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Is there a flow meter that will be reliable, even with moderate particulate in the air stream?

Q: One of my processes has a dust-collection line to eliminate nuisance dust. The same line is tapped for a supporting process in the room. The problem we have experienced is that the dust collection adversely affects the product yield if flow rates are too high. We don't have a way of quantifying an acceptable flow range. I would like to install a flow meter so we can determine such a range and make it part of the operation procedure. The line is about 4 inches in diameter. I've looked at a few flow meters, but would like your thoughts on which type would be best suited for this application. I want to find something that will be reliable, even with moderate particulate in the air stream. Of course, the more economical the solution, the better the chance it will have of happening.

A: A flow nozzle or flow venturi (differential head meter) with a multivariable transmitter (d/p, pressure and temperature) would provide a mass flow with a turndown of 5:1. You could use just a d/p transmitter to get volumetric flow. In either case, the sensing lines need to be purged with an inert gas such as nitrogen to keep particulates out of the lines. I favor a flow nozzle or venturi over an orifice plate to avoid the problems of orifice edge wear and buildup of particulates on the plate. Also, the straight run requirements are less and the turndown (rangeability) and accuracy is slightly better.
 
If much greater flow turndown (rangeability) or accuracy is needed, you could go to a Coriolis flow meter, but this meter is much more expensive than a differential head meter for your line size. Additionally, a Coriolis meter requires relatively high gas pressures (high gas densities) to perform properly and the pressure drop across the meter is much larger than the pressure drop across a flow nozzle or venturi tube.
 

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