Ships Slow Down – Piston Rings, Liners, Corrode Faster

By George Backwell at May 12, 2012 09:27
Filed Under: Company News, General

Ocean-going ships, presently in an excess for the amount of cargo to be lifted, increasingly steam at slower speeds in order to save expensive fuel oil bunker costs; better that than to be laid up reckon shipowners.

Photo courtesy of Maersk Line

The problem is that large marine diesel  engines are not designed to operate below 85% power for long periods without harmful effects; effects best ameliorated by getting lubricating oil of the right specs. After a quick look at lubricating oil solutions, a handy little device to check cylinder performance is spotlighted.

Marine Diesel Engines & Slow Steaming


Experts at Castrol Marine drew on OEM reports and their own engine performance tests to analyse the effects of slow steaming on engine performance, finding that the oil-feed rate as well as a lower engine operating temperature had a bearing on the amount of corrosion caused on piston rings and cylinder liners.

At lower loads, the cylinder oil’s feed rate is reduced, making less available BN (Base Number) constituents to neutralise acids and reducing the protective oil film thickness. This can mean lubricants degrade, increasing the potential for acidic corrosion and increased wear rates. Lower engine operating temperatures that come with slow steaming also further increase the risk of cold corrosion.

The conclusion drawn by Paul Harrold, Castrol’s Technology Manager, Marine & Energy Lubricants: “Higher BN lubricants provide greater neutralisation and hence better corrosion protection across the fuel sulphur range during slow steaming.”

While 40 BN cylinder oil suits vessels predominantly operating in ECAs, cylinder oils of 70 BN and above are better suited to those vessels regularly slow steaming, to ensure piston ring packs and liners remain in good condition.

With slow steaming likely to cause damage to cylinder components it seems apposite to mention a portable, inexpensive gadget made in Sweden  – Prisma Teknik’s ‘Bohman DEC-Tester'  –  which provides a simple, yet useful tool for  assessing cylinder condition.

Bohman DEC-Tester

Prisma Teknik’s ‘Bohman DEC-Tester': Photo courtesy of Prisma Teknik

Air (at standard 6 − 8 bar) is injected into the cylinder via the DEC-Tester, which the manufacturers say is an improvement on similar methods as it controls the pressure as well as the flow of air.

The unit is connected to the indicator valve of a cylinder whose piston has been cranked near TDC and will immediately show a reading on a scale 1 – 10. For example, a recently overhauled cylinder might show a reading of 1.5, another unit with a reading of 7.4, would indicate the need for investigation, while a reading at the top end of the scale normally suggests serious valve problems exist.

The tester can be used on both auxiliary and main diesel engines with a bore diameter of from 160 mm to 460 mm for both continuous follow-up of cylinder condition as well as for troubleshooting.

Worth its 3 kg weight in gold for the solutions it provides, the DEC-Tester comes for just 862 Euros.

 

 

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