Re: Boiler Pressure

The maximum pressure you can operate a boiler at is essentially a function of two things: 1) thickness of the boiler and the condition and size of other structural boiler components (staybolts, etc. ) and 2) the mechanical properties of the materials out of which the things mentioned in part 1 are made out of. There is an equation which relates the properties of the materials of the components, the thickness, and maximum pressure.
As a boiler is used ( or not ) , it continually corrodes. As a consequence, the thickness of the boiler shell changes (always decreasing). This is why you have to periodically measure the thickness of the boiler, nowadays this is down with an ultrasound machine. Essentially what happens after this is someone takes a bunch of measurements, and then an engineer takes the data recorded and analyzes it mathematically and determines a new MAWP (maximum allowable working pressure) for the boiler.
Theoritically speaking, as you make any engine operate over a larger temperature difference, it will be more effecient, hence why locomotive designers wanted higher boiler pressures. This is of course is a compromise as other people have said with weight of the boiler, maintenance costs , and a plethora of other things.
Locomotives tended to use lower pressures than other "modern" steam powered machinery in stationary and marine applications. This is largely because the need for staybolts in conventional locomotive boilers severely limited the pressures which could be used. Those applications used water tube boilers which often had no staybolts to restrict their pressures.
Regards,
Trevor Hartford