SUBJECT : Understanding Split Mechanical
The split seal is the one seal that every consumer wanted. The
main reason that people continue to use packing is that no one wants
to take the pump apart just to fix a leak. Taking a pump apart
involves several problems:
- The mechanic must have enough skill to put it back together
again, insuring that all tolerances, balance and fits are correct.
In a world of multi- craft workers that skill is rapidly
- You must go through a complete realignment between the pump
and the driver. That can take hours.
- In some instances insulation has to be removed to move the
pump. This can involve serious hazardous materials disposal
- In many facilities seal replacement involves multiple crafts.
An electrician to blank out the motor, a pipe fitter to remove the
piping, a rigger to bring the pump back to the shop, a mechanic to
fix it and several work orders to reverse the process when the
pump goes back.
- When the pump is disassembled, to replace the seal,t he
bearings are often replaced at the same time. More often than not
seal replacement means a complete pump overhaul.
- Often, the system has to be sterilized if the pump is
disassembled. This can involve many hours of heating, flushing,
The following illustration shows a modern split seal mounted
between the face of the stuffing box and the bearing case. Note that
the seal is an extension of the stuffing box and is not installed in
a typical "outside seal" configuration. In other words, as the seal
faces wear they move away from the solids in the product and not into
them. You'll also note that the elastomer always moves to a clean
surface, as the faces wear. This is a very important feature if there
are solids in the sealing fluid.
The first successful split seals were used on the atomic submarine
main propeller shafts back in 1954. They proved to be as reliable as
solid seals, but they were very expensive because of a lack of good
technology for cutting the lapped faces or joining the O-ring seals.
There are three accepted methods of joining the split elastomer
- Vulcanize the components together around the shaft. This is
the method that was used on the atomic submarine Nautilus. Its
only limitation is that you're not able to manufacture small
diameter rings because the stock must go around the shaft and then
through the vulcanizing tool. Present technology limits this
technique to shaft diameters larger than six inches (150
- Install extra elastomers over the shaft and into the seal
assembly. You can then move them out and use them as needed. This
is a good technique, but presents major difficulties in seal
- Use the "ball and socket" design supplied by some
manufacturers, or any other logical design that makes sense to
Gluing O-rings, or any other type of elastomer together is never
acceptable for a dynamic elastomer. The glue creates a "hard spot"
that'll prevent proper sealing.
Early split designs were cut in half by "cut off saws" and
re-machined to concentricity. This involved a lot of handling and
hand finishing operations that added to the original high cost. Today
we use lasers, high pressure water tools, fracturing techniques and
other types of sophisticated machinery to accomplish the same thing
at much lower costs. The fact is that reliable split seals are just
about the same price today as any balanced, O-ring, cartridge seal
made out of the same materials. Spare part kits are considerably
cheaper than comparable solid seal spare parts.
Depending upon the brand and size of split seal that you select,
the temperature, speed and pressure limits are just about the same as
any other balanced O-ring mechanical seal. The major difference comes
in the sealing of vacuum. Some splits seals need a positive pressure
to hold the faces together, so if you intend to use the seals in
vacuum service, this type must be turned around sputting atmospheric
pressure on the sealing side. Stationary split seal do not experience
market for split seals is the same as for other mechanical
seals, but they also have several markets open to them that other
seals cannot satisfy. As an example:
- The pump is leaking. You can break off, or cut off the present
gland and install a split seal while the leaking seal remains in
the stuffing box. The pump can be back on stream in about an
- You can install the split seal in a fire pump and leave the
packing in place. This way you will probably not violate any fire
- In most designs you're installing the seal closer to the
bearings. If you install a carbon bushing in the stuffing box of
the pump you stop most of the bad affects of shaft bending and
deflection. Even if you don't use the carbon bushing you're still
better off being located closer to the pump anti friction
- Mixers and awkward locations. The savings are huge! In some
instances you have to take the roof off of the building to remove
the motor before you can pull the pump.
- Vertical and horizontal split pumps. You don't have to rig a
special lifting device and you only have to change one seal
instead of two.
- Shallow stuffing boxes. The seal installs outside the
conventional stuffing box but unlike other seals it does not seal
backwards. The seal gland is an extension of the present stuffing
box. CAUTION: some split seal designs
are actually "outside" type seals that move the seal faces into
the entrained solids as the seal face wears.
- Any time "down time" is expensive, split seals must be
considered as the only sensible solution outside of installing two
seals in all of your pumps.
- The pump is located in a dangerous area (radiation is a good
example) and it's important that the personnel spend as little
time in the area as possible.
- If you want to measure the savings in electricity between
packing and a mechanical seal, the split seal is your only choice.
Take an amperage reading with the packing in the pump and when
you're satisfied you know the power being consumed by the pump,
pull the packing and install the split seal. The difference in
electricity consumption should pay for the seal in less than
- You can install a split seal in a pump that has had the shaft/
sleeve damaged by packing or a mechanical seal, and save the
shaft/ sleeve replacement cost. As you can see in the first
illustration the seal static elastomer sits on a non-damaged
portion of the pump shaft/ sleeve.
- Large diameter shafts are a natural for split seals. Shaft
damage is expensive and pump disassembly is a big problem. Many
times the packing is left in the pump and the leakage tolerated
because of the problems of installing solid seals on shafts larger
than four inches (100 mm.). Split seals changed all of this and
shaft damage can be eliminated entirely.
- If you mount a split seal on a split sleeve you can change
side entering mixer seals with out emptying the mixer. See the
seals are the easiest way to convert any packed pump to a mechanical
seal. You should convert packed pumps for any of the following
- Leaking product can be costly.
- Leaking product can present a personnel hazard and a
- Leaking product can be a pollution problem.
- Seals consume less power than packing.
- There's no adjustment needed to compensate for wear.
- In many cases the flushing water can be eliminated.
- Shaft damage or wear can be eliminated. This'll allow you to
use a solid shaft that'll resist shaft deflection and
- Leaking packing, and the water hose that is needed to clean up
the area, are the major causes of premature bearing failure.
- Leakage causes a "house keeping" problem.
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