Consider a problem where a tank holds compressed natural gas, compressed to approx. 250 times atmospheric conditions. Now release the tank valve letting some of the gas out. Assuming a small timestep, I can approximate the energy lost as 1/2 mv^2, where m is the mass released during the timestep, and v is the (assumed constant for small timestep) velocity of exiting gas. Can I equate this to the change in stored energy in the tank during this timestep with reasonable accuracy? If so, can I model the stored energy as E = PV where P is pressure in the tank and V is the volume. Thanks alot.