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MARINESHELF

Tuesday, October 23, 2012

VALVE ROTATION

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              gives more even seat and face temperatures.

              reduces effect of small leakages as it prevents single area  overheating, this gives less  chance of concentrated corrosion and erosion.

              minimises build up on sealing faces.

              ideal for engines burning heavy oil



There are two main methods currently in use, either vane rotators or mechanical rotators.



Vane rotators



              Simple method  with no moving parts.

              May cause back pressure and turbulence in the gas duct.

              Possibility of inducing vibration

Rotacaps (mechanical valve rotators)

 Operation of Rotacap

              with valve in closed position bellville washer is loaded against body.

              as valve opens washer is compressed and load is transferred to ball.

              further increase in pressure and ball moves along ramp causing rotation.

              as valve closes the washer bears against the body relieving the pressure on the balls and the ramp springs return them to original position.

 A typical profile gauge for checking  seat wear and valve flap burn down. The gauge normally clamps onto the valve stem for datum position.





Valve cooling

              reduction in temperature especially advantageous with HFO.

              reduces distortion.

              reduces stem temperatures allowing smaller guide/stem clearance.

              allows use of cheaper materials and reduces need for hardening.

              more necessary on large valves than small since more heat to be removed even though temperatures are similar.

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