we have only one half a liter (500 cc ) in the stuffing box,
we would have to flush in 9,9 * 2 or 19,8 liters / minute to prevent
a temperature rise in the stuffing box.
The amount of heat generated by a properly installed balanced
mechanical seal is insignificant.
The amount of heat generated by packing varies with the type of
packing and the individual packing the pump. On the average you will
find that packing generates six times the heat of a balanced
HEAT GENERATED BY FRICTION WITHIN THE PUMP
No pump is 100% efficient. If a pump is rated 60% efficient, that
means that 40% of the power is being converted to heat. In a normal
temperature stabilized pump, running at its best efficiency point,
(B.E.P.) the temperature rise within the pump is calculated from the
following formulas :
A temperature rise of 18° F across the pump or 10° Centigrade is considered excessive. This can occur if the pump is run with a shut or excessively throttled discharged valve.
If you would like to calculate the temperature rise of the liquid in a running pump when the discharge is shut, use the following formula:
HEAT FROM THE AMBIENT CONDITIONS
- If pipes, pumps, valves and other equipment are placed next to
hot boilers or exposed to extreme changes in weather we'll have to
consider this addition or removal of heat in troubleshooting
temperature related problems.
HEAT IN THE PRODUCT ITS
- All fluids are processed within some temperature range. It's
this heat that we will be adding to, or subtracting from. Many
fluids are pumped close to the temperature at which they'll
vaporize, solidify, coke, crystallize etc.
- It's critical that you determine the desired operating range
for your fluid before you make any attempt to alter it.
GENERATED BY PARTS RUBBING TOGETHER
- Rotating parts rub against stationary parts when the pump
shaft experiences deflection. L3/D4
explains this problem in great detail.
BY THE BEARING SEALS
WHAT AFFECT CAN ADDITIONAL HEAT HAVE
ON THE LIQUID IN THE PUMP?
- These seals add heat at the worst possible location. Grease or
lip seals will also cause shaft wear at the point where the seal
material rubs the rotating shaft.
CORROSION RATE OF THE LIQUID WILL INCREASE :
- A general rule of thumb is that all chemical reactions double
with a eighteen degree Fahrenheit rise in temperature (10 degrees
Celsius). Corrosion is a chemical reaction and therefore corrosion
increases with temperature. This is the best reason for converting
any acid pump from packing to a mechanical seal.
CRITICAL TOLERANCES WILL
- Critical tolerances include: Wear ring clearance, seal face
loading, throttle/ thermal bushing clearance, bearing
interference, impeller/ case clearance, pump/motor alignment,
- A general rule to remember is that each inch of stainless
steel will grow 0.001" of an inch for each 100 degrees Fahrenheit
temperature rise. In the metric system it grows 0,001 mm. per
millimeter for each 100 degree Celsius rise.
- Open impellers must be set to a specified clearance from the
pump case or back plate. A 0.015" ( 0,5 mm.) clearance would be
typical. If you increase this clearance 0.002" (0,05 mm.) the pump
will lose 1% of its pumping capacity.
- In closed impeller applications, the general rule is that each
additional 0.001" (0,03 mm) of wear ring clearance will decrease
pump capacity by one percent.
- Unfortunately all materials do not grow at the same rate and
in the same direction. As an example: steel grows about 60% to 70%
less than stainless steel and most mechanical seal faces grow at
about one third the rate of stainless steel. This is important to
remember when you set critical settings and interferences. It's
also one of the main reasons we should do everything we can to
keep down excessive temperature rises within the system.
- This also explains why we have less trouble with mechanical
seals and bearings in equipment that runs continuously, as opposed
to intermittent service equipment that goes through many
SOME MECHANICAL SEAL
FACES CAN BE DESTROYED.
- Many of the popular carbon/ graphite seal faces have binders
and impregnates that can be melted or otherwise destroyed by
excessive heat. Some of the lower cost carbons will blister when
sub surface air expands because of elevated temperature. This is
the main reason I have advocated unfilled carbon/ graphite seal
faces at all of my Rotating Equipment Seminars.
- Plated and coated hard faces are subject to heat checking and
cracking if improper bonding methods have been used. I do not
recommend plasma spray processes for this reason.
- Some of the cheaper ceramic faces can be cracked with as
little as a 100 degree Fahrenheit (55° C.) temperature
differential across the seal face.
