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Dynamic Design
Dynamic mechanical seals are designed with spring elements mounted in the 
dynamic (rotating) part of the seal system. Tolerances exist on the various
parts of the seal and pump, for example on the retaining ring, spring(s), face
of the stuffing box, shaft, housing gasket and the machined cover relative to
the shaft axis. As the result the stationary face of the seal is never at right
angles to the shaft axis.
The spring elements of a dynamic seal rotate with the shaft so that the springs of the
rotating face must adapt to the slightly eccentric stationary face twice per revolution.
In other words, the seal has to make a continuous back and forth axial movement in
order to keep the faces together.
This explains why mechanical seals were once known also as axial mechanical seals..
The arrangement just described has its problems, as the following calculation indicates:
A pump operating at 3000 rpm round the clock is affected by the following axial
movements on the mechanical seal parts:
3000 rpm x 2 axial movements per revolution x 60 minutes per hour x 24 hours per day
= 8.6 million axial movements per day!
These immense kinetic loads acting on the mechanical seals cause considerable vibrations
which alone result in a far shorter seal life. Other effects include scoring of the shaft
sleeve, faster hysteresis of the spring(s) and more frequent opening of the seal faces.
All these problems ultimately result in the seal failing.
Scoring Of The Shaft Sleeve
Dynamic seal designs with O-ring, PTFE wedge or similar sealing elements require the
use of shaft sleeves on pump shafts.
As already mentioned, the dynamic secondary sealing element on the sleeve (O-Ring,
wedge, V-Ring etc.) has to make two axial back and forth movements per revolution in order
to keep contact between the faces and to ensure sealing.
The sleeve is usually made of stainless steel. Its chromium content combines with the oxygen
in the air to form a chromium oxide layer which should provide protection for the surface
but is polished away by the extreme axial movement of the secondary seal.
The re-released chromium content of the steel forms once again with the oxygen in
the air the oxide layer and the process repeats itself over and over again. Some experts
believe that the extreme axial movements leave the chromium no opportunity to
form a protective oxide layer, exposing the steel to continuous erosive corrosion.
Whatever the case may be, the result is the same: The sleeve suffers considerable scoring,
which causes the secondary seal to leak or stick to the sleeve surface and the seal faces will
open (seal failure).
Even if the mechanical seal itself is still in perfect condition, the pump has to be dismantled
in order to repair or replace the shaft sleeve and in most cases the mechanical seal
as well.
The fact that the pump component destroyed by the mechanical seal is more
expensive than the seal itself is most annoying!
Stationary Design
Stationary mechanical seals are designed with spring elements mounted in the
stationary part of the seal system. The tolerances existing on the various parts of
the pump and seal are the same in the case of stationary mechanical seals as for
dynamic types.
The main difference however is that the stationary spring elements compensate the misalignments
ONLY ONCE.
The springs remain practically still and therefore cause no additional vibrations. Of course
it is still possible, even with this design, that the mechanical seal will move axially if the shaft is bent.
This remaining movement is only very slight however and will be compensated in nearly all cases
by the elasticity of the elastomer (O-ring).
This contrasts sharply with the dynamic design which requires two compensating movements
per revolution. There is no doubt therefore that stationary mechanical seals work more
reliably than dynamic mechanical seals.
The success and reliability of the stationary design principle has long been proven in motor
vehicles, washing machines, aircraft, ships, submarines, agitators, flue gas desulfurization
pumps, high-speed pumps, etc.
Thanks to their reliability, stationary mechanical seals have been practically the sole choice in
all these applications for many years. With a stationary mechanical seal, e.g. DEPAC Type 191
or 196, no dynamic secondary seal is mounted on a shaft sleeve and therefore no scoring of
the sleeve is possible.
Existing sleeves can stay in use, but only to provide protection from corrosion for the shafts
which are usually made of standard steel. This also applies to cases in which the
sleeve is used in addition to tighten the impeller to the shaft.
In applications with stationary mechanical seals the shaft or sleeve no longer serve as a
wearing parts.
