SUBJECT: Controlling the temperature in
the stuffing box 4-6
Many fluids are adversely affected by a change in their
temperature, and when this change takes place, seal failure is almost
sure to follow. The failure can take several forms:
- One of the seal components can be destroyed. The elastomer,
seal faces, or metal parts will almost always be altered at some
degree of change in ambient temperature.
- Coated hard faces can "heat check" (crack).
- Carbon fillers can melt and pits can form in carbon/ graphite
faces as trapped air expands and blows out pieces of the
- Hydrocarbons can solidify (coke)
between the seal faces and pull out pieces of carbon also, causing
small pits that will prevent you from conforming to fugitive
- Carbon graphite faces can lose their lubricating ability at
cryogenic temperature and chip on the outside diameter as
vibration takes place.
- Elastomers can take a "compression set" and crack at elevated
- Cold temperatures can cause elastomers to harden.
- The liquid can crystallize, restricting seal movement and open
- The liquid can vaporize between the faces forcing them
- The viscosity of the fluid can change either restricting seal
movement, or making the fluid less of a lubricant.
- The liquid can solidify causing the seal to become
- The liquid's corrosion rate will double with an 18°
Fahrenheit (10° C) rise in temperature.
- The liquid can convert to a film between the sliding seal
components, restricting their movements. The magnetite that forms
in hot water is a good example of this.
- A film can form on the seal faces causing them to
- Lapped seal faces can distort and go out of flat at elevated
or cryogenic temperatures.
By keeping the stuffing box temperature within specified limits
you can prevent all of the above from happening. These limits vary
with each fluid, but they can be obtained from any one knowledgeable
about the fluid that has to be sealed.
A balanced mechanical seal
incorporating the following features and installed at the proper
compression, is your best insurance against a significant rise in
stuffing box temperature:
- Proper face balance. 70/ 30 is the most common to 5000 fpm.
(25 Meters per sec.)
- Low friction face materials. Carbon/ graphite vs. a silicon
carbide hard face is the best.
- The correct spring compression to control face loading.
- Faces with good heat conductivity. Tungsten carbide and
silicon carbide have excellent thermal conductivity compared to
most other hard face materials.
- A small cross section carbon/ graphite face press fitted into
a metal holder is better than solid carbon/ graphite for removing
heat from between the lapped faces.
Sometimes, that is not good enough, so occasionally you'll have to
come up with some additional method of controlling the temperature in
the stuffing box area and between the lapped seal faces.
THE HEATING / COOLING
If your pump does not have a heating/ cooling jacket installed,
one is usually available from the pump distributor. If possible try
to select an oversized stuffing box with a cooling/ heating jacket
cast around it. This jacket can be used to heat a product, cool a
product, or keep the product within close temperature limits. When
using the jacket there are several important things to keep in
- Install a carbon bushing into the bottom of the stuffing box
to act as a thermal barrier. The clearance over the shaft should
be about 0.002" per inch diameter of shaft (0,002 mm/ mm of shaft
diameter). The length should be at least 3/8 inch (10 mm).
- Dead end the stuffing box. In other words no discharge, flush
or suction recirculation lines connected to the stuffing box. Many
hot fluids contain lots of solids; the dead ending feature will
allow you to centrifuge the fluid in the stugging box and clean it
- If you elect to use water as the jacketing fluid make sure
that it's not hard water because it will form a layer of calcium
on the walls of the jacket restricting the heat transfer.
- This jacket is also used to cool the rotating shaft in hot
applications. If the cooling is lost the heat will conduct back to
the bearings causing their premature failure.
- Steam is an excellent medium to control heating or cooling. A
regulating valve can be installed on the discharge side of the
jacket for precision pressure control which will, in turn, control
the stuffing box temperature within narrow limits. A mixing valve
proportioning steam and water is another method of controlling
temperature within precise limits.
THE QUENCH OR DRAIN
Quench and drain connections are available in American Petroleum
Institute (API ) type glands. The quench and drain connection (Q) is
used in conjunction with a close fitting, non sparking disaster
- When using the quench for temperature control keep in mind:
- Excessive fluid will be directed towards the bearing case.
Be sure to use only small amounts of steam or water. It would
be wise to replace the existing grease seals with mechanical
bearing seals if you elect to use quench as your primary
temperature control method.
- The drain connection should be connected to a suitable tank
to save the condensed steam.
- The quench connection can be used to vent a volatile
product to a flare where it can be burned.
- The quench fluid can also remove any solids that have built
up outboard of the seal, as well as remove any vapors that
might leak across the seal faces. This is an important feature
when sealing products that can crystallize at the seal faces
and on the outboard side of the seal.
- The drain connection is used to direct the major amount of
failed seal leakage away from the bearings or any personnel in
the area. It should be connected to an appropriate tank for
THE FLUSH CONNECTION
In temperature control applications we flush in cooled product to
control the stuffing box temperature. If you use the pump fluid
cooled, or cooled finished product you'll have no problem with
THE DUAL SEAL
Another method of providing temperature control is to utilize two
seals with the correct temperature liquid circulating between them as
a barrier or buffer fluid. Look at the following illustration:
- When using a dual seal for heating/ cooling be sure to bring
the fluid into the bottom of the seal gland and out the top to
insure that the void between the seals is full of fluid. This is
an excellent method of controlling the temperature at the seal
faces if you are experiencing an over-heating problem.