- Pressed in carbons and hard faces can become loose in their
holders. This has caused some seal manufacturers to glue in seal
faces and as you can imagine, not a very satisfactory
- Some seal face designs can go out of flat with very little
temperature differential. This is very critical in cryogenic
(cold) applications and we often have to lap the seal faces at
cryogenic temperatures to prevent them from distorting in
ELASTOMER (THE RUBBER
PART) LIFE CAN BE DRASTICALLY SHORTENED
- Heat will cause elastomers to take a compression set and if
enough heat is added the elastomer will probably become very hard
and crack. All elastomer compounds have a rated operating
THE PRODUCT CAN CHANGE FROM A
LIQUID TO EITHER A SOLID OR A GAS.
- Water becomes steam. Glue, paint and all kinds of polymers
with odd sounding names can solidify. Oil changes its viscosity,
caustic and sugar syrups crystallize and the list goes on and on.
Centrifugal pumps and mechanical seals can handle liquids, they
have problems with vapors and solids.
- If a Cryogenic fluid evaporates across a mechanical seal face
it can freeze any installation lubricant that might have been put
on the face and either tear up the carbon, or break the hard
- The easiest product to pump or seal, is a cool, clean,
lubricating liquid. Heat can cause that liquid to vaporize,
crystallize, solidify, carbonize, build a film on surfaces, become
- The finest lubricating oils will not work when the oil breaks
down to form first varnish then coke. The bearing oil will start
to do this if the oil gets above 240 F.(115 C.). Remember that a
properly installed bearing is running about 10 degrees F. (5 C)
hotter than the oil temperature. You can only guess what kind of
temperature rise we get in improperly installed bearings. You
should also remember that lubricating oil and grease have a useful
life of thirty years at 30°C. and the life of the lubricant
is cut in half for each 10°C. rise in temperature above that
- Pipe strain causes the shaft to be displaced from the center
of the pump assembly. Rubbing, premature seal / bearing failure
and misalignment are always the result of this problem.
THE WASTING OF COSTLY ENERGY.
- The energy we pay for can be used to move fluid in your
process or heat it up. The pump's job is to move fluid not
generate heat. If you want to add heat to a liquid there are far
more economical and efficient methods of doing so.
- Cavitation is defined as cavities or bubbles in the liquid. A
major cause of cavitation is caused by heating the incoming liquid
beyond its vapor/ pressure point.
CHANGING THE VISCOSITY OF THE BEARING OIL
TO LOWER THE AMOUNT OF HEAT BEING GENERATED WITHIN THE
- Heat lowers the viscosity of the bearing oil causing
increasing wear. As the oil heats up it will change state, first
forming a varnish coating and then turning into a black coke
PUMP SHAFT PACKING
- With the development of the split mechanical seal in the early
nineteen eighties pump packing has become almost obsolete. Packing
a pump shaft is like driving your automobile with the emergency
brake engaged. A balanced mechanical seal will generate six times
less heat than a good set of packing. This saving in electricity,
or what ever form of energy you are purchasing will more than pay
for the seal in less than two years. A 50% return on investment
should get the attention of any accountant.
THE MECHANICAL SEAL.
- Use only the balanced type with low friction faces. Be sure to
set the face load properly and remember this has to be done when
the pump is at its' operating temperature. A cartridge or split
seal is the only way to set face load. Back pull out pumps
(A.N.S.I. or I.S.O. ) present a special problem because the seal
is installed in the shop and the initial open impeller setting is
almost always made at the piping. Those designs that adjust to the
back plate are the exception.
- Open impellers have to be adjusted to keep the pump running
efficiently. The seal must be repositioned each time the impeller
is moved. Again, cartridge or split seals are your only
- Be sure to vent vertical stuffing boxes to prevent air from
being trapped in the stuffing box. Good seals have this vent
located in the seal gland.
- Make sure dual seals have the barrier fluid circulating either
by convection, a pumping ring, or through a forced circulating
- Check that the environmental controls are functioning
properly. Cooling jackets stop functioning when calcium builds up
on the jacket wall. Condensate or steam are good alternatives if
you have problems with hard water.
