What is gland packing

A stuffing box used for shaft sealing may be one employing gland packing or using mechanical seals. Easily replaceable without opening the whole pump, gland packing 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 packing 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 Seal

A seal is a device that prevents the leakage of fluid or gas from a system. Gland packing is a type of seal that uses a soft material, such as rope or cord, to create a seal between a rotating shaft and a stationary housing.

The main difference between gland packing and other types of seals is that gland packing does not have a physical barrier between the shaft and the housing. Instead, the pressure of the packing against the shaft creates a seal. This makes gland packing a less durable type of seal, but it is also less expensive and easier to install.

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.

Difference between gland packing vs seal

Gland packing is typically used in applications where the cost and ease of installation are more important than durability. For example, gland packing is often used in pumps and valves. Seals are typically used in applications where durability is more important, such as in high-pressure or high-temperature environments.

Where gland packing is used

• Pumps
• Valves
• Rotating machinery
• Ships
• Pipelines
• Aircraft

Where seals are used

• High-pressure pumps
• High-temperature pumps
• Chemical processing equipment
• Power plants
• Food and beverage processing equipment

 Advantages of Gland packing over Mechanical Seal

Although considered old and low-end technology, the advantages of the gland packing over the mechanical seal 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 packing 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 packing very fast.

• Whipping of the shaft due to worn bearings.

• Consistent cooling water supply in case of pumps with a lantern ring.

What is the purpose of gland packing in a valve?

The purpose of gland packing in a valve is to create a seal between the valve stem and the valve body. This prevents the fluid that is being controlled by the valve from leaking out. A flexible substance that can be compressed to form a tight seal often makes up gland packing. Asbestos, graphite, and polytetrafluoroethylene (PTFE) are the three materials that are most frequently employed for gland packing.

 

Gland packing is used in a variety of valves, including gate valves, globe valves, and ball valves. It is also used in pumps and other rotating equipment. Gland packing is a relatively inexpensive and easy to maintain sealing method. However, it is not as durable
as mechanical seals and can wear out over time.

 

What are disadvantages of gland packing?

Gland packing is a type of mechanical seal that uses a braided rope or cord to create a seal between the rotating shaft of a pump and the stationary housing. It is a relatively inexpensive and easy-to-install seal, but it has some disadvantages,

Disadvantages of gland packing

Leakage: Gland packing is not as leak-proof as other types of seals, such as mechanical seals. This is because the packing can wear away over time and allow fluid to leak out.

Requires frequent maintenance: Gland packing needs to be tightened and lubricated regularly to prevent leaks. This can be a time-consuming and labor-intensive process.

Not suitable for all fluids: Gland packing is not suitable for all types of fluids. It is not recommended for use with abrasive or corrosive fluids.

Can damage bearings: If gland packing is not properly maintained, it can damage the bearings in a pump. This can lead to premature bearing failure and costly repairs.

Higher energy costs: Gland packing requires more energy to operate than other types of seals, such as mechanical seals. This is because the friction between the packing and the shaft creates heat, which must be dissipated.

Lost product due to leakage: Even small leaks of fluid from a pump can lead to lost product and contamination. This can be a significant problem in food and beverage processing plants, pharmaceutical manufacturing facilities, and other industries where product purity is critical.

Hazardous fumes: If the fluid being pumped is flammable or toxic, the fumes from a leaking gland packing can pose a safety hazard.

Why is gland packing used

Gland packing is used to create a seal between a rotating shaft and a stationary housing. It is a type of mechanical seal that is used in a variety of applications, including pumps, valves, and rotating machinery.

Gland packing works by compressing a soft material, such as rope or cord, around the shaft. The pressure of the packing against the shaft creates a seal that prevents fluid from leaking.

Gland packing is a relatively inexpensive and easy-to-install sealing method. However, it is not as durable as other types of mechanical seals and requires more frequent maintenance.

What type of  gland packing used in the ship

Gland packing in a ship is a type of mechanical seal that is used to prevent water from leaking around the propeller shaft. It is made of a soft material, such as rope or cord, that is compressed around the shaft. The pressure of the packing against the shaft creates a seal that prevents water from leaking.

Gland packing is a vital part of a ship’s propulsion system. If the gland packing fails, water can leak into the shaft seal and cause the propeller to lose power. This can lead to a loss of steering and propulsion, which can be a serious safety hazard.

Gland packing is typically made of a braided material, such as flax or hemp, that is impregnated with a lubricant. The lubricant helps to reduce friction between the packing and the shaft and prevents the packing from becoming brittle.

Gland packing should be inspected and replaced regularly to ensure that it is in good condition and preventing water from leaking. The frequency of inspection and replacement will vary depending on the type of packing and the operating conditions of the ship.

Commonly Gland packing used in ships

Flax: Flax is a natural fiber that is strong and durable. It is also relatively inexpensive.

Hemp: Hemp is another natural fiber that is strong and durable. It is also resistant to water and chemicals.

Graphite: Graphite is a synthetic material that is slippery and resistant to wear. It is often used in combination with other materials to improve the performance of gland packing.

PTFE (Teflon): PTFE is a synthetic material that is slippery and resistant to chemicals. It is often used in high-pressure applications.