One of theassumptions taken while deriving Bernoulli's equation is that the fluid is incompressible. Compressible flows are usually high speed flows with Mach numbers greater than about 0.3. Bernoulli's equation is sometimes valid for the compressible flows or gas flows. read more
For Bernoulli's equation to be valid for gases, there should be no transfer of kinetic or potential energy from the gas flow to the compression or expansion of the gas. In other words the compressible gas should behave as an incompressible fluid for Bernoulli's equation to be valid. read more