Flow Transmitter Orifice Sizing and Uncertainty Analysis

Flow Transmitter Orifice Sizing and Uncertainty Analysis Popular

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Flow Measurement Orifice Plate Sizing & Sensitivity Analysis Rev 1.zip

Flow Transmitter Orifice Sizing and Uncertainty Analysis


Fuel gas flow transmitters rely on a variety of measurement techniques, the simplest (and cheapest) of which probably is the orifice plate. The pressure transducers used by flow transmitters are typically calibrated by mapping the minimum to maximum orifice plate pressure drops to the typical 4-20 mA signals it transmits. At zero flow the transmitter would issue a 4mA signal. As a rule of thumb, the minimum flow would therefore be calibrated to approximately 30% above the minimum signal (8.8 mA) whereas the maximum flow would be calibrated to approximately 70% to 80% of fullscale (14 to 16 mA).

For the case at hand, the information is as follows:

  • Maximum fuel gas flow rate = 1500 Nm3/hr
  • Minimum fuel gas flow rate = 350 Nm3/hr
  • Required pressure drop at maximum flow = 5 kPa
  • Required pressure drop at minimum flow = 0.5 kPa

The attached Flownex project enables the design engineer to easily size and evaluate such an orifice plate for minimum and maximum flow conditions. Flownex’s Designer is used to find the required flow rates by searching for the “Volume flow based on ambient conditions” in the upstream pipe component whilst setting the Ambient temperature to the Normal Temperature value of 0°C in the Solver settings. Note that the so-called Normal and Standard conditions are not globally standardised. Once the maximum flow has been found, the Flownex Designer is also used to determine the orifice size to achieve the required measured orifice plate pressure drop.

Flownex also enables the design engineer to evaluate the influence of temperature and pressure variations (and anything else of interest) on the uncertainty of the orifice plate measurement via the use of Flownex’s Sensitivity Analysis capability.

For the case studied, it is shown that the bulk of the uncertainty originates from pressure and temperature variations. If the calculated mass flow uncertainty of 3.7% is not acceptable, pressure and temperature correction has to be implemented in the transmitter. The orifice plate manufacturing tolerance contributes less to the overall uncertainty but is relatively simple to improve. It is also shown that in typical flow measurement instrumentation, the pressure transducer and transmitter uncertainties are negligible in comparison.

Not only is Flownex capable of designing large, sophisticated flow networks, but it is also able to focus on a single element of design such as a simple orifice plate and then analyse that element in incredible depth. This versatility of Flownex is unparalleled amongst similar products and must make Flownex an indispensable software tool amongst process design engineers.

The full case study discussion document is available on the Flownex® web page in the Case Studies section: