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Steam Engine Slide Valve

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Howard_S

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I am building a version of Elmer Verburg's MF4 slide valve. I will not be using steam only compressed air. Verburg states "Try for freedom on this part so it will just barely float on the seat as steam pressure holds it down". Doesn't the exhaust pressure counteract the pressure in the steam chest leaving the valve to not seat well, since in his design the valve nut slides freely on the valve? Other designs I have seen the valve rod holds the valve close to the Valve plate.
I should point out that I have been at machining for 6 months so I am a beginner.
 

f350ca

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The back side to the valve is always open to the exhaust port, which is at atmospheric pressure. Hence you always have a pressure differential between the steam chest and atmosphere to hold the valve seated.

Greg
 

T Bredehoft

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Welcome, Howard, I applaud your involving your self in a steam engine, They are very interesting projects. Lots of finicky pieces.
 

Howard_S

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The back side to the valve is always open to the exhaust port, which is at atmospheric pressure. Hence you always have a pressure differential between the steam chest and atmosphere to hold the valve seated.

Greg
Greg
Thanks for your thoughts, but isn't the pressure actually higher under the valve? When the piston 1/2 inch diameter and 1 inch stroke has to push the air out through a 1/16 inch diameter bore to the valve where the volume under the valve is tiny compared with the piston volume of air, so the pressure should be quite high. This is my simple thinking and I may be wrong so please tell me where I went astray.
Howard
 

f350ca

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See where your coming from Howard, but the force pushing the piston to exhaust the air is coming from the higher pressure air on the other side of the piston. That pressure is the same or slightly less due to flow restrictions as the air in the steam chest thats pushing down on the valve and the area under the valve is still ported to atmosphere.
If you tried to drive the engine with say a motor, then yes the pressure on the exhaust side would be higher than the steam chest and the valve would unseat.
Got me thinking! If you hold your finger over the exhaust outlet and turn the crank, they won't build pressure, you can't feel compression. Your reasoning is exactly why, the air coming out of the exhausting side of the cylinder lifts the valve and the air comes around to the intake side.
Compound engines, where the exhaust from the high pressure cylinder is feeding the low pressure one would create a whole different ball of worms that would be way above my pay grade. lol

Greg
 

Howard_S

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See where your coming from Howard, but the force pushing the piston to exhaust the air is coming from the higher pressure air on the other side of the piston. That pressure is the same or slightly less due to flow restrictions as the air in the steam chest thats pushing down on the valve and the area under the valve is still ported to atmosphere.
If you tried to drive the engine with say a motor, then yes the pressure on the exhaust side would be higher than the steam chest and the valve would unseat.
Got me thinking! If you hold your finger over the exhaust outlet and turn the crank, they won't build pressure, you can't feel compression. Your reasoning is exactly why, the air coming out of the exhausting side of the cylinder lifts the valve and the air comes around to the intake side.
Compound engines, where the exhaust from the high pressure cylinder is feeding the low pressure one would create a whole different ball of worms that would be way above my pay grade. lol

Greg
Greg
Thanks for your patience, I finally have the concept and agree. thanks
 
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