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DMI Reconditioning of Valve Spindles, Seats and Housings

Total Reconditioning Services Using Advanced Welding Technology
Worldwide Exchange Service With Over 500 Valves and Seats in Stock
12 Month Garantee
Lloyd's Approved Weld Procedures

All current large bore 2-stroke marine diesel engine designs now operate with the "uniflow" scavenge system which incorporates an exhaust valve in the cylinder cover.

The exhaust valve, seat, and valve housing operate in an increasingly hostile environment with new engine designs.

Expensive exotic materials are necessary to combat the high temperatures and their cost is a significant proportion of the price of new exhaust valves.

Reconditioning is therefore a highly cost effective way of prolonging the life of components: but only if the reconditioning is carried out under clearly defined and controlled conditions.

Cost savings obtained by reconditioning can very rapidly be lost by a failure. Inadequate welding technology and techniques can result in disintegration of the valve which could cause catastrophic damage to the cylinder unit.

Higher exhaust gas temperatures and velocities result in erosion of the exhaust valve housing and DMI has established procedures for repairing these and protecting them against further erosion.

The increasing popularity of multiple 4-stroke diesel engines in ferries and cruise ships has resulted in rapidly increasing demand for reconditioning components for these engines. DMI offers a wide range of services including reconditioning of cracked cylinder heads and detachable exhaust valve cages.

2 - STROKE EXHAUST VALVES AND SEATS

In order to withstand the extreme combustion temperatures generated in high performance 2-stroke diesel engines, exhaust valves must be manufactured from sophisticated alloys, containing a high proportion of nickel.

Surface temperatures on the combustion face of the valve can reach 900°c and at these temperatures even exotic materials erode.

The traditional material for the manufacture of 2-stroke exhaust valves is stainless steel with a stellite layer inserted in the seating surface.

Later generation uniflow Sulzer engines are fitted with an exhaust valve manufactured from a solid nimonic alloy forging.

The high material cost of these components makes reconditioning a very attractive commercial proposition provided that it is carried out in a controlled way.

Stainless steel exhaust valves manufactured by OEM licencees in different parts of the world may have variations to the metallurgical specification of the material. Failure to identify the precise material and apply the appropriate weld procedure can result in disastrous failure by cracking resulting in pieces of the valve dropping into the combustion chamber. DMI's weld procedures include careful identification of the material composition before starting to recondition to ensure that the final product has a life expectancy equal to new.

DMI's procedures include building up and re-profiling to original manufacturers' templates of the combustion face and the upper face of the valve by advanced synergic MIG weld equipment. The re-stelliting of the seating face is carried out by the Plasma Transfer Arc process which ensures a high quality weld with minimum dilution of the base material. The valve stem is carefully measured and if any wear exists it is reconditioned using a range of processes including chrome plating or flame spraying.

Nimonic exhaust valves have an extremely long life but are more complex to recondition.

DMI's European factories are unique in Europe in being approved by New Sulzer Diesel to recondition exhaust valves for the RTA range of engines.

Collaboration with New Sulzer Diesel in the early days of development of this range of engines resulted in DMI perfecting a reconditioning technique using plasma transfer arc to build up the seating face of the valve. Wear to the chrome plated stem can be counteracted by removing the existing chromium and re-chroming and grinding to drawing size. Carefully controlled DMI procedures are also applied to reconditioning exhaust valve seats for all types of engines by removing the existing stellite layer, building up the sub-strata followed by re-stelliting and machining and grinding to original profiles. B&W exhaust valve seats can be converted to the later "chamber type" design if required.

EXHAUST VALVE HOUSINGS 2 - STROKE ENGINES
Cold corrosion can occur on the internal surfaces of exhaust valve housings if cylinder temperatures are allowed to drop below recommended levels. A combination of high sulpher fuel, high ambient humidity and low exhaust gas temperatures can then result in the formation of high levels of sulphuric acid leading in turn to severe corrosion of the surfaces of the housing. A DMI repair procedure involves removal of the corroded area by grinding back to sound base metal followed by re-building using specially selected nickel electrodes and re-profiling to original dimensions. Additional protection can be added if required by applying a tin nickel lead coating over the entire inner surface to provide complete protection against corrosive attack.
EXHAUST VALVE CAGES 4 - STROKE ENGINES
Most exhaust valve cages can be reconditioned by one of two means. Cages with damaged seats can be reconditioned by machining back to sound base material on the seat area, building up and reprofiling. Subsequently the seating area is re-stellited and re-machined to original dimensions. Where it is suspected that the internal cooling passages have become blocked an alternative repair procedure is available which involves removing the entire nose of the cage and replacing it with a new steal forging which is welded into place by sophisticated DMI weld procedures. Valve cage reconditioning is available for a wide range of engines including Pielstick, Stork and MaK.
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