Liquids in gas pipelines are recognised as a major issue. Natural gas processing to remove water vapour, carbon dioxide and hydrogen sulphide requires the injection of liquids into the gas stream. If these are not removed in a consistent and effective manner, they can cause significant damage to compressors and other plant in the gas transmission system. Rapid detection and identification of liquids at custody transfer points in the gas distribution network allows buyers and sellers of gas the tools necessary to take rapid operational decisions to minimise contamination of gas pipelines. Therefore, in situ measurement to identify the substance offers operators a rapid determination, and possible diagnosis, of the process problem without taking samples and waiting several hours, or sometimes days, for results. The identity of the liquid can be crucial as it may vaporise in the system or remain in a liquid phase, depending on the nature of the liquid, in which case direct user intervention may be required.
Until recently, no device has been able to make this measurement, given the operational limitations. One possible instrument solution to this problem was to use Raman spectroscopy as the technique is non-invasive, does not require a sample to be taken from the pipeline for laboratory analysis and has become cost-effective in recent years. However, the Raman effect is very weak, and thus conventional systems cannot provide sufficient signal to noise on the time scales required, given the environmental constraints. As part of the operating criteria, any instrument used to identify the nature of an unwanted liquid must not interfere with the gas stream and must be mountable on any pipeline. The instrument must also interface with existing infrastructure. Gas pipelines are often buried, and can only be accessed from a standoff pipe, which can be up to 2 m in height, therefore any device must be capable of making observations from at least this distance. This fact does have the advantage that the optical surfaces are remote from the liquid contamination flowing in the main pipeline, which should help lower maintenance requirements. Given the weak nature of the Raman response, it is imperative all the photons are collected from the target. This implies the spectrometer must have the highest etendue possible if the measurements are to be successful. Also, the instrument itself must be compact and robust if it is to be mounted on pipelines around the globe. Due to the multiplex disadvantage, the filtering requirements are critical to the instrument’s performance and had to provide a suppression of better than 10 to the 7 if the instrument was to be successful.
To achieve this performance, a Raman instrument using a static Fourier Transform spectrometer, known as a spatial heterodyne spectrometer (SHS), was developed.