| What can be said in favor of these seal types? They are riddled
with potential leak paths. All the components of the seal are supplied either cut, to be
joined on site, or in halves. In addition to the normal fitting problems, you have the
additional task of ensuring that all the seal elastomers are properly butted and sealed,
that the stationary and rotating faces are accurately aligned with the mating half, as
well as with each other, and often the recommended application is an emergency situation
where no alignments are guaranteed on the pump in any circumstances.
The worse application I came across was when this type of
seal was fitted to a hot oil vertical pump removing oil residues from a tar pool. It was
recommended as a viable permanent alternative to packing.
First problem,
it was a vertical pump, the seal was fitted in place of the packing but there was no
stuffing box vent.
Second problem,
it was a pump handling hot oil which formed a tar substance when cool, leaked oil hardened
around the seal.
Third problem,
the pump had sleeve bearings, fine with a packed gland, which probably added to the
stability of the shaft, but not so good for this type of seal.
Fourth problem,
the natural leak paths through the seal meant that the seal could never be truly
stabilized and made reliable. The proper recommendation for this pump was to use a double
cartridge seal with a vent and circulating steam trace or, if not viable, carbon fibre
packing.
The pump leaked badly after short service and caught fire. I
remain amazed that anyone could recommend such an application as a permanent repair.
The only time I could recommend such
a seal is as an emergency repair for a leaking gland. It is not a
suitable seal for permanent use. They can be recommended as a fix
for a leaking packing gland. The split seal is mounted on the shaft
external to the gland and runs against the stationary which is
mounted on the gland driver. The pump should be taken out of service
and overhauled at the first opportunity.
Leak paths
A split seal means that all the parts must be
capable of being fitted to a shaft without dis-assembly of the shaft assembly
itself. The seal pictured here is one of the best on the market.
But look at it carefully, note all the split
components, the o-rings, seal seats, body components ... before it can effect a good seal
on the pump unit it has to fight its own construction. Imagine being in the plant
late at night (isn't that just the time when emergencies arise?) its dark, the hand lamp
is always dropping out of sight, you're tired, under pressure, the damn nuts are stiff,
and the night production manager is yelling across your back every five minutes to get his
plant back up and running. To cap it all you have to ensure that all these fiddly
bits of seal, all with their own leak path are fitted exactly, exactly
mind you, in place. What are your chances?
OK so consider that seal. The seal head comes in two
halves, the O-rings are not rings, the metal parts are split. The stationary parts are
also split. The stationary rubber boot, ceramic face, the clamping arrangement are all in
two halves to facilitate fitting around the shaft.
The design must ensure that the sensitive face components are
fitted precisely and the elastomeric components mate together to prevent leakage. The
positioning of the seal on the shaft at the correct fitting length has to be accommodated
and finally the design has to cope with the results of earlier leakage and any resulting
corrosion of the shaft. Given that seals work in a hostile environment of vibration,
mis-alignment, shaft deflection, abrasion (shall I go on? ), the split seal has even more
obstacles to overcome to provide successful service.
Most Plant Operators with pumps sited in crucial areas of their
process have stand-by pumps and redundant capacity. Ok so sometimes plant is run at full
capacity and sometimes even the redundant capacity is brought on-line in these situations,
as it was the case in most of the plant in Saudi Arabia during the Gulf War, at times like
these a split seal could be a process saver. During normal operations though' attend to
the maintenance of your stand by plant and do not rely on a quick fix.
|
Pump
Tutorial CDROM $19.95
