Orifice Calculation

There should be some good orifice data out there (I have used some for refrigerant designs with much less ΔP). Alternatively, you could start with a small orifice and progressively drill it out until you reach a satisfactory size. Or even a needle valve, which could be adjusted if conditions change later on.

Your job doesn't look too complicated for flow measurement. More complicated part is pressure reduction from 340 to 1 kg/cm2. You said "Pressure difference is 319 kg/cm2" , I think it shall be 339 kg/cm2. You may require multistage reduction with reliable controls with safety relief system because of hazardous nature of gas.

I think installing flow measurement orifice in downstream in low pressure piping may give better result as temp & pressure correction factor will be less. Installing in upsteam pipe may have less hardware cost due to smaller size. However, you can consult instrumentation engineer or supplier for correct location.

If this is going into a furnace like the reformer in an ammonia plant shouldn't it be part of the burner management system and be a metered and controlled flow with at least a control valve and probably a block valve as well? Are you trying to use the gas as an SCR ammonia source? In either case, I'm not sure that just an orifice will suffice.

When you go on the internet you can download a number of orifice plate calculation formulas.

Myself I would suggest you use the ISO 5167 or AGA formulas which you can purchase real easy.

There are a number of variables that you would need as for example, the viscosity of the gas, reynolds number etc.

Also you will need the true or average density for the calculations.