GLAND PACKINGS SELECTION GUIDE
Gland packings are a traditional and still popular way of solving dynamic sealing constructions. This type of seal damps leakage between the moving part and the housing and is essential for the efficiency of mixers, pumps, valves and other industrial equipment in rotary motion. For decades, the sealant was simply a plant fibre saturated with grease placed in a gland between the pump casing and the rotating shaft. The advantage of this solution is still simplicity of construction and gradual signalling of wear and tear by increasing leakage.
Modern pumping systems have an extremely wide range of materials to meet the high requirements in terms of chemical resistance, shaft speed, pressure and temperature. Which packing is best suited to meet the user's expectations under certain operating conditions? What will be the most economical? And what is the possibility of long, uninterrupted work? This guide will briefly outline today's high-performance sealants that need to be considered for most applications, as well as the selection criteria that need to be applied to make the best possible decision.
Main selection criteria
There are four main criteria for the selection of packing: chemical resistance or pH level, shaft linear speed, pressure in the flow system, medium temperature. The packing is designed to radially compensate the gap on the rotating shaft, thus controlling the leakage of the medium and preventing the loss of valuable process fluids and unnecessary emissions into the environment. An additional criterion to consider is the rotational movement of the shaft. The rotation of the shaft generates significant amounts of heat due to friction and can cause excessive wear of the packing box components. It is important to select the type of packing that can withstand the required working conditions and is chemically resistant to the working medium.
Temperature – Tmax
In the case of dynamic seals, consideration of the Tmax – maximum temperature is essential for the performance of the sealant. Most materials have a positive temperature coefficient of friction, which means that when a critical temperature is exceeded, the friction starts to increase exponentially. This phenomenon usually leads to overheating and burnout of machine parts and, in the case of seals, to grilling and loss of sealing properties. Temperature of the medium is also not the same as the packing temperature, as the gland operates at a higher temperature due to the friction. A safety margin of at least +50°C is assumed for centrifugal pumps operating without an additional gland cooling system. A good solution is the use of materials resistant to burn-out, based on graphite and carbon fibres. The temperature issue can also be solved structurally by an additional sealant flushing system. However, it does not change the fact that the selection of packing in terms of temperature range is of key importance and its omission may lead to immediate failure of the device.
Pressure and the pV Coefficient
The soft structure of the sealing material does not degrade under the influence of only just factor, i.e. pressure. In dynamic sealants, i.e. pump and valve gland seals, sealant wear is usually caused by friction. This friction is the result of the simultaneous action of movement and pressure in the working gap. With some approximation we can assume that the rate of wear of the 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 for a given material. The pV factor is the product of the pressure and linear velocity that can occur simultaneously in a seal using this material. The pV parameter characterises the sealing materials in terms of abrasion resistance and does not generally specify the permissible pressure ranges, which depend to a large extent on the design of the glands themselves, but indicates at which pressures p and shaft speeds V the sealing material will have a comparable service life. Therefore, when assessing the suitability of a packing for dynamic applications, it is good to compare the dynamic load factors – pV – and not only the permissible operating pressures
The pH Level
In order to ensure that the packing does not degrade due to the chemical action of the pumped medium or fluids used for flushing or cleaning the system, the maximum pH range to which the packing may be exposed must be determined. As we know, fibre materials, especially natural fibres, rapidly degrade under the influence of corrosive liquids, i.e. in acidic or alkaline environments. Therefore, determining the pH range to which the seal will be exposed is crucial for its durability. It is good practice to use materials with a full pH range of 0 – 14, which excludes natural fibres, of course, but still leaves a lot of material space to choose from and guarantees a large margin of safety.
The movement between the shaft and the packing can lead to wear in two ways; by abrasion of the sealant material and by the generation of friction heat and thus thermal degradation of the packing. These phenomena intensify with the increase in VL linear velocity of the shaft; therefore, it is of key importance to determine the working conditions of the packing. Most manufacturers of pumps and equipment specify the shaft speed in rotary motion, determining the number of revolutions per minute (rpm). To calculate the VL linear velocity in the working gap between shaft and packing, use a simple formula: shaft diameter x 3.14 x rpm; to obtain an SI compliant result in m/s the values in metres should be used for the calculation and the result needs to be divided by 60.
