Gland
Packings
A stuffing box used for
shaft sealing may be one employing gland packings or using mechanical
seals. Easily replaceable without opening the whole pump, gland packings give
stiff competition to mechanical seals, but the packings have to be carefully
selected to be successful.
Gland Packing
Gland packing is used extensively for
the sealing and restriction of leakage of the working fluid along the stem in
valves and along the shaft in the case of pumps and also for stationary duties
like manhole cover sealing. The technology has developed over a period. Originally
old ropes and natural fiber products were used for sealing ... with varying
degrees of success. The technology has now progressed to such an extent that
now several combinations of artificial fibres and cutting-edge materials are
used, and gland packings are even used in harsh applications like
nuclear environments and in the handling of corrosive products.
Gland packing installation is not only
a science but is also an art as its success depends on the skill of the
operator installing it. The life of the gland packing greatly depends on how
they were installed, no matter what grade of packing you use. Great prudence is
required in the adjustment of the gland packing as a minimum leakage is to be
allowed for the cooling and the lubrication purposes. An uninitiated engineer
would invariably over-tighten the glands, leading to the burning of the packing
and scored shaft and shaft sleeves.
Applications of Gland Packing
Gland
packing are used for sealing in the following applications:
·
Stationary applications like tanks hatch cover sealing,
manhole covers sealing, etc.
·
Used for reducing leakage along the stem in globe valve, gate
valve, and ball valves.
·
Used for reducing leakage of the working fluid in
reciprocating pumps.
·
Used for reducing leakage of the working fluid in rotating
pumps like centrifugal pumps, and screw and gear pumps.
· In propeller shaft sealing in lifeboats and on old generations of merchant ships.
All the above are entirely
different applications, and hence the type of gland packing to be used also
differs. Selection also depends on the nature of the fluid to be handled like
temperature, pressure, corrosiveness, and suspended solids, etc. A correct
choice of gland packing has to be made by the marine engineer.
Gland Packing Vs. Mechanical
Seals
There are two types of
stuffing boxes used in the centrifugal pumps and other rotary pumps aboard
ship. One type uses a mechanical seal, and the other type uses gland packing.
Nowadays almost all new ships have mechanical seals on all the rotating pumps due
to very strict pollution laws like Marpol 73/78 and others. Any fluid leakage must
be further collected, treated and filtered, with oil stored for discharge to
shore reception facilities. Water is to be discharged according to the
regulations by approved filtering equipment under 15 parts per million
guideline. It amounts to a lot of work, and any marine engineer would vouch for
that.
However, in certain
applications, gland packing is still used where a mechanical seal may not work
properly. In certain corrosive environments, for example, where a mechanical
seal may corrode and fail, a gland packing may serve better. In applications
like globe, gate, ball valves, and valve cock’s gland packing is used because
mechanical seals require a rotary motion for successful sealing action. It is
for this reason that reciprocating pumps use gland packing. Sometimes a ship
owner may want gland packing to be installed on his ships to cut costs. Also
where skilled man power is not available, it is easier to open the gland to
change the gland packing, than to open the whole pump for changing the
mechanical seal. Also mechanical seals are very expensive as compared to gland
packing. Old ships use gland packing extensively. However, a certain amount of
skill is required to cut the joints and, in their installation, and removal, as
otherwise it is difficult to get consistent results and long life.
The Advantages of Gland packing over Mechanical Seals
Although
considered old and low-end technology, the advantages of the gland packings
over the mechanical seals are as follows:
·
It is an
extremely reliable sealing method.
·
It is very
simple to install and maintain.
·
It tolerates
poor mechanical conditions like off-center shafts and worn-down anti-friction
bearings better than mechanical seals.
·
Works better in
abrasive media and corrosive environments than mechanical seals.
·
Reduces the
stock holding as one size packing can be used in all similar sized pumps.
·
It is very cost
effective in down time as very little time is required to change a gland
packing, as little as fifteen minutes.
·
They are less
expensive than mechanical seals.
·
The whole pump
is not required to be opened for changing the gland packing, however in
mechanical seals the pump has to be opened up.
·
The gland
packings are not fragile, and any amount of mishandling would not destroy them,
unlike the mechanical seals.
Properties of Good Gland Packing
To be successful in its
duty and to attain the objective of successful sealing and trouble free operation
the gland packing must have the following properties.
· Anti-friction
properties. The gland packing basically rubs along the shaft and stationary
along the stuffing box side. If the gland packing has friction, it would score
the shaft or the shaft sleeve and also would heat up and fail and even burn due
to consistent overheating.
· Chemical
resistance to the fluid being contained. If the gland packing reacts with the fluid,
it is supposed to seal it would later disintegrate and be flushed away leading
to leakage of the fluid which is not desirable.
· Temperature
resistance. The gland packing should be able to resist the working temperature
of the fluid being sealed without failing.
· Compressibility
and resilience. The gland packing should be able to compress and confirm to the
shaft under the force of the gland flange and when the tension is released it
should come back to its original shape. The latter is important as when the
gland nuts are loosened the gland packing should spring back releasing the
shaft.
· Retention of
lubricants. All the gland packings are coated with lubricants like graphite,
grease, petroleum products etc., also external lubricant is applied before
insertion. The gland packing should be able to retain this grease for
anti-friction properties.
· Should not score
the shaft. Normally on pumps a shaft sleeve is inserted over the portion where
the gland packings are inserted, and which is replaceable. The gland packing should
be nonabrasive to avoid the scoring on the shaft sleeve. Of course, on a long
interval this is not possible, but it should at least sustain till the next
overhaul
· Should not
contaminate the fluid being sealed. In some applications like food and
pharmaceutical industries the gland packing should not contaminate the fluid,
while in others it does not matter.
·
It should be
non-corrosive to avoid damage to the shaft and the housing.
·
It should be
wear resistant to last a long time.
·
It should retain
its property over a time period.
Different Types of Gland Packing (Materials used)
There are many different types of gland packing
depending on the type of applications such as for valve stem sealing, centrifugal
pump shaft sealing, reciprocating pump shaft sealing, static duties like
hatches sealing and pressure vessels manholes sealing, etc. Gland packings are
made from the following basic substances and are a combination of these basic
materials and are sometimes reinforced with metal wires for extreme conditions.
·
Jute
·
Flax
·
Hemp
·
Cotton
·
PTFE
·
Aramid
·
Wrapped metal foils
·
Graphite fiber
·
Carbon fiber
·
Glass fiber
·
Poly acryl nitrile fibers
All the gland packing are
a combination of the above-mentioned basic materials and great research is done
by the manufacturers to develop new and more efficient types of packing. The
packing made of a combination of graphite, carbon, glass, etc. are used for
harsh applications whereas gland packing made of materials like Jute, flax,
hemp, and cotton are used for light applications like sea water, potable water
etc. The compound PTFE is versatile and can be used for a wide variety of
applications.
Preconditions for a Successful
Gland Packing Job
No
matter how many times you change the gland packing and how well you have
done the job, the success of the gland packing job done would also depend on
the following factors.
· The trueness of the shaft. A shaft that is bent would eat the
gland packings prematurely and there nothing you can do except change the
shaft. You must make it a point to check the trueness of the shaft during each
overhaul using a dial gauge.
· Concentricity of the shaft with the stuffing box bore. The
shaft might be true but if not aligned properly would again destroy the gland
packings in no time.
· Surface roughness of the shaft. If the shaft or the sleeve is
pitted or unevenly worn it would again damage the gland packings very
fast.
· Whipping of the shaft due to worn bearings.
· Consistent cooling water supply in case of pumps with a
lantern ring.