Pressure drop of orifice plate

You might want to post your set up as not much info here to provide any help. It typically provides a good answer when its done right in my experience at any rate.

upload your files please.

OK there's a couple of weird things here. For starters why are you setting your exit to such a low pressure? If you just want the pressure drop as a function of flow rate then set your outlet to atmospheric. Secondly, and this is the real issue, look at your mesh across the orifice:

2 cells, barely! You're never going to get any useful results out of this mesh. You need to read up on Flow Sim basics, especially mesh creation. That is the single most important aspect of pre-processing.