The heat of friction is generated when the seal is pressed against the outer surface of the rotating shaft sleeve to form a seal. The higher permitted VL linear velocity of the packing is due to its lower friction coefficient and good impregnation, as well as higher thermal resistance of the packing itself. Also, homogeneous materials such as expanded graphite and PTFE are better in maintaining the lubricating film on the surface of the sleeve than fibrous materials. In the case of valve glands, the VL linear velocity parameter is not very important, while in pumps, especially centrifugal pumps, it is crucial for the service life of the seal. Therefore, when evaluating packing for pumps, special attention should be paid to this parameter, the better it is, the longer packing operation can be expected. It can be assumed that gland packing intended for centrifugal pumps should meet the condition of the VL linear velocity at the level of 15 m/s.
Unfortunately, these are not all the criteria that need to be considered before choosing the right gland packing. There are still important issues to consider in relation to the condition and design of the equipment to be sealed, hygienic conditions, the risk of contamination of the medium and economic issues.
Condition of equipment
Attention should be paid to any deficiencies of the mechanical parts. The most common problems are sleeve wear and run-out, but the gland must also be inspected for surface condition and corrosion. The condition and smoothness of the sleeve surface is of key importance for the proper functioning of the packing and makes it possible to achieve satisfactory tightness even at lower pressure of the gland, which has a key impact on the level of friction in the working gap, temperature and rate of packing wear.
The design of the gland itself, such as the presence of spacing rings, flushing systems, cooling or barrier liquid systems, is also an important issue in the selection of packings. These systems protect the seal against abrasive particles, reduce friction and temperature and thus extend the seal life several times over. The use of a higher- performance packing also saves a significant amount of flushing water used in these systems. Glands with a spacing ring can even operate without leakage, provided that the intermediate pressure is properly selected and graphite packing is applied from the atmospheric side.
The variety of materials that can be used in the construction of packings is enormous, as is the huge spread of prices of these materials. Packing made of hemp rope saturated with grease can be 10 times cheaper than packing based on high carbon fibres. Undoubtedly, success is ensured by the right choice of packing in terms of working conditions, but also it is necessary to have knowledge and own experience of how individual solutions work in a given application. More expensive material will not guarantee longer seal life, the basis is to carry out a practical test. Certainly, the calculation should take into account the costs of downtime and packing replacement, which can often exceed the cost of purchasing even the most expensive gland packing.
Base components of sealants
Of the great variety of sealing materials, only five basic ones are really important for the construction of packings: natural fibres, flexible graphite, PTFE, aramid and carbon fibres. These materials cover more than 90% of the market demand. Each of them has its own characteristics, which predestine it for specific applications and make some even irreplaceable material. Another issue in the construction of packings are impregnates, their task is to seal pores, improve sliding properties and heat dissipation. The most effective impregnates are PTFE dispersion and graphite, which can occur in various forms, as flake graphite or ready-made dispersions in oil. Other popular but less effective impregnates are typical lubricants such as petroleum jelly, oils, paraffin.
Cotton, ramie, flax and hemp are natural plant fibres, traditionally used in gland seals. A particularly good solution is the combination of ramie natural fibre with PTFE impregnation. This allows us to achieve excellent mechanical properties, resistance to abrasive wear and at the same time elasticity and low friction. Such packing ensures tightness at lower pressures of the gland, but it is neutral and can be used in hygienic conditions. Unfortunately, as well as other organic substances, natural fibre is easily influenced by biological and chemical factors and the temperature range is very limited.
Operating parameters: Tmax 120°C / pH 5 – 9 / VL 6 m/s
Characteristics: natural and ecological material with low cost.
Areas of application: Packing made of natural fibres is most often used for clean water, in circulation pumps, brine where the temperature is limited and there is no significant risk of chemical influences, in particular these are pumps and compressors with a water ring, sealing of boat drive shafts, hydroelectric power plants turbines.
Flexible graphite is produced from flake graphite by intercalation and expansion of crystals. Then, the expanded mass is rolled into a thin film or formed into a yarn, which is the basis for the production of packings and sealing strips. Flexible graphite inherits the best features of its precursor, such as very good thermal and chemical resistance, thermal conductivity and self-lubricating properties, as well as gaining softness and flexibility, which make it a predestined material for use as a sealing material. Graphite packings are fully resistant to heat shocks and the risk of overheating of the gland, guaranteeing low friction in all conditions, even after the liquid lubricating film has disappeared and other lubricating additives have worn out. Therefore, for graphite packing the maximum linear shaft speed is practically unlimited, limited only by the pump design.
