A stuffing box used for shaft sealing may be one employing gland packings or using mechanical seals. Easily replaceable without opening up the whole pump, gland packings give stiff competition to mechanical seals, but the packings have to be carefully selected to be successful.
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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 of time. Orginally 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 fibers 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.
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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 life boats 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.
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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 has to 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 a 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 cocks 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.
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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.
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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 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.
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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.
Wrapped metal foils
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.
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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.
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The Lantern Ring and its Purpose
A lantern ring is an annular ring with channels for the passage of water that is inserted between the gland packing. Fluid is fed from an external pipe to the lantern ring for lubrication and the cooling of the packings. It is used in applications where a number of packings are there and it would be difficult of the intermediate packing to get fluid for lubrication. It is also used where the fluid to be sealed is contaminated with abrasives or is corrosive and a clean fluid is required for lubrication. While installation it is crucial that the holes in the lantern ring meet or are aligned with the holes in the pipes. It is very important to inspect and align the lantern ring during each overhaul of the pumps.
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In the next article, "A Guide to Gland Packing Part 2," we discuss how to select a gland packing for different applications, how to cut gland packing, how to remove gland packing, the standard sizes of gland packings available commercially, how to measure the length of the packing required, how much gap should be kept between the ends, how to insert the gland packing in the stuffing box, and how to run in the new gland packings.
A guide to Gland Packing discusses the different uses of gland packings in the engine room, how to measure, cut, remove and insert the gland packing,their properties, selection, the need of lantern rings, the cautions and the precautions in the use of gland packing in Engine room of a merchant ship