Once the physics of steam engines began to be understood, engineers were able to focus their minds on how to get more power for the same fuel, or the same power from less fuel.
A simple way to get more useful power from a steam engine is to lag the boiler. Coal which was used to produce waste heat could be used to produce more steam. This gives more miles per ton of coal. Of course, the railway engineers knew that they could carry on using the same amount of coal over the same mileage and go for higher speed. Which they did. And then some.
Behind these improvements in steam engine design lay an indisputable core fact of thermodynamics. Shovel in more coal and the steam pressure rises. Stop shovelling and the steam pressure drops. This simple fact can be observed at any steam rally or preserved steam railway. It is what any physicist would expect to follow from the laws of thermodynamics.
Improving the insulation does not raise the temperature. But it does significantly reduce the amount of fuel needed to maintain a given temperature, which is why we insulate boilers and buildings, fridges and freezers to this day.
If you improve the thermal insulation and then put in exactly the same amount of energy as before then the temerature rise appears to be caused by the insulation. That is wrong. Temperature rise can only be caused by an energy input. Thermal insulation allows the engineer to get a machine up or down to the desired temperature using less energy.
If a fireman keeps shoveling coal into the firebox of a steam engine without watching the steam pressure, after a while the safety valve will lift. The safety valve was invented by altruistic engineers to keep firemen and engineers from blowing themselves up. In the history of the steam engine there have been many instances of firemen, engineers and at least one cleaner screwing down the safety valve. The crew would do it to get more speed. The cleaner did it to stop the noise as the steam blew off near him. Bad idea!
Letting off steam
Letting off steam is a common expression. If we do not vent our anger in some mild way such as shouting, we may blow up. If you keep pushing a persons buttons, they may get so angry as to cause you some damage. Thermodynamic systems are a lot like you and me: sooner or later, if the input keeps up, they blow off steam.
You can take a steam engine and just keep shoveling in coal. The pressure rises. Screw down the safety valve and you will soon come to know what 'tipping point' means. A tipping point in any system is the point at which the system switches from one stable state to another state which may, or may not, itself be stable.
The tipping point for a steam boiler is the point at which the steam pressure, from having been closely and predictably related to the amount of fuel going into the firebox, drops suddenly and dramatically to zero.
This drop in temperature and pressure is an automatic and irreversible event. The laws of thermodynamics prove conclusively that when the pressure in a boiler exceeds a critical threshold the pressure doesn't rise any more. It drops. To zero. Suddenly. Dramatically.
Now, the stored energy can't just vanish. It must be used to do some sort of work. Which it does. The heat energy is used up in stressing every rivet and tube in the boiler far beyond its design limits. Component failure is inevitable.
A free piece of safety advice:
When a boiler explodes - try as hard as you can to be elsewhere.
Source: Wikimedia - photographer unknown.