Avoiding liquid condensates within transmission pipelines is essential to ensure reliable process operations. A maximum permitted hydrocarbon dew point temperature (HCdp) is therefore stipulated in natural gas quality specifications. This parameter is measured within the processing facility where entrained liquids are removed and HCdp is reduced as well as all custody transfer points along the transmission supply train.
Hydrocarbon dew point can be determined by two methods: Estimation through equation of state calculation of a detailed gas composition or by applying a cooled-mirror condensation technique to determine the HCdp by direct measurement.
Accuracy of the calculation method is primary dependant on the sensitivity of the gas composition analysis. Processed natural gas retains trace quantities of heavier hydrocarbons which are predominant in determining the phase transition of the natural gas from wholly gaseous to containing a proportion of liquid hydrocarbons. This transition occurs at the HCdp temperature, for any defined pressure. Quantities of mid-order alkanes such as nonane, decane, undecane and dodecane, present in single figure mol ppm and sub-ppm concentrations, are of greatest significance in inducing the formation of liquid condensates during process temperature reduction or pressure change. Failure to correctly account for these heaviest component traces would result in large variations in calculated HCdp compared to the results from a full gas analysis or direct measurement. A variation of +/-19.4°C in calculated HCdp may be attributed to heavy hydrocarbon characterisation1. Both overestimation and underestimation of HCdp may occur during process operations, depending on the validity of the characterisation method compared to the actual complete gas composition. Specifically, for the purpose of HCdp calculation, ISO 23874 requires a gas analysis of each individual hydrocarbon quantified to sub-ppm sensitivity up to and including C12 dodecane. Only the most advanced process GC has such capability, such as the LDetek MultiDetek 2 using patented plasma emission detectors. It is more commonplace for such analysis to be carried out in specialist laboratories. This requires a natural gas sample to be taken and transported. Care is required to ensure a representative sample is extracted and the validity of the composition maintained through to the point of analysis, for which advice is available in ISO 10715 and API Manual of Petroleum Measurement Standards Chapter 14.1.
Direct condensation measurement of HCdp offers the opportunity for on-line process measurement. The cooled-mirror method is a fundamental principle for dew-point measurement. Periodic measurements of natural gas HCdp using manual, visual dew point instruments of this type has been common practice for many decades. Opto-electronic detection enables automation of the cooled-mirror principle for on-line analysis. A metallic mirror with etched surface enables the transparent film of low surface tension HC condensate to be detected with high sensitivity to <5mg/m3 hydrocarbon liquid content in gas2. Such direct HCdp measurement is dependent on the complete natural gas composition including all the minute traces of heavier HC components which combine to form the HCdp when the gas is cooled. A true HCdp is therefore determined, accurately monitoring any variation in process gas quality and ensuring the gas received/delivered at custody transfer conforms to the required gas specifications.
Sign up to one of our Industry newsletters and you’ll receive our most-recent related news and insights all directly to your inbox!
Sign Up