TYPES AND PROPERTIES OF GLAND PACKINGS

Glands for soft packings are still a popular way to seal equipment in rotary motion. For centuries, packing was simply a lubricated fiber rope installed in a chamber between the pump body and the rotating shaft. The advantages of this solution are simplicity of construction, gradual signaling of wear with increasing leakage, and, above all, the possibility of adjustment at any time, without interrupting the operation of the device. The basis of this solution is an changeable ring set of braided packing. The variety of packing types can be overwhelming, because most suppliers offer them up to dozens of types. On the surface, this may be considered "form over substance", but given that the packing should be optimized based on several main selection criteria, such a variety of packing types may even be justified. The availability of solutions in this area is very wide. Most suppliers provide immediate supply in the entire cross-section of packing sizes and types. Under these circumstances, the questions impose themselves: what is the significance of the materials and braiding styles? What are the substitutes for these materials? Which solution will be economically optimal?

STRUCTURAL MATERIALS

Only five basic components are the most important for the construction of packings: natural fibers, graphite, PTFE, aramid and carbon fibers. These materials cover more than 90% of market demand. Each of these materials has distinctive characteristics that predestine it for specific applications. Another issue is impregnates, their task is to close pores, reduce friction, improve sliding properties. The most effective impregnates are PTFE dispersion and graphite, which can be found in various forms, as flake graphite or ready to use dispersions. Other popular but not so effective impregnates are typical lubricants, such as oils or vaseline.

1.1 NATURAL FIBERS

Cotton, ramie, flax and hemp are natural plant fibers traditionally used in gland seals. Their important feature is that under the influence of temperature they do not melt but only slowly carbonized which protects the seal from the risk of
sudden failure. The most popular solution is cotton packing with graphite, which significantly improves the sliding properties and resistance of this packing. Also a good solution is to combine ramie fibers with PTFE impregnation. This allows us to obtain excellent mechanical properties, resistance to fast wear and at the same time flexibility and low friction. Unfortunately, natural fiber-based solutions are easily affected by biological and chemical agents.

Operating parameters:

Tmax 120°C / pH 5 - 9 / VL 6 m/s
Features: natural and ecological material with low production and disposal costs.
Areas of application: Natural fiber packing is most often used for clear water, circulating pumps, salt brine where temperature is limited and there is no significant risk of chemical influences, in particular, these are pumps and water ring compressors, seals for boat drive shafts, hydroelectric power plant turbines.

1.2 FLEXIBLE GRAPHITE

Flexible graphite is intercalated and thermally expanded crystalline graphite. This proces give softness and elasticity to graphite flakes, which is a naturally occurring mineral. Flexible graphite inherits the best characteristics of its precursor, such as high thermal and chemical resistance, thermal conductivity and self-lubricating properties. The expansion proces makes it flexible and useful for sealing applications. Graphite packing is completely resistant to thermal shock and the risk of overheating of the stuffing box, and guarantees low friction under all conditions, even after the liquid lubricating film has disappeared and other lubricating additives have been wear and tear.

Operating parameters:

Tmax 450°C / pH 0 - 14 / VL 40 m/s
Features: high heat resistance, self-lubrication and low friction.
Areas of application: Graphite is best suited for dynamic seals of centrifugal pumps for water, petroleum compounds and other chemicals, especially under high speed and high pressure conditions. Graphite also forms the basis of seals for power fittings in the high pressure and temperature range.

1.3 EXPANDED PTFE

PTFE is the material with the highest chemical resistance among materials commonly used in technology. At the same time, it has high thermal resistance and a particularly low coefficient of friction. Two types of PTFE are used for the production of sealing materials: a white version, with 100% purity, and a black version, modified with graphite. Pure white PTFE is designed for hygienic applications and has FDA approval and EU 10/2011 health quality certificate. PTFE with graphite provides better sliding conditions and better heat dissipation, thus guaranteeing a longer service life and can be used in a higher range of both pressures and linear shaft speeds.

Operating parameters:

Tmax 260°C / pH 0 - 14 / VL 15 m/s with graphite 25 m/s
Features: high chemical resistance, low friction and easy installation.
Areas of application: Gland seals for pumps, mixer plungers in the chemical, pharmaceutical and food industries. As for graphitized PTFE, it is a universal sealant with a very wide range of applications, but with the exception of highly contaminated, abrasive or easily crystallized liquids.

1.4 ARAMID FIBERS

Aramid is a synthetic fiber with a characteristic yellow color, excellent strength and abrasion resistance. Aramid is known, first and foremost, for use in ballistic shielding, such as helmets and bulletproof vests, so in sealing technology it is used in the case of for liquids contaminated with solid particles, abrasive and easily crystallized media. In dynamic seals, aramid fiber must be impregnated with a lubricant for high efficiency and durability, or hybrid braided with a material with good sliding properties such as PTFE. There are three predominant patterns in hybrid braiding, they are: corner style for piston pumps and valves, zebra style for centrifugal pumps and mixers, and ladder style for universal applications with high abrasion resistance.

