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In November 2010, the Baltic ferry MS Silja Symphony installed PureDry, a new high-speed separator from Alfa Laval with the capability to recover re-usable fuel from waste fuel oil.

Symphony's Chief Engineer Mats Göras said that since commissioning, the PureDry unit has recovered more than 150 cubic metres (cbm) of oil, which has been returned to the bunker tanks for re-use. "With bunker oil at today's prices, this has meant a significant reduction in fuel costs for us," commented Göras, "we have also cut our costs for landing waste oil."

MS Silja Symphony is a 58,377 grt cruise ferry, delivered in 1991 and owned by the Tallink Group. The ship is equipped with four Wärtsilä-Vasa 9R46 diesel engines for propulsion, rated at 32.5 MW, and auxiliary engines rated at 7.3 MW. In service on the Helsinki-Stockholm route, Silja Symphony consumes some 50 tonnes of fuel oil every 24 hours, sailing approximately 50% of the time.

The PureDry high-speed separator was installed onboard in November 2010 and went into full operation in January 2011. Waste fuel oil from settling and day tank bottom drains, fuel oil filters, fuel oil separators and the diesel engines is separated from other waste oils and collected in a dedicated waste fuel oil tank. PureDry recovers the fuel oil fraction from the waste fuel oil and returns it to the fuel oil bunker tank.

Significant savings

Göras is clearly impressed by PureDry's performance. "We produce roughly 8 m3 of waste fuel oil every 6 days. Since the PureDry was commissioned we have recovered more than 150 m3 of re-usable fuel oil and saved a significant amount on the ship's fuel bill.

"To be sure that the recovered oil was within bunkering limits, we sent samples to DNVPS for analysis and they confirmed that it was suitable for use," he said.

The PureDry process is said to reduce the volume of waste fuel oil by 99%, producing 13-15 kg of "super-dry" solids every 24 hours. Göras said: "We land these solids as dry waste, along with oily rags, used filter cartridges, etc. It's not a problem as they don't require any new special means for disposal."

With PureDry there are no oil losses and no additional wastes are generated. The separated water, with an oil content of less than 1,000 ppm, is pumped to the bilge water system.

Savings on waste oil disposal

Göras pointed out also that, in addition to cutting the fuel bill, Silja Symphony has achieved significant savings on waste oil disposal. "We pay 7.5 EUR/tonne to land waste oil in Helsinki, regardless of the mix of oil and water. PureDry has reduced the amount of waste oil landed by about 450 cbm per year."

Locating the PureDry onboard was not a problem - it replaced an Alfa Laval MSPX sludge treatment system that treated all oily waste streams. Göras said he liked the simplicity of the PureDry concept. "The MSPX required much more attention from the crew."

He sums up by saying that PureDry really benefits his operating budget. "We save money on fuel and on waste oil disposal, and very few man hours are required to keep the unit operating. This is the way we would like to see oil separators designed in the future."

Paradigm shift in separator design

According to Alfa Laval, the new PureDry generation represents a paradigm shift in high speed disc stack separator solids discharge design. There is no aperture in the bowl and no sensitive hydraulic system installed to actuate solids discharge.

A patented, spiral-shaped device called the XCavator transports the super-dry solids to the base of the machine where they exit into a container below the machine. There are just two main moving/rotating parts - the separator insert including the XCavator, and the outer bowl shell.

Simple maintenance

PureDry is supplied with an Exchange Kit, which includes a new separator insert (rotor and disc stack), a new XCavator, and a Consumables Kit. After one year, the crew replaces the separator insert as simply as replacing the cartridge in a filter, and the XCavator. The used parts are returned to the nearest Alfa Laval Service Center, and the ship orders new exchange and consumption kits.