NWH Nimonic Valves Successfully Cope With Extreme Situtations And Improve Valve Lifetime

For many years the German firm Markisches Werk Halver (MWH) has installed Nimonic valves in medium- speed diesel engines when standard valve spindles were no longer able to meet today's requirements.

Both the trend toward higher peak pressures and the increasing use of residual fuels led to higher mechanical, thermal and corrosive stress as well as to additional strain on the valves due to noncombustible residues in these fuels.

In view of this situation, MWH faced the task of finding a material with such properties that it could stand an increased thermal stress (mechanical characteristics under bigher temperatures) and show a better resistance against corrosion.

In the course of MWH tests with different material combinations, Nimonic proved to be the most appropriate material.

Nimonic is a nickel base alloy with high chrominum content, which is also used for gas turbine parts.

As Nimonic is much more expensive than traditional materials, MWH decided to use bimetal valves for economical reasons. The heatexposed valve head is made out of Nimonic, while the valve stem is of standard valve steel having good sliding characteristics.

MWH proved by means of tests and applications that Nimonic is able to cope with the required extreme situations: hot corrosion at the head could be eliminated and corrosion damages on the seat minimized.

According to MWH, in some engines which are running at high temperature levels, the use of Nimonic valves is absolutely necessary to improve valve service life. Nimonic valves ensure an increase of TBOs (Time Between Overhaul) and consequently, lower operation costs.

The good results achieved with Nimonic exhaust valves on fourstroke engines convinced one of the leading manufacturers of two-stroke engines, Sulzer Diesel AG, to use Nimonic exhaust valves exclusively for the RT/RTA series. The engine manufacturer expects to reach lifetimes of 30,000-40,000 h with these exhaust valves.

For free literature containing full information on Nimonic valves from MWH, Circle 14 on Reader Service Card

Maritime Reporter Magazine, page 31, Sep 1990

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