Metal Ferrules
have a number attractive attributes . . .
Metal has the least permeability to air
Metal fittings and ferrules can effectively seal over the largest pressure and temperature range
Metal fittings and ferrules are extremely rugged and can take a lot of abuse
Metal fittings can be and remade many times (if not over tightened)
Problems arise . . . however, because of there hardness, and failures can occur:
Improper matching of ferrule and seat designs (avoid using/mixing generic fittings and ferrules)
Over tightening can cause irreversible damage to the fitting as well as the ferrule.
Improper installation (sinserting hoving tubing into fitting too far and/or over tightening)
can cause the end of the tubing to bulge in the fitting, making it difficult or impossible to remove.
Seizing of the ferrule to the fitting after long periods at elevated temperature.
Inability to seal over fine-lined striations in the metal tubing itself and scoring of the ferrule material
Whilst not ideal sandpaper can be used with care to improve imperfections in tubing
Rotating the tube is far bett than length wise
Soft Metal Ferrule
material (e.g.,
Siltite
-SGE
and Aluma-Seal
Restek) is now availble
which can be used for fused-silica column connections,
They can deform slightly so that they seal with less force than is necessary with hard metal ferrules
otherwise impossible with glass or fused silica.
They are soft enough to be used with fused-silica capillary columns.
They are a little more forgiving of imperfect surfaces than hard metal ferrules.
They have very low (or no) air permeability and can be used over the full range of GC temperatures.
Unlike polymeric ferrules, soft metal ferrules do not shrink and therefore do not need periodic retightening.
They are specially coat to reduce the risk of seizing in fittings.
Soft metal ferrules seize onto the column upon first use, so connections must be done with some care.
These
soft metal ferrules
are particularly suited for MS transfer lines or capillary column connections
where one does not typically need to trim the swaged column end regularly.
Even though they deform slightly soft metal ferrules should still be used with fittings that have the proper
corresponding taper design. To benefit from using this style ferrule, spare replacement parts such as
a new baseplate for the inlet or a special reverse nut for transfer line connections.
Glass and Fused Silica capillary
connections - -
Most ferrules require close tolerance matching of Tubing OD and ferrule ID
Hard ferrules have no give and tend to "crunch" the tubing
Polymeric Ferrules are softer and mor forgiving where applicable
Graphite probably priveds the best all-round ferrule materiial
For glass and ceramic tubing
. . . reducing Graphite Ferrules overcome many adaptions of metric tube size
and Imperial size fittings.
Care is needed to select tubing as round as possible is important in obtaining a "re-doable" and effective seal.
With care Graphite Ferrules can re reused athough extrusion into the thread of the Nut does occur making
them difficult to remove.
For Reducing Ferrule application end-pressure on the tubing is a limitation with Graphite Ferrules
- the tube can be forced out the back of the nut
. . . graphite after all is a lubricant and glass is smooth . . . of course
Extreme application with large OD tubing may benefit by centreless grinding of the glass tube / ceramic
and/or grooving or roughening of the glass surface
Glass tubes to 2" diameter and ceramic tubes to 1000decC are possible - although th graphite does tend to
oxidise and burn away in contact with air Caution PRESSURE LIMITATIONS Low Pressur HIgh Flow
applications are OKthrough . . .
with CARE !
. Enclosed in an inert atmosphere temp to >1500degC are possible ( ceramic tubes )
- often there are Just NO Alternatives to Graphite ferrules
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TelNo : 03 9762 2034 . . . in AUSTRALIA
