Should you use a Coriolis flowmeter as a backup system? Plus a search for information about diaphragm gas meters

A backup system may not be the best use of a Coriolis meter, and a century-old gas meter technology that still has some life in it.

By Bela Liptak

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This column is moderated by Béla Lipták, automation and safety consultant, who is also the editor of the Instrument and Automation Engineers’ Handbook (IAEH). If you have an automation related question, send them to: liptakbela@aol.com

Q: Coriolis for orifice backup? I am working at RCSPL, a design and detailed engineering company for the oil & gas sector in India. The conditions of my application are these: Service –fuel gas line; Pipeline size – 2 in.; Flow rate – 1,000 lb/hr.

Presently an orifice meter (concentric square edge) measures the fuel gas flow. The client wants a Coriolis flowmeter in series with and upstream of the orifice plate to provide it with backup. Are there any problems that we should consider concerning this installation? If not, please advise how the installation of this Coriolis flowmeter should be made? 

R. Muthuganesan
muthuganesanr@gmail.com

A1: The Coriolis flowmeter is superior to the orifice-type sensor because it directly measures the mass flow, has low maintenance, requires practically no straight pipe runs and provides both better accuracy and better rangeability than does the orifice-based flowmeter. The orifice is a volumetric flow detector and therefore requires pressure, temperature and density compensation to determine the mass flow, requires more maintenance, longer straight pipe runs, and both its accuracy and rangeability are lower. For these reasons, I would not use the Coriolis meter as a backup, but would use it as the primary sensor.

As to installation, I would prefer to install the Coriolis meter downstream of the orifice, because the Coriolis has practically no upstream straight pipe run requirement while that of the orifice is substantial, particularly if an upstream restriction is present, such as a smaller-than-pipe-size Coriolis meter. In your case, in the 2-in. pipe, the Coriolis will probably be 1 or 1.5 in. Its preferred installation would be in a vertical pipe with upward flow direction (Figure 1). This should be in a location where there is no vibration and the pipe does not transmit much compression, tension or shear force onto the meter.

Finally, I should note that while this addition will provide a backup, still, when the two meters disagree, you will not know which reading is the right one, although the probability is that it will be the Coriolis. You should provide block and bypass valves, so when one meter is removed for any reason, you can continue operation using the other.

Béla Lipták
liptakbela@aol.com

A2: Based on my experience, both types of flowmeters are suited for the described application. There is nothing special in the installation requirements of the Coriolis instrument. The supplier of the instrument will give you any requirements/recommendations, if such exist.

Avihu Hiram
Avihu@HiramEng.com

A3: In general you will have difficulty reconciling the measurements between the Coriolis (basically a mass flow device) and the orifice plate. The conversion from mass to volumetric flow on a fuel gas line (gas has low density) will not be accurate due to the density measurement not being accurate for lower densities. 

This is also a problematic configuration because there will be discrepancies between the two readings, and the user will have to continually justify them. Also this does not account for any inaccuracies in the orifice reading. 

I am not sure that the client understands "backup," since the measurements will disagree, and the client will not know which is to be taken as the good one.

You have not really given enough information about the process. We need to know:

• Gas pressure, temperature and density;

• Allowable pressure drop (you now have additional instrumentation in the line);

• Piping configuration (e.g., is there a pressure regulator in the gas line?);

• Whether this is feeding a fuel gas header or an individual piece of equipment (e.g. fired heater); and

• Reliability requirements.

There are straight-run requirements for an orifice plate (more for upstream). It would be better to place the Coriolis downstream of the orifice because it will introduce significant flow profile disturbance for the orifice. You will need at least a 40-pipe-diameter straight pipe run from the Coriolis if installed upstream of the orifice plate. (You could probably get away with five to 10 diameters if the Coriolis is downstream.)

You can go to the vendor websites and find the sizing procedure for the Coriolis meters. There you will find data on size, required pressure drop, etc.

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  • As stated in Answer 3 above (problem of low densities), a coriolis may not work well in this particular application. It depends on fuel gas composition. Please be aware that refinery fuel gas can be hydrogen rich at times and can have very low molecular weight and hence low gas density. Check with your vendor and get confirmation whether their meter will work with very low molecular weights such as hydrogen rich gases.

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