Standard cubic foot

A standard cubic foot (scf) is a unit used both in the natural gas industry to represent an amount of natural gas and in other industries where other gases are used. It is the unit commonly used when following the US Customary System, a collection of standards set by the US National Institute of Standards and Technology. Another unit used for the same purpose is the standard cubic meter, customary when using SI units.[1]

Definitions

A standard cubic foot defines a standard unit of molecular quantity for gases, and not of volume, as the name suggests. In spite of the label "standard", there is a variety of definitions, mainly depending on the type of gas. Since, for a given volume, the quantity is proportional to the pressure and (absolute) temperature, each definition fixes base values for pressure and temperature. The ideal gas law allows then to compute the quantity per unit of volume for actual pressures and temperatures.

Natural gas

Since natural gas is an imprecise mix of various molecular species, chiefly methane but with varying proportions of other gases, a standard cubic foot of natural gas does not represent a precise unit of mass, but only a molecular quantity, expressed in moles.

In the US Customary System, one standard cubic foot (scf) of natural gas is the amount of natural gas contained in a cube whose edges are one foot long at a base temperature of 60 °F (288.706 K) and base pressure of 14.73 pounds per square inch [psi] (101.560 kPa).[1]

The standard cubic meter of gas (scm) is used in the context of the SI system. It similarly defined as the quantity of gas contained in a cubic meter at a temperature of 15 °C (288.150 K) and a pressure of 101.325 kPa (1 atm).[1]

Converting between these two different standard volume units is not straightforward, because the base temperature and pressure used are different. A standard cubic foot in the US Customary System is approximately equivalent to 0.02833 standard cubic meters in the SI system.

In the natural gas industry, where quantities are often expressed in standard cubic feet, large multiples of standard cubic feet are generally not expressed with metric prefixes, but rather with prefixes based on roman numerals, where the s for "standard" is often omitted. Common units of gas volumes include ccf (hundred standard cubic feet), Mcf (thousand standard cubic feet), and MMcf (million standard cubic feet).[2] The M refers to the Roman numeral for thousand, while two M's represent one thousand thousands, or one million. Bcf (billion standard cubic feet), Tcf (trillion standard cubic feet), Qcf (quadrillion standard cubic feet), etc., are also used.

Compressed or liquefied gases in refillable cylinders

The National Conference on Weights and Measures, a US-based non-profit organization working in cooperation with the US National Institute of Standards and Technology, has defined a set of standards in a regulation entitled the "Uniform Regulation for the Method of Sale of Commodities".[3] This regulation defines a standard cubic foot, for compressed or liquefied gases in refillable cylinders other than LPG by, "A standard cubic foot of gas is defined as a cubic foot at a temperature of 21 °C (70 °F) and a pressure of 101.35 kilopascals [kPa] (14.696 psia)".[3]

Industrial gases

Yet other definitions are in use for industrial gas,[4] where, in the US, a standard cubic foot for industrial gas use is defined at 70 °F (21.1 °C) and 14.696 psia (101.325 kPa), while in Canada, a standard cubic meter for industrial gas use is defined at 15 °C (59 °F) and 101.325 kPa (14.696 psia).

Converting actual volumes to standard volumes

An actual volume can be converted to a standard volume using the following equation:[5][6]

Vs = Va × Fp × Ft × (Fpv)2

Where,

Vs: Standard Volume
Va: Actual Volume ( sometimes shown as Vr for Registered Volume )
Fp: Pressure Factor ( sometimes shown as Pm for Pressure Multiplier )
Fp = Absolute Pressure / Standard Pressure = (Line Gauge Pressure + Atmospheric Pressure)/Base Pressure
Ft: Temperature Factor ( sometimes shown as Tm for Temperature Multiplier )
Ft = Absolute Standard Temperature / Absolute Line Temperature = [ 273.15 + Standard Temperature(deg C) ] / [ 273.15 + Line Temperature(deg C) ] or [ 459.67 + Standard Temperature(deg F) ] / [ 459.67 + Line Temperature(deg F) ]
Fpv: Super Compressibility Factor (often omitted or shown as equaling 1)

Example: How many standard cubic feet are in 1 cubic foot of gas at 80 °F and 50 psig? (assuming we have 13.6 psi atmospheric pressure and ignoring super compressibility)

Vs = 1 cu ft * [ ( 13.6 psi + 50 psi ) / 14.73 psi ] * [ ( 60°F + 459.67°F ) / ( 80°F + 459.67°F ) ]
Vs = 4.16 scf

See also

References

  1. 1 2 3 James E. Gallagher (2013). Natural Gas Measurement Handbook. Elsevier. p. 10. ISBN 978-0-12-800000-7.
  2. ↑ "Category:Glossary". PetroWiki. SPE International. Retrieved 21 November 2017.
  3. 1 2 "Uniform Regulation for the Method of Sale of Commodities" (PDF). Uniform Laws and Regulations in the Areas of Legal Metrology and Engine Fuel Quality (2017 ed.). National Institute of Standards and Technology. November 2016. p. 120. doi:10.6028/NIST.HB.130-2017. Retrieved 21 November 2017.
  4. ↑ Compressed Gas Association (1990). "Handbook of Compressed Gases" (3rd ed.). Chapman & Hall. pp. 12–13. Retrieved 17 Nov 2017.
  5. ↑ "PART VIII: Provisions Specific to Gas". Electricity and Gas Inspection Regulations. Justice Laws Website. Government of Canada. Retrieved 18 July 2014.
  6. ↑ "Introduction to Gas Metering" (PDF). Introduction to Gas Measurement. Barchard Engineering. Retrieved 18 July 2014.
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