SUBJECT : The dual seal arrangement
Dual seals are recommended for a variety of purposes that
- To prevent a costly product from leaking.
- To prevent a dangerous product from leaking to the
- To prevent a pollutant from escaping to the atmosphere.
- As a back up seal to prevent costly down time when the first
seal wears out or fails. This is an important element in any
predictive maintenance program.
All of the above are sensible reasons for using two seals in a
pump, but In this paper we'll be considering the use of dual seals as
an environmental control to prevent the sealed product from either
opening the seal faces, or damaging one of the seal components, the
two reasons any mechanical seal ever failed.
We can use the dual seals to:
- Control the temperature at a seal face.
- Prevent a pressure drop across a seal face.
- Eliminate atmospheric conditions outboard of a mechanical
- To break down the pressure in a high pressure application, by
inserting an intermediate pressure between the seals. Two lower
pressure seals can then be used to seal a high pressure fluid that
would normally require a very expensive high pressure mechanical
- To provide a lubricant if one is needed to prevent "slip
stick". This is almost always the case when you are sealing a
Dual seals can be of either the rotating or stationary version and
can be installed in four different configurations.
- Back to back or facing in opposite directions
- Tandem or facing in the same direction
- Face to face or facing towards each other
- Concentric or one inside the other.
The fluid that circulates between the seals is called barrier
fluid if it is higher than stuffing box pressure. It is called buffer
fluid if it is lower than stuffing box pressure. It can be circulated
between the two seals by:
- Natural convection using a convection tank. Insulated piping
coming from the top of the gland to the convection tank, and
finned piping coming out of the tank will aid convection if heat
removal is a problem.
- A pumping ring can be installed between the seals for those
instances where natural convection is not sufficient to remove the
heat being generated between the faces. This is very necessary
when oil is used as the barrier fluid. Oil has a low specific heat
and poor conductivity, making it a poor choice as a barrier fluid.
Most of the newer cartridge dual seals come equipped with a built
in pumping ring.
- Introducing the fluid between the seals, from an external
source. Be sure to bring the fluid in at the bottom of the gland
and out the top to prevent an air pocket from forming in the
The following illustrations describe the rotating version (the
spring or springs rotate with the shaft) of these dual seal
configurations. You should be aware that a stationary version is also
available from any of the major seal companies. You should also
- Use only the hydraulically balanced version of these seals to
prevent the generation of excessive heat between the seal
- Two way balance is always desirable in any dual seal
application to allow you the option of using either a high or low
pressure barrier fluid and to prevent the seal faces from opening
if either the system or the barrier fluid pressure
- Be sure to select seal faces with good thermal
- Try to locate any elastomers away from the seal faces if
possible. Elastomers are very sensitive to heat.
First we will look at the back to back version of a rotating
The rotating back to back version would be your worst possible
choice. Here are some of the reasons :
- This configuration requires a higher barrier fluid pressure
between the seals. This means that an inner seal leak will cause a
dilution of your product. There will be no visible evidence of
this happening unless someone notices a change in the product
concentration or tank level.
- In operation the outboard seal is carrying the higher
differential pressure and should be the first seal to wear out or
fail. When this occurs the barrier fluid pressure will drop and
the inner seal can blow open. In other words, if the seal works as
designed, both seals will fail at the same time.
- High barrier fluid pressures are hard to maintain because of
pressure fluctuations and varying system pressures. Water hammer
and pressure surges are not that uncommon.
- A reversing pressure can blow the inner seal open. Seals
should shut with pressure. They should not "blow open" when
something goes wrong.
- If a connection in the barrier fluid system is ruptured the
inner seal can blow open, dumping the pump contents to the
environment. The second seal would be of no use.
- Note the snap ring holding the inner stationary face against
the end of the stuffing box. This part is missing in just about
every application I have ever seen. Without this snap ring, higher
process fluid pressure can over compress the inner seal spring
force moving the stationary face into the rotating face, causing
massive face wear and very high rubbing temperatures.
- A common version of this seal utilizes spring loaded dynamic
O-rings. O-rings should be placed in O-ring grooves, they should
not be spring loaded. The Durametallic CRO seal is typical of that
- This version is known as the "double fretter" in the sealing
industry. It will groove the shaft in two places just beneath the
O-rings. See another section of this series for a further
explanation of shaft fretting.
- This seal is often used in slurry applications. Centrifugal
force will throw the slurry into the inner faces causing excessive
carbon wear. The slurry will then pack in front of the moveable
face preventing it from moving as it tries to slide forward to
compensate for normal face wear, thermal growth, most impeller
adjustment and shaft end play.
Tandem is the next version. This is the configuration you find in
most Oil Refinery applications.
Here are a few comments about this version:
- It takes the most axial space of the four types and as a
result is seldom found in the process industry, although newer
designs are being made shorter as a result of computer finite
- You need two glands and this adds to the cost as well as the
axial space required.
- A low pressure buffer fluid is circulated between the seals,
eliminating the possibility of product dilution.
