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Comparing Ultrasonic Flow Measurement with Orifice Flow Measurement
How to confirm that the upstream ultrasonic flow measurement is equal to downstream two orifice flow measurements

We have an Ultrasonic Flow Meter installed on a Natural Gas Line. This line divides into two branches and each branch has an orifice flow meter installed in it. The reading of ultrasonic flow meter is not coming equal to the sum of the readings of Orifice meters. I know that there is a difference of accuracy between Ultrasonic and Orifice but how close can we get? And what should be the strategy to match it to the best possible extent?

We do not have a live density value available here. Only Temperature and Pressure Transmitters are available.

In Ultrasonic Flow meter, predefined gas data are used for example:

1) Relative Density
2) Heating Value
3) % CO2
4) % N2

We can calculate the Compressibility Factor from here. Shall we use the same compressibility factor in the Orifice Meter equation to get the mass flow?

What model of sonic meter, how many beams, etc. and are you involved in custody transfer measurements covered under contract?

The Ultrasonic Meter is Elster Instromet QSONIC 4. It has four paths. It is kind of a custody transfer requirement.

Regards

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A meter like that can be used to calibrate the orifice measurements.

1. You have to remove the plates and check the bores, and edge quality, etc. and replacing as necessary.

2. Check your upstream requirements, you'll need some 40 diameters of undisturbed straight pipe flow with 10-15 downstream, for decent measurements. Have your orifice plate supplier run certified meter calculations, and recalibrate the d/p meters.

3. Get the best estimate stream data you can on a molar of volumetric basis. The meter supplier can usually do the necessary gas calculations as long as you have properly calibrated pressure and temperature measurements.

With all that you'll get the accuracy to 2%. Beyond that you need flow tubes that have been pre-calibrated. That takes you to the 1% inaccuracy range.

You can also, check your orifice plates against the ultrasonic meter, but this requires valving off one of the streams.

I'm looking for more of a calculation point of view. The biggest problem for me is that the live density value is not available. There are some fixed values as mentioned in my first post configured in Ultrasonic Flow Meter through which it is calculating compressability factor by which it can calculate mass flow rate.

If I can find that density of ultrasonic flow meter, I can put the same density in Orifice flow equation to get the orifice flow on that density. What do you say?

No issue there, if you have your nominal composition and P/T the calc is pretty simple,

If contractural transfer the gas purity or Sp. Grav at some standard condition, is part of the contract.

The orifice equations straight forward, getting the right data is the tough part!

Both ultrasonic flowmeter and orifice flowmeter can give you fiscal category measurement uncertainty (i.e., 1.5% of the volumetric flow measurement range.) But to achieve that accuracy you need to do many things correctly. In most of the countries, normal operational flowmeter accuracy is defined as 3%. So if the reading from both of your ultrasonic flowmeter and orifice flowmeter are within +/-3% of the range, that should be acceptable.

Please note that ultrasonic flowmeters measure fluid velocity and hence volumetric flow independent of any fluid property and composition, just depending on the pipe dimension and pulse transmit time between the transmitter probe and receiver probe.
Ultrasonic flowmeters also can measure sound velocity independent of fluid properties. Since sound velocity depends on the fluid density, the ultrasonic flowmeter has the capacity to calculate density of the fluid from the sound velocity.

Please check the operating manual of the ultrasonic flowmeter or contact manufacturer, they can tell you how to retrieve data of the fluid density from the ultrasonic flowmeter. You can then use this density in the orifice flowmeter calculation.

Can you please tell your location details: country, ambient temp and pressure and humidity?

Temperature can go upto 52 degC
Humidity usually remains high from 60% to 100%