Although the pressure section of this publication is not intended to be a primer on pressure calculation, the following formula is used sufficiently frequently, but is obscure enough, that it has been included.
When a gas is compressed, energy is stored in it. If the energy is released in an unfavorable way, it will cause damage. Stored energies in excess of 100 kJ are considered high hazard. Sometimes it is helpful to think of stored energy in terms of grams of TNT. One gram of TNT contains 4.62 kJ of energy.
Vh = The volume of the vessel.
Ph = The absolute pressure of the vessel.
Pl = The absolute pressure to which the vessel would drop if it burst. Generally this would be one atmosphere (14.696 psi or 101,300 pascals). A pascal is a newton per square meter.
g = The adiabatic exponent or ratio of specific heats, Cp/Cv. The value is 1.666 for monatomic gases such as argon and helium; 1.4 for diatomic gases such as nitrogen, oxygen, hydrogen, and air; and variable for polyatomic gases such as methane, water, and carbon dioxide but generally very nearly 1.3.
Note that the bracketed value is dimensionless but that Ph Vh is not. Therefore, the length units used in Ph and Vh must match.
Example:
The gas is air (g = 1.4)
Vh = 1.0 stere (1.0 cubic meter) or 35.3 cubic feet
Ph = 10 atmospheres (150 psi) gage or 11 atm absolute or 1.1 mPa
Pl = 1 atmosphere or 100 kPa or 14.7 psi
A N-m is a joule, so the stored energy is 1.4 MJ, which is equivalent to somewhat more than half a pound of TNT. 