SUBJECT: Writing a set of seal
It is extremely important for any modern process
company to have a good set of seal specifications. When written
properly the specifications will:
seals by operating conditions.
- You can use rotating seals (the spring rotates
with the shaft) to 5000 feet per minute (25 Meters per sec.) as
measured at the seal faces.
- You must use stationary seals for speeds
greater than 5000 feet per minute (25 Meters per sec.) as measured
at the seal faces. Stationary seal are usually a much better
choice for all speeds because they're not sensitive to
mis-alignment between the pump and the driver and other forms of
- Specify balanced seals for stuffing box
pressures of one Torr vacuum to 400 psi. (28 bar) Note: this is
not discharge pressure.
- Use heavy duty seal designs for greater
pressures. These designs incorporate:
- Back up rings to prevent elastomer
- Thicker cross section components to prevent
- New configurations have become popular
since the introduction of finite element analysis
- Lower spring pressure at the seal
- A different balance ratio. Especially when
sealing fluids with a specific gravity below 0.4.
- Specify two way balance for dual seal
applications. This will prevent opening the inner seal if barrier
fluid pressure is lost, or if there are fluctuations in stuffing
- Use o-rings to published temperature
- Use non elastomer (metal bellows) seals for
temperatures outside these limits. Petroleum products being the
exception, because of "coking". In these applications seal chamber
cooling is necessary.
- Specify excessive motion designs for mixers,
agitators, sleeve bearing equipment, etc.
- These designs incorporate:
- Wider hard faces so that the thinner
carbon/ graphite face has more radial movement
- More internal clearance in the seal
- Axial movement capability without
compressing or extending the spring or springs and affecting
proper face loading.
- Any time you pump:
- Dangerous products
- Costly products
- If down time is very expensive.
- Fluids that give off fugitive
- These dual seals can be used in the following
- Rotating face to face
- Rotating tandem
- Rotating concentric
- Stationary back to back
- Stationary concentric
- Stationary tandem
- Do not use dual seals in the following
- Rotating back to back
- Stationary face to face
- Stationary tandem if the first stationary
face is positioned in the bottom of the stuffing
In each of these configurations the sealed product
is at the inside diameter of the seal faces and sliding components.
Solids in the fluid will cause problems as centrifugal force throws
the solids into the seal faces and restricts the free movement of the
sliding and flexing components.
The inner seal should be pressure balanced in both
directions to prevent opening of the inner seal if barrier fluid (or
system) pressure fluctuates or is lost. Two way balance can easily
compensate for vacuum or reversing pressure.
Clearly identify all of the
seal materials, otherwise it'll be
impossible to troubleshoot a premature seal failure.
- Carbon/graphite. Only unfilled grades are
acceptable with the exception of cryogenic service, where a
special self-lubricating grade is necessary. Do not use a carbon
/graphite if you're sealing petroleum products. The small pits
that form in the face will not allow the seal to pass a fugitive
- Hard faces. Alpha sintered silicon carbide,
nickel base tungsten carbide and 99.5 Ceramic are all satisfactory
faces for most applications. Keep in mind that 99.5 ceramic is a
poor heat conductor
- Elastomers. Use the o-ring configuration. Any
of the acceptable and available compounds are satisfactory. The
o-rings must not be spring loaded or glued in any manner. When
using Viton®, specify those grades that can be used with water
- Metal components. Any corrosion resistant
material compatible with the pumped fluid and pump components is
acceptable. The springs or metal bellows should be manufactured
form Hastelloy "C" or any other acceptable material not subject to
chloride stress corrosion.
features in any mechanical
- Centering ability. You want to start out with
the narrow softer face in the center of the wider hard face so
that the seal can compensate for maximum shaft deflection.
Centering can be accomplished by:
- Piloting the inside of the stuffing
- Piloting the outside diameter of the
stuffing box face.
- Shim stock at the inside diameter of the
stationary face and center off he shaft.
- Cartridge centering clips.
- Anti clogging features:
- Springs out of the fluid.
- The dynamic elastomer moves to a clean
- Centrifugal force throws the solid
particles away from the lapped seal faces and flexible
- Non fretting designs with no dynamic
elastomers touching the shaft or sleeve. Solid shafts should be
specified to help resist shaft deflection at start up and when
running off of the pump curve best efficiency point
- Built in environmental controls where
- Flush/ recirculation/ vent
- A vent and drain/ quench
- Disaster bushing for bearing failure and
- The smallest cross section possible to
maximize the room in the narrow stuffing box. You need this room
for radial shaft movement and for some place to centrifuge heavier
solids in the liquid.
- Cartridge designs for open impeller adjustment
and ease of assembly.
- Be sure the cartridge is sealed to the
shaft or sleeve at the inboard end. Outboard sealing can let
solids penetrate between the seal sleeve and the shaft making
the seal removal difficult
- Split designs for easiest installation and to
avoid re-alignment between the equipment and its driver.
- Glued elastomers are never acceptable. The
glue will cause a hard spot to form in the elastomer causing
the dynamic seal to leak.
- Vibration damping, especially with metal
- Slotted or other forms of non-dedicated glands
for maximum flexibility.
- Rotate the fluid in the stuffing box rather
than have the seal components rotate through the fluid. This is
especially important in metal bellows designs because of their
thin plates. Abrasive fluids can cause severe wear in a short
period of time.
Present and more restrictive future legislation
will discourage the shipment of seals to outside repair facilities or
back to the manufacturer because of personnel hazard problems. Try to
select designs that can easily be cleaned and the critical parts
replaced at your facility. The following parts must always be
- The carbon/ graphite face. Relapping is not
acceptable because trapped solids and lapping fluid will be forced
into the soft carbon. Machining or grinding a blank carbon is not
acceptable because the blank will not have enough density when the
outer layers are removed.
- The springs, because they work harden with
time and corrode at a faster rate than the other metal
- All of the elastomers (rubber
- Set screws. They're not manufactured from
hardened material because of corrosion resistance and if used
several times, will "mushroom" and lose their holding
Relapping of the hard face is acceptable as long
as there are no cracks or chips visible.
® E.I.DuPont Dow elastomer
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