Hi,
Reducing the 16" line to 4" for Flow element and then again increasing back to 16" is confusing me.
source is natural gas and going to compressor suction. this will not create a problem for compressor and as well back pressure developing and more over condensation due to sudden expansion in compressor suction.
Please put some light on this problem.
Thanks

That size ratio would usually be indicative of too large a suction line for the available flow, or too small a flow element for a typical 16" flow capacity. The Beta ratio for metering at 25% is extreme. As I recall, it should be generally in the range of 0.5-0.7

If it ain't broke, don't fix it. If it's not safe ... make it that way.

For an AGA-3 Square Edged Orifice Meter, the uncertainty is pretty constant between 0.36 and 0.72 (many people read the uncertainty data as the sweet spot being 0.3-0.76, but I'm happier in the narrower range). Uncertainty increases dramatically above and below that range.

Swedging down from 16" to a 4" meter is probably the last option I would have taken for any gas flow that I can imagine. That says to me that their pipeline is flowing at separator-magnitude velocities which is rarely a good idea.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

Hi David,
Could you please explain uncertainty, what is this and how you correlate with this type of applications.
Thanks
10815L

Uncertainty means that you don't know.

If an analog instrument has a scale of 0-10,000 (you pick the units) and major divisions of 1,000 units, with minor divisions of 500 units then if you see the needle between 5000 and 5500 you have to guess at a value more precise than either 5000 or 5500. That guessing is the uncertainty. Typically acceptable values are half of the smallest increment (i.e., on this instrument any reading that is not divisible by 250 is implying less uncertainty than really exists and valid readings would be 5000, 5250, or 5500).

For electronic instruments we have lost these rules and I often see a 0-10000 unit instrument with an uncertainty of +/-2% of full scale (+/- 200 units) report a value of 5634.67821 units. In reality you know that the reading is between 5400 and 5800, but to imply that you know to 5 decimal places would be laughable if it weren't so sad.

For Square Edged Orifice measurement you have uncertainty in the pressure, differential pressure, and temperature instruments. You have uncertainty in your gas analysis. You have uncertainty in the ID of the meter tube and the bore of the orifice. There is even uncertainty in the constants used in the flow calculation. Combining all of these uncertainty values is described in API 14.3 Part 4 and is quite complex arithmetic. At the end of it there is a graph that shows total uncertainty to be a function of β ratio, nearly linear, and very horizontal in a range between 0.36 and 0.72. Outside this range the uncertainty increases slowly for a little while (which is where the wider range I mentioned above comes from) then turns sharply upward.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

Hi David,
Thanks for your reply, I can guss this line is on boil off gas from tank and then going to compressor suction. Engineer just bring to me only line and p&ID with out any further details.
I will check further process parameters and will see the situation for this flow element problem.
Thanks
10815l