SUBJECT : The sealing of liquid slurries
A slurry is defined as: Solids suspended in liquid that cannot be
dissolved by controlling the temperature and/ or pressure. The solids
may or may not be abrasive. It does no good to try to identify the
number of solids or their size because no one knows how these numbers
relate to slurry related seal problems. Whenever you deal with
slurries there are several problems you must consider:
- The slurry can clog the flexing parts of a mechanical seal,
causing the lapped faces to open as a result of both shaft and
- If the slurry is abrasive it can wear the rotating components.
This can be a serious problem with thin plate metal bellows
- The pump rotating assembly will go out of balance as the
slurry wears the impeller and other rotating components. This will
cause excessive moving of the seal components.
- The pump will lose its efficiency as critical tolerances wear
rapidly. This can cause vibration and internal recirculation
problems. The wear will also cause the need for frequent impeller
adjustments that will cause problems with mechanical seals
It's generally believed that the main problem with slurries is
that they penetrate between the lapped seal faces and cause damage.
Although this is true, it's also true that they cannot penetrate
until the seal faces open.
Seal faces should be lapped flat to within three helium light
bands. That is a distance just a little bit shy of one micron.
Compare this to the fact that the smallest object that can be seen
with the human eye is forty microns in size, and you'll appreciate
the technology used in the manufacture of mechanical seals. As a
matter of comparison, look at a common coffee filter. It filters out
particles larger than ten to fifteen microns.
All of this means that the seal is in fact a superior filter and
as long as you can keep the two lapped faces in contact, there's
little chance for solids to penetrate the faces and do any type of
There are three approaches to the sealing of solids :
- Design a seal with non clogging features.
- Create a clean sealing environment for the mechanical
- Do a combination of both
Let's look at each of the approaches, and in the process learn a
sensible method of sealing a slurry.
Build a seal with non clogging
- Take the springs out of the sealing fluid.They cannot clog if
they're not in the slurry.
- Make sure the sliding or flexing components move towards a
clean surface as the seal faces wear.
- Take advantage of centrifugal force to throw the solids away
from the sliding/flexing components and lapped seal faces.
- Use a non stick coating to prevent the slurry from sticking to
the sliding components.
- Use only balanced seal designs. Additional heat generated at
the seal faces can cause many products to solidify, coke, and
crystallize creating even more solids problem.
- Metal bellows designs can be used, but they must have extra
thick plates to resist excessive wear. Extra convolutions will
have to be provided to compensate for the higher spring rate
caused by these thicker plates. Rotating the abrasive fluid with
the bellows can be a big asset. Some commercial designs have this
Create a clean sealing
Give the seal as much radial room as possible. You can either bore
out the packing chamber or install a large bore sealing chamber. Try
to give yourself at least 1 inch (25 mm. ) radial space if possible.
The more room you can provide for the seal the better off you're
going to be.
Try to remove the solids from the sealing area. There are a number
of techniques for doing this. Some work and some do not. Let's look
at each of them. First we will look at the solutions that do not work
very well and comment on their problems :
- Bad Solution #1. Connect a filter in the line from the pump
discharge to the stuffing box. Since the discharge is a higher
pressure the flow of liquid through the filter will clean up the
fluid and then there will be clean liquid flowing to the stuffing
- Comment : The problem with this idea is that the filter
will clog and no one will clean it.
- Bad Solution #2. Install a cyclone separator into the line
instead of a filter. Connect it between the pump discharge and
suction with the third (the center) port connected to the stuffing
- Comment: This idea is just as bad. The cyclone was never
intended to be a single pass device. They work well if used in
a bank of several filters but there is not enough pressure
differential between the suction side of a pump and the
stuffing box for them to be effective.
- Bad Solution #3. Install the seal outside the stuffing box so
the springs will not be located in the dirty fluid.
- Comment: The problem with this idea is that as the seal
faces wear they must move forward and in doing so they will
move into the dirty fluid. The result will be that the movable
face will hang up in the solids and the faces will open.
