Canalta Double Block and Bleed Orifice Fitting video
Video above: Live demonstration of Canalta's Double Block and Bleed Orifice Meter at TUV NEL in East Kilbride
The orifice meter is a generic DP (differential pressure) meter that measures mass flow like all DP meters by cross-referencing the physical laws of the conservation of mass and energy. The orifice meter therefore uses the generic DP meter mass flow equations featured in ISO 5167:2003 and API 14.3 (AGA 3).
The discharge coefficient Cd used within the mass flow equation is a function of pipe inner diameter D, the orifice plate bore d and the Reynolds number Re. The calculation is stated in ISO 5167-2:2003 [i] section 18.104.22.168 and is called the Reader Harris-Gallagher (RHG) equation. The availability of the RHG equation means that…
The operator does not need to calibrate (or periodically re-calibrate) an ISO or AGA compliant orifice meter to have low flow rate prediction uncertainty in service.
The mass of industry data publically available from ISO and AGA compliant orifice meter installations, has enabled the standards boards to accurately predict the discharge coefficient Cd and therefore the performance of an orifice meter, without the need for calibration.
This cannot be said for alternative technologies like USM (Ultrasonic Meters) which require initial laboratory calibration and then re-calibration at approved laboratories at yearly (or agreed periodic) intervals. This requires the USM to be withdrawn from service and transported to one of only a few laboratories able to mimic gas flow close to the actual process flowing conditions for calibration.
A gas meter is the financial cash register from point A to point B, so the level of measurement or the measurement uncertainty is one of the most important factors of the meter.
The orifice meter has a competitive flow rate prediction uncertainty, and what follows is worst case statements based on the respective standards.
- Ultrasonic meters to ISO 17089:2010 required for fiscal metering have an uncertainty of ≤ ± 0.7%. These devices require calibration.
- Turbine meters are calibrated to AGA 7 to predict volumetric flow ≤ ± 0.5%. The density uncertainty is typically ≤ ± 0.4%, so the root sum of squares (RSS) of these uncertainties gives a turbine mass flow rate uncertainty of ≤ ± 0.7%. These devices require calibration.
- Orifice meters have a mass flow prediction uncertainty ≤ ± 0.7%. Orifice meters require no calibration only manufactured in accordance with the dimensional compliance of either API 14.3 or ISO 5167.
Ultrasonic and Turbine meters typically have electrical outputs whereas the orifice meter output is a pressure output. With the advent of increased accuracy DP transmitters the above mass flow prediction uncertainty ≤ ± 0.7% is most likely conservative and a modern orifice system using modern high accuracy DP transmitters could have a lower uncertainty, and still without the need for calibration.