SUBJECT: Two way hydraulic
There are several reasons why you might want to
invest in the higher cost of installing two seals in your pump, or
some other pieced of rotating equipment:
- The product is dangerous.
- A seal leak could cause a pollution
- The product is very costly.
- Unscheduled down time is too
- You need fugitive emission
- It is a sensible way to institute a predictive
maintenance program for mechanical seals.
Please look at the following diagram and you'll
see a major problem with manydual seal applications:
In a typical dual seal application, barrier fluid
is circulated between the seals at a pressure at least one atmosphere
(14.7 psi, or 1 bar) higher than stuffing box pressure. This presents
some operating problems:
- Since systm pressure can vary, the barrier
fluid pressure must be kept at one atmosphere higher than the
maximum stuffing box pressure and that pressure is very hard to
predict because of pressure surges, cavitation, and water hammer
in the system.
- Barrier fluid pressure can vary depending upon
its source. Shop water is notorious for pressure
- If the system pressure exceeds the barrier
pressure, the inner seal can blow open.
- If the barrier fluid piping or fittings are
damaged or leaking, the inner seal will blow open allowing the
product to escape to atmosphere. Remember that you purchased the
second seal to prevent that possibility.
- Some mixer applications alternate between
pressure and vacuum.
- If the outside seal wears out, or fails
prematurely the barrier fluid pressure will drop, and the inner
seal will blow open. In other words, if the seal works properly,
both seals will fail at the same time.
Hydraulically balancing the inside seal will not
help because with a standard 70-30 balance the seal is hydraulically
balanced in only one direction. You can check the paper
8-1 in this series if you are not familiar
with standard seal balance.
In that paper we learned how to balance a rotating
seal (the spring loaded face rotates). Now we'll see how a stationary
seal (the spring loaded face does not rotate) is balanced. Please
refer to the following diagram:
If the higher pressure is coming from side "A" you
have the classic 70-30 hydraulic balance, but if the higher pressure
is at "B" you have a 30-70 balance and the seal faces will
In 1939 a solution was found for this very real
problem, and it was called "two way balance". Look at the following
diagram, it appears to be very similar to the previous diagram with
two major exceptions.
- The dynamic 0-ring is placed in an odd shaped
- The stationary seal face is wider than the
previous version. The O-ring cross section represents 40% of the
area of the seal face with 30% of the face area outside and inside
of the O-ring area.
If the higher pressure is at side "A" the O-ring
"C" is pushed against gland "D" and any sliding action of the seal
face will take place at the inside diameter of the O-ring. In that
case 70% (40%+ 30%) of the face area would see the pressure and 30%
would not. The classic 70-30 balance.
Reverse the pressure to side "B" and the O-ring
will bottom against the seal face and all sliding, or flexing will
occur on the O-ring outside diameter, allowing 70% (40%+ 30%) of the
seal face area to see the pressure and 30% would not.
In other words you would have the same 70-30
balance regardless of the direction of the higher pressure. This
presents some very real advantages over non-two way balanced
- You can elect either a higher or lower barrier
fluid pressure. It's your choice. With a lower barrier fluid
pressure you can just about eliminate the possibility of product
- A rupture in the barrier fluid pressurizing
system will not open the inner seal face, allowing the product to
escape to atmosphere.
- Water hammer, cavitation, or pressure surges
will not blow the seal open. The faces always shut with the higher
pressure, the way they're supposed to.
Are there any disadvantages to this design? Yes, a
- A wide seal face is required restricting the
use of the seal to mostly mixer applications because of the
additional radial room needed. There seldom is enough room in the
typical centrifugal pump stuffing box to accommodate the cartridge
version of this design.
- Because one half of the dynamic O-ring groove
is mounted in the gland and the other in the seal face, The O-ring
must slide when the pressure reverses, and in some slurry
applications it will "hang up" unless flushing water is
- Extra cost is usually involved if it is not a
standard seal with your supplier.
In recent years other methods of achieving two way
balance have evolved with very obvious advantages. One of them is
shown in the following diagram:
- A narrow seal face can be used.
- The seal will fit conventional stuffing box
- The dynamic O-ring does not have to slide when
the pressure reverses. This is a very important feature when
sealing slurries, viscous fluids and liquids that can
- Manufacturing cost is reduced.
- Conventional O-ring cross sections can be
The diagram should be self explanatory. You will
note that in this version the hydraulic balance reverses from 70-30
It does not make sense to install two seals into a
piece of rotating equipment without this two way balance feature. Be
sure to specify it in all future purchases and proposals.
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