9 replies to this topic

[farid.k]

guys, this question might be very basic, however i need to clear my mind.

can anyone advise me how to abtain the specic gravity of gas? some say ratio of molecular weight of gas to air and some says ratio density of gas to air at standard pressure and temperature. if i simulate hysys and try calculate the value, both give different values. appreciate to advise me. 

[breizh]

http://www.engineeri...ases-d_334.html

 

Consider this resource to support your work .

 

" The Specific Gravity - SG - of a gas is a dimensionless unit defined as the ratio of density of the gas to the density of air at a specified temperature and pressure. In general conditions according NTP - Normal Temperature and Pressure - defined as air at 20^oC (293.15 K, 68^oF) and 1 atm ( 101.325 kN/m2, 101.325 kPa, 14.7 psia, 0 psig, 30 in Hg, 760 torr), where density of air is 1.205 kg/m³, is used. 

Note that the density of both gases should be evaluated at the same pressure and temperature - and when this happens the SG can be calculated utilizing only the molecular weights ".

 

 

Breizh

Edited by breizh, 15 June 2014 - 05:36 AM.

[fallah]

farid.k,

 

Both methods are right and should be lead to the same result. Please recheck your simulation/calculation again. Note that the gas dencities should be evaluated at the same pressure and temperature...

[farid.k]

first of all, i would like to thank for the prompt reply. i do appreciate so much.

 

refer attached file for the simulation. by right, the molecular weight would not change regardless of the temperature and pressure. however, by using ratio of density, density change with pressure and temperature. if calculate specific gravity of gas at operating pressure (some says should calculate at operating pressure) which is 40 barg, using molecular weight and density, it will gives two different value which is very significant. thats why i become confuse.so i think, better to use molecular weight. constant regardless of temperature and pressure.

Attached Files

•  hysys simulation.JPG   73.6KB   13 downloads

[staffel]

you can refer to the definition of standard condition given above

 

293.15 K 1 atm

 

and calculate densities for your gas mixture at those conditions,

the point is that at those conditions most fluids act as ideal fluids

you can assume ZV=1 and calculate specific gravity as

 

Density Fluid (293.15 K,1 atm) /  Density Air (293.15 K,1 atm)

 

or

 

Mw(fluid)/Mw(Air)

 

(from ideal gas law)

 

in real world many fluids at 293.15 K are well above TC

and ZV may differ from 1.0,

for example ZV for C1 at 293.15 K 1 atm is about 0.998

 

value calculated with PRODE PROPERTIES

 

and you may  get values a bit different from Mw(fluid)/Mw(Air)

[oscarsender]

Farid,

 

The Propylene is in liquid state at 4101 kPa and 32 degC (the density of 504 kg/m3 is too large for a gas phase). The air is still in vapor phase. With this you cant apply the calculation of specific gravity of liquid in relation to gas phase.

Edited by oscarsender, 16 June 2014 - 02:22 PM.

[farid.k]

oscarsender

 

thanks for the reply..yes, you were right. but what if i change the propylene to gas state (increased Temperature to 80 deg C)? when i calculate SG, it is around 3.19.

 

So i think SG of gas shall be based at 20 degree C and 1 atm regardless of how high the operating pressure and temperature.

Attached Files

•  Capture.JPG   111.56KB   6 downloads

[xavio]

farid.k,

 

SG is seldom used for gases, due to potential confusion like you are facing now.

 

For liquids, it is universal to use 1000 kg/m³ water as refence (4 degC), so when I say SG of a liquid at certain temperature is 0.5, people will quickly grab the fact that the density of the liquid is 500 kg/m3 at that temperature.

 

For gases, density depends on pressure as well, even the reference density is pressure-dependent.

The problem is, people do not universally agree on which condition to use as reference.

 

Mr. Breizh has explained that some people use air at NTP as reference, which is 1.205kg/m³.  When you mention the SG of a gas, you should not forget to tell us what reference condition you use, e.g. "compared to air at NTP".

The so-called "NTP" itself is a matter of debate, different organizations use different set of conditions.  Why bother then?

 

I'd prefer to tell you directly what the density of the gas is at the P and T of interest.

 

Good luck.

 

xavio

[farid.k]

xavio:

 

 

then, what is the value that i supposed to put in the control valve sizing for specific gravity?

[xavio]

farid. k,

 

So it is all about sizing a control valve, we can then refer to specific literature.

In ISA-75.01.01, specific gravity of gas is defined as follows:

 

Gg Gas specific gravity (ratio of density of flowing gas to density of air with both at
standard conditions, which is considered in this practice to be equal to the ratio of
the molecular weight of gas to molecular weight of air.

 

Actually Mr. Breizh has answered your question in clear and concise manner. Have a look again at his explanation.

If you only want to calculate SG for this particular application, the SG is the ratio of MW regardless of P and T.

 

Anyway, as I have said, defining SG of gas can be confusing.

If you put the wrong SG on control valve spec, vendor may come out with erroneous result.

When I specify control valve for gas, I always provide P, T, MW, and Z, and let the vendor calculate the rest.

There are variants of sizing formula which do not use SG.

 

Good luck.

 

xavio