The water problem was overcome by cylinder cocks that would open automatically like safety valves at higher than boiler pressure.

Another difficulty with slide valves was lubrication at higher temperature superheated steam. The older oils available didn't work real well with slide valves but were easier on piston, according to some old timers I have talked to. I do notice that the retrofit of piston valve conversion kits seem to coincide frequently with the addition of superheaters to old saturated engines - the 2-8-0's at Georgetown Loop are a great example.

Another possible benefit is the inside admission and outside exhaust - easier than dealing with steam chest gasket problems. Also, rings are easily made and replaced and cages bored much more readily and accurately than milling the valve surfaces flat on the engine.

Dave

irondave@bellsouth.net

A piston valve has another advantage, increase in valve passage volumn. Think of a piston valve as a sliding D valve rotated around a spindle. A large D valve would have a great deal of friction due to its large surface area. There would be far less machanical resistance and thus more efficiency with the self balancing piston valves.
Tom