SUBJECT: Sealing products sensitive to a
change in temperature or pressure. 8-9
Liquids and gases are both called fluids and a fluid can shorten
the life of a seal in two ways;
- It can cause the seal faces to open allowing solids to
- It can damage one of the seal materials. Elastomers are very
sensitive to both temperature and pressure.
In this paper we will be considering how small changes in either
temperature or pressure will cause one or both of these failures to
occur, and learn how to prevent these changes especially when the
pump is stopped and often subject to both temperature and pressure
fluctuations. In another paper in this series we learned that a
change in temperature could:
- Cause a fluid to crystallize, sticking the seal to the shaft
and making the faces open as the shaft moves. Caustic and sugar
solutions are examples of this.
- Cause a liquid to vaporize, blowing the lapped seal faces
apart, letting solids penetrate between the faces or causing
damage as the faces bounce open and shut. This happens any time
water flashes to steam.
- Cause some liquids to become viscous, preventing the seal
faces from staying in contact. Bunker fuel oil becomes very thick
when it gets cold.
- Cause some liquids to solidify, either sticking the seal to
the shaft, preventing the flexible seal parts from moving or
causing the seal faces to stick together. Sugar syrups do this
when they get hot, some fluids do it when they get cold.
- Cause a film to build on the seal sliding components or
between the faces. Oil varnish or "coking" is as typical example
of this problem. Hard water will build a film on the seal sliding
components as the water temperature increases. If the system is
new and has not been passivated (protective oxide film on the
metal surface) Ferric oxide or a similar oxide can build up on the
sealing components. This build up will accelerate with
- Cause a liquid to become a non-lubricant. Water becomes less
of a lubricant as its temperature increases. This lack of
lubrication can cause "slip stick" problems between the lapped
- The corrosion rate of most corrosives increase with a rise in
temperature. A general rule of thumb says that the corrosion rate
of an acid will double with a 18°F (10°C) rise in
temperature. This is the reason we avoid the use of packing in
acid pumps. You will recall the packing generates almost six times
the heat of a balanced mechanical seal
If you are not using a dual seal with a pressurized barrier fluid,
then you will get some sort of a pressure drop across the seal face.
A pressure drop could:
- Cause the fluid to vaporize and blow open the lapped faces. If
this happens several problems might occur:
- Allow sub micron size solids to penetrate between the faces,
imbed themselves into the softer carbon and destroy the lapped
- As the product passes across the faces a cooling occurs,
causing the faces to close. When the faces close, the cycle
repeats its self and the alternating closing and opening will
probably crack the carbon as it bangs against the drive lugs or
chip the carbon face on the outside diameter.
- If the product freezes when it evaporates it could freeze any
oil or grease that was put on the seal face, causing damage to the
carbon. This will also freeze the moisture on the outboard side of
the seal causing ice that can restrict the movement of the seal
(you can see the ice on the shaft).
- Cause the liquid to solidify.
- Paint is a mixture of a solid and a solvent. If the solvent
evaporates, the paint will solidify between the faces. This can
also occur if the suction of the pump is under a vacuum (negative
suction head) because the pump is trying to lift the fluid.
- Some liquids will form a film at the faces if the pressure
If the temperature or pressure of the pumping fluid never changed
we would seldom have any application problems. But since pumpage
pressure and temperature changes are normal (especially at shut down)
we are going to have to become skillful in controlling the
temperature and pressure in the stuffing box area to prevent a
premature seal failure.
THE HEATING AND COOLING JACKET
If you use this technique be sure to check:
- A carbon thermal bushing is installed in the end of the
stuffing box to reduce the heat transfer between the product you
are pumping and the fluid in the stuffing box.
- The cooling jacket must be free from scale and calcium build
up. There are many cleaning products on the market you can flush
through the jacket to insure that it is clean with out having to
disassemble the pump.
- Dead end the fluid; no recirculation lines either into or out
of the stuffing box. Check carefully because some of these lines
can be hidden by insulation. We are trying to trap a small amount
of liquid in the stuffing box that will be easy to either heat or
- The best fluids to circulate through this jacket are steam and
condensate. Shop or city water is generally too hard and will form
a calcium film on the inside of the jacket.
