Re: Boiler Pressure *LINK*

Most of the water tube boilers tried in locomotive practice (at least in the USA) had water tube fireboxes, and fire tube boiler shells. The boiler shell acted as a first stage heater, the steam header that would normally go to the throttle went to a header in the water tube firebox for superheating.
There were several problems with water tube and other high pressure boilers:
1. Most of them were one-off examples; such machines tend to spend most of their time in the back of the shop until someone has time to sort them out.
2. There is a limit to the pressure you can can feed a reciprocating steam engine. Going from slide valves to piston valves raised the limit, but I think you are still looking at less than 500 psi; utility turbo-generators operated at pressures twice that and greater.
3. Steam locomotives are subjected to wide fluctuations of load (as mentioned earlier). They are also subject to a lot of shock from slack couplers and other forces. Tube breakages were common.
Both utility plants and steamships spend most of their time either running flat out or with gradual variations in speed. This is of course not true with steam locomotives; a fire tube boiler could take the swings much better. Steam turbines did not work well for the same reasons; like steamships, steam locomotives also needed a seperate reverse turbine in order to run in reverse.
-James Hefner
Hebrews 10:20a