Conversion of Flow Transimitters Signals to Volumetric Readings 3

[ghamsa]

How can I convert actual field flow transmitter signal read-outs (4-20Am) to volumetric readings, say GPM, without looking up the readings on the DCS display? (flow loop uses orifice plate for delta P)

I am conducting a test and trying to directly use flow transmitter signal read-outs in order to minimize readings errors.

Thanks for helping

 

[FiltrationEngineer]

You just need to know what span the flow meter is set to, say if the flow meter is set to 0-100 gpm for the 4-20mA scale, then 12 mA would be 50 gpm, 4mA is 0gpm, and 20mA is 100 gpm.

You can usually change the span on the flow meter electronics manually.

By the way, if you are reading the flow off of the flow meter display or a PLC HMI, it will probably be more accurate than trying to measure the current directly because most hand held current sensing devices (such as a multimeter) accuracy is a lot lower than a PLC or the display on the unit.

 

[zdas04]

ghamsa,
It might be a bit more complex than that. You need to know the span of the pressure, temperature, and dP instruments. The 4-20 signal on each instrument will then tell you the reading of that instrument. Then you have to have a "meter factor" that deals with the beta ratio of your meter, flow area, etc. From there, the AGA equation for gas will give you MCF/day by multiplying the meter factor times the square root of the product of pressure and differential pressure divided by temperature. You have to convert the 4-20 pressure signal to psia and the 4-20 temp signal to Rankine prior to doing the math.

I've never used a square-edged orifice meter to measure anything that can reasonably be reported in gpm so I can't tell you the arithmetic.

David

 

[dcasto]

Orifices don't care about phase as long as it is just one phase. I like the mass per hour equation the best. Once you know the mass/hour you can convert with the density in mass/volume. The equation is Q= 6.02 * Fb * sqrt (hw * rho)

You can see in the equation you'll be taking the sqaure root of the differntial pressure, if that is what you are transmitting in the 4 to 20 mA reading. So at 50% or 12mA you'll have about 70% of the maximum flowrate.

 

[ghamsa]

Many many thanks to all of you.

 

[gedomin]

Orifice meter output must be scaled into square root.
Example: Flow span from 0 to 1000 gpm, and differential pressure span from 0 to 100 in H2O.
49 in H2O differential pressure corresponds to sqrt(49/100)*1000 = 700 gpm.
During the pen recorders era, this reading was close to the center, at 49% of the chart.