Read Also: Scraping the Bottom of the Barrel-Part 3
Q: What is the difference between the orifice calculations described by AGA 3 and ISO 5167? For what kind of services should we calculate by AGA 3, and what applications should be calculated using ISO 5167?
A: AGA3 (aka API MPMS14.3) is specifically for metering of natural gas. ISO 5167 is generic for liquids or gases. For natural gas, the question to be asked is "what does the sales contract call for?" They both give similar answers, but not exactly the same ones.
Ian H. Gibson
A: The recommended services for AGA 3 include applications where orifice plate-based flow measurement is used for natural gas or other hydrocarbon gases. ISO-5167-2 is recommended for liquid, air, vapor or other non-hydrocarbon gases. ISO-5167 could be used for hydrocarbon gases where custody transfer calculations are not required.
AGA 3 is intended for calculation of natural gas and other hydrocarbon gas flows through orifice plates, or considering a more detailed set of variables, including the gas expansibility factor, the gas relative density that depends on the molar composition of the gas, gas compressibility factors, etc. ISO 5167-2 developed a calculation for fluid in general that considers the expansibility factors for air, steam and natural gas. ISO-5167-2 can also be used for all gases, liquid and vapors fluids.
A: My experience is that the end user decides which of the several standards to use. It is typically an industry-specific thing. In general, the gas industry uses AGA. The ISO equations have been nearly the same. (And I could be out of date here. Periodically, the different organizations talk about reconciling any differences). If there is a purchase or sales contract involved, you need to look at that.
In fact, AGA, ISO and ASME equations are all so close in their results that those differences are normally much smaller than the uncertainities caused by non-ideal installations. For example, a few years ago there was much discussion about which gas expansion factor equation to use. I ran the calculation and did a 3D plot of the differences, and it showed that, yes, there were differences, but they were so small that they made no real difference.
So ask the client. If they care, then use whatever program is available, but I suggest that you document what you did. You do want to use current standards, not old ones.