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Pump packing is an age-old sealing technique that originally involved the use of fibres and old ropes to pack around the shaft to reduce leaks. Modern packing products are rather more advanced yet still work on the same basis.
Pump Packing Basics: How it Works
Today’s packing materials are made of various fibres that provide it with structure and the strength it needs to withstand fluid leaks. To further aid this, blocking agents fill the gaps between the fibres, and they often play a dual role and act as lubrication to lower friction between the packing and the pump shaft. This allows the stuffing to meld itself to the shape of the stuffing box.
The installed packing then has axial pressure exerted by the gland follower, which is converted to radial force in the packing rings.
This creates friction between the packing and shaft and exerting constant pressure that pushes the lubrication out of the packing. This action provides the lubrication necessary for continual efficiency. However, the amount of lubrication has a finite lifespan – once used up, the packing must be replaced.
Effective packing has five distinct requirements:
- Strength: The pressures exerted on the packing are large. If it’s not strong enough, the packing fibres can be forced through the minute gaps at the bottom of the stuffing box or working their way under the gland. Other wear can occur from the rotating shaft, shaft sleeve or a leaking medium.
- Resiliency: Whatever the type of seal (flange, mechanical, gasket, O-ring, lip seal), it will consist of at least a contact face and an elastic element.
- Thermal conductivity and temperature resistance: Frictional heat builds up as the shaft operates. This causes wear to all elements of the packing. The better its thermal conductivity, the less this happens. The integrity of the shaft or shaft sleeve is also subjected to friction and wear. Therefore, reducing friction is vital for the longevity of both asset and packing.
- Sealability: Leaky packing equals inefficient operation. The whole purpose of packing is to create a seal, so it must be flexible and malleable enough to conform to all the irregularities of the stuffing box. It must also be non-porous – an aspect that’s addressed with blocking agents and the addition of other particles to decrease porosity further.
- Chemical resistance: Without this, packing materials will prematurely wear. This makes it essential to use a packing material with a lubricant suitable for the pH rating of the liquid / chemical that’s being sealed.
Because of the many different packing requirements, traditional packing has been a balancing act to address various needs. This results in a trade-off between resilience and friction. The use of aramids provides the robust elements necessary but creates a high level of friction. Conversely, PTFEs are low friction but not as tough, meaning they can consolidate and extrude.
In severe applications, such as with slurries, this has historically meant regular repacking and the use of flushes to ensure efficient operation.
Pump Packing in 2021: DualPac® Advantages
In 2016, Chesterton sealing and rotating machine specialists made a significant technological breakthrough that effectively addressed this issue. By utilising a novel braiding technique, they combined two fibres, exploiting the positive aspects of both aramids and PTFE.
The patented DualPac® 2212 Packing combines the thermal resistance, sealability and shaft-friendliness of meta-aramid with the resiliency and strength of para-aramid. This has resulted in a unique packing solution that uses an innovative braiding technique and provides packing with completely different fibres on both sides. This stands out from traditional methods that typically have both materials present on both sides.
DualPac offers multiple advantages and has been proven in many real-world situations. For example:
- A European paper plant had tried many packing options on its three centrifugal pumps, including a commonly used heat resistant, strong synthetic fibre that burned out within eight weeks. Installing DualPac® 2211 Packing has seen successful pump operation for over eight months with minimal shaft wear.
- In Mexico, a gold mine sealing a cyanide pump with 60% solids was only achieving packing longevity of 3-5 days with constant adjustments. Their aim was for 14 days of continuous operation before having to renew the packing. Installing DualPac® 2211 Packing increased the MTBF to 25-35 days, with failure being more equipment-related than packing.
- A tars sand mine in a remote location operated a tailings pump that was frequently leaking. The packing gland was being adjusted 8-10 times per 1000 hours of operation. Changing to DualPac® 2211 Packing equated to only a single adjustment per 1000 hours.
- A Bulgarian power plant with abrasive slurry was experiencing a two-week MTBF and constant adjustments due to leakage and excessive sleeve wear. The shaft sleeve was being replaced every three months. Switching to DualPac® 2211 Packing increased the packing life to 3 months and the sleeve to 12 months.
The savings made and increased efficiency after installing DualPac® 2211 Packing technology has been welcomed in a wide variety of applications.
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