Equipment Reliability Improvements in the Pulp and Paper Industry: Part 2 of 3
Uncontrolled Water Leakages
Following on from Part 1 in this series, which discussed the issues and solutions surrounding effective bearing lubrication, we move on to another direct cause of Mean Time Before Failure (MTBF) – uncontrolled water leakage from the stuffing box.
The pulp and paper industry routinely uses high flush water rates to overcome the inadequacies of gland packing and mechanical seals. Flush in / flush out is common. However, not only is this expensive and common in today’s plant operations, but it is also wholly inefficient.
Let’s look at the effects of high flush consumption:
- Much of the flush water leaks from the stuffing box and ends up in the bearing housing, and this causes the bearing to fail prematurely, leading to unreliable equipment.
- The flush water mixes and dilutes the pumped product. This routinely leads to reheating to evaporate the flush water – a significant and unnecessary annual cost.
- Flush water expelled from the stuffing box enters the drainage system and must be treated at the plant’s wastewater facility. This has a negative monetary and environmental impact.
The solution to these issues is to have more effective sealing systems in place. Successful solutions have already been designed, tested, proven and are in use. So why aren’t more plants taking advantage?
The answer isn’t a simple one, but the following are some of the major reasons:
- An unawareness of what’s on offer.
- The mistakenly perceived time and cost to upgrade.
- An attitude of ‘this is how it’s always been done’.
All these reasons are generally due to misinformation. So, let’s investigate the advances in technology that are changing the landscape of equipment reliability in the pulp and paper industry.
Solutions to prevent water leakage from stuffing boxes
Effective methods of preventing unnecessary leakage include:
- Improving the mechanical packing: Advanced fibres and new construction methods reduce the speed of degradation. Couple this with a stuffing box environmental control device and you can drastically, or even eliminate, the amount of flush water required.
- Using advanced seal technology that can run flush-free: The use of single or split mechanical seals with an environmental controller device can remove the need for a flush in a wide variety of mill applications.
Let’s look at both in more detail.
Many factors influence the longevity of packing. Design, use, and maintenance are key – and virtually all of these can be improved using different fibres, knitting techniques, and enhanced installation methods.
Conventional flushed packing systems have many flaws, with their inefficiencies impacting the performance:
Figure 1 – Conventional flushed packing system
- The distribution of the gland load transfer to radial load between each ring is inefficient after the first 3 rings (Figure 1).
- The level of sealing (radial load) decreases dramatically as the compressive load decreases.
- The final 2 rings have minimal radial load. This allows the pulp to leak between the packing and sleeve, causing a grinding effect on the sleeve and damaging both (Figure 3).
- After a few tightenings of the gland bolts, the lubricants are washed away from the last 2 rings. This volume loss requires further gland tightening to maintain proper loading on the bottom rings.
- The above adjustments increase gland load, creating a higher radial load on the top rings. In turn, this causes sleeve damage, packing glazing, or burning, (Figure 2), while resulting in the dislocation of the lantern ring, potentially obstructing the flush flow completely.
- The gland load transfer becomes inefficient and is drastically reduced past the lantern ring.
- This results in insufficient radial load, allowing stock to penetrate beneath the packing and cause premature wear.
Figure 2 – Conventional flushed packing system after a few adjustments
Figure 3 – Damaged sleeve
Installing packing in this way isn’t only maintenance-heavy but also leads to a dramatically reduced MTBF.
The solution is to use advanced fibre technology with new braids and special weaving techniques, as well as environmental controller devices, along with different installation methods.
Research into the traditional stuffing box geometry (3 ring/lantern ring/2 ring – with the final 2 rings closest to the process fluid) has shown that the final 2 rings not only provide no sealing performance but might actively be a cause of sleeve damage and lantern ring alignment. Moving to a 3 ring/lantern ring configuration improves sealing and reduces flush water usage (Figure 4).
Figure 4 – Using an environmental controller can further reduce flush water needs
Furthermore, replacing the lantern ring with an environmental controller, such as the EnviroSeal SpiralTracTM further reduces flush water needs. It also reduces the level of solids that reach the packing.
The result? Improved packing life and increased MTBF.
While a seal’s performance is affected by a variety of factors depending on the process, the most common factors are a result of the following:
- Paper stock dries and solidifies on the seal face due to frictional heat, starving it of lubrication.
- The traditional soft face/hard face combination can’t cope with any abrasives in the pumped fluids, reducing seal life and increasing leakage.
- Seal face imbalance and designs that place seal springs inside the process fluid reduce the seal’s ability to cope with solids and reduced lubrication. This imbalance generates increased friction and a resulting raised temperature. Internal springs then become clogged and fail prematurely.
- Fibreline and liquor processes in particular, produce cavitation, vibration, and ‘off curve’ running, due to system upsets and stop/start operation, resulting in seals to constantly adjust. This causes excessive wear and reduced MTBF.
The most common fix is to use a flush. However, this is only as reliable as the external supply system for flush water – and we’re looking at ways to reduce the need for flushing.
The key is to use a third-generation mechanical seal that avoids the seals drying out, offers effective heat dissipation, and reduces abrasive wear. This, combined with an environmental controller, such as the SpiralTracTM, manages all of this.
SpiralTracTM removes the abrasives from the stuffing box and provides additional cooling to the mechanical seal. It also doesn’t require a flush unless the consistency is over 3%. Even so, if this is needed, only very small amounts are needed – typically saving 70%-90% on traditional flush requirements.
Increasing seal reliability and reducing flush needs are paramount requirements for plants to reduce their overall costs and meet environmental standards. Significant gains can be had by making these relatively small adjustments.
Chesterton is at the forefront of this technology – find out more at https://www.chestertoncustomseal.com.au/partners-chesterton-products/mechanical-seals/
In Part 3 of this series, we’ll tackle system energy savings through the application of the latest advanced techniques.