Just as with vessels and tank farms, pipeline issues include
manually-entered data and error handling. Some gas
composition information continues to come from samplers and
errors due to pressure, temperature, and other failures must be
addressed. For a pipeline, the FLOWCAL application can
recalculate volume and energy quantities due to restated
pressures, temperatures or other anomalies and create a full
API 21.1 and 21.2 compliant audit trail.
Balancing gas and liquids
Once all gas and liquid volumes have been measured, a mass
balance can be performed to validate the measurements and
identify any issues. The below example uses gas and liquid
measurements:
The calculation of mass for a gas meter can be done using
the volume and the relative density of the gas being measured.
Mass = volume x relative density of the gas x density of air
Figure 1 shows a FLOWCAL screenshot where it has
calculated the mass for an orifice meter.
When mass% analysis is provided, the mass for each
chemical component can be calculated as follows:
Component mass = total mass x mass% / 100
It is also possible to calculate the mass for a liquid meter.
Figure 2 shows a FLOWCAL screenshot for liquid mass.
Once the total mass and component mass for the gas and
liquid meters have been calculated, it is then possible to put
them into a balance calculation and balance on component
mass and total mass. This can be seen in Figure 3.
Balancing allows operators of the various processes
involved – vessel, tank farm, liquefaction or regasification plant,
and gas pipeline – to be confident in the tracking, auditing and
verification of their natural gas and LNG contracts.
References
1. ‘Qatargas’ Q-FLEX Arrives in the United States’, AME Info.
2008-06-24; via Wikipedia, retrieved 2 August 2008.
2. K. Helminski, ‘Reducing Measurement Uncertainty’,
LNG Industry
, April 2014, pp. 47 - 51.
Figure 4.
Balancing inventory across the entire process,
between the gas pipeline and LNG vessel, presents a major
challenge.
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