9 replies to this topic

Dear All,

I have a question about specific gravity and its relation with temperature/pressure.

As you know the Specific Gravity - SG - is a dimensionless unit defined as the ratio of density of the material to the density of water at a specified temperature or can be expressed as:

SG = = ρ / ρH2O (3)

where

SG = specific gravity

ρ = density of fluid or substance (kg/m3)

ρH2O = density of water (kg/m3)

If we have a component for example propane in two difference conditions for example @ (20 oC / 15 barg) and (-42 oC / 0.01 barg) then please clarify we need to consider the density of water at above-mentioned conditions for calculating SG for propane?

Thanks

[gvdlans]

No, see http://en.wikipedia....elative_density

Normally water at 4 degrees Celsius is used as reference material.

[kkala]

In the work I use specific gravity of liquids as specified by gvdlans and wikipedia. Looking at W. L. Nelson (Petroleum Refinery Engineering, 4th ed, 1958, p.21), you can find a slightly different definition:
"The specific gravity is the ratio of the weight of a unit volume of oil to the weight of the same volume of water at standard temperature. Unless otherwise stated, both specific gravity and API gravity refer to these constants at 60 0F. An exception is the use of 77 0F (25 0C) in connection with asphalts and road oils".
Since water density at 4 0C, 60 0F, 77 0F is 1000, 999, 997 kg/m3 correspondingly, Nelson's definition can hardly make any difference in practical calculations.
It is noted that specific gravity in many European books (e.g. of physics) is defined as weight of a unit volume and is not dimensionless, expressed in kN/m3, KGforce/m3, etc. It is ρ*g (g=gravity acceleration), for both liquids and gases. However the American concept of specific gravity (as above) is widely used now, especially in Engineering companies.
Speaking of the specific gravity of gases, what wikipedia says ("often measured with respect to dry air at a temperature of 20 °C and a pressure of 101.325 kPa absolute, which has a density of 1.205 kg/m3") seems to contradict with the formula that follows (RD=Mgas/Mair). Hopefully somebody can clarify it, over here gas density is used (not gas specific gravity) in the relevant data sheets.

[S.AHMAD]

In the work I use specific gravity of liquids as specified by gvdlans and wikipedia. Looking at W. L. Nelson (Petroleum Refinery Engineering, 4th ed, 1958, p.21), you can find a slightly different definition:
"The specific gravity is the ratio of the weight of a unit volume of oil to the weight of the same volume of water at standard temperature. Unless otherwise stated, both specific gravity and API gravity refer to these constants at 60 0F. An exception is the use of 77 0F (25 0C) in connection with asphalts and road oils".
Since water density at 4 0C, 60 0F, 77 0F is 1000, 999, 997 kg/m3 correspondingly, Nelson's definition can hardly make any difference in practical calculations.
It is noted that specific gravity in many European books (e.g. of physics) is defined as weight of a unit volume and is not dimensionless, expressed in kN/m3, KGforce/m3, etc. It is ρ*g (g=gravity acceleration), for both liquids and gases. However the American concept of specific gravity (as above) is widely used now, especially in Engineering companies.
Speaking of the specific gravity of gases, what wikipedia says ("often measured with respect to dry air at a temperature of 20 °C and a pressure of 101.325 kPa absolute, which has a density of 1.205 kg/m3") seems to contradict with the formula that follows (RD=Mgas/Mair). Hopefully somebody can clarify it, over here gas density is used (not gas specific gravity) in the relevant data sheets.

Dear kkala

There is no contradiction whether we use density or molar mass (molecular weight) since density and molecular weight is interrelated by the GaS Law PV=nZRT or we can rewrite the gas Law in density term PM = ZRT x density where M is the mole wt. At low pressure and temperature Z the compressibility factor is close to 1 and when we take the ratio of gas density to that of air then the density ratio is equal to the molecular weight ratio.

Regards

[djack77494]

Any use of the term "specific gravity" requires that the reference conditions be carefully defined. For liquids, the reference is water and, being essentially incompressible, pressure is not a significant variable. Obviously that is not true for gases where we need to define T, P, and whether or not we're considering it to be an ideal gas. Not a problem but it means we have to be quite careful with our definitions. That's why I prefer to talk about densities rather than specific gravities, especially when discussing gases.

Edited by djack77494, 04 February 2010 - 10:24 AM.

[shan]

Any use of the term "specific gravity" requires that the reference conditions be carefully defined. For liquids, the reference is water and, being essentially incompressible, pressure is not a significant variable. Obviously that is not true for gases where we need to define T, P, and whether or not we're considering it to be an ideal gas. Not a problem but it means we have to be quite careful with our definitions. That's why I prefer to talk about densities rather than specific gravities, especially when discussing gases.

Don’t be “over correct” on the engineering terms. If you tell people 0.65 specific gravity hydrocarbon gas, they know you talk about typical natural gas. If you say 0.05 lb/ft3 hydrocarbon gas at the standard conditions, nobody knows what you are talking about.

Basically, the gas specific gravity reflects molecular weight ratio of the considering gas with air.

[mbeychok]

Dear All,

I have a question about specific gravity and its relation with temperature/pressure.

As you know the Specific Gravity - SG - is a dimensionless unit defined as the ratio of density of the material to the density of water at a specified temperature or can be expressed as:

SG = = ρ / ρH2O (3)

where

SG = specific gravity

ρ = density of fluid or substance (kg/m3)

ρH2O = density of water (kg/m3)

If we have a component for example propane in two difference conditions for example @ (20 oC / 15 barg) and (-42 oC / 0.01 barg) then please clarify we need to consider the density of water at above-mentioned conditions for calculating SG for propane?

Thanks

[mbeychok]

19120180:

You would find it most instructive to read this article in the online Citizendium encyclopedia:

Click here ==> Hydrometer

In particular, note the definition of specifc gravity (for liquids or solids) in the "Scales" section.

[kkala]

There is no contradiction whether we use density or molar mass (molecular weight) since density and molecular weight is interrelated by the GaS Law PV=nZRT or we can rewrite the gas Law in density term PM = ZRT x density where M is the mole wt. At low pressure and temperature Z the compressibility factor is close to 1 and when we take the ratio of gas density to that of air then the density ratio is equal to the molecular weight ratio

Dear S.Ahmad,
What you say is correct concerning the RD formula of wikipedia, actually meaning that reference air shall be at same conditions as the gas. Wikipedia writes that specific gravity is same as relative density, but then:
"The relative density of gases is often measured with respect to dry air at a temperature of 20 °C and a pressure of 101.325 kPa absolute, which has a density of 1.205 kg/m3. Relative density with respect to air can be obtained by RD = ρgas/ρair ~ Mgas/Mair" (symbols modified a bit).
Let us assume dry air at 200 0C and 1 Atm a (101.325 kPa a). Its density is calculated at 28.96/22.414*(273.15/473.15)= 0.7459 kg/m3 (ideal gas).
So specific gravity or relative density of mentioned air is 0.7459/1.205=0.619 (often measured....), or 28.96/28.96=1.000 (RD formula).
Probably specific gravity and relative density of gases should be defined as separate quantities.

[JMW]

Exactl.
Relative density is a term used instead of specific gravity where the reference gas may be any other gas and not necessarily air.
A typical application for online relative density meters is natural gas supply where quality can vary.
It doesn't help that the Emerson 3098 relative density analyser is referred to by them as a Specific Gravity analyser when it is more correctly a Relative density analyser since the reference gas is not usually air.
Manual here: http://www2.emersonp...ages/index.aspx