Gland packing or mechanical seals? You decide... U

sed to prevent fluid leaking from the shaft region of pumps and other rotating equipment, the significance of seals can easily be underestimated, yet the impact they can have on operational efficiency, maintenance, repair budgets and downtime can be huge. In fact, it’s fair to say that the reliability of a pump rests on the quality of these relatively small components. Advanced engineering has led to a step-change in the design and efficiency of mechanical seals since they first arrived on the market more than 80 years ago – from the highly durable materials they are made from, such as tungsten and silicon carbides, to the engineering techniques they employ. Gland packing is one of the oldest, and arguably outdated, types of sealing on the market today, but its comparatively low cost, and force of habit, means companies across many industries continue to use it. Traditional gland packing, whose

Mechanical seals require no ongoing maintenance, and no closely packed materials means no wear to the shaft or shaft sleeves, which can be so costly to repair or replace

central sealing principle comprises a compressed, lubricated material which is literally packed around the pump shaft to physically stuff the gap, dates back to ancient Greece and the only real advancement has been to replace rope- based packing with more durable synthetic materials, which are better equipped to reduce friction and withstand heat. Gland packing needs to press tightly against the rotating shaft on a pump to achieve the most efficient seal, and the friction this causes gradually wears away the packing, allowing leakages to occur. In an ideal scenario, correctly adjusted gland packing should be maintained with a leakage rate of one drop per minute of sealed product per 25mm of outside diameter of shaft, which equates to 450 litres a month or 5,400 litres a year on a 50mm diameter shaft. The pump shaft itself is impacted over time by

Why do companies still use gland packing when there are so many benefits to using mechanical seals? David Amory of AESSEAL comments

friction, as a groove gradually gets worn into it; and the risk of overheating must also be mitigated, meaning the gland packing must be flushed with large volumes of water to keep it cool. Combining this with the leaking product could realistically lead to 60 drops per minute. The pump may also require more drive power and, therefore, energy consumption, to turn the shaft. Constant maintenance to offset the negative impact of gland packing is therefore a necessity.

MECHANICAL SEALING SOLUTIONS So why do companies continue to choose gland packing over mechanical sealing solutions for their pumps? The most common reasons are the cheaper initial cost and short-term convenience. Yet take a step back to consider the long-term benefits of ‘more expensive’ mechanical seals and the perceived benefits of using gland packing quickly diminish. Mechanical seals have no visible

leakage which, with just 0.002% of water contamination to lubrication oil estimated to reduce bearing life by almost half, should alone make the case for an upgrade. It’s well known that disposing of waste can be more expensive than the original manufacture costs. With properly specified and installed mechanical seals, effluent disposal costs are eliminated, product is protected from dilution and airborne pollution is reduced. Mechanical seals require no ongoing

maintenance, and no closely packed materials means no wear to the shaft or shaft sleeves, which can be so costly to repair or replace.

To understand the scale of return on

investment by upgrading to mechanical sealing solutions, take the example of a global food and beverage manufacturer which was experiencing a MTBF (Mean Time Between Failures) of just two months on a vertical mixer processing starch at its starch and sweetener processing plant in Poland. The gland packing on the mixer

was leaking and causing seal and, subsequently, bearing failure. Costs to the company were almost £5,000 a year in packing replacement and maintenance, in addition to the cost of lost production. The vertical mixer also operated under

high pressure, generating vibration. This, combined with the pressure of the packing, led to premature wearing of the shaft axial. So, seal and seal support systems manufacturer AESSEAL took into account the 800rpm rotation speed, the maximum radial movement of almost 1mm, and the vibration, when designing a new solution. The gland packing was replaced with a cartridge double mechanical seal specifically designed for mixers, where radial shaft movement caused by vibration is common. The inboard seal faces were made from highly resistant tungsten carbide, and the seal was designed to accept radial movement up to 1.5mm, with outboard faces made from tungsten carbide and carbon. Labyrinth style bearing protector seals, which exclude moisture and other contaminants, were also installed. The new seal and support system

eliminated the problems of leakage and wear and were still working problem-free three years later – an increase in MTBF of 1,700%. The company saved £3,450 within the first year on packing replacement and maintenance costs and achieved a return on investment within four months. Total savings to date are £10,300. The figures are unequivocal – and show that by looking beyond the price tag and rethinking ingrained maintenance practices, an upgrade to mechanical sealing solutions can create a seismic shift in terms of operational and financial efficiency.

AESSEAL 24 MARCH 2019 | DESIGN SOLUTIONS 

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