SUBJECT: Causes of overheating
in cartridge mechanical seals 7-4
Too much heat can cause multiple problems with
- The elastomer (rubber part) can be
- Some seal faces can be damaged.
- Carbon-graphite faces can pit as trapped air
expands within the carbon, or the product carbonizes and pulls out
pieces of the seal face.
- Plated faces can heat check and crack causing
rapid carbon face wear.
- The filler in some carbon /graphite compounds
can melt or oxidize at elevated temperatures.
- Critical dimensions can change causing the
lapped seal faces to go out of flat and leak prematurely
(especially fugitive emissions).
- The sealed product can change state and
- Vaporize between the faces opening
- Crystallize on the moving components,
restricting their movement.
- Change fluid viscosity restricting the
ability of the seal to follow run out.
- Solidify, making the seal
- Build a film on sliding components and the
lapped seal faces.
- Carbonize or coke restricting the seal
movement and opening the lapped faces.
- Corrosion always increases with increasing
Some heat problems are not seal design, or seal
- An inefficient heating or cooling jacket on
- A layer of calcium or some other similar
product has built up on the jacket walls, interfering with the
- The coolant is flowing too rapidly through
the cooling jacket.
- A thermal bushing was not located in the
bottom or end of the stuffing box.
- If steam is being used as the coolant, the
pressure is too high.
- The fluid is not "dead ended" in the
stuffing box. There is either suction or discharge
recirculation of the pumping fluid.
- Clearance between the seal outside diameter
and the stuffing box bore is not sufficient.
- The shaft material is conducting the product
heat to the cartridge static elastomer and other components. As an
example: carbon steel conducts heat much better than a stainless
- The dual seal convection tank is not
- The convection tank is running
- The dual seal barrier or buffer fluid has
been shut off.
- The quench has failed.
- The product has a low specific heat and poor
conductivity. Oil is a good example of such a product.
- The seal faces were over-compressed during the
- A wrong installation measurement was
- The mechanic did not read, or understand
the print dimension.
- The pump sleeve moved as the impeller was
tightened on the shaft.
- The measurement was taken at the wrong
place. The stuffing box face is the only safe reference
The cartridge seal design has a major affect on
heat generation and heat sensitivity:
- Unbalanced seals generate more heat than
balanced mechanical seals.
- Two hard faces generate more heat than
carbon/graphite vs. a hard face.
- Silicone carbide and tungsten carbide
dissipate heat faster than 99.5 ceramic or
- The location as well as the grade of the
elastomer can be critical in temperature sensitive
- In dual seal applications, convection systems
are not as efficient as pumping rings or forced circulation of the
barrier fluid system. When oil is used as a barrier fluid forced
circulation or the use of a pumping ring is mandatory.
above problems are not unique to cartridge seals, there are, however,
some problems that are unique:
- Pushing the seal gland along the shaft and
against the stuffing box face can over compress the seal because
of the friction between the shaft and the cartridge sleeve static
elastomer. In dual seal applications the inner seal can over
compress as the outside seal looses some of its compression. Be
sure to reset the spacing device (usually "clips" of some type)
prior to locking the seal to the shaft.
- Some open impeller pump designs (Flowserve or
Duriron as an example) adjust to the back plate rather than the
volute. Be sure to reset the cartridge seal after the impeller
- Cartridge set screws can slip on a hardened
sleeve. The system pressure can then over compress the
- Higher pressure applications, or water hammer
can move the set screws and over compress the seal
- Be sure to re-tighten the adjusting nuts after
making the impeller micrometer adjustment on those pump that uses
that type of adjustment method.
- Make sure the centering-positioning clips are
in place when installing or resetting the seal for proper face
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