Best Practices for Measuring Dew Point in Transmission Gas

Protecting pipelines and gas power systems

The unpredictability and complexity of climate change is making a global impact, affecting individuals, business operations, and government focus. 

For example, the transition to a decarbonized energy system has been underway for some time, with long-term planning and structured roll-out programs already in place; however, they’ve been given fresh impetus following recent extremes in seasonal temperatures, combined with the continuing gas and oil supply crisis caused by Russia’s invasion of Ukraine. 

One consequence is increasingly fragmented supply sources for natural gas networks. We’re seeing a rise in the use of biomethane, hydrogen, liquid natural gas (LNG) and shale gas, combined with the natural gas from long-standing on- and off-shore gas fields. 

Navigating transmission gas specifications 

The progressive diversity of supply highlights the importance of accurate monitoring and control of the products that are added to transmission networks, and of applying consistent and internationally agreed criteria.  As these are used for both quality control and managing charging mechanisms, they are often referred to as ‘tariff specifications’ in North America, where criteria are generally set by the pipeline operators; in comparison, Europe fosters a common ‘custody transfer specification’, set out in the EASEE-gas Common Business Practice 2005-001/01 standards. 

In each case, the different sets of specifications define the maximum concentrations of gases, liquids and chemical elements such as hydrogen sulphide, sulphur, oxygen, hydrogen and water – allowable in gases that are injected into the transmission network.  Gas quality measurements are typically made at system entry points, cross-border points (in Europe) and at blending stages, using either fixed monitoring equipment or, as is more often the case in Canada and the United States, portable instruments. 

Achieving gas and liquid equilibrium 

Hydrocarbon and water dew point are two of the key measurement criteria for transmission gas quality. These factors are important to prevent the risk of liquid dropouts during distribution and use.  There is the risk of hydrocarbon condensates pooling at low points in pipelines, which can create a breeding ground for microbial corrosion. Another risk is the presence of downstream liquids affecting the efficiency of gas turbine generators, with the threat of damage to turbine blades, pre-ignition flashbacks, and increased NOx emissions.  Similarly, elevated moisture content in transmission gases can lead to the formation of solid crystalline hydrates, and accelerated corrosion in steel pipes and system components. 

The measurement criteria for hydrocarbon dew point are commonly based on the cricondentherm condition. This is the maximum temperature at which a particular composition of gases can exist in two states simultaneously – at both liquid and vapor phases. Hydrocarbon dew-point temperature varies with gas pressure as a retrograde curve, effectively creating a phase envelope encircling the region where gas and liquid are in equilibrium; this can be seen in the graphics below, which indicates where the gas occurs as 100 % vapor on one side of the hydrocarbon dew-point curve (at higher temperature), or 100 % liquid (at lower temperature) on the outside of the hydrocarbon bubble line. 

Moisture vapor content in gas composition dictates water dew-point temperature, which increases with pressure, and the maximum aqueous condensation temperature occurs at the fullest pipeline operating condition. 

In Europe, although the harmonized specification requires hydrocarbon dew point to be below -2 °C at pressures that can range up to 70 bar, normal practice involves measuring the cricondentherm point at 27 barg. In comparison, Canadian requirements stipulate hydrocarbon dew-point measurements at maximum pipeline pressure, or a defined pressure above the cricondentherm condition, although this may not be where the highest hydrocarbon dew point occurs. 

Further variation in approach can be found with the criteria for water dew point: the European specification requires measurements to be -8 °C at maximum pipeline operating pressure, while in North America it is common to use maximum moisture content rather than dew point as the appropriate reference. 

A new generation of dew-point instruments 

Ensuring that dew-point measurements are precise, consistent, and repeatable depends on the latest generation of advanced analyzers.  For example, our Condumax II and recently launched CDP301 offer a choice of fixed on-line or portable devices.  

Condumax II chilled mirror technology has been proven in natural gas installations around the world, while the CDP301 is ideal for use throughout North America.  Its robust, compact size, and freedom from the need for any coolant gas cylinders, makes it easy to carry to site and for measurements to be taken by unskilled operators. Also, an integral camera and recording unit allows readings to be analyzed by off-site specialists.  

With almost 50 years’ experience developing innovative precision instruments, we are the application experts in dew-point measurements for all natural gas applications. If you would like to discuss your requirements, please contact our team today.

If you found this interesting, why not check out the following blog posts?

Moisture Measurement for Natural Gas Liquid Extraction

Moisture Measurement and Liquified Natural Gas

Moisture Measurement for Hydrogen and Natural Gas Blending

Moisture Measurement in Biomethane