- Remember that steam will act as a coolant with hot oil
- The steam temperature can be controlled by the use of a
regulator on the outboard side of the jacket. The temperature of
steam is directly related to its pressure.
- You can use a mixer valve that will blend the steam and
condensate to give you a very precise control over the stuffing
- The main advantage of this control is that it lets you
regulate the stuffing box temperature when the pump is shut down.
That far out weighs the disadvantage of having to provide
circulation to the jacket.
- Be sure to bring the coolant into the bottom of the jacket and
out the top. This will insure that there are no bubbles trapped to
restrict heat transfer.
- Because you are "dead ending" the fluid, centrifugal force
will throw the solids away from the seal components and very soon
the seal will be in a clean environment at exactly the right
- If your pump is not equipped with a jacket, one is probably
available from the pump manufacturer or an after market
THE QUENCH AND DRAIN CONNECTION is
used to heat or cool the outboard side of a single seal and wash away
any product the might leak across the faces or build up outboard of
- Use only low pressure steam or water at connection "D". You do
not want these products to penetrate through the disaster bushing
and get into the bearings. This is another reason to replace those
bearing grease or lip seals with either a labyrinth or a positive
- The non sparking disaster bushing has two functions:
- To direct most of the seal leakage to a drain where it can
be collected, or a flare where it can be burned.
- To prevent the shaft from hitting the seal if you have a
bearing failure. If the product burns this could cause a fire
or an explosion. In any case the damage would be severe without
this non sparking disaster bushing.
- A steam line hooked up to this connection can be used to
put out a fire in the stuffing box area. All you need is a
solenoid valve and a melt switch that will open the solenoid
when it senses high temperature (same as a fire sprinkler
A DISCHARGE RECIRCULATION CONNECTION
connected between the pump discharge and the stuffing box can
be used to pressurize the stuffing box area with the discharge
pressure available at the pump.
- Do not aim this connection at the seal faces or sliding
components. The abrasive action of entrained solids can injure the
lapped faces or destroy a seal component. Thin wall metal bellows
seals are very sensitive to this abrasive action.
- The high velocity fluid can also interfere with the seal
movement so be very careful how you make the connection.
THE DUAL SEAL is another way to
control either temperature, pressure at the seal faces, or both at
the same time. Take a look at the following diagram:
- Circulate a fluid at the correct temperature between the
seals. You can cool the area, heat the area or hold the
temperature at precise limits if that is desirable.
- Bring the fluid in the bottom and out the top to avoid air
- You can pressurize between the dual seals to prevent a
pressure drop across the seal faces.
- If you use the two way balanced version of a dual seal you can
choose either a higher barrier or lower pressure buffer fluid
between the seals.
- Fill the system and convection tank with anti-freeze and you
will prevent ice from forming out board the inner seal. This can
happen any time you seal a product that can freeze the moisture in
HERE ARE A FEW MORE CONSIDERATIONS ABOUT
CONTROLLING PRESSURE AND TEMPERATURE IN THE SEAL AREA:
- A cooler in the line between the pump discharge and the
stuffing box is not a good method of controlling stuffing box
temperature because it functions only when the pump is running,
and many problems with crystallization, solidifying, becoming
viscous, etc. occur when the pump is shut down.
- Flushing the system between batches seldom cleans the stuffing
box area and the mechanical seal.
- Flushing the stuffing box with an outside fluid is the
universal environmental control. You can always replace the fluid
that is giving you trouble by flushing in a clean liquid at the
right temperature and pressure. It will cause product dilution,
but maybe you can flush in finished product or a fluid that is
compatible with the fluid you are trying to seal.
- Heat tracer lines are often used in piping systems, but are
seldom placed on the stuffing box. Maybe you will find it
practical to trace and insulate the stuffing box for your
- There is little need to lower the pressure in the stuffing box
area. If you find that the stuffing box pressure is to high for
your mechanical seal, you are better off purchasing a high
pressure mechanical seal that will satisfy your application.
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