Volume to mass conversion for ideal gas – gas compression engineering – eng-tips gas city indiana post office


The problem that I keep having with this topic is that too many people believe that STP is only one thing (the one they are using). Every contract, every government agency, every processing plant PLC programmer has the ability to assign a different number to STP. The U.S. EPA has 5 different sets of values for STP in their various regulations. That is correct, if I calculate emissions under Subpart OOOO I have to use a different value for both standard pressure and standard temperature than if I’m calculating exactly the same parameter under Subpart W. It is very common for a gas sales contract to use a different value for STP than you have to use to report to the state. For a well in New Mexico on one of the reservations I have to be prepared to report standard volume to the state at one value, the tribe at another value, the BIA using a third value, and to the company’s internal production system using a fourth. It is all very doable, but it requires considerable rigor in knowing the pressure/temperature basis for the value you store in your internal system.

We bought a cheep RTU one time that did not let you input a local atmospheric pressure or a pressure base. We had it installed and running of a couple of months before I tried to reconcile the system material balance. It took considerable effort to nail down that particular bit of programming stupidity. When I did, I replaced the RTU with a different brand and did a presentation at an international measurement conference where I didn’t mention them by name but I included a photograph of their box. I hope I had a hand in their going out of business.

The Sm3 vs Nm3 discussion is a really slippery slope. Everyone thinks that they have THE definition and that there is only one. There is not only one, there are dozens. I get around most of the nonsense by defining SCM as "the volume in cubic meters that a mass of gas would occupy at 100 kPa(a) and 15C" (or whatever temperature and pressure the parties agree to). There is simply zero benefit to defining two separate imaginary conditions, it is simply a self-deluding method to keep from doing the job properly (and always ends up with someone spending more effort reconciling where things went so horribly wrong). If I’m doing serious Engineering calculations I get to mass as quickly as possible and duck into actual volume flow rate as necessary to get velocities, etc. I reserve SCM and SCF for contractual/regulatory things. As bad as Nm3/Sm3 are, they are not as bad as the Canadian e3m3 to refer to 1000 standard or actual cubic meters with no agreed upon way to tell the difference.

I hate the Oil & Gas use of Roman Numeral "M" (1000 in Rome) and then using "MM" (2000 in Rome) for one million. Nothing about that makes mathematical or historical sense. When I see MMSCM or MMnm3 I just want to cry. I reserve any m^3 or ft^3 for actual conditions. Not everyone follows that (obviously) so I’ve started being really careful to say MACF or kACM, but even that leads to confusion too often. Therefore I’m on a one-man crusade for this industry to stop being stupid. My chance of success is exactly equal to zero, but that didn’t stop Don Quixote.

I picked that example because it has an extra interested entity. I rarely see a single pressure/temperature base for all involved entities. When I am asked to review a gas-sales contract (which I seem to be doing 5-6 times a year) the first thing I do is go to the definitions section and look for a definition of "standard". I don’t care what values are in there, but if that definition is missing I stop the process until I have an agreement from the parties. It is amazing how often the people negotiating a contract don’t know what their company uses for internal reporting. I know it is pretty small beans (the difference between 14.606 and 15.025 is 2%, on a 100 BCF gas sales contract that is a mere 2.5 BCF (or $7.5 Million, chump change), but it can really be an ugly discussion in court.

I looked at one international company and found that their annual report program just accessed the production data from their subs and added it together even though it was stored at 9 different pressure/temperature bases. Fixing it did not make a material difference in their economic performance, but it got rid of a half dozen footnotes trying to justify numbers not adding up.