Exhaust Valve Explained

B & W exaust valve
Two stroke cross-head engine have a single exhaust valve mounted in the center of cylinder head. Exhaust valves open towards inside in the cylinder,such away that the gas pressure inside cylinder ensure positive closing & helps to avoid  building up carbon on the valve seat.The valve opening and closing is controlled by cam mounted on the camshaft.
Modern 2 stroke cross-head engines work on hydraulically operated air spring  exhau.st  valves. Now a cam operate hydraulic pump instead of the push rods in earlier versions. The pump which displaces Oil from the engine LO system operate a piston in the exhaust valve which push valve to open. Here in placeof the mechanical spring there is  an “air spring”. Air at 7 bar through a non return valve goes  to the underside of the piston attach to the valve spindle. when valve open, air underneath is then compressed. When hydraulic pressure is released, the expansion of the compressed air assist in the closing of valve.
Air also taken down to guides of the exhaust valve, this not only keep it cool but also prevent exhaust valve from leaking.
The valve spindle fits with the valve rotator. The kinetic energy in the exhaust gas rotates the valve by a small degree  when it  pass. This helps to keep valve at  even temperatures & so help reducing the build-up.
Exhaust valve consists of a valve housing and a valve spindle. The valve housing is of cast iron and arranged for water cooling. The housing is provided with a bottom piece of steel with stellite welded onto the seat. The spindle or valve stem is made of heat resistant steel with stellite welded on to the seat. The housing is provided with spindle guide or valve guide. The exhaust valve housing is connected to the cover with studs and nuts tightened by a hydraulic jack. The exhaust valve is opened hydraulically and closed by a set of helical springs.
The exhaust valveis subjected to hot gases, and the temperature resistance of its seat and body is therefore crucial. In new designs, Nimonic (Nickel,Chromium,Cobalt)valves combined with proper seat cooling have yielded excellent service behavior and long life times. When exhaust valves are not centered in the combustion chamber, heating will not be symmetrical on the valve lid. For this reason, an automatic rotating device may be fitted causing the valve to rotate slowly, thereby avoiding localized heating.
Material Properties:
  • High tensile strength and creep resistance properties.
  • Resistance at high temperature properties.
  • Resistance to high temperature corrosion.
 Design Characteristics: 
  • Bore colling for efficient cooling system i.e as low as 327 degree Celsius.
  • Valve rotational mechanism.
  • Heat and corrosion resistance material used.
  • Hydraulic push rod with controlled valve landing speed.
  • Air spring
  • Guide bush is sealed by pressurized air.

Author marineGuru


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