Eng-Tips is the largest engineering community on the Internet
Intelligent Work Forums for Engineering Professionals
-
Congratulations waross on being selected by the Tek-Tips community for having the most helpful posts in the forums last week. Way to Go!
scfh to Nm3h 1
- Thread starter egebull
- Start date
-
1
- #2
egebull:
There are many different sets of temperature and pressure used to define scfh and to define Nm[sup]3[/sup]h. The best way to convert any gas volume from one set of temperature and pressure conditions to another set of temperature and pressure is to use the combined gas law as given below. It does not matter what you call each volume (scfh, Nm[sup]3[/sup]h or anything else) ... all that matters is the temperature and pressure of each volume:
V[sub]2[/sub]/V[sub]1[/sub] = (Z[sub]2[/sub]/Z[sub]1[/sub])(P[sub]1[/sub]/P[sub]2[/sub])(T[sub]2[/sub]/T[sub]1[/sub])
where:
V = gas volume
P = pressure
T = temperature
Z = gas compressibility factor
In most cases, the gas compressibility factors are not needed unless you want extreme accuracy.
As for the many different temperature and pressure reference conditons used to define "standard" or "Normal" volumes, read this online Wikipedia article:
Whenever you use a term like "scfh" or "Nm[sup]3[/sup], you must state the temperature and pressure that applies to that term.
Milton Beychok
(Visit me at www.air-dispersion.com)
.
There are many different sets of temperature and pressure used to define scfh and to define Nm[sup]3[/sup]h. The best way to convert any gas volume from one set of temperature and pressure conditions to another set of temperature and pressure is to use the combined gas law as given below. It does not matter what you call each volume (scfh, Nm[sup]3[/sup]h or anything else) ... all that matters is the temperature and pressure of each volume:
V[sub]2[/sub]/V[sub]1[/sub] = (Z[sub]2[/sub]/Z[sub]1[/sub])(P[sub]1[/sub]/P[sub]2[/sub])(T[sub]2[/sub]/T[sub]1[/sub])
where:
V = gas volume
P = pressure
T = temperature
Z = gas compressibility factor
In most cases, the gas compressibility factors are not needed unless you want extreme accuracy.
As for the many different temperature and pressure reference conditons used to define "standard" or "Normal" volumes, read this online Wikipedia article:
Whenever you use a term like "scfh" or "Nm[sup]3[/sup], you must state the temperature and pressure that applies to that term.
Milton Beychok
(Visit me at www.air-dispersion.com)
.
The most important thing to understand about standard volumes is that they are representing a fixed number of moles - not a volume!
Then also remeber never take anything for granted! Always state your standard conditions (typically 60 def F 0 psig or 0 deg C, 1 atm). But these may vary. Event the US units!
If you have 1 SCF (60 deg F, 0 psig) then you have 0.0269 Nm3
Best regards
Morten
Then also remeber never take anything for granted! Always state your standard conditions (typically 60 def F 0 psig or 0 deg C, 1 atm). But these may vary. Event the US units!
If you have 1 SCF (60 deg F, 0 psig) then you have 0.0269 Nm3
Best regards
Morten
egebull:
MortenA forgot to say that 1 scf at 60 deg F and 0 psig equals 0.0269 Nm3 only when the Nm3 is defined as being at 0 deg C and 1 atmosphere (absolute).
As stated in the Wikipedia article that I referred to earlier, there is no universally accepted set of reference temperature and pressure for scf or Nm3 or any other named volume.
As emphasized by both MortenA and myself, you must always define your reference conditions of temperature and pressure for scf, Nm3, acf, icf, etc., etc.
Milton Beychok
(Visit me at www.air-dispersion.com)
.
MortenA forgot to say that 1 scf at 60 deg F and 0 psig equals 0.0269 Nm3 only when the Nm3 is defined as being at 0 deg C and 1 atmosphere (absolute).
As stated in the Wikipedia article that I referred to earlier, there is no universally accepted set of reference temperature and pressure for scf or Nm3 or any other named volume.
As emphasized by both MortenA and myself, you must always define your reference conditions of temperature and pressure for scf, Nm3, acf, icf, etc., etc.
Milton Beychok
(Visit me at www.air-dispersion.com)
.
i acutally wanted to stress that - but my mind must have wandered.
almostbroken
Petroleum
The main difference between Nm3 and Sm3 is temperature. Nm3 is at 0C (32F) while Sm3 is at 60F (16C). Both are taken at atmospheric pressure.
almostbroken
See thats the problem! Nver take anything for granted! Even though SCF is defined at 60 def F=15.667 deg C - the SM3 is usually defined at 15 deg C!
Do you think im wrong? Check out this reference at wikipedia:
As you can see - ISA,ISO 13443, EEA and EGIA prefers the 15 deg C! But SPE apperanely now recommends 1 bar as the reference pressure where most other prefers 0 barg or 1.01325 bar!
Best regards
Morten
See thats the problem! Nver take anything for granted! Even though SCF is defined at 60 def F=15.667 deg C - the SM3 is usually defined at 15 deg C!
Do you think im wrong? Check out this reference at wikipedia:
As you can see - ISA,ISO 13443, EEA and EGIA prefers the 15 deg C! But SPE apperanely now recommends 1 bar as the reference pressure where most other prefers 0 barg or 1.01325 bar!
