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History of the mechanical seal (sponsored whitepaper)

19th August 2021 - 15:30 GMT | by Industry Spotlight

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In the early 1900s - around the time that naval vessels were first experimenting with diesel engines - another important innovation was emerging at the other end of the propeller shaft line.

This whitepaper is brought to you by Wärtsilä.

Across the first half of the twentieth century the mechanical seal became the standard interface between the shafting arrangement inside the ship’s hull and the components exposed to the sea. The new technology offered a dramatic improvement in reliability and lifecycle compared to the stuffing boxes and gland seals that had dominated the market.

The development of mechanical seal technology continues today, with a focus on enhancing reliability, maximising product lifetime, reducing cost, simplifying installation and minimising maintenance. Modern seals draw on state-of-the-art materials, design and manufacturing processes as well as taking advantage of increased connectivity and data availability to enable digital monitoring. 

Wärtsilä Shaft Line Solutions (SLS) has been at the forefront of this advancing technology for decades. Nowhere is its continuous investment in product quality more evident than in the evolution of the Wärtsilä Enviroguard M seal, the most widely used and highest performing mechanical seal across naval applications worldwide. Wärtsilä’s Enviroguard M seals have now been updated to further improve performance and value for naval customers. Highlighting the new features and developments shows exactly how far this technology has progressed.

Before Mechanical Seals

Mechanical seals were a remarkable step forward from the previously dominant technology deployed to prevent seawater from entering the hull around the propeller shaft. The stuffing box or packed gland features a braided, rope-like material that is tightened around the shaft to form a seal. This creates a strong seal while allowing the shaft to rotate. However, there are several disadvantages that the mechanical seal addressed. 

Friction caused by the shaft rotating against the packing leads to wear over time, resulting in increased leakage until packing is adjusted or replaced. Even more costly than repairing the stuffing box is repairing the propeller shaft, which can also be damaged by friction. Over time, the stuffing is likely to wear a groove into the shaft, which could eventually throw the entire propulsion arrangement out of alignment, resulting in the vessel requiring dry docking, shaft removal and sleeve replacement or even shaft renewal. Finally, there is a loss of propulsive efficiency because the engine needs to generate more power to turn the shaft against the tightly packed gland stuffing, wasting energy and fuel. This is not negligible: to achieve acceptable leakage rates, the stuffing must very tight. 

The packed gland remains a simple, failsafe option and is often still found in many engine rooms for backup. Should the mechanical seal fail, it can enable a vessel to complete its mission and return to dock for repairs. But the mechanical end-face seal built on this by boosting reliability and reducing leakage even more dramatically. 

Early Mechanical Seals

The revolution in sealing around rotating components came with the realisation that machining the seal along the shaft – as is done with packing – is unnecessary. Two surfaces – one rotating with the shaft and the other fixed – placed perpendicular to the shaft and pressed together by hydraulic and mechanical forces could form an even tighter seal, a discovery often attributed to engineer George Cooke in 19032 . The first commercially applied mechanical seals were developed in 1928 and applied to centrifugal pumps and compressors2 . 

The US company John Crane was the first to apply mechanical seals to vehicles. In 1949, the company invented the first automotive mechanical seal. Wärtsilä acquired John Crane-Lips in 2002. Although the John Crane name did not transfer with the business, Wärtsilä gained several important brands, notably the Deep Sea Seals mark which is still known and respected in naval and commercial shipping. The US headquarters of John Crane was eventually to play a crucial role in the rise of Wärtsilä’s Enviroguard M seal when the company launched an ambitious push to serve the US Navy. As US legislation requires products to be supplied by an American company, John Crane served as a launchpad into the world’s biggest navy1 . 

But the American expansion only came in the early 1980s. Long before then, as early as the 1950s, many of the conceptual designs and application guidelines that are in use today had been developed. Commercially available designs included both rotating and stationary flexible elements, balanced and unbalanced hydraulic loading, rubber and metal bellows, and a wide variety of spring designs and types. Various secondary sealing elements are available and the evolution in seal face materials has given rise to highly specific surfaces dependent on the application2 .

The Wärtsilä Enviroguard M seal

The history of Wärtsilä’s best-selling seal series, Enviroguard M, exemplifies the modern evolution of the mechanical seal in marine applications. Originally developed in the 1970s and at first known simply as the M-Series seal, the latest iteration of the range shows how several crucial features – face material, cooling/ flushing and condition monitoring – have developed over the years into today’s state-of-the-art stern tube seal. 

Face material 

The materials used for the stationary and rotating surfaces that press together to form the primary seal are among the most important considerations for mechanical seals. A balance must be struck between the cost of materials and their longevity, to prevent wear-down over time. Where water replaces oil lubrication, the face material needs to be able to cope with higher levels of abrasion, especially in waters containing sediment or other contaminants. This is particularly important for naval vessels operating in challenging environments, including coastal and tidal waters which may carry a high sediment load. 