Operating parameters:

Tmax 260°C / pH 3 - 12 / VL 10 m/s
Features: high mechanical strength and resistance to abrasive media.
Areas of application: Aramid is indispensable in abrasive conditions or strong dynamic interactions. Typical applications in pumps for crystallizing media or media contaminated with solid or abrasive particles, such as waste water, hydro-transport, drainage systems, and slurries and pulps in the food or paper industries. Aramid rings are an ideal solution for the closing rings in sets, where they protect the sealing packet against penetration of solid particles and extend the life of the seal.

1.5 CARBON FIBER

Carbon fiber is a black, lightweight fiber with high strength. It is resistant to high temperatures, friction and chemical influences, which is crucial in the construction of dynamic seals. The properties of carbon fiber are similar to graphite but due to its fibrous structure it is more durable and easy to braid. The material successfully withstands 900°C in a reducing conditions, so despite its high cost, it is still gaining importance in sealing technology.

Operating parameters:

Tmax 600°C / pH 0 - 14 / VL 20 m/s
Features: high thermal resistance and mechanical strength.
Areas of application: Repairs and service of industrial and energy fittings, especially closing rings in sets made of graphite die-formed rings – GDR. Carbon fiber packing is indispensable in chemical processes at high temperatures and for sealing chemical media pumps with strong dynamic impact.

1.6 IMPREGNATES

Packings are produced by braiding fibrous materials and in this way flexibility and strength is obtained. The fibers in packing are naturally porous and highly permeable. Various types of impregnate agents are added to seal them. In addition, impregnate also aim to: reduce friction, improve heat dissipation, soften structure
and facilitate assembly. Traditional impregnates are tallow, oils and greases, but these are only used in economical natural fiber sealants. The most effective impregnates are dispersions based on PTFE and graphite. The addition of graphite ensures low friction in critical conditions up to 600°C, increases conductivity and greatly improves heat dissipation from the friction zone. PTFE dispersion guarantees the maintenance of low friction for a long time at temperatures up to 300°C, in addition, PTFE can maintain sterility and causes no risk of staining.

BRAIDING STYLES

The packings are braided in the form of square profiles from many single wefts, it can be up to 36 strands of yarn. Homogeneous sealants are most common, but hybrid braids and various types of metal or aramid reinforcements and cores are also used.

2.1 HOMOGENEOUS PACKINGS

Homogeneous square profiles are produced from 3 mm to 50 mm with a tolerance of ±10%, For small sections up to 6 mm, a twice weave is used, for 8 - 10 mm a triple weave, above 12 mm a fourfold weave. Larger sections, above 20 mm, are made using the multiple braid over braid method. Trapezoidal and rectangular braids are also made for the special needs of customers. Trapezoidal – adaptive braids are designed to compensate the profile after the packing is formed into rings.

2.2 HYBRID PACKINGS

Hybrid sealants are braided from at least two types of material. Most often, these are materials that strengthen the sealant or give them new functionality. There are several types of hybrid weaves, they are corner, zebra, ladder and twoside. Hybrid braid is especially important for aramid packings, which should be combined with a soft material that reduces friction.

2.3 CORE PACKINGS

A popular solution for covers and manholes seals is to add an MVQ elastomeric core to the packing. This facilitates the laying of the packing and causes a radical increase in flexibility. For high-speed centrifugal pumps packings, graphite cores are used to improve heat dissipation from the friction zone and protect the packing from overheating.

2.4 PACKINGS WITH METALLIC REINFORCEMENT

A special class are graphite packings with metal reinforcements intended for power fittings, which are an alternative solution to GDR rings (Graphite Die-formed Rings). Light reinforcements in the form of wires made of high-nickel alloys are usually used, but there is also a special graphite yarn braided with metal mesh – GTR type is used. The second solution is recommended especially for end-rings in worn-out valves, where clearances and inaccuracies exceed the assumed standards. Packings with the addition of metal reinforcements are intended only for valves and static seals.

MAIN SELECTION CRITERIA

Choosing the right type of sealant has a key impact on the reliability and service life of seal operation. There are four main criteria for packing selection, these are: temperature, pressure in the flow system, type of medium and pH, linear shaft speed.
The packing is designed to radially compensate the gap on the rotating shaft, thereby controlling the leakage of the medium and preventing the loss of process fluids and thus harmful emissions to the environment. In addition, the possibility of mutual interference of these quantities and the nonlinearity of certain tribological phenomena must be taken into account. Such as the effect of velocity and pressure on temperature rise, or the positive temperature coefficient of friction for most materials.

