KROHNE Summit 8800 Vol 2 - Manual (Page 32)

32 www.krohne.com 08/2013 - MA SUMMIT 8800 Vol2 R02 en
SUMMIT 8800 CONFIGURATOR 04
3 .7 .2 Line and base density
An alternative way is to rewrite the equation in terms of density as the density of gas is ρ = m/V
The result is therefore: P = ρ *R/M*T or P = ρ*Rspecific*T
Therefore Vb= Vm* ρm/ ρb
This version can be used when the meter or line density is measured, e.g. when using a densi-
tometer.
The base density will typically be calculated using the AGA 8 formula or via the relative density/
specific gravity.
3 .7 .3 Relative density/ specific gravity
The terminology specific gravity is mostly used in US related specifications. This is referred to as
ratio between gas density and air density.
In the rest of the world this is called relative density, where specific gravity is the density ratio
between a fluid and water. In this document it is further referred to relative density.
The base density can be derived from the relative density or specific gravity as follows:
The relative density or specific gravity is: db= ρb / ρair or ρb = db* ρair
The relative density can be calculated from the composition, e.g. via a GC
3 .7 .4 Mass and energy
The mass and energy can be calculated from the volume (or the volume from the mass) using:
Mass flow rate: qm= qbc* ρm
Energy flow rate: qe= qn* Hs
Where Hs is the heating value. Two types can be used:
• The superior heating value, also known as higher heating value or higher calorific value
or gross calorific value is referring to the energy produced when gas is burned and all flue
gases/vapours are cooled down to ie 1 bar pressure and 25°C.
• The inferior heating value, also known as lower heating value or lower calorific value or net
calorific value.indicates the energy produced considering the burning of the gas without cool-
ing down the vapours.
3 .7 .5 Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system and is important for
steam. It includes the internal energy, which is the energy required to create a system, and the
amount of energy required to make room for it by displacing its environment and establishing its
volume and pressure.
As enthalpy cannot be measured, the enthalpy difference is normally used. It deals with the
vapour of gaseous phases of liquid and is the energy required to turn the liquid into gases.
In most cases, the IAPWS or International Association for the Properties of Water and Steam is
followed.