Comparison of gas network calculations with positive pressure/absolute pressure

A simple gas pipe whose end node lies 19m higher than the initial node serves as a demo network. The medium is in the piping and the pressures is calculated.

As calculation default an initial pressure of 1035.4 mbar (absolute) and/or 22.4 mbar (positive pressure) is set. The ambient pressure at the initial node therefore amounts to 1013 mbar.

The atmospheric pressure is assumed constant with 1.2 kg/m³, the gas density with a standard density of 0.73 kg/m³. The initial node is at a height of 339 m, the end node at a height of 358 m.

In the calculation with absolute pressure, sisHYD determines a pressure of 1034 mbar for the end node, the calculation with information of positive pressure determines a positive pressure of 23.3 mbar.

The calculated values appear contradictory at first, but.

In the case of the absolute pressure calculation, the pressure in the gas line is proven with 1034 mbar. This pressure results due to the static difference of pressure between K1 and K2, which is calculated as

The absolute pressure in the pipe therefore reduces from 1035.4 mbar to 1034.01 mbar.

The calculated positive pressure can be converted with the ambient pressure at the respective node to an absolute pressure by

pAbs = pOverpressure + pEnvironment

The air pressure of the environment is calculated at

pEnvironment =

pEnvironment = 1013 mbar – 2,23 mbar = 1010,77 mbar

Thus the absolute pressure of the gas in the piping amounts to

pAbs = 1010,77 + 23,23 mbar = 1034,0 mbar

The influence of the static difference of pressure shows clearly that the reference height for the pressure indications is crucial in positive pressure calculations.