3.1 TEMPERATURE TMAX

In the case of gland packing, the maximum operating temperature Tmax is of fundamental importance for the selection of the right material. The temperature of the medium is not the same as the operating temperature, as the gland operates at a higher temperature due to friction. A safety margin of at least +50°C is assumed. A good solution is to use overheat resistant materials based on graphite and carbon fibers. The temperature issue can also be solved by design, through an additional flushing or cooling system for the seal. However, this does not change the fact that the selection of the sealant in terms of temperature range is crucial and its omission can lead to immediate failure of the device.

3.2 PRESSURE AND PV FACTOR

The soft structure of the sealing material does not degrade under the influence of only one factor - pressure. In dynamic seals, which are the stuffing box seals of pumps and valves, sealant wear usually occurs due to friction. This friction is the result of the simultaneous action of motion and pressure in the working gap. With some approximation, we can assume that the rate of wear of a dynamic seal is directly proportional to the product of the linear velocity VL and the pressure of the medium p and inversely proportional to a certain dynamic load factor pV, characteristic of the material. The coefficient pV is the product of the pressure and linear velocity that can simultaneously occur while maintaining the expected seal operating time. The pV parameter characterizes materials in terms of abrasion resistance and generally does not determine the permissible pressure ranges, which depend largely on the design of the stuffing boxes themselves, but it does indicate at what pressures and shaft speeds the packing material will maintain a comparable service life. Therefore, when evaluating the suitability of packing for dynamic applications, it is a good idea to compare dynamic load factors – pV and not just allowable operating pressures.

3.3 TYPE OF MEDIUM AND PH LEVEL

The packing may degrade due to the chemical influence of medium. As we know, fiber materials, especially natural fiber, degrade rapidly when exposed to corrosive liquids, that is, in an acidic or alkaline environment. Therefore, determining the pH range to which the seal will be exposed is crucial to its durability. It is good practice to use materials with resistance in the full pH range of 0 - 14, this excludes natural fibers but still leaves a considerable space of materials to choose from and guarantees a large margin of safety.

3.4 SHAFT SPEED VL

Shaft motion causes wear of the packing by abrasion, but motion and friction also cause a temperature increase that affects its degradation. These phenomena intensify as the linear speed of the shaft increases, so the VL parameter is crucial in determining the working conditions of the packing. Frictional heat is generated when the seal is pressed against the surface of a rotating shaft or protective sleeve. The higher permissible linear velocity VL of the packing is due to its lower coefficient of friction and good impregnation and also to the higher thermal resistance of the sealant itself. In the case of sealants designed for valves, the VL parameter is of little importance, while in pumps, especially centrifugal pumps, it is crucial to the durability of seals. It can be assumed that gland packing designed for centrifugal pumps should meet the condition of linear velocity VL on level of 15 m/s.

SECONDARY CRITERIA

These are not all the criteria that should be considered before choosing the right packing. There remain important issues to consider related to the condition and design of the equipment to be sealed, hygienic conditions, the risk of contamination of the medium, and economic aspects.

4.1 CONDITION OF EQUIPMENT

Note any deficiencies in mechanical parts. The most common problem is bushing wear and shaft run out, but the stuffing box also needs to be inspected for surface condition and corrosion. The condition and smoothness of the surface of the bushing is crucial to the proper operation of the packing and causes satisfactory sealing to be achieved even at lower gland pressures, which has a key effect on the level of friction in the working gap, the temperature and the rate of wear of the packing.

4.2 BARRIER FLUID SYSTEM

The design of the stuffing box itself, such as the presence of spacing rings, flushing, cooling or barrier fluid systems, is also an important consideration in sealant selection. These systems make it possible to protect the seal from the ingress of abrasive particles, reduce friction and temperature and, as a result, extend the life of the seal several times. The use of higher-performance packing also saves a significant amount of water used for flushing in these systems. Spacer ring glands can even operate leak-free, provided the intermediate pressure is properly selected and graphite packing is used on the atmospheric side.

4.3 HYGIENIC CONDITIONS

Drinking water, cosmetics, food, paints, pulp. High sanitary standards in the food, pharmaceutical and cosmetic industries narrow very much the circle of materials permitted for use in sealing. In general, it can be assumed that the basic tips is the FDA guidelines, which allows the use of PTFE and aramid sealants in hygienic applications, but the European market is governed by EU Regulation 10/2011, which requires detailed laboratory testing of migration into model fluids for each type of material separately. Another issue is that of the risk of staining, as in the production of paper or paints and varnishes. In these cases, seals based on graphite, carbon fibers and packings impregnated with graphite, grease, tallow, etc. should be avoided.

