Two? Since I’m quoting from movies, it’s not like I know what I’m talking about, but a valve for cleaning sludge out of a boiler could easily be outside the cabin.
Opening the bottom blow valve on a boiler that still has a fire under it or hot brick work will not cool the boiler but will add to the chance of it blowing.
You can surface blow a firing boiler but never, never give a firing boiler a bottom blow. It can interrupt circulation of water around the tubes and crown sheet, and lead to a failure.
The reach rods for both blow valves are in the cab. Have you ever been ridding a steam train when it stops in an out of the way location and they blow a lot of what looks like steam off to one side? That is really water that is being blown off and some of it flashes into steam as it leaves the boiler.
The blowdown valves dump liquid (and silt and scale) from the bottom of the boiler. This is far less effective at cooling the boiler than releasing steam from the top area, because the steam is replaced by the liquid boiling, which requires a lot of latent heat.
If anyone is curious what a blowdown looks like:
Sorry for the delay, but I was out of town for a few days.
Correct, but at least on locomotives there is always a blowdown pipe on each side of the bottom of the firebox. I don’t know if all (or any) would also have a way to perform a top blowdown.
Far from never, doing a bottom blowdown is SOP, albeit only a few seconds at a time, the purpose being to blow sediment out of the boiler from where it accumulates. The area or the boiler at the bottom of the firebox is called the mud ring for reason.
What we were talking about, though, was an emergency procedure where you have no water to add to a boiler. In that case, yeah, you want to dump the fire first before dumping the water, assuming you want to. Me? I’d pull the fire and keep an anxious eye on the pressure gauge. If the pressure continued to rise because of the latent heat in the firebrick I’d let the safety valves do their job. This would definitely make the water level drop so if it disappeared from the bottom of the glass (meaning exposing the crownsheet is imminent), then and only then I’d open the blowdown valve(s), quite possibly leading to a ruined crownsheet, but at least at zero pressure. No kaboom is nice.
It could be, but as you can see in allyn’s video, cranking open a hand valve inches from a howling stubby pipe is unpleasant at best. Adding the linkage to make the valves openable from inside the cab is trivial.
And you can see from the video why you want two. If you’re sitting in a station and just have to blow down the boiler, you wanna pick the side away from the platform. More typical is to do so when you’re on the road somewhere as in this video on the Cumbres & Toltec. Even in the middle of nowhere, you’ll have a favorite side to blowdown to.
I am fortunate enough to be a trained steam locomotive fireman (or called stoker in some countries), and it’s encouraging to see accurate information above… the 'dope lives up to it’s reputation. 
Scr4 above mentioned that “the boiler cannot get hotter than the boiling point of water”… that’s not true, because the boiler is under significant pressure (up to 250psi or more in some cases), and thus the boiling point is raised quite a lot.
As to the original question, which really has already been answered, what I do to “cool the boiler” in the case the pressure is reaching the safety valve opening pressure, is to inject water. There is an upper and lower operating range for the water level in the boiler, so in the event we’re delayed from starting, and I have a hot fire and full pressure, I will inject some water to cool the boiler a bit, lower the pressure and prevent the safeties from opening (which is a waste of fuel, considering only 5-10% energy from the coal you burn actually turns into steam you can use).
Injectors themselves are fascinating devices - they use steam from the boiler, to force cold water past a check valve into the boiler…the most efficient device on a steam loco, approaching 95% efficiency!
cheers,
Ok now I see where you are going with opening the blow down valve.
Low water and everything in me screams no don’t do than and why would you do that. Because as the water drops it would expose more of the crown sheet and fire tubes to heat with out cooling. But if you can blow it down fast enough 1st you could drop the pressure, it would take time to get the boiler almost empty and a major drop in boiler pressure. Another saving factor is it the crown sheet did fail and the boiler explode there would be less water in the boiler to flash into steam. Meaning the explosion would be less powerful. Also the crown sheet is normally made of thicker metal then the fire tubes. So with the lower level a tube or two could fail before the crown sheet. Smaller area of explosion and more directed.
Well, he’s technically correct. It’s just that if you’ve got a boiler at 300-pounds psi the boiling point is a touch above 400-degrees F, not 212.
I’ve always thought they were kind of magical which is why I included a link to the Wiki article in my earlier post. I didn’t mention them specifically because my piece was long and rambling as it was but included it as extra credit, so to speak.
It should be mentioned that running low on water is an extremely rare occurrence. Like an airplane crash, a long string of improbable events have to converge likely including inattention on several peoples’ parts.