- A convection tank can be installed between the seals, but it
will seldom do an adequate job of lowering or raising the barrier
fluid temperature. In almost every instance forced circulation
will be necessary if you need any degree of heating or cooling.
Convection tanks are satisfactory for removing the heat generated
by balanced seal faces, but that's about all.
- A convection tank can been used with an installed cooling
coil and a pumping ring built into the mechanical seal.
- The amount of barrier fluid circulation needed will be
determined by the seal size, speed, and stuffing box pressure.
Your seal supplier will gladly supply this information.
- Water should be selected for the barrier fluid whenever
possible. Oil is a poor choice because of its low specific heat
and poor conductivity.
THE HEAT EXCHANGER
The normal procedure is to install the heat exchanger in the
discharge recirculation line connected between the pump discharge and
the stuffing box. If you elect to use this method be careful of the
- This can be dangerous in hot water applications because a leak
in any of the fittings will direct high pressure, hot water into
the atmosphere and some one may be standing close by and become
- Many hot fluids also contain solids that will clog up the heat
- The temperature control is effective only while the pump is
running. Many seal failures occur at start up because of lack of
proper temperature control while the pump was idle.
- If you want to use this method, and only a small amount of
cooling is necessary, a commercial automotive, automatic
transmission cooler can be used effectively in many
- A heat exchanger can be used with a pumping ring. In this
application low pressure fluid is circulated out of the top of the
stuffing box, to the heat exchanger, and then back to the seal
through the bottom connection on the seal gland.
- Vertical pumps require venting or the seal will trap air in
the stuffing box, causing high heat at the seal faces. To vent the
stuffing box properly, connect a suction recirculation line
between the seal flush connection and the pump suction. Vertical
pump applications also present a problem for dual seal
applications. You'll need to provide some method of venting air
trapped at the outside seal.
- Carbon/ metal composites are a good choice for heat
dissipation across the carbon seal face.
- Try to avoid seal faces that are thermally isolated by
- Silicon carbide is a good choice for the hard face because of
its excellent thermal conductivity feature. Use the alpha sintered
type to avoid chemical compatibility problems
- If you elect to use anti-freeze as a barrier fluid between two
seals, do not use the automotive brands as many of them contain an
anti-leak chemical that will clog up the mechanical seal. Water is
the best barrier fluid because of its high specific heat (1.0) and
good conductivity. Oil is a bad choice because of its low specific
- If you must use oil, try to select a heat transfer
- Heat pipes should have application in stuffing box cooling,
but their application experience is very limited.
- Try to select seal designs that have the elastomer positioned
away from the seal faces. The elastomer is the one seal component
that is very sensitive to temperature change. Because elastomers
usually have poor thermal conductivity, cooling one side of the
elastomer has a minimal affect on the other side.
- Unfilled carbon/ graphite seal faces are absolutely necessary
in higher temperature applications. Less dense seal faces
experience trouble when air trapped below the surface of the
carbon, expands and blows out pieces of carbon from the center of
the seal face. The exception to this is high temperature oil that
will coke at the seal faces and pull pieces of carbon away. These
resultant pits will cause problems if you are trying to meet
fugitive emission standards.
- In those pump designs where the open impeller is adjusted back
against the back plate (Flowserve), any impeller adjustment tends
to over-compress the seal faces, causing high heat. Proper face
load is essential to long seal life so cartridge designs should be
specified any time you use open impellers in high heat
- Keep in mind that the pump cooling jacket is also used to cool
the shaft that is conducting heat back to the bearings. If you
have a high heat application, you might consider a stainless steel
shaft because of its poor heat conductivity compared to steel.
Some bellows seal manufacturers tell customers that they no longer
need stuffing box cooling and the result is premature bearing
- A centerline design pump is always desirable in hot
applications to prevent pipe strain at the pump suction and damage
to the close clearance wear rings. Instead of supporting the
volute at the bottom this design bolts the pump feet to the sides
of the volute, allowing the volute to expand both up and down. The
wet end of your pump can be modified to this configuration, or a
new wet end can be purchased.
- If the seal is going to be used in a hot oil application, do
not hydrostatically test the seal with water or a water based
fluid. Moisture trapped in gaskets, elastomer clearances, and
other small crevices will flash when it comes into contact with
the hot oil, causing a potential damage to the equipment, seal
and/ or the people in the area.
- In cryogenic applications it is not practical to heat the seal
area to protect the elastomer. A non-elastomer seal with a special
self lubricating cryogenic carbon is your best solution to this
application. Be aware that the moisture laden atmosphere can
freeze on the out board side of the seal restricting the seal
movement as the faces wear. In most cases a dual, non elastomer
seal with a non freezing barrier fluid between the two seals is
going to be your best choice.
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