Hello-
I work in a plant where we meter both NG and COG flows.
First you need engineering sheets listing what your max. DP equates to in Flow. This is vital and should have been provided to you by the O-Plate vendor, or your Project/Plant Engineer. Someone someplace sat down and calculated the DP for the flow range they wanted and bought the plates.
Next, how accurate of a reading do you want? The NG supply pressure greatly effects the flow reading (as well as temp) and should be compensated for-- Considering the cost of NG nowadays I would use a "multi-variable" Flowmeter. Foxboro makes a model now (I've used them all at this point) that can be calibrated rather easily compared to other models. Your DP to flow calculations are entered into this device and the 4/20ma output is compensated flow. Square-rooting is also done there.
Another choice would be a stand-alone flow computer that do this and output a compensated flow signal. KEP makes several (we use the Supertrols) and Prosoft makes a flow computer card for the CLogix. They do however require separate process meters for DP, Pressure, and temp.
If accuracy is not an issue, the easiest solution is to calibrate your flowmeter for straight (linear) DP, then square-root and scale the value to flow in the PLC. I typically use this formula--
Flow coming in (counts): 0/4095; LIN
Range needed: 0/5000 CFM; SQR
(SQR(Flow*0.02442)/10)*5000
The 0.02442 figure is 100/4095, this reranges the counts to 0/100. The SQR function then yields a 0/10 square-rooted range, and dividing by 10 yields a 0/1 range that can be multiplied with the max range to give you the square-rooted range.
You may also want to add totalizers to your flow as most companies are fairly shrewd about NG consumption and it would be a handy tool for them to see how much NG each boiler uses in a month's time.
Good Luck!