GLAND
SEALING
Gland sealing is the standard
form of sealing adopted for many slurry pump
applications due to it's relative robustness,
gradual failure mode and ease of maintenance.
ANATOMY OF A GLAND SEAL
A gland seal is comprised of a
chamber (Stuffing Box) which houses stationary
sealing components such as Lantern Rings, Neck
Rings and Gland Packing. The chamber allows for
flushing water to be feed into the sealing area
through a feed hole. Passing through the centre
of the chamber is a shaft which may have a
sacrificial wear sleeve which rotates against
the stationary packing in the sealing chamber or
stuffing box. Pressure is applied between the
packing and shaft sleeve through a gland
follower which when tightened compresses the
packing, this forms a sealing line between the
sleeve and the packing, between the pressure in
the pump and atmosphere outside the pump.
Naturally this friction creates
heat for which the the purpose of flushing water
is to do what it says, flush and cool the
sealing line between the stationary and rotating
parts. Within a slurry pump which can pump not
only solids laden product but also acidic or
alkaline solutions allowances need to be made to
reduce not only the effects of friction between
the sleeve and packing but also corrosion and
erosion.
FAILURE MODES OF GLAND SEALS
There are 3 main forms of attack
on gland seals in slurry pumps that lead to
sealing failure. These can be either individual
or combined effects.
1. Corrosion - Generally caused
by a hyper saline or chemical environment with
incorrect materials selection. Apart from direct
chemical or oxidisation effects on materials
crystallization around sealing surfaces can
compound failure through parts erosion.
2. Erosion / Wear - Normally
caused due to contamination of the sealing
chamber by slurry being pumped through
inadequate flow and pressure of sealing water,
can also be caused through fluid crystallization
or through excessive force applied between
sealing surfaces through over tightening of the
gland follower.
3. Friction - Normally caused
through over zealous gland adjustment to achieve
near zero leakage. However this then causes a
breakdown of the flushing water function in
cooling of the gland. All gland sealed pumps are
designed to leak and should be allowed a slow
trickle or fast drip of flushing water from them
to facilitate cooling and flushing or the
sealing line.
The failure cycle of a gland
seal is generally progressive due to the
robustness of the design which has inherent
redundancy, gland seal failure it is rarely
instantaneous. Energy is the underlying form of
failure, physics tells us that energy follows
the path of least resistance. Within a gland
that is stressed due to any combination of the
above conditions energy is being transferred and
dissipated across the sealing components, this
energy can be in the form of chemical,
potential, kinetic etc. associated with either
the fluids or solids in the chamber. So
naturally the fluids / solids will seek to
release or transfer their energy to the weakest
component in the chamber being the packing. This
is exactly what a gland seal is designed to do,
the packing is the main sacrificial element in
the chamber and as such is changed out more
often than the other components.
However over time gland packing
has been enhanced to the point that special
material such as Kevlar, Carbon Fibres and
Teflon have been incorporated into it's design,
this has resulted in packing being much more
stable and able to resist wear or to dissipate
the energy into other areas of the sealing
chamber, namely the secondary sacrificial
element being the shaft sleeve.
Shaft sleeves along with lantern
and neck rings are probably the second most
changed out components of a gland sealing
system. historically sleeves have been made out
of alloys that are harder wearing than the gland
packing so that they last longer. But as packing
has evolved in strength and design with
resultant longer lifespan sleeves have either
been changed with packing cycles or enhanced
through new materials, coating systems or a
combination of both. Enhanced sleeves which
offer hard coatings for wear resistance can then
outlast new generation packing and offer
enhanced service life across the sealing line.
However many coating systems have their own
inherent design flaws and weaknesses which if
not supported by sufficient feed of flushing and
cooling water can lead to accelerated failure of
the gland seal.
For more information on failure
of coated sleeves please refer to our
CIS Sleeve page.
FAILURE MODE REDUCTION
Measures to reduce the effects
of gland seal failure modes include.
1. Sealing Configuration -
Ensuring that you have selected the right
sealing configuration for duty and process
conditions. On this point there are many
aftermarket products available that offer
enhancements to pump sealing over original
design, each offering needs to be assessed on
it's claims and merits taking into account not
only the pump duty but process conditions.
2. Flushing Water - Ensuring
that the gland has the correct arrangement of
parts with sufficient clean flushing water at
the right pressure and flow. Over 90% of sealing
problems can be traced back to insufficient feed
of clean flushing water at the correct pressure,
with correct gland adjustment.
3. Materials Selection -
Selection of the right materials to suit the
duty conditions of the pump and the availability
of flushing water.
Stuffing Box - In
chemical duties an inert material needs to be
used however most chemically inert materials are
not hard wearing so a compromise material may
need to be selected that gives a balance of wear
life and chemical resistance. For wearing duties
harder materials can be used but you will need
to be mindful that the harder the material then
generally the lower it's mechanical strength and
subsequent pressure capacity. For Chemical and
hard wearing applications you need a material
that is wear and chemically resistant. For this
environment Slurrytech have developed the SB-WRC
(Stuffing Box - Wear Resistant Carbide face),
this seal is constructed from a chemically
resistant alloy Stuffing Box with a hard wear
face coating of WRC (Wear Resistant Compound)
exposed to the slurry side of the chamber.
