Calculating Discharge from a Pipe

P= 2kg/cm2 = ~ 28.5 psi

A= 3.356 in2 (for standard weight pipe, SCH40)

In reality, in order to get a accurate answer for flow rate, you have to take into account the length of your pipe. Friction/pressure loss will occur over length. Another factor (which may not make much difference depending on your application) is water temperature, because temperature changes viscosity. When you include those variables, you can fairly accurately predict your flow. I would give you a ball park number, but i do not want you to make an uneducated design error.

I can refer you to some information you may find usefull:

http://www.psig.org/papers/2000/0112.pdf

Also you did not state which type of pipe you are taling about. SCH 40, SCH80, SCH 160 ect. these factors will change the area of your pipe, and in turn your flow rate.

When you can provide the appropriate information, an appropriate answer can be given.