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section (pipes, valves, feeding and withdrawal points, etc.) and can
reproduce the flows, flow velocities, different gas qualities and gas mixtures
on the basis of defined measuring points (pressure, gas quality and volume)
at any point within the network. Based on the calculated flow velocities and
the gas quality determined at fixed points, the quantities involved in mixing
processes at the point of confluence can be calculated and the quality and
quantity of all mixing situations can be identified. However, the
reconstruction of the combustion characteristics at any point in the system
can only be correct if all aspects of the following parameters are measured.
All volumes fed in or discharged.
.
.
Pressures for the monitoring system.
All gas compositions at the feed-in points.
.
.
All valve positions and operation modes of reducers and compressors.
Furthermore, exact knowledge of the pipeline system (pipeline lengths, soils,
diameters, inner surface roughness, etc.) is required.
This method is often applied in connection with supra-regional
transmission pipelines and regional distribution gas supply networks with
few feed-in and withdrawal points. In low-pressure distribution networks
the method is difficult to apply because back-mixing in interconnected
networks, flow reversals or stagnant gas quantities (no flow due to lack of
gas withdrawal) cannot be described mathematically. Despite that, the
method was applied successfully to the Lu¨ chow distribution network in the
Wendland area in 2011. The companies E.on Ruhrgas and E.on Avacon
(local network operator) created a mathematical image of an end
distribution network with a total of four feed-in points (among these was
one biomethane feed-in point) in a computer-based heating value
reconstruction model. They performed constantly recurring gas analyses
at defined points within the network and referenced the analytical results to
the results calculated by the computer-based method. The accuracies
required under calibration legislation were met without exception. In light of
this, E.on Ruhrgas filed an application for registration of this method by the
Physikalisch Technische Bundesanstalt (PTB). (The PTB based in
Braunschweig, is the German federal authority responsible for legal
metrology under the auspices of the Federal Ministry of Economics and
Technology and the Germany National Metrology Institute providing
scientific and technical services.) The main advantages and disadvantages of
the computer-based heating value reconstruction method are summarized in
Table 16.4.
Accuracy of the heating value measurements of 0.8% of the specified
restricted measuring range required by calibration legislation can be
maintained by such systems. This was proven by several system installations
inspected and tested by the PTB (Office of Weights and Measures).
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