BRUSSELS, (PPI Magazine) -Advances in double mechanical seals and tank systems helped achieve a return on investment of fewer than 12 months for a South African paper mill
Fig. 1- a) Colder water is supplied to the mechanical seal, which is in tur n heated up by the seal faces. This ‘warmer' water then rises back into the vessel, raising the bulk temperature of the vessel, b) The vessel then loses heat to the atmosphere which results in the denser colder water flowing back to the bottom of the vessel and back to the seal faces.
Water is a fundamental and valuable resource within the pulp and paper industry, and makes up a significant proportion of the total cost of running a plant. Aside from the financial implications of using large volumes of water, environmental considerations are becoming more important. Reducing water consumption and accurate water-balance planning over the long term are therefore critical to the success of a business.
The industry is developing at a dynamic pace with growing environmental awareness, changing customer expectations and greater competition all driving innovation around ways to save water. The benefit of investing in mechanical seals for pumps and other rotating equipment is not just about reducing water usage, though. Traditional packing causes shaft damage and leakage, which affects surrounding machinery and means that the equipment has to be regularly stripped down and repaired. Specifying the right sealing solution can reduce energy consumption, too, by allowing the rotating shaft to rotate more freely, compared to traditional packing, which can be like operating a piece of equipment with the brakes on.
Ineffective mechanical seal support
Previous practices of supporting packing and mechanical seal systems using quench-to-drain flushing resulted in the use of large amounts of water, and, in some cases, produced water effluent that had to be dealt with, adding more cost. When using quench-to-drain, the packing is flushed with water to keep the packing lubricated and cool. With mechanical seals, pressurized plant water is taken and injected into the ‘flush' port of the seal in an effort to keep contamination away from the seal faces. The two drawbacks of this approach are a) high water consumption and b) the high cost of removing the injected water by evaporation further along in the manufacturing process.
To put all this into context, typical water consumption can be more than 14 liters (3 gallons) a minute, which totals 7.9 million L/yr (1.7 million gallons) per pump. All these different kinds of quench-to-drain flushing systems, although still commonly used on both gland packing and single mechanical seals, go hand in hand with high water consumption, high maintenance costs and can lead to poor equipment reliability.
Double mechanical seal with continuous loop water management
To address and overcome these issues, the industry has developed a range of modular systems that bring about a step change in mechanical seal support methodology. These systems are maintenance friendly and require no external compressed air or gas pressurization. They are also largely self-regulating and self-operating, and do not require any manual intervention for refilling.
All of the drawbacks of quench-to-drain flushing can be eliminated with this kind of double mechanical seal and support system. A double seal has two sets of faces: one set sealing to the process fluid and one set sealing to atmosphere, with a barrier region in between the faces, Fig. 1.
A properly designed seal support system (tank or ‘seal pot') supplies a clean, cool liquid - usually water - to the barrier space between the seals, at a higher pressure than the process fluid in the pump. This differential pressure forces the clean barrier fluid across the faces, forming a stable fluid film.
As the mechanical seal faces generate heat, the hot water in the barrier zone of the seal rises to the tank. The tank radiates heat to the atmosphere and the cooler, denser water sinks back down to the seal. This process utilizes the thermosiphon effect, and it enables the tank to provide the mechanical seal with a constant supply of fresh, cool, clean, pressurized water for the fluid film. No water leaves the system, which means it instantly cuts water use by as much as 99.9%
These systems are maintenance friendly and require no external compressed air or gas pressurization, and have no moving parts. They are also largely self-regulating and self-operating, and do not require any manual intervention for refilling.
Case study - Mondi Richards Bay
Mondi decided to replace quench-to-drain flushing and lubricating systems fitted to mechanical seals at its Richards Bay (South Africa) mill with AESSEAL®systems incorporating a continuous loop water management design. High water usage, concerns over MTBF and energy consumption had prompted the company to look for a better sealing solution.
The replacement system, incorporating double mechanical seals and continuous loop water management system, has cut water usage at the plant by more than 63 million L/mo, helping to conserve this scarce resource in northern KwaZulu Natal. The replacement design was supplied by Easy Coat, the AESSEAL®agent for Richards Bay. The system comprised of 167 type-SW2 and SW3 water management systems that use recycled water to cool, lubricate and flush mechanical seals in the plant.
The SW2 system uses an integral vessel to store flushing water for continuous recycling. The system is connected directly to the plant water line which becomes its fluid and pressure source. The pressure is adjusted so that the barrier fluid pressure within the system is maintained at 1 bar above that within the stuffing box. This results in a pressure differential that keeps harmful products away from mechanical seal faces and increases seal and pump reliability.
The barrier fluid is circulated to and from the double mechanical seal by the thermosyphon effect which minimizes water wastage and provides more efficient cooling than the once-through quench-to-drain design. The SW3 system is supplied with finned tubing as standard so that it can be used on high heat applications and Mondi now uses a mix of SW2 and SW3 systems according to process product temperature.
Investing in the continuous loop water management system has helped Mondi cut yearly water consumption by over 750 million liters at the plant and achieve a return on investment of fewer than 12 months. There has also been a 95%+ reduction in unplanned stoppages and no incidents relating to the seal system.
It was water scarcity in the Richards Bay region that gave AESSEAL®and Easy Coat the opportunity to present Mondi with a water saving solution. Mondi's own environmental department at the mill supported the proposal because Easy Coat was able to show how it had cut water usage at other Mondi locations over the last 12 years, whilst achieving a MTBF of over six years.
The 167 competitor cooling and lubrication systems to be replaced were consuming an average of 63,210 kiloliters of water every month. With the cost of water calculated at R 2,50 (£0.13) per kilolitre, excluding the effluent treatment costs, water savings of R 158,025 (£8,217) per month will deliver a return on investment in the new AESSEAL®systems in under a year, and saving more than 4.5 billion liters of water in the next six years.
As can be seen in this example, double mechanical seals do work. Not only that, but they are collectively eliminating the use of billions of gallons water each year, while simultaneously improving the MTBF of the pumps.
Rob Adam has been working in the mechanical seal industry for the last 35 years and is currently sales director, AESSEAL® SubSahara Africa, www.aesseal.com.