Calculating Flow Measurement with Orifice

At our Gas compressor facility, we used to calculate final Flow throughput of the station with Orifice Metering System. On discharge line there is an Orifice plate to create Differential Pressure (DP) along with flow transmitter to transmit corresponding DP signal, a static pressure transmitters and RTD (temperature sensor) installed and above signals are transmitted to DCS.

Since 1985, these static pressure, temperature and Differential Pressure across the orifice readings are calculated in DCS and we achieved results in FLOW in accordance with (ISO-5167) and AGA-3 Orifice metering calculations.

Due to unavailability of orifice spares (like retaining rings, gaskets etc.) we could not check orifice condition till date, and we need to confirm whether our flow calculations is still valid and appropriate or require any tuning in the form of calculations, formulas etc. Please help me to calculate correct flow with under mentioned process conditions. (Please note that under mention gas compositions / calculation parameters are set from 1985)

Fluid: Natural Gas
Max. Flow: 500000 Nm3/hr
Inlet Pressure (static): 79.6 barg
Temperature: 60C
Differetial Pressure: 2500 mmH2O (98.32 inH2O)
Pipe Line: 20 inches 900lbs
Pipe inner diameter: 18.50 inches
Beta ratio: 0.6
Coefficient of discharge: 0.59768
Compressibility factor: 0.9
Upstream Density: 60.00743 Kg/m3
Mass Rate of flow: 115.13472 Kg/s
Isentropic Exponent or Specific Heat Ratio: 1.3
Dynamic Viscosity: 0.01
Reynolds (Ro): 8314.3 J/Kmolk
You'll need to specify the meter taps used, the latitude of your meter station, and its elevation above mean sea level. Additionally, is a weep hole for drainage used? is it a square-edged plate, materials of construction?

First and formost you need to check the meter plate.
If you can stop gas compressor, isolate the suction and check your orifice diameter. Sometimes orifice is eroded and the diameter become larger. Stainless steel orifice should last longer. If you found that orifice diameter is different then Adjust parameter in your DCS calculation.

Also don't forget to calibrate DP transmitter.

RTD is difficult to be calibrated, so I suggest you replace the RTD with new RTD. Sometimes the resistance of the cable influence the measurement. Make sure your RTD is in correct measurement. Then if you still doubt about the result, you can calculate by yourself by put your parameter into AGA3 formula.

Or you can buy some flow calculation instrument device such as Bristol or Floboss. But, again, the calculation is depending of your field instrument (DP and RTD). So if your DP trans and RTD is correct then you will get correct measurement (you just need to adjust some AGA3 calculation parameter).

Hope this help
Actually i have gone through with AGA, but I found it some what complicated. What I need from this forum is to share some simple AGA calculations, formulas. Is there any simple calculation method which privide actual flow in simple steps.