|
Know how they work!
Soft packing stuffing boxes and pump glands appear so simple and
are so common-place that we continue to cope without taking a few moments to understand
why they fail or how to improve their performance.
When I first worked as a maintenance technician it did not occur
to me to wonder why it was that the metal shaft sleeve wore away at almost the same rate
as the fiber packing. Later, charged with the responsibility of advising maintenance
engineers I was faced with worn out shaft sleeves on a daily basis. The type of
fiber made
no difference, the metal sleeve became damaged against hemp, cotton, aramid, teflon, and
especially asbestos and, later, the asbestos substitutes. It appeared to make no sense to
have a soft material damage a metal surface so badly as to cause grooves and ridges in
what was to become a familiar pattern.
The packing fibers available come lubricated with a variety of
greasy materials or the packings are made of inherently lubricated materials such as
teflon, or carbon fiber, often loosely braided materials contain fillers such as the
silicone filler in braided teflon packing, and the search for the answer as to why the
shaft becomes worn starts here.
Fillers, grease, graphite grease, tallow, and the other
materials used serve two functions in packing materials.
Now take the packing material and place it in the pump stuffing
box.
Cut to size, the packing pieces are eased down the shaft to the
neck ring. The cut ends are staggered to prevent leakage through them, the lantern ring is
placed in position, the final three pieces of packing tamped in place by the gland plate
and the work is done.
The next stage is to adjust the gland to ensure that it leaks.
The leakage rate is controlled by the pressure exerted by the gland plate on the end of
the packing set and the leak is allowed to develop along the shaft / packing interface to
provide a cooling medium, removing the friction heat generated by the rotation of the
shaft in the packing set. Two further things happen here. The grease in the packing melts
slightly and is washed away by the flow of liquid along the shaft, and wear at the surface
of the packing begins. A cycle is beginning which leads to the destruction of the packed
gland as an effective leakage control device. The packing volume decreases as the
lubricant is lost. Inevitably this causes the leakage rate to increase. As the rate
increases more material is lost until the gland is tightened to reduce the leak to a
minimum.
In very few cases can an engineer claim that the fluid passing
through his pumps is not contaminated by dirt particles. Iron oxides, chromium oxides,
grit, aluminum oxides, mica, and many other minute contaminants will exist in all system
fluids. These solids, being denser than the pumped fluids, will be centrifuged and
concentrated at the outer edge of the volute casing at just the point where the lantern
ring tapping is sited. This contaminated fluid is then passed, at pressure, directly into
the pump gland. Whatever material is used to seal a stuffing box if it is cooled by fluid
contaminated by solids its surface will change producing an effective grinding surface. To
improve the life of the stuffing box gland, contamination from the cooling water has to be
avoided..
The gland packing has been wearing away. Through loss of
lubricant it has lost volume, and the packing surface is exposed. The fluid passing
through the gland, providing a cooling stream, is contaminated with various oxides and
grit. The flow is increasing. A passing engineer notices it and takes appropriate action.
The gland plate is tightened, pressure is exerted on the packing material to make it
deform to reduce the clearance between it and the shaft. For a moment the flow of fluid is
stemmed and the packing clamps down on the shaft trapping any solids moving through the
gland at that moment. The interstices of the packing fill up with debris. The packing
surface is now beginning to be converted from its original state into one consisting of
oxides. The shaft sleeve itself may be contributing. Stainless steel ss316 or ss304,
continually polished by the action of the packing replaces its surface of chrome oxide
instantaneously, oxide which is taken up into the packing material. The build up of oxides
on the surface of the packing changes the nature of the gland dramatically.
Our engineer has adjusted the gland plate and reduced the flow
of fluid leaking out of the pump gland. The packing set has deformed to reduce the leak
path. The deformation is not uniform. The action of the gland plate is to provide a force
directed along the shaft which has to be translated into a radial force to effectively
deform the packing. The friction at the outer edge of the stuffing box possibly
supplemented by vulcanization of the packing material with the metal surface of the
stuffing box, prevents the packing from sliding easily. Consequently, the first two rings
of packing, experiencing the most force, are unable to transmit the axial force evenly
down the length of the stuffing box and invariably this results in an over-tightening of
this area of the gland in order to effect sufficient pressure throughout the gland.
Combine the over-tightening of the front end of the gland with
the oxide impregnated gland packing and we are beginning to re-shape our shaft sleeve. But
there is more to come.
As the rest of the packing set is adjusted by the overtightened
first two rings the lantern ring is gradually pushed down the shaft. The packing pieces
between the neck ring and the lantern ring are squeezed allowing the lantern ring to move
further until in extreme cases it is cut off from its fluid supply. The gland is failing
fast. Cut off from its coolant the gland can now overheat, causing rapid failure. Often
before this occurs a partial repacking of the gland has taken place. New packing pieces
have been put into the gland replacing the badly worn first three rings. But their life is
limited because the rings placed into a worn stuffing box need to be deformed to
accommodate the increased radial width of the stuffing box, and the cycle continues until
the shaft sleeve is destroyed. .
The purpose of the lantern ring is to provide a balancing
pressure within the packing set and to allow the cooling water to flow evenly around the
gland. The pressure within the gland is greater than the pump suction pressure but less
than the discharge pressure of the pump and is easily calculated from
SP+ (DP-SP)/4
Where
SP = Suction pressure
DP = Discharge pressure.
Fluid is taken from a tapping in the volute casing
and piped directly into the lantern ring. This is a convenient pressure source readily to
hand and self contained within the pump unit but consider the action of the pump impeller.
Rotating at high speed the impeller acts as a very efficient centrifuge. Any dirt
particles entrained in the fluid will be flung to the outer limits of the volute casing,
leaving the less dense fluid clean until the streams re-unite at the impeller throat on
their way out of the pump. As all the particles of dirt are at the periphery of the
impeller clean fluid exists at the impeller center.
The stuffing box pressure is greater than the
suction pressure of the pump, but less than the volute pressure. The state of the fluid
within the volute casing at the back of the impeller is relatively clean having been
centrifuged by the spinning action of the impeller. To prevent contamination of the gland
is therefore, a simple matter of reversing the flow of fluid through the gland, using the
volute casing pressure to produce a flow back through the gland to the suction side of the
pump. Leakage will be controlled in the same way as before but the gland, being supplied
with clean fluid, will no longer be subject to contamination to the same degree as before
and a longer interval between adjustments and replacements of the gland packing can be
expected.
|
Pump
Tutorial CDROM $19.95
