Save Billions of Gallons of Water and Increase Uptime with our Double Mechanical Seals

Water is becoming more scarce and more expensive. Still, large quantities of water are essential to most mineral mining and extraction processes. Accurate water-balance planning over the life of the mine, and reduction of overall water consumption, are critical to a mine’s success.?

This article presents a technology for sealing slurry and process pumps that is proven to save millions of gallons of water per pump per year, while also increasing the MTBR (Mean Time Between Repairs) of the equipment with an ROI (Return on Investment) that is typically 6 months or less.

Water Pumps and Sustainability

The growing scarcity of water on a global level, its fundamental importance to the mining industry, and the need to reduce water use have been stated by many experts such as Mohamed ElBoradei (Head of the IAEA)1, and companies such as Newmont Mining2, Barrick Gold3, Rio Tinto4, and Potash Corp5. Planning, sourcing, permitting, pumping, tracking, reporting, and disposing of process water consumes a significant portion of most mines’ operating budgets, especially those that operate in arid regions of the world.

Eliminating water consumption from any part of the process is a worthy consideration for most mine operators. One area to consider for major reductions is the supply of gland water to the packing-on process and slurry pumps. The mining industry is one of the most arduous and expensive industries for the maintenance of rotating equipment. Not only must it deal with abrasive and corrosive applications, but it also must accommodate historical “run-to-failure” maintenance practices and the difficulty of operating in remote locations.

A common mining industry misconception is that the only way to achieve a reliable seal on these tough pumping applications is through the use of gland packing. However, gland packing goes hand in hand with high water consumption, high maintenance costs, poor equipment availability, and large production losses.

Mechanical seals were introduced to the mines a few decades ago, however, end-users were not properly educated and for years have been supplied the wrong seals, wrong materials, and wrong (or no) seal support systems. This contributed to the general misconception that mechanical seals do not work in the mining industry.

Mechanical seals DO work in the mining industry!

Not only do mechanical seals work, but they are also collectively eliminating billions of gallons of wasted gland water each year, while simultaneously improving the MTBR of the pumps. Thousands of double mechanical seals are now operating successfully around the globe in some of the most remote and difficult phosphate, platinum, gold, potash, copper, and other mineral extraction operations. This has been made possible by simply following the golden rule of sealing: “Maintain a stable fluid film”.?

Problems with Packing

Gland packing has been the traditional method of sealing pumps for nearly 100 years. Packing is generally available, relatively low-cost per unit, and most mechanics are familiar with its use.?

However, there are some inherent drawbacks:

• Requires large amounts of gland water for cooling and lubrication. (A typical slurry pump requires 12 gpm (gallons per minute) of gland water, which equates to 6 million gallons per year
• Requires more energy (than a mechanical seal) to turn the shaft
• Wears quickly and requires a high level of maintenance
• Damages the shaft sleeve, due to friction, requiring frequent sleeve replacement
• Sprays gland water or process liquids directly onto the pump’s bearing housing when it leaks, resulting in premature bearing failures
• Causes corrosion that requires frequent repainting from gland water leaking on the pump and pump base

A Partial Solution: Single Mechanical Seals

The key to successful sealing is to maintain a cool, clean, and stable fluid film between the faces. When a single mechanical seal is used, the liquid being pumped (the pumpage) becomes the fluid film. This works fine when the pumpage is a clean liquid such as water. However, when the pumpage is a slurry, the abrasive nature of the slurry can quickly damage the seal faces and result in component failure.

A single mechanical seal incorporates two flat faces, one stationary and one rotating, running against each other with a fluid film between them providing lubrication. Without a stable fluid film between the faces, they would be in full contact known as “dry running”, which would lead to rapid heat build-up and component failure. In this case, an external flush of clean liquid (typically water) can be injected on the process side of the single seal to force the slurry away, and surround the seal faces with a cool, clean liquid.?The primary drawback of this arrangement, known as API Plan 32, is the injection of water into the process at a higher pressure than the stuffing box pressure. This is problematic on tailings pumps in series, where the final discharge pressure can reach several hundred psi, and special pump systems must be installed and maintained just to supply this high-pressure flush water.

The typical flush injection can waste several million gallons of clean water per year.?If the process is hot and the injected flush water is cool, large amounts of energy must be added to raise the temperature of the flush water. If the process is sensitive to dilution, even more energy must be added to evaporate the flush water from the process.

The Complete Solution: Double Mechanical Seal and Support System

All the drawbacks of both gland packing and single seals described above can be eliminated with a properly chosen 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 the atmosphere, with a barrier region in between the faces.

A properly designed seal support system 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. Thus, there is a pressure differential that 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 is known as a “thermosiphon”, and it enables the tank to provide the mechanical seal with a constant supply of fresh, cool, clean, pressurized water for the fluid film, with no moving parts!

A well-engineered solution will cut seal water consumption (per pump) from an estimated six million gallons of water per year with a traditional flushed seal or packing, to less than 10 gallons per year, for a 99.999% reduction in seal water usage.

Summary: Double Mechanical Seals Reduce Mine Water Footprint and Improve Uptime

Packing has several drawbacks when used to seal rotating shafts on pumps. Perhaps the biggest drawback is the requirement for millions of gallons of gland water per pump per year, for cooling and lubricating the packing.

Double mechanical seals and tank systems eliminate all the problems associated with packing and can greatly reduce a mine’s water footprint, while also reducing the manpower required to care for the packing and increasing the uptime/availability of the equipment.?In those cases where the process is sensitive to dilution, double mechanical seals can save millions of dollars per year in lost products.

Not just any mechanical seal arrangement will accomplish the above goals. The pump owner must select a robust double mechanical seal and then maintain a clean, stable fluid film across the seal faces. This is accomplished by using a self-filling, maintenance-free tank support system that maintains the seal barrier fluid pressure at 15 to 30 psi over the pump fluid pressure.

Thousands of double mechanical seals and tank systems are in operation worldwide in Mining and other industries, with a total estimated savings of over 25 billion gallons of water each year.

Note:?1 billion, as used in this article, refers to one thousand million, or 109.