flow calculation software for orifice plate

Did you google your question? I found several...

http://www.pipeflowcalculations.com/orifice/index.htm

You can do this with flow analysis software. The software we use is Flowmaster v6.5. It will calculate your D/P (which I think might be Pd, Pressure drop). You would need to provide lots more information before an analysis could commence, such as:-

a) Temperature of fluid.

b) Viscosity of fluid.

c) Velocity of fluid.

d) Edge characteristics of orifice.

e) Head pressure of fluid.

f) And on, and on!

You can do rough calculations with a plethora of cheapo' stuff. Just use google!

efunda.com has an easy to use orifice plate calculator,

The equations are quite simple and you don't need special software. Just get the standard desired (ie ISO or API). They spell out all the equations, where to get factors and everything.

My motto is "If you can not do the calculation by hand, never rely on a computer for an answer"

You could try the FlowCalc32 orifice plate calculator