PACKING INSTALLATION INSTRUCTIONS
Glands with soft packing are traditional and still primary means of sealing industrial equipment in rotating motion. They are characterized by simple design, low manufacturing cost, ease of assembly and disassembly, and minimal risk of sudden failure. Users may select suitable sealing material and optimise the solutions to match pressure, temperature and type of service. Despite its seemingly outdated concept, soft packing has a number of advantages, since in the majority of cases its wear-and-tear accumulation is currently signalled, a device with an expanding leakage may continue to operate, the service can easily foresee and schedule packing replacement, the replacement is quick and downtime short, the replacement packing can be quickly delivered or is in own stock. Furthermore, soft packing’s service life has radically increased through the use of high performance flexible graphite, PTFE and aramid based materials, and its durability is currently equal to that of mechanical seals, while still superior to them in economic terms. Soft packing’s durability is even greater, because in modern pump gland solutions packing protection systems are used that consist of setting a lantern ring and providing a flushing fluid inside the sealing package thereby relieving the seal and preventing penetration of solid particles. More advanced solutions include also a particle deflector, autonomous system of barrier fluid, lubricating, flushing, cooling or sometimes a seal heating system.
1. SAFETY CONSIDERATIONS
Packing itself contains no substances that under normal use could be harmful to health or the environment. However, during installation security measures should be always taken as appropriate for the device’s operation. Prior to packing installation the device must be switched off and disconnected from the mains, with de-pressurised system, closed valves, and it must be made sure that no moving parts would endanger the operator. If a device operates with hazardous substances, the system should be preflushed, and the operator should use appropriate personal protective equipment.
As regards waste and worn-off soft package components, it should be taken into account that polymeric materials, including PTFE, aramids, silicones and most of elastomers, are resistant to biodegradation and can last long in the environment. Therefore, this kind of waste must be handled with care, and when dismantled must be disposed of or recycled by the packing supplier. It must never be burned or heated above 340ºC. In high temperatures they may release dioxins, furans, fluorine compounds and other harmful fumes. This also applies to seemingly safe materials, plant fibre or expanded graphite based , which, even if made of safe and natural materials, also may contain harmful compounds captured from the service.
2. DISPOSAL OF WORN-OFF PACKING
When removing worn-off packing, precautions should be taken so as not to scratch or otherwise damage the gland surface. Where service is clean and safe, after loosening a gland the worn-off package is easily pushed out by pressure in the system. In other cases a special extractor with a corkscrew shaped tip should be used. Extractors are screwed in at least two opposing locations in the old package, so that no seizure occurs, and then worn rings are pulled out evenly on the corkscrewing principle. Both when dismantling, as well as installing a new packing package, it’s recommended to use specialized tools to facilitate the work, minimize the risk, and save time and money. Gland should be cleaned of corrosion and the service’s crystallized residues. Shaft or shaft sleeve should have clean and smooth surface without carbon deposits, nicks or cracks, any unevenness and cracks can lead to very rapid wear of the packing. In an extreme case, the shaft should be welded and ground, or the shaft sleeve replaced. Newer packing solutions include a protective shaft sleeve with high hardness, which can be replaced or repeatedly sanded.
3. GLAND INSPECTION
JQuality and condition of the surfaces dynamically mating with packing has a significant impact on the packing wear rate. Because glands without the protective shaft sleeves are already rare, the term “shaft” will mean both a shaft and a shaft with a sleeve. Requirements for the shaft are similar to those for plain bearings, i.e. surface roughness at the fine grinding level Ra≤0.63. Roughness of the remaining elements of the interface with packing does not matter, surface quality at the precise machining finish Ra≤5 level is sufficient. PACKING INSTALLATION INSTRUCTIONSTo avoid excessive shaft abrasion, suitable surface hardness of 60 HRC should be provided. For packing with a low friction coefficient, such as those PTFE based, these requirements are lower, and for flexible graphite the minimum hardness is not required at all, because the shaft is covered with a graphoid film, and friction from the shaft transforms into laminar friction inside the graphite, like in greases. However, any kind of packing can absorb abrasive particles from the service flow and thus indirectly wear the shaft. The gap between the shaft and housing should not exceed the assumed standards, or at least of 0.5mm. For particularly worn-out devices, wherein this value may be exceeded, sealing rings can be used, made of packing with high mechanical strength, or packing reinforced in its corners with carbon or aramid. Shaft runout should be kept in the standard range and not exceed 0.1 mm or 1/100 of the packing width. If shaft runout is larger, it can be prevented by using a packing with elastomer core, which, with its greater flexibility, may take over vibrations off the shaft, and consequently enhance the packing durability despite more difficult operating conditions.
4. GENERAL RULES FOR PACKING SELECTION
Issues related to packing selection for specific operating conditions can be divided into two groups. The first group are the parameters which are treated arbitrarily, such as service type and degree of its aggressiveness pH, temperature range and the right size (in short MTR – service, temperature, size). Their usefulness’ assessment must also take into account the fact that some services can react with, or possibly dissolve, the packing components. These criteria must be strictly observed, otherwise the packing will be destroyed or worn-off very quickly.