This reduces production stoppages, cuts repair times, saves expensive sleeves
and greatly improves operational efficiency.
Dynamic mechanical seals are designed with spring elements mounted in the dynamic (rotating) part of the seal system. Tolerances exist on the various
parts of the seal and pump, for example on the retaining ring, spring(s), face
of the stuffing box, shaft, housing gasket and the machined cover relative to
the shaft axis. As the result the stationary face of the seal is never at right
angles to the shaft axis.
rotating face must adapt to the slightly eccentric stationary face twice per revolution.
In other words, the seal has to make a continuous back and forth axial movement in
order to keep the faces together.
This explains why mechanical seals were once known also as axial mechanical seals..
The arrangement just described has its problems, as the following calculation indicates:
A pump operating at 3000 rpm round the clock is affected by the following axial
movements on the mechanical seal parts:
3000 rpm x 2 axial movements per revolution x 60 minutes per hour x 24 hours per day
= 8.6 million axial movements per day!
These immense kinetic loads acting on the mechanical seals cause considerable vibrations
which alone result in a far shorter seal life. Other effects include scoring of the shaft
sleeve, faster hysteresis of the spring(s) and more frequent opening of the seal faces.
All these problems ultimately result in the seal failing.
Scoring Of The Shaft Sleeve
Dynamic seal designs with O-ring, PTFE wedge or similar sealing elements require the
use of shaft sleeves on pump shafts.
As already mentioned, the dynamic secondary sealing element on the sleeve (O-Ring,
wedge, V-Ring etc.) has to make two axial back and forth movements per revolution in order
to keep contact between the faces and to ensure sealing.
The sleeve is usually made of stainless steel. Its chromium content combines with the oxygen
in the air to form a chromium oxide layer which should provide protection for the surface
but is polished away by the extreme axial movement of the secondary seal.
The re-released chromium content of the steel forms once again with the oxygen in
the air the oxide layer and the process repeats itself over and over again. Some experts
believe that the extreme axial movements leave the chromium no opportunity to
form a protective oxide layer, exposing the steel to continuous erosive corrosion.
Whatever the case may be, the result is the same: The sleeve suffers considerable scoring,
which causes the secondary seal to leak or stick to the sleeve surface and the seal faces will
open (seal failure).
Even if the mechanical seal itself is still in perfect condition, the pump has to be dismantled
in order to repair or replace the shaft sleeve and in most cases the mechanical seal
as well.
The fact that the pump component destroyed by the mechanical seal is more
expensive than the seal itself is most annoying!
Stationary Design
Stationary mechanical seals are designed with spring elements mounted in the stationary part of the seal system. The tolerances existing on the various parts of
the pump and seal are the same in the case of stationary mechanical seals as for
dynamic types.
The main difference however is that the stationary spring elements compensate the misalignments
ONLY ONCE.
The springs remain practically still and therefore cause no additional vibrations. Of course
it is still possible, even with this design, that the mechanical seal will move axially if the shaft is bent.
This remaining movement is only very slight however and will be compensated in nearly all cases
by the elasticity of the elastomer (O-ring).
This contrasts sharply with the dynamic design which requires two compensating movements
per revolution. There is no doubt therefore that stationary mechanical seals work more
reliably than dynamic mechanical seals.
The success and reliability of the stationary design principle has long been proven in motor
vehicles, washing machines, aircraft, ships, submarines, agitators, flue gas desulfurization
pumps, high-speed pumps, etc.
Thanks to their reliability, stationary mechanical seals have been practically the sole choice in
all these applications for many years. With a stationary mechanical seal, e.g. DEPAC Type 191
or 196, no dynamic secondary seal is mounted on a shaft sleeve and therefore no scoring of
the sleeve is possible.
Existing sleeves can stay in use, but only to provide protection from corrosion for the shafts
which are usually made of standard steel. This also applies to cases in which the
sleeve is used in addition to tighten the impeller to the shaft.
In applications with stationary mechanical seals the shaft or sleeve no longer serve as a
wearing parts.
This reduces production stoppages, cuts repair times, saves expensive sleeves
and greatly improves operational efficiency.
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