Lets consider a fluid element in side a pipe. the face toward the pipe opening constantly facing the atmospheric pressure .and the other side is facing the pressure created by pump . The pressure difference is capable of drive the fluid element. Hope this will clear your question.

Hi Satya Sagar! The fluid pressure you are referring is the static pressure ,which is normal to the direction of flow.Assume the fluid stream coming out of the pipe as 'virtual stream/ tube of fluid'. The moment the pipe ends, the stream tube begins. Now the static pressure exerted by the atmosphere on the fluid at any cross section of that stream should be equal to the static pressure exerted by the fluid. In a way its a simple force balance.The fluid pressure you mentioned about is only the external static pressure that would be applied on the fluid stream.There is no conversion/dissociation of any sort. Now, instead of atmosphere, if you pump/discharge the fluid into a vessel with a different ambient pressure, then the static pressure at the exit would be equal to that corresponding ambient pressure. For more detailed understanding you might look into a some terms like 'Back Pressure', 'Under Expansion','Perfect Expansion', 'Over Expansion' of jets.

I agree with Surya that this is only about static pressure. The dynamic component (kinetic energy) at the outlet of a pipe will dissipate in a virtually infinite volume (the atmosphere), so the pressure increment will be virtually zero.

For the fluid to come out of the pipe into the atmosphere, it needed that the internal pressure of the fluid to be greater than atmospheric pressure (p0): dp = p - p0>0.

dp can be composed of these terms based on the generalized Bernoulli equation: the change in pressure due to the forces of the viscosity; change of the hydrostatic pressure (gravitational field); change in the pressure due to the change of the fluid velocity with radius of pipe if it changes; change in pressure for acceleration of the fluid if the fluid velocity changes with time.

The fluid pressure coming out of a pipe into the atmosphere has the atmospheric pressure as long as the fluid has a Mach number which is less than one. In such a case. the expansion waves (which travel at the speed of sound) can go upstream and accomodate the pressure. Otherwise, they remain at the pipe exit and you may end up in having at the exit a oressure higher than the atmospheric one. This is typical of convergent nozzles that are fed with an absolute pressure more than about double than the atmospheric pressure, but other situations do exist. Another condition so to have the fluid pressure coming out of a pipe equal to the atmospheric pressure is that the fluid streamlines have to be straight and parallel among each other, so that pressure gradients do not exist.