Convert Vent Gas Volume to Mass and Mass to Volume
February 20, 2015

People often ask, how do I convert a volume (e.g., standard cubic feet) of natural gas to pounds (mass) and pounds of natural gas to volume? This is important in communications and reports involving vent gas, VOC emissions and vapor recovery units. Below are some equations and examples to use for the conversion with a more detailed explanation later in this blog topic.

Conversion of natural gas volume to weight (mass) requires the volume of gas in standard cubic feet and the molecular weight of the natural gas. Below is a sample calculation with equations to use. Converting from volume (SCF) of natural gas to pounds (lbs) of natural gas

 Volume of gas: 50,000 standard cubic feet (SCF) at 60°F and 14.7psia Molecular weight (MW) of natural gas: 20 lbs/lb-mole Ideal gas law conversion factor: 379.3 SCF/lb-mole  (see Reference 1) Weight (mass) of natural gas: 2636.4 lbs      (see Equation 1) Weight (mass) of natural gas: 1.32 tons    (see Equation 2)

Equations Used:

Weight in lbs of total gas = (SCF)*(1 lb-mole/379.3 SCF)*(MW of gas in lbs/lb-mole)              Equation 1

Weight in tons of total gas = (lbs of natural gas)(1 ton/2000 lbs)                                                   Equation 2

Converting from pounds (lbs) of natural gas to SCF volume of natural gas

 Mass of gas: 10,000 lbs Molecular weight (MW) of natural gas: 20 lbs/lb-mole Ideal gas law conversion factor: 379.3 SCF/lb-mole  (see Reference 1) Volume of natural gas: 189,650 SCF              (see Equation 3)

Equation Used:

Vol. total gas, SCF = (lbs natural gas)*(379.3 SCF/lb-mole)÷(MW of gas in lbs/lb-mole)   Equation 3

The ideal gas law conversion factor used above is based on the relationship of 1 lb-mole of an ideal gas occupies approx. 379.3 SCF at standard conditions of 60°F and 14.7psia (see Reference 1). Typically, for air quality regulatory calculations for natural gas and its components, an ideal gas is assumed and compressibility factors (Z-factors) are not used.

Discussion

Oil company field operations and engineering departments think in terms of the volume of gas since the volume produced will be normally be reported as standard cubic feet (SCF) at 60°F and 14.7psia (see Reference 1 below). Air quality regulations customarily express limits based on the mass amount (e.g. pounds or tons) of the gas emitted to the atmosphere.

If you are going to communicate effectively with field operations, then you need to be able to convert between pounds (lbs) of gas (weight) to SCF of gas (volume) and also from SCF to lbs.

Below is some more useful information to help you do the conversions that can be used for methane or natural gas mixtures.

Natural Gas Pressure Units

• psia = pounds per square inch absolute
• psig = pounds per square inch gauge; 0 psig = 14.7 psia (most common units for gas pressure in oilfield). If gas units are in psig, then to obtain psia, add 14.7 to the gauge pressure to get the absolute pressure (psia).

Natural Gas Standard Conditions

For oilfield operations, natural gas at standard conditions normally refers to gas volumes measured at:

• 60°F and 14.7 psia

If you have 1000 SCF of a natural gas, it is based on the natural gas at standard conditions of 60°F and 14.7 psia – even if the actual temperature and pressure of the gas produced was higher. If you have actual conditions of pressure and temperature, a conversion from actual gas temperature and pressure is needed to convert the gas volume to standard conditions.

Most production reports will already be in SCF so no conversion should be needed.

Conversion of Gas Volume to Standard Conditions

Below is a conversion of actual conditions of 1000 cubic feet of natural gas at 100°F and 100 psig to standard conditions.

V1 = desired value of gas volume at standard conditions of 60°F and 14.7 psia

• P1 = 14.7 psia
• T1 = 60°F
• V2 = 1000 cubic feet (actual)
• P2 = 100 psig (actual)
• T2 = 100°F (actual)

The formula to convert from actual conditions to standard conditions uses the relationship from the Boyle’s and Charles’s laws:

P1*V1÷T1 = P2*V2÷T2                          Equation 4

To use Equation 3 above, you must convert the actual pressure (P2) to psia (by adding 14.7 to the psig value) and the actual temperature (T2) to degrees Rankine, °R (by adding 459.67 to the actual temperature in °F).

Solve for V1 = (P2*V2*T1)÷(P1*T2)     Equation 5

Solving for V1 (standard conditions volume) using Equation 5:

V1 in SCF = [(100 psig + 14.7)*(1000 cubic feet)*(60°F + 459.67)] ÷ [(14.7psia)*(100°F + 459.67)]

V1 = 7232 SCF

References

1. Compendium of Greenhouse Gas Emissions Estimation Methodologies for the Oil and Natural Gas Industry, August 2009
2. GPSA Engineering Data Book
3. EPA NSPS OOOO (40 CFR 60 Subpart OOOO)
4. EPA Greenhouse Gas Reporting Rules (40 CFR 98 Subparts C and W)

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