Shaft Sleeves - Sealing
sleeves rotate with the pump shaft against
stationary rings of packing in the stuffing box.
Basic material grades of sleeves are in hardened
stainless and are generally very robust, pumps
running these sleeves normally have gradual
failures of the sealing assembly. New generation
sleeves are available with a variety of hardened
coatings and application processes for these.
Most coated sleeves suffer from a disassociation
of material properties between the substrate and
the coating which can lead to rapid failure of
the gland seal. Slurrytech CIS sleeves are
designed to offer a harder wearing surface that
is infused into the substrate to avoid
traditional failure modes of most coating
systems. Please see
CIS Sleeve page
for further information on our sleeves.
Gland Packing - Today's
modern gland packing comes in more varieties,
wraps and material combinations than has ever
been available in the past. the key rule with
packing is to ensure that you match the packing
for the chemical, wear and gland materials being
used as well as taking into consideration gland
water availability and pressures. All these
factors effect how well not only the packing but
the sleeve and other components will hold up in
duty conditions. Unfortunately there is no one
type suits all conditions design of packing
available.
At Slurrytech have designed our
own general range of packing which consists of
Kevlar woven corners for strength, braided
Teflon walls for reduction of friction and a
bound graphite core for lubrication and solids
take up.
All glands in slurry conditions
will suffer from solids contamination over time,
we have designed our packing with this in mind
so that it has the capacity to take up and
absorb contaminants rather than bind them up
between the sleeve and the packing. Our grade of
packing works equally well for alloy or ceramic
coated shaft sleeves and is suitable for a wide
range of pH levels and pump pressures.
EXPELLER
SEALING
Expeller seals are designed to
reduce bypass pressure from the pump into the
gland or rotary sealing area of the pump. This
is achieved through a combination of centrifugal
force and component tolerances.
Expeller seals rely on pump
speed and the tolerance between the housing and
expeller face to maintain performance. One of
the main drawbacks of expeller sealing is the
compromise that needs to be made between pump
performance and sealing performance.
In most typical slurry pumps you
can either adjust the pump impeller forwards for
reduced internal recirculation and improved
efficiency at the impeller end which will result
in reduced expeller performance as it opens the
sealing face gap. Or you can adjust the impeller
backwards for better expeller performance.
Once you have the right balance
the pump generally gives reasonable performance
for both the wet end and sealing end. However
depending on materials used for the expeller
ring the sealing performance can deteriorate
rapidly. As the Expeller Ring (housing that the
expeller runs in) wears it creates turbulent
flow patterns and increased tolerances which
reduce the effectiveness of the expeller itself
and also leads to accelerated expeller wear.
Pumping a wearing medium the expeller sealing
area is continually subjected to wear which will
lead to it's eventual failure.
As with Gland seals most
expeller seals have an outboard seal for when
the pump is stationary as the expeller only
works when operational. This is usually a gland
seal with reduced packing rings or alternatively
with V-ring seals. As per gland sealing many of
the same considerations need to be made when
selection and supporting this type of sealing
device.
Slurrytech have recognised the
shortfall in expeller seal design and have and
can develop customised solutions to enhance
expeller seal life and performance.
EXP-WRC - This is a
standard expeller seal which has been modified
to take an internal coating of Wear Resist
Compound (WRC). this design is currently being
used at many Mineral Sands operations and is
being trialled at gold mines.
Benefits - Increased wear life,
maintaining of expeller performance, maintaining
of expeller geometry due to reduced attrition
and wear of expeller ring, easy to rebuild and
repair with additional WRC, reduced waste, lower
costs. When fitted with Ceramax wet end parts
pump and seal life is significantly increased.
Drawbacks - None noted.
EXP-SiC - This is a
standard expeller seal which has been modified
to take a Silicon Carbide internal lining. This
design is currently being used at some Mineral
Sands operations.
Benefits - Significantly
increased wear life over standard materials,
increased wear life over WRC expeller ring,
maintaining of expeller performance, maintaining
of expeller geometry due to reduced attrition
and wear of expeller ring, can be rebuild,
reduced waste, lower life costs. When fitted
with Ceramax wet end parts pump and seal life is
significantly increased.
Drawbacks - Higher initial cost.
Full EXP-SiC expeller ring, substrate is
SAF2207 duplex stainless for corrosion
resistance. Sleeve is Slurrytech CIS on SAF2205
substrate.
Ceramax lined pumps with EXP-SiC sealing.
MECHANICAL
SEALING
For
mechanical seals SLURRYTECH use and recommend AES
sealing devices and technology.
AES HDDSS
(Heavy Duty Double Slurry Seal) is a plug and
play item that does not require any
modifications to the pump, any special adaptor
parts or modified impellers, simply remove the
existing gland or expeller seal and fit the
HDDSS in it's place.
Some
sealing applications may require external
support equipment for seal flushing.
Please feel
free to contact either Slurrytech or your local
AES agent for further information.
