How to improve safety and gas quality while minimizing operational risk
One of the key measurement parameters to ensure safety, gas quality and operational efficiency of natural gas processing and distribution systems is hydrocarbon dew point (HCDP).
Often referred to as ‘hydrocarbon liquid drop-out’, HCDP indicates the temperature at which heavy hydrocarbon components begin to condense out of natural gas as it is cooled at constant pressure. A higher HCDP normally indicates that the gas contains a greater proportion of mid-order hydrocarbon components, such as ethane, propane and butanes, together with increased trace concentrations of the heaviest hydrocarbon vapors associated with the oil in the production gas reservoir.
If these heavier hydrocarbon components are allowed to condense into a liquid form, they can create a number of problems:
For a more detailed introduction to hydrocarbon dew point, please read our blog post, What is Hydrocarbon Dew Point?
What can cause high levels of HCDP in natural gas?
The most common cause of high levels of HCDP stems from inefficient or incomplete processing of natural gas.
In its raw state, gas extracted from a well contains a complex mix of hydrocarbons. The heavier elements are normally condensed out by reducing the temperature of the gas, either by an external cooling system or using the Joule-Thompson process, where the gas is expanded to cool it below the condensate point. Once the condensates are removed, the gas is recompressed prior to transmission. Any problems with this process will, however, leave heavier elements entrained in the natural gas or present as condensates (often referred to as ‘natural gasoline’), which are carried downstream. In each case, the result is the same: a level of HCDP that exceeds the contract or tariff specification.
HCDP issues generally manifest themselves during the distribution process. This is normally due to pressure changes as the gas passes into a regional network or at a city gate, or where there is a lowered transmission temperature. These issues can lead to hydrocarbons condensing out of the natural gas at different – and often unpredictable – stages of the distribution network.
How to ensure natural gas quality
From the description above, it is clear that the quality of natural gas being delivered to its final destination depends on correct processing and distribution plus, most importantly, accurate and reliable HCDP measurement and monitoring procedures. In turn, these rely on the use of advanced sensors and analytical instruments.
There are several ways to measure HCDP. These include the use of a US Bureau of Mines’ Dewscope, which is a slow, manual method of taking spot measurements that requires the skills of an experienced operator if accurate and repeatable results are to be achieved. Alternatively, a gas chromatograph or a proprietary fiber-optic evanescent wave spectroscope can be used. These are, however, expensive options and limited in value.
Gas chromatographs, for example, can theoretically be used to measure HCDP through equation of state. Achieving an accurate value requires extended analyses, which need to include the heaviest trace hydrocarbons determined by ppm sensitivity; this is generally beyond the capabilities of the on-line process instruments normally used in gas metering systems.
Best-practice HCDP measurement
By comparison, best practice in the natural gas sector is to use a fully automatic dew-point analyzer, such as our advanced Michell CD603 – Condumax Dew-Point Analyzer.
The system uses a variation of the chilled mirror dew-point sensor combined with direct photo-detection of hydrocarbon condensates, to produce exceptionally high levels of accuracy, to within ± 0.5 °C at a resolution of 0.1 °C.
Designed to be used as an on-line system, CD603 has a sampling cycle time of under ten minutes, is self-cleaning with no need for purge or cooling gases, and is fully certified to IECEx, ATEX, UKEX and cQPSus standards.
The use of CD603 automatic cooled-mirror instruments is key to the successful operation of natural gas processing, transmission and power generation operations. With each CD603 being backed by worldwide technical support and calibration services, you have total peace of mind, with optimum systems efficiency and natural gas quality at all times.
We are the world’s leading experts in moisture monitoring and dew-point measurement. We have eight different technologies covering all moisture applications, backed by unrivalled technical and customer support. To learn more, talk to one of our application specialists today.
Related Information
Related Blogs
Critical Quality, Safety and Reliability Measurements in Natural Gas
Related Products
Condumax Dew-Point Analyzer – Michell CD603
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