Release the tension of 1 string out of 3 throughout the whole piano. Then realease the tension of a second string out of 3 throughout the whole piano. And last release the tension of yet another string throughout the piano. Only then you can completely release the total tension and remove all the strings of the piano. This way, you wont disturb too fast the 40,000lb of pressure on the plate.
I've scrapped dozens of upright pianos over the years and always take a cutting wheel to the strings. Quickly zip them apart, never had an explosion, never had an injury, never been a problem. Sure it can be scary because conventional thinking says wire under tension can snap and whip and fly around causing damage but I've never found that to be the case when cutting piano wires. Not the safest thing to be doing I understand, however I've never had an issue with it. Just don't stand over the strings or rest your arm on them while cutting. Stand back and keep your distance as best you can.
And remove the strings evenly, not just all left to all right. Across all the piano strings there can be 20,000lbs of pressure. Each string can be bearing 200lbs.
To add to that, an piano tuner who had worked pianos all his life, told me that the proper way to destring a piano without having an explosion is to take one section do a quarter turn with each peg skip over the next section and continue quarter turn on each peg... and so on until you have completely loosened the strings and that is how you will have no mishaps. So using a power drill to begin with would also be unsafe as well, until you e completely loosened the strings
It was an excellent idea to take all of the tension off of the strings. Then you were able to remove the tuning pins. After you unscrew the plate, you can lift it out carefully. Best to use a crane. If not, get a number of people to carefully lift it out.
Imploding rather than exploding. I have had in my career as piano technician 2 or 3 cases of snapping frames while tuning or raising the pitch. It is an awful and frightening sound but not more than that. As soon as the frame cracks all the pression is off the strings at once so nothing flying around and no one was hurt. It was nevertheless an awful experience because this implosion occured with absolutely no warning.
