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Participant

Join Date: Jul 2012
Posts: 1

### Estimating Orifice Plate Installed Accuracy for Turn Down Operation

07/12/2012 12:23 PM

Is there any work done on estimating installed orifice plate for turn down operation. Many time after we selected a plate for a classical 3:1 turn down assuming known accuracy, we would face a question from the field on how much we can stretch the accuracy statement we assumed in design.

On the other hand, we run orifice calculations to select a plate good for 10:1 turn down typically focus on design case and allowable pressure loss, not on the extreme turn down case. How do we estimate the uncertainty in turn down cases?

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Commentator

Join Date: Jul 2010
Posts: 69
#1

### Re: Estimating Orifice Plate Installed Accuracy for Turn Down Operation

07/12/2012 1:55 PM

I've read that the API MPMS 14.3.1 has an uncertainty estimate for Natural gas (methane), but I don't have a copy to consult.

Guru

Join Date: Jun 2010
Posts: 531
#2

### Re: Estimating Orifice Plate Installed Accuracy for Turn Down Operation

07/12/2012 4:10 PM

I'll wade in here and maybe Iris will bail me out. The easy answer to your first sentence, is yes, dating back to the 1920's and earlier. Neglecting the change in orifice coefficient, a large part of the error has to do with the difficulty of measuring the differential pressure. This little chart shows ΔP vs flow:

 % Flow Differential 100 100.00 95 90.25 90 81.00 85 72.25 80 64.00 70 49.00 60 36.00 50 25.00 40 16.00 30 9.00 25 6.25 20 4.00 15 2.25 10 1.00

The 10% range of full-scale flow from 85% to 95% uses 18% of the Δp full range, while the 10% range from 15% to 25% uses only 4% of the Δp. Considering the accuracy and resolution of the Δp measuring device, it would seem that the error at 20% flow would be about 4 times that at 90%, and gets much worse at 10%.

Parallel meter runs can be installed, and I have seen 2 Δp cells with different ranges installed on the same orifice. These can be switched or have their own indicators/recorders.