Slurry centrifugal pumps "TL"
- "TLK",
 "SL"
- "SLK", "FL", "D" - "TD"
- "SD"
Single-stage centrifugal pumps with open anti-clogging impeller
with wear plate.
Drive options include flange for internal combustion engine,
tractor PTO gearbox or bareshaft versions for electric motors
and engines.
Available with a chopper device and manual or mechanical priming
system.
These pumps are mainly suitable for pumping sewage and animal
dejections, also with fibers and solids in suspension, in installations
requiring high operating pressures; furthermore they can be
used in irrigation systems too.
Use
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Performances
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 :
o Finished, clean product or one of the mixture's clean ingredients
o A compatible fluid.
o A solvent.
o An additive that is going to be added down stream and could
be injected into the stuffing box location.
o Clean water.
o 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 flushing fluid.
" 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 are 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 particles.
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 are trying to remove.
" 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 are 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:
o You are 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 faces.
o You are sealing a Duriron pump where the impeller adjusts to
the back plate.
o You are sealing double ended pups where the stuffing boxes are
at suction pressure.
o 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 a normal 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
chamber.
" 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 in the end 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 are 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
adjustment.
" Closed impeller pumps will have to be disassembled and
the wear rings changed when the clearances become excessive. If
you are 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 slurries :
" Kaoline (China clay) is a product that is 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.
o 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.
From: Mc Nally Institute
www.mcnallyinstitute.com
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