4.4 ECONOMIC ASPECTS

The variety of materials means that the price spread of packings can be very large. Aramid fiber packing can be many times more expensive than hemp rope saturated with grease, but the former solution usually provides many times longer service and is more economical finally. Undoubtedly, the right choice of packing ensures success, but you should also have your own experience of how each solution performs in a given application. Not necessarily a more expensive material will guarantee longer seal operation, the basis is to conduct a practical test. Certainly, the cost of downtime and packing replacement should be included in the calculations, which can often exceed the cost of purchasing even the most expensive packing.

OTHER OPTIMIZATION OF FACTORS

5.1 TYPE AND SPECIFICITY OF APPLICATION

When choosing packing and, in fact, for optimizing this choice, the type of application should be taken into account. In this case, the specific operating conditions of the equipment should be considered, such as high shaft speed in centrifugal pumps, large friction surface in reciprocating pumps, high pressure in valves or high radial stress in mixers. There is no universal sealing material, it is the specific characteristics of the packings construction that predestine it for specific applications, such as the presence of metal reinforcements in valve sealants or aramid reinforcements in sealants for abrasive media. In the technical data sheets, the performance parameters are given taking into account the specifics of the application, this allows you to quickly assess which sealant should perform best in which applications.

5.2 QUALITY OF BASE MATERIALS

The range of gland packings is very wide, many items in the range have their substitutes and the prices of seemingly the same material can differ significantly. This situation is caused not only by the difference in production and sales costs. Seemingly the same materials or rather the same fibers can differ dramatically in internal structure, which will affect the performance and suitability of seals made from them. This problem is especially true for expensive fibers. Carbon fiber is a very capacious term, and there can be both advanced fibers with high module and thermal resistance under this term, as well as carbonized acrylic fibers, whose strength does not necessarily exceed the level of natural fibers. Undoubtedly, one should be guided by the quality and performance of the materials. This simple principle allows you to reduce risks and save resources. Although you have to pay a higher initial cost associated with the purchase of a better material, later you save on its operation. But the biggest benefit lies in reducing the risk of failure and the frequency of its replacement. As we know, the operation of replacing the packing, means taking the device out of service, which involves much higher costs than the cost of buying the packing itself.

MOST COMMONLY USED TYPES OF SEALANTS

Despite a very wide range, with most suppliers offering at least 20 to 30 types of packings, there are really only a few that have market relevance. When looking for the optimal solution, it is first necessary to consider the most commonly used ones, following the Pareto 80/20 principle that only 20% of the studied objects affect as much as 80% of the resources. In our case, this 20% is only 6 types, which, in a quantitative assessment, cover more than 80% of the demand for high-performance packings.

6.1 GRAFOPAK GRA 450

Packing made of flexible graphite with currier fibers and inhibitor. It is a typical packing for dynamic applications, excellent sliding conditions, good thermal conductivity, high resistance to temperature and chemical influences. Graphite packing guarantees reliability and good economic performance but requires careful installation and good condition of equipment. Grafopak GRA 450 has the largest share of the packing market, it is more than 30% of all applications.

6.2 TEFAPAK PUR 200

The white packing made of high-purity PTFE. This material, may not have as good sliding properties as graphitized PTFE but for that it is neutral, does not cause the risk of staining and, above all, provides hygienic conditions. Tefapak PUR 200 has FDA approval and EU 10/2011 health quality approval and is the most popular of the packings in hygienic applications with a market share of about 20%.

6.3 TEFAPAK GRF 260

Packing made of graphitized PTFE. This material combines all the best features of packing for dynamic applications, such as low friction, flexibility, resistance in the most desirable ranges, ease and reliability of installation. Tefapak GRF 260 is the most versatile yet easy-to-use sealing rope, with a market share of 15%.

6.4 GRAFOPAK IGP 600

Packing made of high-purity expanded graphite, reinforced with inconel wire and equipped with a sacrificial anode system. This packing, in fact, is a metal-graphite composite with a structure resistant to simultaneous high pressures and temperatures. Grafopak IGP 600 packing is used exclusively for seals in power fittings, so its market share of all packings does not exceed 8%.

6.5 ARAMPAK AP 300

Braided packing made of continuous aramid fibers with a characteristic yellow color. AP 300 packing is impregnated with PTFE dispersion to seal pores, reduce friction and improve flexibility. The aramid fiber is characterized by particularly high mechanical strength, resulting in excellent abrasion resistance. The market share of aramid packing is about 8%.

6.6 ARAMPAK ZG 320

The hybrid, braided packing made of aramid and graphitized PTFE with a characteristic yellow-black zebra pattern. Arampak ZG and also XG, AG, and XP are a special class of packings with high resistance to abrasive and highly contaminated media. The hybrid braiding of aramid with PTFE improves sliding properties, flexibility and heat dissipation, while maintaining high strength. Arampak ZG 320 packing has a market share of 6%, but taking into account other aramid hybrid styles for abrasive media, this share rises to more than 12%.