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Specific Fuel Consumption CalculationBy: Ijaz Ahmad Abid

Oct 09, 2010

 Application of Temperature and Pressure Correction Factors in Gas Volume

Measurement

COMBINED BOYLE’S – CHARLES’ GAS LAWS

Temperatures & Pressures vary at Meter,Readings must be converted to Standard conditions for correct volume measurement

Introduction:Most gas meters measure the volume of gas at existing line conditions of pressure andtemperature. This volume is usually referred to displaced volume or non corrected volume m3.The value of the gas (i.e., heat content) is referred to in gas measurement as the standardvolume or volume at standard conditions of pressure and temperature. It is called correctedvolume, the unit is Nm3 (Normal cubic meter)

Since gases are compressible fluids, any change in pressure or temperature will have an effecton the measured volume by the meter and the shown volume will be changed. This is the causeof bringing the measured volume at standard volume or corrected volume.

Note: Standard conditions are based on the pressure exerted by the air around us and

base temperature. This value is normally 1.0325 bar (14.696 psi) absolute pressure at sea levelfor pressure and 15 C° or 288.15 K° (15+273.15) for temperature.

This fact is referred to as Boyle’s Law which states,

“Under constant temperature conditions, the volume of gas is inversely proportional to the

ratio of the change in absolute pressures”.This can be expressed mathematically as:

P1 V1 = P2 V2

Charles’ Law states that,“ Under constant pressure conditions, the volume of gas is directly proportional to the

ratio of the change in absolute temperature” . Or mathematically,

V1 / T1 = V2 / T2

Combining Boyle’s Law and Charles’ Law gives the relationship as under,

For gas operated equipment especially Gas generators, the Specific Fuel Consumption SFC iscalculated. The unit of SFC is Nm3 / KWh (the gas required to produce 1 KWh electric).

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This is true since the gas meter; however, will only measure the displaced volume and thecounter on the meter will only register displaced volume M3 in a specific time period. If the gas iscompressed to one-half of its volume, the measured volume will exactly be double than theprevious one with in specific time period.

For these reasons the gas counter reading or non corrected volume must be corrected tostandard volume (Nm3).

To obtain the real value of the gas that has passed through the meter.Following formula will be used,

Here;V1 is non corrected volume m3

V2 is corrected volume Nm3 

So

We can simplify more

“C” is correction factor

Correction factor can be found with following formula

Gas BillGas bill is calculated by MBTU (millions BTU)

The heat value in gas is expressed as GCV (gross calorific value), the unit is BTU/ft3.GCV can differ at different places, it can vary between 900 and 1100 BTU/ft3.

To obtain MBTU following formula will be used,

1m3 = 35.31 ft3 

Gas bill amount = MBTU*unit price

To calcu late the SFC in Nm3 / KWh,

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 Abbrev iations,P1 = Gas pressure Avg. (Abs) in BarV1 = Gas volume displace in specific time period in M³

T1 = Gas Temperature in Kelvin (C° + 273.15)

V2 = Corrected gas volume based on standard gas pressure and temperature in Nm3 

(Based on standards ICAO's ISA, ISO 13443, EEA, EGIA) 

P2 = Standard gas pressure (1.01325)

T2 = Standard gas temperature in Kelvin (C° + 273.15)

Table 1: Standard reference conditions in current use

TemperatureAbsolute

pressure

Relative

humidityPublishing or establishing entity

°C Bar % RH

0 1 IUPAC (present definition)0 1.01325 IUPAC (former definition), NIST, ISO 10780

15 1.01325 0 ICAO's ISA, ISO 13443, EEA, EGIA

20 1.01325 EPA, NIST

25 1.01325 EPA[14]

25 1 SATP[15]

20 1 0 CAGI

15 1 SP] 

20 1.013 50 ISO 5011[18]  

°F psi % RH

60 14.696 SPE, U.S. OSHA, SCAQMD

60 14.73 EGIA, OPEC, U.S. EIA

59 14.503 78 U.S. Army Standard Metro

59 14.696 60 ISO 2314, ISO 3977-2[25]  

°F in Hg % RH

70 29.92 0

AMCA, air density = 0.075 lbm/ft³. This AMCA

standard applies only to air.

Note: The Base Pressure and Base Temperature values can normally be found in contract with

gas supplier.