Best regards
Morten
KernOily:
Firstly, the combined gas law equation that I gave in my first response above includes compressibility factors Z.
Secondly, it depends on what you mean by "no good". What degree of accuracy do you define as "no good"? In my experience, using the combined gas law equation can be used up to 1000 psig or more and worked fine in the design of refinery process units that operated at those pressures. Where did you come up up with that cap of 60 psig?
Milton Beychok
(Visit me at www.air-dispersion.com)
.
Firstly, the combined gas law equation that I gave in my first response above includes compressibility factors Z.
Secondly, it depends on what you mean by "no good". What degree of accuracy do you define as "no good"? In my experience, using the combined gas law equation can be used up to 1000 psig or more and worked fine in the design of refinery process units that operated at those pressures. Where did you come up up with that cap of 60 psig?
Milton Beychok
(Visit me at www.air-dispersion.com)
.
There are a couple of small errors above that I'd like to correct for anyone who reads this later.
The real key to this is that the only place that the "standard" conditions are laid out clearly and unambigiously is in gas-sales contracts. If you are dealing with gas sales then you had better read the contract. If you are doing a plant balance then as long as you are consistent then you can pick your own values (within reason).
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
First, "atmosphere" is a local phenomena that defines the abaolute pressure when a gauge reads zero (where I live the atmospheric pressure ranges from 11.2 psia to 12.5 psia depending upon which hill or valley you're in). For standard conditions I've seen 1 bara (100 kPa, 14.5 psig), or 101.325 kPa (14.696 psi), but "atmosphere" is meaningless. Second, the multiplier you gave only works if both reference pressures are the same. If for example, the scf reference uses 14.73 psia (which is the reference recommended by GPA and API in the hydrocarbon measurement standards) and the nm3 reference uses one bar then the multiplier increases to 0.0272. Not a huge change, but it is over 1% error.
The real key to this is that the only place that the "standard" conditions are laid out clearly and unambigiously is in gas-sales contracts. If you are dealing with gas sales then you had better read the contract. If you are doing a plant balance then as long as you are consistent then you can pick your own values (within reason).
Man I wish it were that simple. I've gotten data in nm3 that referenced 0C, and data that referenced 16C and data that referenced 15C. Same with sm3. Pressures are just as far over the map. When I'm dealing with SCF, I assume the Oil & Gas "standard" of 14.73 psia, but a couple of states mandate 15.025 psia and my "assumption" bites me in the butt (you can't do velocity calcs on SCF). When I'm dealing with nm3 or sm3 I don't assume anything.
I've found that you are reasonably safe using the ideal gas law (without the "Z") up to about 2 atmospheres (call it 36 psia where I live, 44 psia by the seashore) with pretty good quick and dirty results. Going all the way to 60 psig is just too much error on real gases.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
As David and i agree upon: Never assume - always state your reference conditions (not only in sales contracts but also in engineering work). Its just two lines and it can avoid a lot of hassle later on if a deviation in definition is spotted at a later stage.
While there seems to be more confusion re. the "metric" definition it appears that even for SCF there exists deviations.
Jest remember that at the end of the day the std volume is just another word for a number of moles.
Best regards
Morten
While there seems to be more confusion re. the "metric" definition it appears that even for SCF there exists deviations.
Jest remember that at the end of the day the std volume is just another word for a number of moles.
Best regards
Morten
djack77494
Chemical
There may be a bit of harping going on about the need to state the reference temperature and pressure, but it is absolutely worthwhile. Engineers and scientists NEED high levels of confidence and precision, so please resist the forces of inertia, waste a little paper, and note the reference conditions near the place you introduce "standard" or "normal" conditions.
Doug
Doug
Actually I think the hour unit 'official' abbreviation is h, this listed among the abbreviations for SI units.
The 60 psig rule of thumb is just that. I got it from Campbell. It works fine for oil patch work, for the gases I deal with every day (HC gas mixtures with high CO2 fractions) within an 'acceptable' margin of error. It produces results that correlate well with field data read from store-bought pressure gauges as well as pressure transmitters calibrated on Friday afternoon by oil patch instrumentation techs who are on their way out of town to go hunting or 4x4'ing. If 44 psia works for you, right on brother.
The 60 psig rule of thumb is just that. I got it from Campbell. It works fine for oil patch work, for the gases I deal with every day (HC gas mixtures with high CO2 fractions) within an 'acceptable' margin of error. It produces results that correlate well with field data read from store-bought pressure gauges as well as pressure transmitters calibrated on Friday afternoon by oil patch instrumentation techs who are on their way out of town to go hunting or 4x4'ing. If 44 psia works for you, right on brother.
I get it. I do an awful lot of "good enough" stuff too. The problem I keep running into is when most of the rules of thumb were first proposed (mostly prior to 1980), gas was treated by the industry as a waste product and didn't get much attention. I've found that gas above 2 atmospheres (gauge) behaves very much different from gas below one atmosphere (gauge). I keep finding empirical equations that were the cornerstones of our business at 400 psig that aren't even close at 10 psig. So I'm writing my own low pressure rules of thumb.
David
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
David
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
- Status
Similar threads
- Locked
- Question