what
is involved in a normal seal repair? Let's take
a close look at a typical sequence
1.
Inspect and troubleshoot the various components
for evidence of corrosion, rubbing, wear or
damage. This type of failure analysis should be
a normal part of repairing seals. Why put in
another seal until you have learned why the
present seal failed? The troubleshooting
sections of these web pages will help you with
this determination.
2. Disassemble the seal and throw away the
following components:
- The carbon/graphite face.
- The elastomers and gaskets.
- The spring or springs.
- The set screws
- Any drive lugs or anti rotation pins.
- Cracked or broken hard faces.
- Cracked or broken bellows.
3. Clean the remaining components such as the
sleeve, face holder, adapter, etc.
4. Inspect the cleaned components to be sure
they meet the manufacturer's original
dimensions, tolerances, and finish.
5. Some metal-carbon composites have to be
stress relieved after assembly to remove
manufactured stress that will prevent the carbon
from remaining flat during the storage and
shipment period. Watch out for seals that are
used in cryogenic service. They should be lapped
at their cryogenic operating temperature.
If you intend to insert a carbon into a metal
holder, you will be better off pushing it in
with an arbor press. If you try to expand the
holder and shrink it to the carbon you will have
problems maintaining face flatness.
6. Reassemble the components using only new
parts that have been supplied by the
manufacturer. Be sure to use original equipment
parts because many seal manufacturers design
components that have been created from "finite
element analysis" or similar techniques. Solid
hard faces can be relapped if they are not
damaged in any way. Plated or coated faces must
be stripped and re-plated before lapping.
7. Check the assembled seal for flatness. A
vacuum check should be performed on the movable
assembly to insure there is no leak path through
or around any of the components. If you elect to
check cartridge mounted double seals with an air
test, avoid immersing the seal in water. If the
seal was later installed in hot oil, or a
similar application, the water trapped in
gaskets and small crevices would flash to steam
generating possibly dangerous pressures.
8. Repackaging varies with the manufacturer.
Here are some points to consider:
- The packaged seal should be able to
survive a thirty nine inch drop (one meter)
without injury to any seal component
including the lapped seal faces.
- A protective coating (Silicon is a
common one) should be put on the lapped
faces.
- A new set of installation instructions
and an installation print should be placed
in the box with the rebuilt seal. A box is
always superior to "bubble packaging."
- The box should be clearly labeled as to
its contents. The use of only a part number
leads people to open the box, often causing
unnecessary damage to the mechanical seal
while it is being handled.
During the rebuilding process you should keep
in mind that troubleshooting a rebuilt seal is a
frustrating experience. A trained troubleshooter
is looking for evidence of rubbing, damage and
corrosion. You should be careful to clean up
evidence of any of these symptoms prior to
repackaging the seal, to avoid confusion the
next time the seal failure is analyzed. |
Teflon