Sealing options for meeting VGP grow

07 May 2014

Thordon’s Compac bearing system uses seawater as the lubrication medium in place of oil.

A growing range of sealing solutions are available which enable ship operators to meet the US Vessel General Permit conditions; the latest options are described by Wendy Laursen.

Since 19 December 2013, all commercial vessels over 79ft (24m) in length operating in US waters have been required to use environmentally acceptable lubricants (EALs) in all oil-to-water interfaces unless deemed technically infeasible to do so. There are a range of options available for VGP compliance, and Wärtsilä is trying to raise awareness globally about the more environmentally sound solutions that are similarly available for shipping sectors and regions where regulations are not as strict as the US.

The company recently launched its Bio Seal Ring product as an option for compliance with VGP requirements for newbuilds and retrofit. It is claimed to be the first seal on the market that works with EALs for a lifetime of at least five years. Conventional seal rings are recommended to be changed at two and a half year intervals, so the extended operating life expectancy has a significantly positive impact on drydock scheduling and related costs.

Wärtsilä has also announced that its Airguard and Oceanguard propeller shaft sealing systems can continue to utilise mineral oil since they meet VGP guidelines. With these systems there is no oil-to-sea interface. An air chamber or separation space within the seal captures any water or oil leakage, which is then transferred to inboard tanks for monitoring and further treatment. This is said to completely eliminate oil drips or leakage into the sea. The EPA requires these designs to be functioning normally, which can be assured by proper operation and maintenance according to Wärtsilä’s guidelines. In case of system failure, both systems prevent any reasonable possibility of oil leakage, which is the second criterion for the continued use of mineral oils.

In section 2.2.9 of the VGP it is recommended that all newbuild vessel operators endeavour to use seawater-based systems for their stern tube lubrication to eliminate the discharge of oil from these interfaces to the aquatic environment. Wärtsilä supplies a wide range of water lubricated seals and composite bearings and has a huge reference list globally. For operators wishing to utilise water technology, the company offers oil to water stern tube conversions as part of its range of products and services designed to promote environmental sustainability. 

Upgrade and retrofit options are available, says Edwin Magesh, Wärtsilä’s area sales manager for seals and bearings in the Middle East and Asia. “The conversion process involves removal of seals, bearings and pipe work from the vessel. Journals are shrink fitted to the shaft and the shaft is then coated to prevent corrosion. The stern tube is also coated and then water lubricated composite bearings are installed followed by the inboard seal.”

Why convert to sea water lubrication? asks Mr Magesh. “It provides zero oil leakage risk obviously, but there are other benefits. Our system reduces complexity as there is no lube oil required and therefore no lube oil testing required. It also eliminates the need to repair shaft seals following rope entanglement, and it prepares shipowners for possible future legislation and rewards such as reduced port fees as a result of green ship certification.” Ice class solutions are also available with a closed loop water lubricated system with anti-freeze technology included.

Canadian company Thordon Bearings states that it is a concern for shipowners that oil-based EALs are still considered a pollutant under the Oil Pollution Act of 1990 (OPA ‘90) and US Clean Water Act (if there is a sheen). Any discharges of oil-based EALs still require reporting of the discharge to the US Coast Guard, as well as having clean-up and remediation costs.  Even though biodegradable lubricants may be deemed non-toxic by OECD testing, their presence on the water surface is a threat to seabirds – the hydrophobic nature of oil causes bird plumage to absorb the oil readily, thereby decreasing a bird’s insulation, waterproofing, and buoyancy leading to death from hypothermia and starvation. 

According to Canada’s Migratory Birds Convention Act c. 22, 1994.  - 5.1 (1): "No person or vessel shall deposit a substance that is harmful to migratory birds, or permit such a substance to be deposited, in waters or an area frequented by migratory birds or in a place from which the substance may enter such waters or such an area."

Thordon’s Compac bearing system uses seawater as the lubrication medium in place of oil. Seawater is taken from the sea, pumped through non-metallic Compac propeller shaft bearings and returned to the sea. The bearing is an elastomeric polymer alloy designed in a 2:1 L/D configuration in the aft bearing. To promote early formation of a hydrodynamic film between the shaft and bearing, the lower (loaded) portion of the bearing is smooth, while the upper half of the bearing incorporates grooves for flow of the water lubricant/coolant. The Compac system typically includes Compac bearings, shaft liners, a 'water quality package' for removing suspended solids, Thor-Coat shaft coating and a forward seal. The Thor-Coat propeller shaft coating provides 10-year corrosion protection against seawater. This toughened, two-part epoxy coating is claimed to be much more flexible than existing shaft coating products.

Thordon claims that over 600 commercial ships are now using its seawater-lubricated system that uses no oil – meaning full compliance with the VGP. Existing ships can be converted to seawater-lubricated propeller shaft systems as these systems typically fit in the same space as an oil lubricated system. Several companies have converted their existing ships from oil lubricated systems to seawater lubricated shafts saving hundreds of thousands of dollars per year on stern tube lubricants and aft seal maintenance costs, says the company. 

SKF Blohm + Voss Industries manufacturers the Airspace sterntube seal, and the company has received the required third party verification from DNV GL confirming that the Simplex-Compact Airspace seal excludes any possible contamination of sea water under normal operating conditions, and UK distributer Simplex-Turbulo says this means VGP compliance. SKF Blohm + Voss Industries can now make the assurance that a complete stern tube system equipped with an Airspace seal can continue to be operated using mineral oils.

The Airspace seal is claimed to be the only seal on the market that regulates the pressure in the air chamber in order to prevent any air escaping into the seawater, and consequently the Airspace seal avoids the risk of escaping air carrying with it the oil particles which may be present in the air chamber.

IHC Sealing Solutions, part of IHC Merwede, offers the Supreme range of stern tube seals. The bronze aft seal uses one oil and two water-repellent nitrile butadiene rubber (NBR) lip seals to effectively separate water and oil. The forward seal design contains two oil-repellent lip seals and is made of cast iron housing parts. Both seals use a chrome steel liner with an option to use an unique HML fusion layer to significantly increase liner lifetime. Company testing has shown that liner life can be stretched to up to 20 years, depending on operating conditions, and customer experience shows virtually no wear after five years. Supreme seals come with all the regular options available in the market such as net cutters, wire winders, cathodic protection and dirt barriers.

The Supreme SeaGuard builds on the standard solution with the addition of an extra lip seal module. This module serves as a back-up seal in case of leakage. That way, docking and maintenance can be postponed. The Supreme Ventus is also an enhancement on the standard design and has an air system and an internal drain tank to guarantee zero emissions and minimal wear. The air pressure inside the seal is effectively matched with the outside water pressure to minimise wear and avoid leakage, and all fluids that enter the seal are drained into a drain tank.

MAN Diesel & Turbo offers a range of stern tube alternatives for both oil and water lubrication which are tailor made and finalised in cooperation with the shipyard. The standard stern tube is designed to allow thermal expansion/contraction of the stern tube and this decreases the necessity for close tolerances of the stern tube installation length. As an option the stern tube can be installed with a press-fitting and bolted to the stern frame boss. The stern tube is then supplied with 5mm machining allowance for yard finishing. 

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