A significant cause of inaccurate measurement where rich gases are most likely to be found is the error caused by two-phase flow through the meter run. All gas flow-meters used in custody transfer are designed for single phase gas flow only and will generate significant errors when liquid hydrocarbons occur in the flowing stream (up to 5 % with ultrasonic meters. Additionally, gas sampling systems are designed to reject liquids prior to analysis in gas chromatographs, so the total energy content will be under reported if some of the heavy hydrocarbons have dropped out into the liquid phase.
The Danalyzer 770XA C9+ gas chromatograph with the Hydrocarbon Dew Point (HCDP) application can help avoid this source of error by calculating the HCDP at the flowing pressure. If the HCDP is below the temperature of the flowing stream, the stream will be single phase only as all the hydrocarbons will be in the gas phase. However, if the HCDP reaches the flowing temperature, the heavy components will begin to drop out into the liquid phase and the flowing stream will become two-phase –resulting in inaccurate flow measurement and gas sampling.
The Danalyzer 770XA C9+ gas chromatography can calculate the HCDP at flowing pressure,which can then be compared to the stream temperature. If the HCDP gets too close to the Stream temperature, an alarm can be generated to warn the operator of impending two phase flow conditions.