Flow Meter Before or After Backpressure Valve

[RJB32482]

Hi,
I am going to install a Corolis Flow Meter From Emerson Products that is going to measure flow of about 0.8 GPH. I was going to place it before a backpressure valve having a pressure differential around 20 psig. The line after the backpressure valve is around 25 ft long. Will placing the meter before or after the valve hav any effect on its measured flow? I was going to put it before since having higher pressure at the flowmeter helps improves its accuracy.

Thanks.

 

[zdas04]

I think you're on the right track. Measuring low-pressure fluids (especially gases, but liquids also) generally adds to your accumulated uncertainties. Also, even a Corolis meter has some pressure drop through it. If your downstream pressure is critical then adding a device with a measurable pressure drop downstream of the control valve increases the complexity of the control process slightly.

Neither of these issues are show stoppers, both are pretty small impacts so if there is a compelling reason to put it downstream (e.g., equipment access) then you won't be doing a terrible thing. In other words, if all other things are equal, put it upstream, if not then put it where the other issues dictate.

David

 

[JJPellin]

In systems where we use a Corolis meter, our controls engineers are very concerned about the presence of any gas. Apparently this affects that accuracy of the meter greatly. Someone please correct me if I have my facts wrong. But if there is any chance of gas flashing out of solution as a result of the pressure drop across the control valve, this could be another reason to put the meter before the valve.

 

[zdas04]

JJPellin
When the Corolis meter first hit the market, they flogged it as a final solution to the age-old question of how to measure multi-phase flow. That marketing crap quickly went by the wayside. The meter is detecting the momentum of the flow stream and using that to infer a volume flow rate. If flow is constant then it "knows" density. If density is constant it "knows" mass flow rate and will infer a volume flow rate from the constant density.

Your controls guys have it right for a system that is intended to measure liquid. The introduction of a small amount of gas will introduce a significant error in density and therefore the momentum measured will result in the inference of the wrong volume.

On the other hand, there are a significant number of these gadgets working well in gas service. In gas service they are more forgiving of small water incursions, but a slug of liquid will still give you interesting answers.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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