Gas flow using DP and orifice?

dginbuffalo

Member
Join Date
Dec 2010
Location
Buffalo,NY
Posts
630
Hello,

what is the equation to measure gas flow across an orifice plate? i have a customer who has an orifice plate, DP transmitter, pressure transmitter and RTD installed. If i get the makeup of the gas (methane), can i calculate the flow with the above info?
 
Without going to ASME's Fluid Meters standard, http://en.wikipedia.org/wiki/Orifice_plate looks to cover the subject fairly well (you gotta love Google). You will need to assume a discharge coefficient and verify some of the other assumptions that are made.

Not sure how easy these calculations will be if you are using a PLC ...
 
The DP transmitter provides a volumetric flow rate when square rooted (as a percentage of max flow at design conditions), the RTD and pressure transmitter provide a temperature and static line pressure for converting volumetric flow to an inferred mass flow.

Remember that the temperature in the formulae is in Kelvin units and the static line pressure is an absolute pressure value, not a gauge pressure value. At low line pressures, the use of gauge pressure can produce very erroneous calculation results.
 
just buy a flow computer and be done with it. I know that is not what you asked but that is the best way to do this. the cost of your time to do this and the 3 transmitters will be more than buying a flow computer. sorry, Ive seen enough problems with these pieced together solutions to never recommend doing it this way.
 
Not to mention that if you are doing Custody Transfer with this set up a PLC is usally not acceptable. I think the ScadaPAK PLC's are approved for Gas Measurement.

I agree a Flow Computer is the best way to go and it adds a little CYA if you have a measurement discrepancy.
 
... the cost of your time to do this and the 3 transmitters will be more than buying a flow computer....

I disagree. I've done this successfully and accurately dozens of times. The caution is that, like anything else (including flow computers) the programmer has to understand the physics and program the math properly. Once they have accomplished that then they reduce their incremental expense on future projects to essentially zero, since they need the same sensors for a flow computer or the Rosemont unit or whaterver. All they need to do is add a few lines of code to their program! (Using orifice differential pressure, temperature, and line pressure are mandated by the physics!)
 
I disagree. I've done this successfully and accurately dozens of times. The caution is that, like anything else (including flow computers) the programmer has to understand the physics and program the math properly. Once they have accomplished that then they reduce their incremental expense on future projects to essentially zero, since they need the same sensors for a flow computer or the Rosemont unit or whaterver. All they need to do is add a few lines of code to their program! (Using orifice differential pressure, temperature, and line pressure are mandated by the physics!)

most single run flow computers have all of the sensors needed built in and no programming is necessary outside of entering gas analysis variables, orifice size, and pipe id. the widely accepted industry standard is to use a flow computer, a device specifically made, tested, and universally approved for the application. Not to mention you get approval for custody transfer and most offer some form of archived EFM data along with calibration control built in.
 
You can measure your flow using following basic formula

Mass Flow (Kg/Hr) = C*Sqrt(stand.gas density)*Sqrt(dP) * Sqrt(1/(273+Temp))*Sqrt(Atm.Press+Pross.Press) where:

stand. gas deinsity in kg/m3 (for air @ 0°C, sea level = 1.292)
T - Process temp. deg.C
dP - differential pressure across orifice in Pascals (1"wc=249 Pa)
Atm.Press in Pascals (at see level=101325 Pa)
C = Orifice Cross Area(m2)* 3600*Sqrt(2)*Flow coeff.

Notes:
1.Flow coeff. for orifice may vary widely depend on multiple conditions, including shape of edges, material, roughness. Try to start with something between 0.6 and 0.65.
2. If you process pressure is not really much different from atmospheric (+/- 10"wc) you may not use last multiplier (one with pressure parameters) as its not significantly change the final value. The process temp. is much more important for the calculations.
 

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