Stern tube seals from Cedervall & Söner are proving effective at preventing water ingress along the propeller shaft. Following positive feedback from users, the company has extended its guarantee on its latest design, the SeaQual seal, from one to three years.
According to managing director Ingemar Lundgren, seals have been installed on 250 vessels, including platform supply ships, geophysical ships, and shuttle tankers. Demand has been particularly strong for offshore newbuilds - the company’s latest such order is for four new supply vessels for Swire Pacific of Singapore.
SeaQual has a fully split design, which means it can be fitted and dismounted easily without the propeller shaft having to be removed. This is particularly advantageous for applications involving controllable pitch propellers such as supply vessels, Lundgren points out. Larger vessels can be trimmed to allow retrofitting while afloat with the shaft above water. For smaller vessels, a diving team can perform fitting and inspection of the seal underwater.
Cedervall & Söner's SeaQual stern tube seal has helped extend intervals between propeller shaft surveys.
Like other Cedervall seals, SeaQual is designed to tolerate axial and radial shaft movements. Its most significant feature is a built-in pressure equalizer. This takes the form of rubber bellows that compensate for pressure pulses along the shaft in the seal area, ensuring that the stern tube is pressurized only by static pressure from the gravity tank.
Such pressure pulses can be very high, and over time can have a damaging effect on conventional seal performance. When shuttle tanker operator Knutsen OAS, one of Cedervall’s leading offshore customers, performed detailed measurements on one of its vessels, it found pressures of up to 4 bar were being exerted on the stern tube seals, compared with only 1 bar from the static tank. Knutsen has installed SeaQual seals on 18 shaft lines and is considering more retrofits, according to Lundgren.
All major classification societies have approved the seal. Lloyd’s and DNV have approved a 15-year interval for shaft surveys on vessels using the seal, offering potential for major cost savings, Lundgren says. The company itself gives owners a guarantee that they will not need to remove the shaft for seal-related reasons for 15 years if they use SeaQual. It is in talks with various parties concerning whether shafts fitted with SeaQual could be left in place for up to 30 years without the need for dismantling.•
For more information, contact Ingemar Lundgren, Cedervall & Söner. Tel +46 31 928 412, fax +46 31 928 401, [email protected], www.cedervall.com.
