Unit of volume used in gas industry 
The volume of reference or the cubic meter normal is often characterized
by the abbreviation m³(n) or (Nm3).
This indication means that the cubic meter is given under the conditions
of temperature to 0°C, under an absolute pressure.
Specific Gravity
The density of gas, relative to air, is called specific gravity.
The specific gravity of air is defined as 1. Since propane gas has
a specific gravity of 1.5, propaneair mixtures have a specific
gravity of greater than 1.
Unit of pressure used in the gas industry 
The bar and the millibar are used for the measurement of the pressures
of gas distribution.
 Low pressure: up to 50 mbar included.
 Average pressure a: from 50 mbar to 0.4 bar included.
 Average pressure b: from 0.4 bar to 4 bar included.
The legal unit of measurement of the pressure, in the international
system, is the PASCAL (N/m2) and the bar.
You will find in the table attached its multiples and submultiples.
Notation:
With:
 Pef = effective pressure.
 P = absolute pressure.
 Pat = Atmospheric pressure.
Calorific value of a fuel 
The calorific value of a fuel is the quantity of heat produced
by its combustion, at constant pressure and under the conditions
known as " normal " of temperature and pressure (i.e.
to 0°C and under a pressure of 1 013 mbar).
The calorific value of natural gas is expressed in kWh per cubic
meter.
The combustion of a product generates, inter alia, of water to
the vapor state. Certain techniques make it possible to recover
the quantity of heat contained in this water of combustion by condensing
it (boilers at condensation)
One thus distinguishes two calorific values:
 The lower calorific value or Net calorific value (NCV) which
supposes that the products of combustion contain the water of
combustion to the vapor state. The heat contained in this water
is not recovered.
 The higher calorific value or Gross calorific value (GCV) which
supposes that the water of combustion is entirely condensed. The
heat contained in this water is recovered.
Il existe, pour la plupart des combustibles un rapport " GVC
/ NVC" entre ces deux coefficients :
 Natural gas: 1.111
 Butane, Propane: 1.087
 FOD: 1.075
 FOL: 1.055
 Coal: 1.052
Since most gas burning appliances cannot utilize the heat content
of the water vapor, gross calorific value is of little interest.
Fuel should be compared based on the net calorific value. This is
especially true for natural gas, since increased hydrogen content
results in high water formation during combustion.
The natural gas makes it possible to recover, by condensation,
the heat contained in the fume without there being problem of corrosion.
The current outputs of the boilers with condensation can reach 105
% on NCV
Thus, while reasoning in kWh NCV, one reaches outputs higher than
100 %. The reasoning in kWh GCV is thus adapted more to energy natural
.
Calorific power and useful output 
The description plates of the generators gas present two, even
three displayed powers.
One finds there nominal useful output nominal, input ratings and
sometimes of the minimal useful output.
The difference between these values is directly related to the
output of the generator.
The nominal calorific flow rate (gas flow rate) represents the
maximum calorific power of the generator.
The physical definition of the output is the relationship between
the power provided (useful) to the water of heating of the generator
and the power absorptive by this one. One takes account of the losses
inherent in the operation.
This output is also called useful output.
Thus, a boiler having a calorific flow rate of 89 kW and an useful
output of 75 kW will have a useful output of 89 % approximately
NCV. This generator will be able to thus feed a network of heating
of a maximum power of 75 kW.
If one wants in practice to refine the calculations, it is possible
to calculate the calorific power and the useful output of the generators.
The absorptive power or calorific power is the product of the volumic
flow rate of gas by the NCV with constant pressure of fuel expressed
in quantity of heat.
 Qv = Volumic flow rate of gas.
 PCI = Lower calorific value of gas.
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