Speaking of slide or D valves, I ran across this interesting comment by John H. White Jr. in his excellent book American Locomotives: An Engineering History. 1830 -1880 (p. 203).
"The ordinary D valve suffered from one major defect. That was the rapid wear of the valve seat and the friction generated because of the great pressure exerted on the top of the valve body by the steam. An ordinary locomotive might have a "load" of from 8 to 10 tons on each valve and expend 25-30 horsepower to work each valve. ..."
"According to a test performed by the Central Railroad of New Jersey in 1886, the rapid wear of ordinary slide valves was given as one thirty-second of an inch for every 6,000 miles. ... The same railroad recorded only one thirty-second of an inch of wear for to 65,000 miles when balanced valves were used."
White goes on to say that a lot of patents and designs for "balanced" slide valves (in which steam would not bear down on the top of the valve) were introduced, but none was universally accepted or adopted, partly because of expense, and partly because of technological advances. "Before a balance slide valve had firmly established itself, it was superseded by the piston valve about 1910."