Another problem with this approach is that centrifugal force
throws the solids into the seal faces and not away from
- Bad Solution #4. Install a double rotating seal in the "Back
to Back" configuration with a higher pressure, clean liquid
barrier between the seals.
- Comment: This is a very common approach to the problem and
has all of the problems associated with installing the seal
outside the stuffing box. In addition to a rapid failure you
will also experience product dilution as the barrier fluid
leaks into the pump.
- Bad Solution #5. Since we are discussing things that don't
work we might as well try two hard faces.
- Comment: Needless to say they will not prevent the faces
from opening and when they do open, experience shows that
you're going to destroy both hard faces. Some seal salesmen may
even try to convince you that the seal faces are designed to
"grind up" the solid particles into a fine powder. In other
words the seal is designed as some type of a "quasi- milling
Now we will look at some methods that do work:
Good Solution #1.
- Flushing with a clean liquid is a good method of cleaning up
the pumping fluid. The amount of flushing you will need depends
upon the design of your seal. If the design has multiple small
springs in the fluid then more flushing will be required. There
are various sources for the flushing liquid :
- Finished, clean product or one of the mixture's clean
- A compatible fluid.
- A solvent.
- An additive that is going to be added down stream and could
be injected into the stuffing box location.
- Clean water.
- A compatible grease can be used with most balanced seals
running at lower speeds
- NOTE : Never introduce live steam into the stuffing box as it
could cause the product to flash and the pump to cavitate.
- Be sure to start with a flushing pressure that is at least one
atmosphere (15 psi or 1 bar) higher than the stuffing box
pressure. You can use a pressure gauge to determine stuffing box
pressure. You can then use a flow meter to regulate the amount of
- With intermittent service pumps it is a good idea to have an
electrician install a solenoid valve with a delay switch that
would allow the flushing fluid to come on thirty seconds prior to
the pump starting and to leave the flushing valve open for a few
minutes after the pump has stopped
Good Solution #2.
- Install an oversize, jacketed sealing chamber and "dead end"
the fluid. "Dead ending" means that there are no circulation lines
coming in or going out of the sealing chamber.
- You can use the cooling jacket to remove the heat being
generated by the seal faces as centrifugal force cleans up the
solids that are present in the small amount of fluid, trapped in
the seal chamber. This solution works exceptionally well with
fluids where temperature control is important. Heat Transfer Oil
is a prime example.
- If the fluid you're sealing is not hot, the cooling jacket
will not be necessary. Some times one shot of clean liquid into
this oversize, dead ended stuffing box, is all that is necessary
to seal even a severe slurry. Needless to say this application
works best on a continuous running pump.
- If the specific gravity of the solids is less than the liquid
they are mixed in, centrifugal force will not work for you. A
clean flush will be necessary in this instance or one shot of a
higher specific gravity compatible liquid.
Good Solution #3.
- If the solid particles are sub micron in size two seals with a
higher pressure barrier fluid becomes necessary. In some instances
you might want to use two hard faces on the inner seal. Kaoline
and some dyes are a good example of products with sub micron size
Good Solution #4.
- Install a large seal chamber on the pump and connect a
recirculation line from the bottom of the stuffing box back to the
suction side of the pump. The size of this line will be determined
by the size and number of solids that you're trying to
- This will cause liquid to flow from behind the impeller to the
stuffing box and then on to the suction of the pump. Fluid
entering the stuffing box, from behind the impeller has been
centrifuged and should be a lot cleaner than the fluid you're
pumping. This solution works well with closed impeller pumps and
those open impeller designs that adjust to the front of the pump
volute. If your open impeller adjusts to the back plate (as is the
case with the Duriron pump) this method is not as effective.
- Do not use this technique if:
- You're pumping close to the vapor point of the fluid, as
lowering the pressure could cause the pumping fluid to vaporize
in the stuffing box and in some cases between the seal
- You're sealing a Duriron pump where the impeller adjusts to
the back plate.
