Gland Packing Material

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. The 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 packing for dynamic applications, it is good to compare the dynamic load factors – pV and not only the permissible operating pressures

The pH Level
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.

Shaft Speed
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 the higher thermal resistance of the packing itself. Also, homogeneous materials such as expanded graphite and PTFE are better at 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.

Condition of Equipment
Attention should be paid to any deficiencies in 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 are of key importance for the proper functioning of the packing and make 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.

Flushing System
The design of the gland itself, such as the presence of spacing rings, flushing systems, and 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.

Economic Issues
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 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 characteristics, which predestine it for specific applications and make some even irreplaceable material. Another issue in the construction of packings is impregnates, their task is to seal pores and improve sliding properties and heat dissipation. The most effective impregnates are PTFE dispersion and graphite, which can occur in various forms, such as flake graphite or ready-made dispersions in oil. Other popular but less effective impregnates are typical lubricants such as petroleum jelly, oils, and paraffin.

Optimization Factors
The type of application must be taken into account when selecting a packing or 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.