Standard cubic foot

"A standard cubic foot of gas is defined as a cubic foot at a temperature of 21 ºC (70 ºF) and a pressure of 101.35 kilopascals (14.696 psia)". ^[1][2] Other base pressures and temperatures are sometimes used as well.

A standard cubic foot is thus not a unit of volume but of quantity, and the conversion to quantity (scf to moles) is straight forward; a standard cubic foot represents 1.19804 moles (0.0026412 pound moles). However, converting between standard volumes is not always straightforward. For example, converting standard cubic feet to standard cubic meters (SCM) is not the same as converting cubic feet to cubic metres (1 cubic foot = 0.0283168466 cubic metres) because the standard temperature and pressure used are different. A standard cubic foot (using the convention of 14.73 psi and 60 degree F ) is equivalent to 0.0283058557 standard cubic meters (using the convention of 101.325 kPa and 15 degrees Celsius ).

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 prefix, 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), MMcf (million standard cubic feet),.^[3] The M refers to the Roman numeral for thousand,two M's would be one thousand thousand, or one million. Bcf (billion standard cubic feet), Tcf (trillion standard cubic feet), Qcf (quadrillion standard cubic feet), etc. are also used.

Converting Actual Volumes to Standard Volumes

An actual volume can be converted to a standard volume using the following equation:^[4][5]

Vs= Va × Fp × Tm × (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)2: Super Compressibility Factor (often omitted or shown as equaling 1)

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

Vs = 1 cubic foot * [(13.6 psi + 50psi)/ 14.73 psi ] * [(60F + 459.67 F)/ (80F + 459.67F) ]

vs= 4.16 scf

References