- A loss of buffer fluid will not cause the seal faces to
The face to face version is next
This face to face version is a compromise between the "back to
back" and the tandem version:
- You normally run a lower pressure buffer fluid between the
faces. If you lose this pressure the seal faces do not open.
- Half the seal is in the stuffing box with the other half
outside. This means the seal does not take an excessive amount of
either axial or radial space.
- Many versions of this seal specify a common stationary unit
with holes drilled in the stationary for circulation. I do not
like this configuration because if you break the stationary face
you could lose both seals. There should be two separate faces
specified for maximum safety.
The concentric version is next, but I do not have an illustration
to show you. In this version we have one of the seals inside the
other sharing a common stationary face. The stationary face is
drilled between the rotating faces to allow circulation of the
- Like the tandem and "face to face" versions you normally
circulate a low pressure buffer fluid between the faces.
- This configuration takes the least amount of axial space, but
requires the most amount of radial space. You will sometimes find
them used on a top entering mixer application, but you will seldom
find them in a pump application because of the radial space
- Because of the common stationary used there is the danger of
losing both seals if you break the stationary face.
The convection tank is a unit you can either purchase or
manufacture your self. When a manufacturer supplies this unit, it
requires a "Boiler Maker Stamp" and a 600 psi rating, making it very
expensive to purchase. You can probably manufacture one for your
purposes at a much lower cost.
You have many choices when it comes to your choice of the buffer
or barrier fluid:
- Anti-freeze is popular in northern climates. Do not use the
commercial brand that contain a chemical used to plug leaks in
- Finished product is always acceptable.
- A fluid compatible with your product is often used.
- You might consider a cleaner or solvent that will be used to
clean the system.
- If a flush is being used in the system you can always use that
as a barrier fluid.
- Once again, try to avoid using any type of mineral, petroleum
or vegetable oil as a barrier fluid. Oil has a low specific heat
and poor conductivity that can cause varnishing and coking
problems between the seals. Some of the newer synthetic and heat
transfer oils can be used if the temperate is not too high.
Now that you have all of the basics under control, we will use a
dual seal to solve some of our common application problems:
Use a pressure higher than stuffing box pressure between the seals
- Prevent kaolin (china clay) or other micron size solids from
penetrating between the faces.
- To provide face lubrication if the product you are sealing is
a non lubricant. This will prevent excessive wear and "slip-stick"
- To prevent a pressure drop across the faces that could:
- Cause a product to vaporize and open the lapped faces
- Cause a fluid to solidify, paint is a good example
- Cause Ethylene Oxide to attack the dynamic O-ring in the
inboard seal. Ethylene Oxide can penetrate the elastomer and blow
out the other side if there is a differential pressure across the
Use a lower pressure between the seals to:
- Create an intermediate pressure in a high pressure
- Use a low pressure, with an anti-freeze as barrier fluid, to
prevent ice from forming outboard of the seal when sealing
products that freeze at atmospheric pressure. You will see ice on
the outside of the pump if you trying to seal one of these
- To be able to identify which seal has failed.
CAUTION Do not split the pressure
between stuffing box and atmospheric pressure. This will put an equal
load on both seals and they will wear out at the same rate.
Circulate the correct temperature fluid between the seals,
especially when the pump is shut down. You are going to have to make
the decision as to what barrier fluid temperature is needed. You can
increase the temperature, decrease it or hold it within narrow
- To prevent a product from crystallizing.
- To prevent a product from vaporizing.
- To prevent a product from becoming viscous.
- To prevent a product from solidifying.
- To prevent a product from building a film on the seal
- To prevent the product corrosion rate from increasing with
Here are a few more considerations:
- With the proper selection of barrier or buffer fluid pressure
you can transfer the hydraulic load to either the inboard or
outboard seal. It is never wise to use the barrier fluid at a
pressure of half the stuffing box pressure because this would
cause both seals to share the load and they would be expected to
wear out at the same time. It is always better to select one of
the seals to carry the load.
- A transmitter can be installed on the convection tank that
will send a signal to a control panel informing the operator which
seal has failed or worn out.
- Some people are tempted to run the outboard seal dry. With the
correct selection of the seal type and the proper materials this
is possible, but not recommended. You should use a convection tank
under some fluid pressure to be able to tell if you have had a
seal failure, and which seal has failed.
- Use the cartridge version of these seals to ease the
installation problem and allow you to make impeller adjustments
and compensate for thermal shaft growth. Be sure the cartridge
sleeve is sealed to the shaft inside the stuffing box, or fluid
will flow between the cartridge and the shaft making cartridge
removal difficult. Some manufacturers seal on the outboard end,
but this is not desirable.
- If you use the stationary version of these seals:
- Back to back is acceptable, the fluid will be at the seal
- Tandem is acceptable if the stationary units are positioned
in the glands.
- Face to face is not acceptable. The fluid will be at the
I.D. of the faces and centrifugal force will work against
- Concentric is O.K. if you can locate a concentric version
of a dual seal.
- If you are going to use a cartridge version of the
stationary dual seal, be sure it has some type of self-aligning
feature to prevent excessive movement as a result of "cocking"
when the cartridge sleeve is attached to the shaft.
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