6 replies to this topic

[go-fish]

Can the ratio of MW of actual gas and air be used to calculate specific gravity at high pressure conditions?

The gas under questions is a vapor from 3-phase separator in crude oil processing which is being compressed in 3 stages to upto 70 barg.

My understanding is to calculate it using actual density (from simulation) at every stage and divide it by 1.205 kg/m3 so that we get different SG under all the three stages. This makes sense as we are compressing the gas, so the SG should increase at every stage.

The ratio of MW can be used only at low pressure under ideal gas behavior. If we use ratio of MW, we will end up with same SG in all stages. Please comment. I seem to share different opinion with my co-worker who proposes to use ratio of MWs for all stages.

[Zauberberg]

Specific gravity is defined as the ratio between actual density of gas (at given p, T) and density of air at standard conditions, which have to be defined (e.g. 60 degF, 1 atm).

As the conditions of gas change - pressure and temperature - specific gravity changes as well.

[JMW]

Possibly it is the relative density you need which is the ratio of the density of the gas to the density of a sample.
You can measure density on line and you can measure relative density online, if this is a critical parameter for control.
(Emerson 3098 relative density analyser and 7812 density: http://www.emersonpr...ex-density.html,
LEMIS Gas density : http://www.lemis-pro...easure=43&pid=6
Sarasota Gas density: http://www.thermosci...CT&searchType=0)

[katmar]

I have to disagree with Zauberberg here. The specific gravity of a gas is defined as the ratio of its molecular mass to that of air. Or, you could say it is the ratio of the density of the gas to that of air, both taken at standard conditions. This is the common usage of the term. It may not be logical, but it is the way it is used. There was a very long discussion about this on the Eng-Tips forum a while back. See
http://www.eng-tips.....cfm?qid=233225

[kkala]

On Feb 2010 there was a discussion in the forum to clarify specific gravity and relative density, also concerning gases. You can look at http://www.cheresour...h__1#entry36104(more details than below). A brief summary, with updates from wikipedia, is as follows.
"Specific gravity" per today's wikipedia is density in respect to water density (the latter usually at 4 oC).
"Relative density" per today's wikipedia is generally same as specific gravity, but the latter often means relative density in respect to water. Gas relative density is usually its density in respect to dry air at 20 oC and 1 Atma (1.205 kg/cm3); and (they say) it is approximately equal to MWgas/MWair. This is not same as the previous definition (some contradiction).
Old European concept of specific gravity is weight per unit volume, and relative density is MWgas/MWair for an ideal gas.
At least for gases we use densities in all data sheets to avoid confusion.

Edited by kkala, 26 November 2010 - 02:37 PM.

[Zauberberg]

Harvey,

Thank you for correction. My understanding of the original post was: which specific gravity to consider when considering actual gas density (at high pressure conditions). By simply dividing Mw of the gas with that one of the Air, we would definitely produce error in the calculation work, as the "actual" specific gravity is much higher than the figure defined at standard/normal conditions.

[kkala]

Once (some 50 years ago) specific gravity meant for a liquid how much "heavier" than water was. If it had a meaning for gases, this was how much "heavier" than air was.
In high school specific gravity became weight per unit volume, generally for liquids or gases. Simply density x gravity acceleration (g). Relative densities of gases in respect to air were "independent of pressure or temperature" and this was proved through ideal gas law.
Books of physics have retained the high school concepts. In Engineering books specific gravity of liquids was often in respect to water of 4 oC. For gases it was once considered as relative densities to air, but this was not widely used here.
American history was somehow different, having insisted (so far?) on the original concepts (those mentioned in the beginning) with some improvements for more precise definitions. American literature on engineering influenced terminology worldwide.
And now what can we do? Issue can hardly cause wrong interpretations, and probably will have been settled internationally in next (say) 40 years. In my opinion one should use density to allow no room for misunderstanding. Go-fish could use actual densities instead of specific gravities, which would give a clear picture.
Rest mentioned quantities do not seem so useful in the future, even though they give a sense of how much "heavier" is the fluid in respect to the reference fluid, or the weight per unit volume (European specific gravity). Will they survive? All these concepts can be derived from density.

Edited by kkala, 26 November 2010 - 05:49 PM.