Flowmeter correction factor 1

[sheiko]

Hello,

I'm not an instrument guy but I have a question concerning the application of correction factors to flow measurements.
During actual plant operation, operating conditions may be significantly different from design conditions (density for liquids, and T, P, Z and molecular weight for gases).
Therefore, the measured flowrates should be corrected to account for the measured process conditions.

I was wondering if the REFERENCE CONDITIONS used in these corrections shall always be equal to the design conditions (vendor data sheet) or if it is possible to redefine them.

For exemple, suppose a gas flowmeter had been designed with a molecular weight of 10 and that the actual measured molecular weight is closer to 25. For sake of simplicity, let's assume actual and design T and P are identical.
Is it sensible to redefine the reference molecular weight to a value close to the actual molecular weight, let's say 20 (so that the correction factor is equal to the square-root of 25/20)?
Or shall we let the reference molecular weight equal to the design molecular weight (from the data sheet) because the primary element had been designed with a molecular weight of 10 (so that the correction factor is equal to the square-root of 25/10)?

In the case it is recommendable to modify the reference conditions, shall we then recalibrate or recalculate the flowmeter accordingly?

Hope the description of my query is clear enough.

"We don't believe things because they are true, things are true because we believe them."

 

[TD2K]

Why not just rerun the meter calculations at the actual (now) MW of 25?

If that's not possible for whatever reason, I'd just use the original meter factor (calculated for a MW of 10) and then correct it using the square root ratio of the MW as a quick correction. However, that's not going to capture other meter factors which, to varying degrees, will also be affected by the change in MWs.

That correction approach is true for all dP style meters I believe, for other meters if you aren't talking about an orifice type meter, the correction factor (eg thermal) could be different. I likely should know that but I don't.

 

[sheiko]

What do you ean by "rerun"?

"We don't believe things because they are true, things are true because we believe them."

 

[TD2K]

Take your existing orifice meter, max dP and gas properties to calculate the maximum meter reading (assuming it's an orifice meter). Then you can use the actual meter dP to estimate the flow.

Same process as if you were sizing the meter today or did I misunderstand something?

 

[ports394]

What kind of meter you using? Is it a orifice/dP?

 

[sheiko]

Yes, an orifice meter.

"We don't believe things because they are true, things are true because we believe them."

 

[dcasto]

In this day and age just use a computer to calculate the flowrate every 1 second with live inputs. A stand alone unot or a subroutine in most all plc or dcs.