"Gas SG can't be the ratio of gas density at actual conditions to air density at standard condition. In that case gas SG closely follows its actual density as the air density at standard conditions is about 1.2 bar."
I am assuming you meant 1.2 kg/m³, and not 1.2 bars for the density of air at standard conditions. Also, please note that I asked for USC units, and not SCI units.
"Did some calculations for 10 gases at pressures ranging from 0 to 100 bar g (more or less close to ideal conditions) and SG values by pressue definition (i.e actual gas density/air density at gas conditions) differ after second decimal when compared with SG values by molecular weight definition."
I ran my own numbers and found a much bigger difference than the second decimal place as you described. Here are my sample calculations, using steam as the example fluids.
Using MW to define SG:
Steam MW: 18.02
Air MW: 28.966
SG = 18.02/28.966 = 0.622
Using density to define SG:
Steam density @ 114.7 psia/400 °F = 0.2338 lb/ft³
Air density @ standard conditions = 0.075 lb/ft³
SG = 0.2338/0.075 = 3.12
As you can see, there is quite a bit of difference between the two values. Just for fun, I wanted to see what the SG of steam at 14.7 psia would be:
Using density to define SG:
Steam density @ 14.7 psia/212 °F = 0.0373 lb/ft³
Air density @ standard conditions = 0.075 lb/ft³
SG = 0.0373/0.075 = 0.497
As you can see, there is a big difference in SG when the vapor is under pressure.
I spoke to a mechanical engineer here, and he stated that the MW version, that everyone on this forum keeps referencing, is only valid for ideal gases.
Densities of gases at high pressures can be approximated by applying the theorem of corresponding states. This theorem states that gases under the same "reduced" conditions exhibit the same deviations from ideal behaviour.
The deviations can therefore be determined using the generalized compressibility factor [z] diagram, knowng the critical properties.
d = MP/zRT = 44[×]100/(0.9[×]0.08206[×]471.2) = 126.44 g/L
The NIST value is 124.64 g/L. The error = 1.4 %.
With the probable exception of NH3 with errors around 7%, the accuracy using the generalized compressibility diagram is better than 5% for pure substances.
1.2 bar is a mistake and kg/cu.mtr is the correct unit. As SG is a dimensionless number, units are not important.
Taking your case of 114.7 psia and 400F steam, the density is 0.2338 lb/cu.ft (Z is 0.958)
The density of air corresponding to 114.7 psia (7.9 bara)pressure and 400F (204.4C) temperature is 5.78 kg/cu.mtr or 0.3608 lb/cu.ft (Z value of air at 114.7 psia and 400F is 1 so you can safely assume that the condition is ideal)
So, SG = 0.2338/0.3608 = 0.648 (air density should be at actual conditions)
By MW definition, it is 18/29 = 0.6206
When you are using MW definition, you are assuming that both gases are at ideal condition. The error for considering steam as ideal at this condition is approximately 7%.
When you take steam at 14.7 psia and 212F (1 bara and 100C), air density is 0.95 kg/cu.mtr or 0.0593 lb/cu.ft
So, SG by density ratio at actual conditions = 0.0373/0.0593 = 0.629 (air is ideal and error of considering steam at ideal condition is about 2%.
ASTM D3588 defines the specific gravity (relative density) as the ratio of the density of a gas under specific conditions of temperature and pressure to the density of dry air, of normal CO2 content at the same pressure and temperature.
In addition, BS 7589, 1996 (which is the same as ISO 6976, 1995) states the relative density is the density of a gas divided by the density of dry standard air at the same specified conditions of pressure and temperature. the term ideal relative density applies when both gas and air are considered as fluids which obey the ideal gas law; the term real relative density applies when both the gas and air are considered as real fluids.
My interpretation of these definitions is the the specigic gravity/relative density do not change with the specified reference conditions of pressure and temperature.
Thanks for the specific definition and referencing your source of information.
Is your interpretation, the same as quarks directly above? Meaning, you don't think that the specific gravity is going to change under different pressure and temperature conditions?
The relative density meter 3098 from Emerson Mobrey is used for fuel gases where the relative density is the density of the fuel gas to the density of a reference gas (not air) at the same temperature and pressure.
This is a commonly used measurement in hydrocarbon gas applications.
I agree that based on the codes quoted quark is correct when he says "Gas SG can't be the ratio of gas density at actual conditions to air density at standard condition". I also agree that using the MW definition only applies to ideal gases.
Folks,
I've been watching this discussion with interest because it's a problem which vexed me a while ago.
I totally agree with the definitions given by athomas236 above. The Gas SG is the ratio of gas density to air density at the same temperature and pressure conditions. The point that I never got clear in my head was whether that means (a) you bring the gas density to "normal" conditions and divide by air density at normal conditions, which is a well known figure. OR (b) divide the actual gas density by the air density to the actual conditions. Now this might not sound like a big deal but try it, especially where the gas is non-ideal and actual conditions are a long way from "normal".
