Hydrocarbon dew point
The hydrocarbon dew point is the temperature (at a given pressure) at which the hydrocarbon components of any hydrocarbon-rich gas mixture, such as natural gas, will start to condense out of the gaseous phase. It is often also referred to as the HDP or the HCDP. The maximum temperature and the pressure at which such condensation takes place is called the cricondentherm. The hydrocarbon dew point is a function of the gas composition as well as the pressure.
The hydrocarbon dew point of a gas is a different concept from the water dew point, the latter being the temperature (at a given pressure) at which water vapor present in a gas mixture will condense out of the gas.
Importance of the hydrocarbon dew point
If the hydrocarbon dew point of pipelined natural gas is too high, some liquids may condense out in the gas pipeline. This not only degrades the heating value of the remaining gas, it increases the potential for problems in the pipeline transmission systems and causes problems for the end users of the gas such as industrial combustion equipment and household gas appliances.
Therefore, the hydrocarbon dew point is universally used in the natural gas industry as an important quality parameter, stipulated in contractual specifications and enforced throughout the natural gas supply train, from producers through processing, transmission and distribution companies to the final end users.
Relation of the term GPM to the hydrocarbon dew point
In the United States, the hydrocarbon dew point of processed, pipelined natural gas is related to and characterized by the term "GPM" which is the gallons of liquifiable hydrocarbons contained in 1,000,000 cubic feet of natural gas at a stated temperature and pressure. When the liquifiable hydrocarbons are characterized as being hexane or higher molecular weight components, they are reported as "GPM (C6+)" meaning hydrocarbons with 6 carbon atoms or more.
However, it should be noted that the quality of raw produced natural gas (before it is purified by processing) is also often characterized by the term "GPM" meaning the gallons of liquifiable hydrocarbons contained in 1,000 cubic feet of the raw natural gas. In such cases, when the liquifiable hydrocarbons in the raw natural gas are characterized as being ethane or higher molecular weight components, they are reported as "GPM (C2+)". Similarly, when characterized as being propane or higher molecular weight components, they are reported as "GPM (C3+)".
Care must be taken not to confuse the two different definitions of the term GPM.
Liquifiable hydrocarbons terminology in metric countries
The natural gas industry in countries using the metric system probably use similar terminology (for characterizing liquifiable hydrocarbons in pipelined gas) expressed in metric units rather than the above units used in the United States. British Petroleum's Miller Gas System delivering gas from the (North Sea to Scotland), for example, uses m³ of liquid per 1,000 m³ of gas to characterize liquifiable hydrocarbons in their pipeline gas.
- Hydrocarbon Dew Point
- White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure (NGC+ Liquid Hydrocarbon Dropout Task Group, October 15, 2004)
- White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure (NGC+ Liquid Hydrocarbon Dropout Task Group, September 28, 2005)
- Miller Gas System