 Having some abrasion resistance is paramount to improving durability and ensuring that seal faces remain responsive to the wear in the opposing face and thus maintain a tight seal. If the face material is too hard, any blemishes that occur to either face – for example if the face is damaged by sediment or by impact - will result in the seal leaking excessively. But with the right materials, the faces will wear just enough to maintain a tight fit however much the surfaces change over the lifetime of the seal. 

In the late 1980s, the face material used in the Wärtsilä Enviroguard M series was changed to remove asbestos from the composite. That material, known as Manetex, was used for many years until the new design. Today the faces are made from an aromatic polyamide or ‘aramid’ composite material from the same family as the bullet-proof fibre Kevlar®. This material has been specially developed to enable the seal to perform in all environments, including brown water conditions.

Redundancy 

Though the mechanical seal dramatically increased the reliability of the stern tube, the rise in naval demands over the years has led to further developments for increased reliability and reduced risk in the event of failure. With the Wärtsilä Enviroguard M seals, a back-up inflatable seal has traditionally provided redundancy, preventing water flow between the internal diameter of the stern tube and the external diameter of the propeller shaft in the event of a failure occurring in one of the blocking methods. 

Building on that redundancy, the double inflatable barrier method mitigates against the highly unlikely event of primary seal failure, enabling afloat repair maintenance without cofferdams or divers. The latest double barrier design is easily accommodated, requiring only a slight increase in the length of the seal assembly in the design stage of the vessel, and significantly reduces the risk of compartment flooding and any subsequent vessel damage. 

The first inflatable seal allows for the shaft to continue operating at reduced speed, even after main seal failure for a safe return to port; the second provides extra redundancy. Either of the seals can be used to keep the vessel moving or, when the shaft line is locked, to enable repairs and maintenance to be carried out afloat without the aid of divers. Minor overhauls of worn parts can also be managed in this way, which reduces the cost and complexity of maintenance by eliminating the need for dry dock.

Cooling and flushing 

Flushing is an important element of mechanical seals in naval applications. The primary purpose is to cool rotating components where constant friction would otherwise produce excessive heat and potentially damage the seal elements. In seals that are lubricated with water rather than oil, another important function of flushing is to prevent the accumulation of sediment or organic matter within the seal assembly. This material could cause early wear or even seize up rotating elements if unchecked.

The use of monitoring systems to control the volume and quality of water used for flushing was a recent step in making cooling seals more efficient. The flushing system itself has also evolved across the Wärtsilä Enviroguard M seal’s history. 

In previous versions of the Wärtsilä Enviroguard M seals, water was channelled through the seal housing to the bearings. In the latest incarnation, a significant upgrade is direct flushing to the composite running face. By separating the bearing flush from the water used to flush and cool the seal, abrasive particles are kept away from the running face and more effective cooling is provided. This is another design feature that increases the lifecycle of the seal and minimises leakage. 

Condition monitoring 

Improvements to connectivity along with the rise of affordable sensor technology and powerful data analytic tools, have affected all areas of vessel machinery operation – mechanical seals are no different. While the inspection of seals remains mostly manual, today digital monitoring systems can alert crew – onboard or even onshore – about potential concerns. These functions can dramatically simplify maintenance and troubleshooting. The latest mechanical seals are designed to incorporate these advantages. 

The renewed Wärtsilä Enviroguard M seals can come with optional temperature sensors for the purpose of identifying sealing interface faults. This optimises maintenance scheduling by telling operators how much running time they have left until an overhaul is needed. In many cases monitoring can alert operators of a condition which can be corrected to prevent premature seal failure. This reduces the risk of unexpected downtime and lowers maintenance costs due to the simplified procedure. 

Although condition monitoring is becoming increasingly important, it should not entirely replace a manual inspection routine. Today the Wärtsilä Enviroguard M seals are built with the option for condition monitoring rather than it being offered as standard. But given the general naval demand for improved reliability and minimised downtime – as well as the very low cost of monitoring compared with the potential expense of seal failure – it is an option that is increasingly being adopted by naval customers. 

Meeting naval demands today

The design iterations discussed above show how the Wärtsilä Enviroguard M seals have evolved as technology has advanced. The driving force behind these advances has been the increasingly stringent requirements of naval forces. These demands come in several forms, from safety, redundancy and shock resistance to ease of installation and maintenance as well as, increasingly, the availability of condition monitoring to enable predictive maintenance and early diagnostics. 

Across all areas, Wärtsilä Enviroguard M has been designed to outperform expectations on the most challenging naval applications. That was the ethos when the range was first brought to the market 40 years ago and remains the same today, making it the leading series of water-lubricated mechanical stern tube seals in the marine market. The latest design upgrades position Wärtsilä Enviroguard to maintain that pedigree and continue outperforming expectations, well into the future

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