- Make sure that the stationary face is centered around the
shaft to prevent rubbing if the shaft is displaced because of run
out, whip, wobble, unbalance, vibration, bending, misalignment
- Check the oil level and change the oil on a regular basis. A
pump running at 1750 rpm is almost the same as running your car at
50 miles per hour. This means that every 2000 hours your pump
shaft travels about one hundred thousand miles. If the pump runs
twenty four hours a day it will run 2000 hours in 83.3 days or
just under three months. Imagine that your pump bearings go 100
thousand miles every three months. At 1500 rpm the pump bearings
travel 150,000 kilometers every 90 days. Check the oil level with
a properly installed oil level gauge, or sight glass, not the dip
stick we find installed on some pumps.
- If the bearings are not fit properly they'll generate
excessive heat. Refer to a bearing chart during your next
installation to insure you have the proper dimensions. The
internal clearance in a properly installed bearing is just a few
ten thousands of an inch (thousands of a millimeter). To do this
properly you'll need an induction coil and a shaft that has been
ground to the proper tolerances. Avoid cooling the outside
diameter of the bearing because it will shrink and generate still
more heat. Cool the bearing oil, never the bearing or the housing
- The bearings should be lasting from twelve to fifteen years.
Most failures are caused by lubrication contamination or high
heat. Improper installation is a major source of high heat
problems, Try to do the job carefully.
- The grease or bearing lip seals should be thrown away and
replaced with labyrinth seals or positive face seals that will not
add heat to the bearing oil or let contaminates into the oil
reservoir. The labyrinth, or positive face seals will not cut or
wear the expensive shaft and as you know, this is a serious
problem with all grease seals.
- Nothing beats insulation for keeping high ambient temperature
away from your pumping fluid.
- More than one maintenance man has built a dog house over his
pump and controlled the temperature within the dog house.
OTHER HEAT SOURCES
MODIFICATIONS THAT WILL EITHER LOWER THE AMOUNT OF HEAT BEING
GENERATED OR LESSEN THE AFFECT OF THIS HEAT.
- Watch out for bypass lines and re circulating lines adding
heat to the suction side of a pump.
- With some parallel pump installations one of the check valves
can see a higher back pressure causing the pump to run with a
throttled discharge and generating more heat.
- A recirculation line from the discharge of the pump back to
the stuffing box will not only add additional heat to the fluid,
but will also increase the amount of solids in the stuffing box.
In almost every case you will be better off connecting the line
from the bottom of the stuffing box back to the suction side of
the pump. Caution: do not do this if you are pumping a fluid close
to its vapor point.
- Check the wear ring or impeller clearance on a regular basis.
As the pump looses efficiency the heat and vibration will
- Pipe strain can cause wear ring contact.
- Use a larger stuffing box for mechanical seal applications.
You can use the jacketed type if you need extra cooling. If you
find there is not enough material to bore out the present box you
can purchase the larger bore box from your distributor or
manufacturer as a spare part.
- If the pumping temperature exceeds 200 F (95° C) convert
the wet end of your pump to a "centerline design" to avoid pipe
strain at the suction side of the pump.
- Convert to a solid stainless steel shaft to lessen the amount
of heat that will be transferred to the bearings.
- Add oil cooling to the bearing case if you're going to see
higher temperatures. Be sure to cool the oil, never the bearing
- Convert to a "C" or "D" frame adapter to avoid misalignment
- Use mechanical seal designs that work better at these elevated
temperatures. Desirable features would include:
- Balanced for low heat generation.
- Split or cartridge for correct installation.
- Carbon/metal composite for better heat dissipation.
- High temperature elastomers or "no elastomer" designs
- Solid rather than a coated hard face.
- Springs out of the fluid.
- Unfilled carbon for density
Excessive heat causes seal and bearing problems. Since the heat
can increase corrosion, destroy seal faces, vaporize the fluid, coke
the oil, solidify some liquids and crystallize others, change
critical tolerances, attack the elastomers, increase the bearing
squeeze, cause misalignment and pipe strain, etc, it would be
ridiculous to try to build a mechanical seal, or bearing capable of
operating in excessive heat.
Most claims for high temperature seals address the problem of
elastomers and ignore those other factors that we have discussed in
detail. This explains the popularity of the high temperature bellows
seal that must be cooled in all high temperature petroleum
applications. There is no magic, but there is a sensible
Do as many of those things we have discussed in the above
paragraphs and if you find that you still have trouble, try to find
some logical method of getting additional cooling to the seal and
bearing oil. We discussed a lot of those options in the above
Heat is always a problem, but now you have the tools to fight
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