When choosing a packing in terms of temperature resistance one should be aware that the allowable operating temperature is not equivalent to the service temperature. Due to friction, a gland operates at a temperature than the service, and thus a margin should be taken of at least 50°C. Where there is high pressure or high rotation, or possibly when gland operates on insufficient leakage, the temperature margin should be even greater.
The second group are the parameters to a large extent relative, such as pressure, flow velocity and application type (abbreviated as pVA – pressure, velocity, application). This parameter group differentiates packings primarily in terms of strength and substantially does not determines the acceptable ranges of application. Soft packing material structure is not destroyed under the influence of only one of these factors, and only their concurrent action causes wear. So it is good, in assessing a material’s suitability for gland packing, to employ dynamic load factor – pV, i.e. product of the velocity and pressure, which might concurrently occur in the application, without causing too rapid degradation. It is a reliable parameter that pretty well indicates at which pressures p and flow velocities V the packing material will retain comparable durability.
Another criterion that determines the packing selection is the type of application. In this case, specific conditions of the device’s operation should take into account, such as high shaft speed in centrifugal pumps, high friction area in piston pumps, high pressure in valves, or large radial stresses in mixers. For most packings the limits are defined of operating parameters for a specific application type, e.g. maximum pressure in centrifugal pumps, in reciprocating, in valves and in static conditions.
Also referred to application type are sanitary requirements for the packing material. In the food, pharmaceutical, and cosmetic industries most of the installations must meet high quality requirements imposed by standards and directives, including European Commission Regulation (EC) No 10/2011 on health standards for materials intended for centrifugal pumps reciprocating pumps valves and disc valves flange or static seals contact with food. In such cases the materials should be used that have been tested on migration into simulants and have a Health Quality Certificate 10/2011. One can be guided by Polish NIH and/or US FDA certificates, or similar, but they are not sufficient criteria to meet current regulations on the EU market.
5. RING PREPARATION
Packing size should be so selected that after its formation the ring can be freely inserted into the gland’s working gap, and at the same time does not leave too much play. In general, device manufacturers indicate the packing size and quantity required for replacement. This also can be easily determined by direct measurement of the gland’s working gap or the shaft diameter and the inner diameter of the stuffing box. Half of the difference between these diameters is the size of the gap. Final packing dimension should take into account a margin for the twist and a slight installation play, generally 10-20% of the thickness, but this depends greatly on the packing type and assembler skills. Most packings are produced in size range from 4x4mm to 30x30mm with the progress of about 20%, giving it ample opportunity to adjust the size because of good flexibility. Whenever in doubt, a size lower in the same line should be chosen, because it is much easier to compensate for excessive installation play by pushing the gland than to repair assembly damage to ill-fitting rings.
In order to better match packing’s profile, it can be slightly rolled out with a cylindrical tool without detriment to its properties, even if the profile will adopt a slightly rectangular shape. It’s good to slightly roll out aramid or carbon fibre based packing to a trapezoidal shape, so that after rolling it in a ring and installing the packaging turn offset the trapeze and aligns the profile. This will cause a greater static pressure on the perimeter, and eliminate free space between packing rolls on the gland’s outside, and most importantly will relive the shaft surface. This assembly reduces sleeve’s wear, accommodates lubricating film on the shaft and consequently prolongs the packing’s service life in the case of application of a packing with high mechanical strength.
Packing rings can be trimmed in several ways: by wrapping the packing around the shaft with the same diameter as the required one, using an instrument with a slider and computing scale, and by trimming sections based on theoretical calculations. Unfortunately, this method is most difficult and unreliable because the length of a section is not calculated by a simple formula, and packing’s after-turn compression should be always taken into account, which varies depending on packing material and structure. A reliable method is to wrap the packing around a shaft, the diameter of which can be easily adjusted to the required one by wrapping a sheet of cardboard or flexible plastic around it. The roll should be slightly larger than the actual shaft diameter, so that the rings’ trim is slightly oversized and consequently the installation play is between the shaft and the package. Packing on the shaft can be easily cut either at rglight angles or obliquely. Cutting at 45° angle is important for better seal of the lock ring, which is recommended for valves. Unfortunately cutting at right angles after rolling the ring does not produce exact contact of the lock surface. To remedy this, after marking the cut location and removing the packing from the shaft, each section should be cut at a slight slant on both sides. Depending on the packing structure and shaft diameter, to obtain proper bevel, the cut should be tilted by ca. 10° to 20° respective to the right angle.
However, the fastest and very precise method of section preparation is to use an instrument with a slider and computing scale. The instrument determines the section lengths based on the calculation slider principle, it takes into account the shaft diameter and packing size and defines cut locations taking into account the bevel for accurate lock setup. Packing should be cut with a sharp knife with a safe handle, by one decisive stroke. In order to maintain the blade’s performance it should be sharpened periodically using tools, the use of which gives rise to small teeth on the blade to facilitate cutting. Many packings include, however, hard fibre or metal reinforcement, and some of them are made of aramid fibres, which are used also for ballistic cloth. For these reasons, cutting them is extremely difficult and normal tools quickly blunt. The ideal solution to this problem is to use a guillotine trimmer for packing, which not only enables easy and precise cutting of every packing type, but also measures section lengths and sets proper cut angle.