Operating parameters: Tmax 450°C / pH 0 – 14 / VL 40 m/s
Features: thermal resistance and low friction.
Areas of application: Graphite is best suited for the packing of dynamic centrifugal pumps for water, petroleum derivatives and other chemicals, especially under high speed and high-pressure conditions. Graphite is also the basis for seals for energy fittings in the high pressure and temperature range.
PTFE is one of the plastics with the highest chemical resistance, withstands strong oxidants, ozone and UV radiation and at the same time has a high thermal resistance and a particularly low coefficient of friction. For the production of sealing materials, PTFE is used, obtained by suspensive polymerization of two types: white version, 100 % pure and black version, incorporated with graphite. PTFE strips are expanded and linearly oriented and then formed into elastic yarn, which is the basic raw material for braiding packings of different profiles. Pure, white PTFE is intended for hygienic applications and has FDA approval and, more importantly, the EU Health Quality Certificate 10/2011 for contact with foodstuffs, required on the European market. Graphite PTFE provides better sliding conditions and better heat dissipation, thus guaranteeing a longer service life and can be used in a higher range of both shaft pressure and shaft speed.
Operating parameters: Tmax 260°C / pH 0 – 14 / VL 15 m/s with graphite 25 m/s
Features: chemical resistance, low friction and easy assembly.
Areas of application: Gland seals for pumps, plunger mixers in the chemical, pharmaceutical and food industries, where sterile conditions or hygienic approvals are required. In the case of graphite PTFE, they can be considered as a universal packing with a very wide range of applications, with the exception, however, of highly contaminated, abrasive or easily crystallising fluids.
Aramid is a synthetic fibre with a characteristic yellow colour, excellent strength and abrasion resistance. Aramid is primarily known for its use in ballistic shields such as helmets and bullet-proof vests, which is why it is used in sealing technology for liquids contaminated with solid particles, abrasive and easily crystallising media. Unfortunately, the aramid fibre easily wipes the shaft's protective sleeve, so it must be impregnated with lubricant and high efficiency and durability in dynamic seals. Another solution are hybrid braided packings with good sliding properties such as PTFE and the use of a hardened protective sleeve of the shaft with hardness of 50 – 60 HRC. Hybrid weave is dominated by three designs: corner weave for piston pumps and valves, zebra weave for centrifugal pumps and mixers, and zebra corner weave for universal use or as extreme closing rings to protect the packing from the influence of solid particles.
Operating parameters: Tmax 260°C / pH 3 – 12 / VL 10 m/s
Features: high mechanical resistance.
Areas of application: Aramid packing is used in sealing of the most polluted or crystallising media pumps, for abrasive media in sewage pumps, hydrotransport and drainage systems as well as in the food and paper industry. Aramid rings are the ideal solution for closing rings in sets of packages with soft packing. In such kits, the outer rings protect the entire package from abrasive particles, reduce friction and mprove seal life.
Carbon fibre, which is still gaining in importance in technology, is light and durable, and above all, it is resistant to high temperatures and abrasion, which is crucial in the construction of dynamic seals. Carbon fibre is produced by pyrolysis and carbonisation of carbon-rich precursors, followed by refining and graphitisation to achieve a high strength and purity of more than 98%. The properties of carbon fibre are similar to graphite, but thanks to its fibrous structure it is possible to braid it and its strength is many times higher than other fibrous materials. What is more, the carbon fibre has a positive temperature coefficient of strength, which means that as the temperature rises, the strength increases and the sliding properties improve. This material successfully withstands 900°C in a reduction environment, so despite its high costs, it is still gaining in importance in sealing technology.
Operating parameters: Tmax 600°C / pH 0 – 14 / VL 20 m/s
Features: thermal resistance and high strength.
Areas of application: Repair and service of industrial and power fittings, where it works perfectly as a substitute for pre-formed graphite GDR rings and can provide the highest standards of tightness, including API standards. Carbon fibre packing is also used for closing rings in sets with graphite and to seal media pumps with a strong dynamic effect, where chemical and thermal resistance is additionally required.