The specific gravity is obtained by dividing the density of a gas by that of a reference gas (e.g., air at predetermined standard conditions) therefore it would change with T,P, as the density would.
I may be wrong, but it seems to me the main purpose of using specific gravities is to avoid using dimensional mass/volume units.
25362,
No I don't believe that this is correct. The Gas SG is not the gas density divided by reference gas (e.g. air) at standard conditions. The definition is as written by athomas236 above. The temperature and pressure at which the densities are quoted must be the same. i.e. the density of the gas and reference gas (e.g. air) must be given at the same conditions and then ratiod. Thus the Gas SG should be a constant.
[By the way, I absolutely hate using Gas SG and firmly believe in quoting densities at defined T&P]
The most important lesson that can be learnt from all this discussion is that there is obviously no universal definition of the SG for a gas. I have stated above that in my experience the SG is the ratio of the molecular weight of the gas to that of air. I know that this is (or was?) a very widely used convention. athomas236 has referenced codes that define it differently, and I accept that he is correct.
The upshot of all of this is that whenever you see the term SG used, you need to check what that particular author intended it to mean.
Unfortunately this is typical of so many things in engineering. We have had many discussions over standard conditions and normal conditions for gases - none of which are really standard or normal. The SG for a liquid can be defined for a variety of different temperatures - for both the fluid and the water. A valve Cv can be based on imperial or US gallons.
So rather than get all uptight over what the "right" definition is, we need to see what the definition is in the context that it was used.
I've seen both definitions for sp. gr. of gases, the one based on the air density at standard (whatever) conditions, and that mentioned by KenA. This discrepancy should be treated, as katmar so rightly says, in the right context. A becoming star for katmar's laudable message.
I have watched this thread with interest as it has developed to over 30 replies and I have to admit to being surprised by the statement about getting "uptight over what the right definition is". The more than 30 replies to this thread suggests that people do want to know the right definition.
For me the important lesson to be learned is, if in doubt, look in codes and standards first.
As per GPSA chapter 23 for Physical Properties:
"Relative Density (also termed specific gravity or gas
gravity) — is defined as the ratio of gas density (at the temperature and pressure of the gas) to the density of dry air (at the air temperature and pressure."
And also:
"The ideal gas relative density is the ratio of the molecular mass of the gas to the molecular mass of dry air."
Hence, when the specific gravity to be used in our calculations specified it as 'std conditions', we can calculate it as ratio of MW. If it is not specified as 'std conditions', we need to calculate it as ratio of density at actual conditions, in this case, the air referenced T and P should be defined or clarified.
Almost always, the gas flows are measured in STANDARD volume per time (Such as standard Cubic feet per minute or Standard Cubic meters per hour.
Standard conditions are at atmospheric pressure and 60F. Therefore compression-induced density changes drop out and you're back to the ratio of moleccular weights.
Compression of the gas is dealt with in the valve sizing equations.
Another reason to give your application engineer FULL data when specifying a valve. Nitrogen with 50 psi Delta P is meaningless. 100F Nitrogen with 250 psi(g) in, discharging to 200 psi(g)out, is vastly more specific.
Katmar is right on this, even if you are dealing with a gas (or one of its components) condensed at the reference conditions, it is the nature of approximate gas law calculations, for better or worse.
Athomas236,
the real lesson is that when two people use the same term each may mean something different and thus it is important, where such differences of interpretation are known, to clarify the meanings.
I recall a pharmaceutical engineer who kept referring to brine solution and it was only when asked if that was sodium chloride brine that he answered that it was ethylene glycol and water.
I took your advice and spoke to a Fisher rep on this subject, since I am referencing Fisher equations. He said that specific gravity is the ratio of the densities of your fluid to air or water depending on your fluid's state. He then stated that the density of air is at standard conditions, regardless of the process conditions of the fluid. I will use this definition, since he sizes valves and regulators for a living, although, I happen to agree with the ASTM definition described by athomas236.
The main problem I had with referencing air at a specific pressure and temperature, is that there is no easy way to look up the density of air at, say 114.7 psia/400 °F. At least, I don't know an easy way.
Thanks to everyone for their prompt responses on this controversial issue.
I have in front of me the Fisher Control Valve Handbook, 3rd Ed, 1999. On page 111 there is a table of terminology that gives the following definition for Gas Specific Gravity.
Gas specific gravity (ratio of density of flowing gas to density of air with both at standard conditions(1), i.e., ratio of molecular weight of gas to molecular weight of air), dimensionless.
(1) Standard conditions are defined as 60[°]F (15.5[°]C) and 14.7 psia (101.3 kPa).
The important part to note is the bit I have bolded in the definition viz. "both at standard conditions". This means that your formula can only adjust the density on the basis of ideal gas behavior by using the temperature and pressure values for the flowing gas. If you can find a formula that uses the actual density of the gas, rather than inferring the density from that of air and ideal gas behavior, that would be much better. Unless of course your gas is at conditions where ideal gas behavior applies, and depending on the degree of accuracy that you need.
