KROHNE Summit 8800 Vol 2 - Manual (Page 31)

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08/2013 - MA SUMMIT 8800 Vol2 R02 en
SUMMIT 8800 CONFIGURATOR 04
We can calculate how much measurements may change to result in 1% change in normalized
volume. Some typical numbers:
• The Volume changes 1% when the pressure changes 1% (e.g. 10 mBar at 1 bar)
• The Volume changes 1% when the temperature change of 3 °C
• The Volume changes 1% when the density change due to pressure or temperature:
• Pressure change of 4 bar or temperature change of 70 °C in a 5 bar pipeline
• Pressure change of 4 bar or temperature change of 4 °C in a 60 bar pipeline
• The Volume changes 1% when the density change due to composition:
• Either 3 % change of methane, 1% change of ethane or 0.5% change of pentane
This means that in gas:
• Correction for Pressure and Temperature is always needed.
• Correction for density is important for high pressure
• Correction for composition is only needed for high pressure
• A Gas chromatograph is only needed for changing composition
The SUMMIT 8800 flow computer calculates the gas volume at these expanded conditions based
on the measured actual flow. For this the pressure and temperature at the location of the meter
are used to calculate flow at “normalised” or “standardised” conditions.
Since most gases are non-ideal gasses, the gas compression needs to be corrected by means of
the compressibility factor (z-equation). This compressibility factor must be calculated at normal
conditions and on line conditions, the division of these defines the correction for a non-ideal gas.
This correction factor can also be determined based on the density determined at base condi-
tions and at line conditions.
Many different formula’s have been developed in the last years to correct the liquid, gas and
steam, depending on the type of product. Some based on a database of gasses, some based on
physical properties, all off them with limited range of validity and associated accuracy. Therefore
the configuration of a flow computer assumes the basic knowledge of formula’s needed. Here
the basics:
3 .7 .1 Equation of state
As gas, the influence can be calculated from “Equation of state”
P*V = n*Z*R*T or V =n* Z*R*T/P where:
P = Pressure
V = Volume
n = Number of moles= mass/molar mass or n=m/M
Z= Compressibility of the gas
R = Universal gas constant = 8.31451 J/mol K
T = Temperature
Z depends on the composition of the gas and is at very low pressure equal to 1.
From here it follows that: Vb= Vm* (Pm*Zb*Tb)/(Pb*Zm*Tm)
This is one of the ways to calculate the volume at base conditions. In this case the compressibil-
ity could be derived from the composition of the gas e.g. from a GC.