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     Gas Compressibility Server 
    The gas compressibility factor (Z) describes the difference between ideal and actual behaviour of a gas. The Pressure/Volume/Temperature relationship for a non-ideal gas becomes


    Where Z is the compressibility factor. In fact, Z should be written as Z(P,T), as it is a function of both pressure and temperature.

    For pressures close to atmospheric, it is common to use a value of Z set to 1.0, and similarly, when the pressure is controlled very close to design setpoint, it is usually adequate to use the fixed design compressibility.

    If, however, the pressure and temperature can vary during normal operating conditions, then there can be a signficant variation in the compressibility factor for a gas. The following plot shows the compressibility factor of a gas typically used for gas lift, varying with pressure for a number of different temperatures.

    Compressibility Curves for a Typical Lift Gas

    It can be seen that at typical subsea conditions of 200 bar and 7°C (280 K) the compressibility is varying considerably with both pressure and temperature.

    The Process IT Development Gas Compressibility Service is an OPC client which allow the calculation of gas compressibility on-line, using process pressures and temperatures. Industry standard equations of state such as Peng-Robinson and Soave Redlich Kwong are used in the calculations.

    The gas compressibility (Z) factors can then be written back to the control system to allow more accurate calculation of gas flowrate.


     Gas Compressibility OPC Server 
    To complement the ZCalc Service, we are also working on development of an OPC server version. This should be available 3rd quarter 2005.


     Gas Compressibility Calculations - Web Service 

    The compressibility calculations using the Peng-Robinson EOS have also been built as Web Service, with a demonstration client shown below (requires Internet Explorer).

    Alternatively, the Web Service can be accessed via its own ASP.NET page. This will be more suitable if you are using a browser other than Internet Explorer.

    Refer to the full definition of the web service for more information.

    Critical Pressure bar A
    Critical Temperature K
    Acentric Factor  
    Pressure bar A
    Temperature K