[modeleng] Re: Flat valves

I'm by no means an expert on this, but I think there are a number of factors at 
play here.
 
Firstly, there's the way slide valves are held down to the port face by steam 
pressure. A 1 foot square valve at 100 psi has 1440 lbs of pressure on it, and 
as a result the friction between the valve and the port face is high. A piston 
valve however is not held onto the port face by steam pressure in the same way. 
It's just the rings in the valve head which are under steam pressure. The area 
of these rings is much less than for the equivalent slide valve, so the the 
friction is less. So for a given steam pressure the friction involved in moving 
a slide valve is higher than that for a piston valve.
 
Secondly, un-superheated steam is likely to be wet. This is as a result of 
pressure drop from the regulator to the steam chest, some of the steam 
condenses. The water droplets help lubricate the valve. With superheated steam, 
even allowing for pressure drop, the steam is dry in the valve chest. Hence no 
help with lubrication.
 
Thirdly, superheated steam is hotter. This increased temperature caused the 
lubricants available at the time to break down. 
 
There amy well be other factors involved here, but I think all these are 
involved. Having seen a slide valve from one of the Fairlies on the Festiniog 
Railway after a lubrication failure at 200psi, you realise quite how much 
damage can be caused by this.
 
John
 
 
 
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