The type of application must be taken into account when selecting a packing or, in fact, when optimising this choice. In this case, the specific operating conditions of the equipment such as: high shaft speed in centrifugal pumps, large friction surface in piston pumps, high pressure in valves or high radial stresses in mixers should be considered. There is no universal sealing material, but the specific design features of packings predestine them for specific applications, such as the presence of metal reinforcements in valve sealants or aramid reinforcements in abrasive sealants. The technical data sheets give the operating parameters taking into account the specificity of a given application, which allows you to quickly assess which packing should work best in which applications.
The size of the gland packing offer is overwhelming, many items from this offer have their replacements and the prices of seemingly the same material can vary considerably. This situation is not only due to the difference in costs of production and sales. Apparently, the same materials or rather the same fibres may differ diametrically in the internal structure, which will affect the performance and usefulness of the seals made of them. This is particularly true of expensive fibres. Carbon fibre is a very capacious term and can be described as both advanced fibres with high strength and thermal resistance, as well as carbonized acrylic fibres, whose strength does not necessarily exceed the level of natural fibres.
Undoubtedly, one of the most important factors of optimization is to have appropriate knowledge and the ability to use it. Let's list and compare the most important operating features of materials used for packing glands in the table. Eliminating the next fields of this table, we can narrow down the selection to the last column, which allows us to assess the potential cost of its purchase. Of course, we can create such a selection table based on other criteria and for any analysed materials.
The most commonly used type os gland packings
Despite the very wide range of products in offer, most suppliers offer at least 20 to 30 types of packings, in fact only a few of them are of real market importance. When looking for an optimal solution, one should first consider the most frequently used ones, according to the Pareto 80/20 principle, that only 20% of the studied objects affect as much as 80% of the resources. In our case, these 20% of facilities are only 5 types, which in the quantitative assessment cover more than 80% of the demand for highly efficient packings.
Graphite gland packing Grafopak GRA 450
Packing made of graphite PTFE. This material combines all the best features of a sealant 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 and easy to use sealing cord, its market share is 20%.
PTFE gland packing Tefapak PUR 200
White packing made of high purity PTFE. This material may not have as good sliding properties as graphite PTFE but it is neutral, does not cause any risk of colouring and above all ensures hygienic conditions. Tefapak PUR 200 is FDA approved and certified for EU 10/2011 health and is the most popular sealant in hygiene applications with a market share of around 15%.
Graphite gland packing Inkograf IGP 600
Expanded graphite packing of high purity, reinforced with Inconel® wire and equipped with a sacrificial anode system for protection against galvanic corrosion. This sealant is actually a metal-graphite composite with a structure resistant to high pressures and temperatures at the same time. Inkograf IGP 600 packing is used exclusively for sealing high-pressure fittings, its market share of all sealants is 8%.
Aramid gladnd packing Arampak XG 340
Aramid packing, braided with graphite PTFE with a characteristic yellow-black strip pattern. Arampak XG as well as ZG, AG, and XP is a special series of packings with high resistance to abrasive media and highly contaminated. Hybrid aramid weave of PTFE improves sliding properties, flexibility and heat dissipation, while maintaining high strength. Arampak XG 340 packing for abrasive media has a market share of about 8%.
The possibilities offered by modern advanced sealing materials should be exploited. In industrial equipment, they usually perform better than mechanical seals, both in terms of operating costs and reliability. The soft seal glands are characterized by simplicity of construction, ease of assembly and minimal risk of sudden failure. Users have the ability to select the right sealing material and optimize the solution depending on pressure, temperature and type of working medium. Despite the seemingly outdated idea, gland packings have a number of advantages, because the wear of the sealant is gradually signalled, equipment with increasing leakage can still work, maintenance can easily predict the necessity and timing of replacing the packing, the replacement operation is fast and the packing to be replaced can be quickly delivered or is in stock at home.
Quality and performance should be taken into account – this is the last advice for users who have to choose the right material for the packing. This simple principle reduces risk and saves resources. Although it is necessary to incur a higher initial cost associated with the purchase of a better material later we save on its operation. But the main benefit lies in reducing the risk of failure and reducing the frequency of packing replacement. As we know, the operation of packing replacement is switching off the device from traffic, which involves much higher costs than the cost of purchasing the packing itself.