First, decide what type of orifice you're designing and download the appropriate Excel Spreadsheet (read the articles for the equations and valid operating conditions)

- Small Bore Liquid Orifice (ASME MFC-14M-2001)
- Pipe diameter less than than 5 cm
- Spreadsheet
- Article describing equations and restrictions
- Large Bore Liquid Orifice (ISO 5167)
- Pipe diameter greater than 5 cm
- Orifice size greater than 1.25 cm, pipe size greater than 5cm and less than 1m, diameter ratios greater than 0.75
- Spreadsheet
- Article describing equations and restrictions
- Small Bore Gas Orifice
- Pipe diameters of less than 5 cm
- Orifice diameter greater than 6 mm
- Spreadsheet
- Article describing equations and restrictions.
- Large Bore Gas Orifice
- Pipe diameter between 5 cm and 100 cm
- Pressure ratios greater than 0.75
- Spreadsheet
- Article describing equations and restrictions

I'll illustrate the calculation process with an example problem (but the principles are the same for the other spreadsheets). All the spreadsheets require Excel's Goal Seek functionality because the calculations are iterative.

We will now find the orifice diameter for a small-bore liquid flow meter under the following conditions

- Pipe diameter: 0.042m
- Density: 1000 kg m
^{-3} - Viscosity: 0.001 Pa s
- Desired pressure drop: 30 Pa
- Flange taps

**Step 1**: Define the parameters as specified above, and also include an initial guess value for the orifice diameter.

**Step 2**: Go to Data > What If Analysis > Goal Seek. Set the difference in the guess and calculated values of the Reynolds number to zero by varying the orifice diameter.

As soon as you click OK, Excel will give the correct orifice size, as well as other parameters, like the flowrate, orifice coefficients, Reynolds Numbers etc.

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