A 500 gal empty propane tank is usually right around 1,000 lbs with the regulator, etc, attached.

The HP/hr idea of computing energy in compressed air neglects the energy that escapes in heating the air as you compress it, then the heat escapes. The Second Law of Thermodynamics applies here.

I've heard of complete ends coming off of rusty vertical compressed air receivers, but not of a horizontal tank like a 500 gal propane tank.