- You're sealing double ended pups where the stuffing boxes
are at suction pressure.
- If the solids have a low specific gravity or density and
float on the liquid.
Compensate for the fact that the rotating
unit will go out of balance.
- The seal faces have to be vibration dampened. O-ring type
seals are equipped with a natural vibration damper because of the
dynamic elastomer that has been installed. Metal bellows seals
have to be provided with some other method. Letting the seal face
holder rub and vibrate along the shaft is an approach used by most
metal bellows seal manufacturers. The logic is questionable.
- Give the seal room to move. Shaft run-out and vibration can
cause the seal rotating components to contact the inside of the
stuffing box unless you have installed an oversized sealing
- Use motion seals if the run out or vibration is excessive.
Unlike pump seals, these seals have much wider hard faces and
extra internal clearances. Most popular designs can compensate for
plus or minus 1/8" (3 mm. ) in a radial direction and 1/8" (3 mm)
in an axial direction.
- Move the seal closer to the bearings. Split seal designs are a
logical choice because most of them come with a stuffing box
extension gland that positions them next to the bearings. A
support bushing, or sleeve can be installed near the face of the
stuffing box to minimize the affects of unbalance, vibration and
shaft whip or wobble. A variety of materials are available for
these support sleeves. Check with your supplier for availability
in your area.
The pump will lose its efficiency and
experience more shaft movement as close tolerances wear.
- If you're using open impellers it will mean frequent impeller
adjustment. In this case a cartridge seal is your best approach as
impeller adjustments can be made without disturbing the seal face
loading. Split seals can compensate for the initial impeller
setting and split seals mounted on a split sleeve will easily
compensate for movement caused by temperature growth or impeller
- Closed impeller pumps will have to be disassembled and the
wear rings changed when the clearances become excessive. If you're
fortunate enough to have adjustable wear rings on your pump then
only an outside adjustment will be needed and the pump will not
have to be put out of service. Cartridge seals can almost always
be reused in these applications because the seal faces were not
separated as the pump was disassembled.
- Remember that with closed impeller pumps the wear rings will
have to be replaced when the normal clearance doubles. A typical
normal clearance would be 0.008" to 0.015" ( 0,2 to 0,4 mm). A
good rule of thumb is that the pump will lose 1% of its capacity
for each .001 inch (0,025 mm.) of wear ring wear.
A few more thoughts about the sealing of
- Kaoline (China clay) is a product that's used in many
industries including paper and pharmaceutical. Its abrasive
particles are less than one micron in size and as a result, will
penetrate lapped seal faces causing rapid carbon and hard face
wear. In this application it is necessary to use two mechanical
seals with a higher clean barrier pressure between the faces to
prevent most of the penetration.
- In addition to one of the recommended solutions mentioned
above, two hard seal faces can also be used as some particles will
always penetrate the faces.
- Using a combination of packing and a split mechanical seal is
proving to be an ideal solution in many applications. With the
seal installed there is no pressure differential across the
packing and therefore the solids do not try to penetrate. Move the
packing flushing line to the bottom of the split seal housing and
flush the packing through this connection instead of the lantern
ring or seal cage. The flushing is necessary to remove the
additional heat being generated by the packing.
- You should be able to cut the flushing fluid volume down to
about one third of the amount you had been using. Since the
packing is not being forced to the shaft only a small amount of
cooling is necessary.
- CAUTION! It is important that
the flushing fluid be kept at a higher pressure than the
stuffing box pressure. If this pressure differential fails it
could force the packing into the rear of the mechanical seal. A
split adapter plate installed between the split seal and the
stuffing box face can prevent the packing from blowing out if
the flushing pressure is lost.
- If you elect to use a rotating metal bellows in a slurry
application, remember that the bellows should rotate the fluid in
the sealing chamber. Most bellows designs allow the thin bellows
plates to cut through the abrasive slurry and experience severe
wear and breakage in a short period of time.
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