FOLDED-PATH GAS ANALYSERS MAKING THE ANALYSER FIT THE PROCESS
In chemical plants, petrochemical plants and refineries, tunable diode lasers (TDLs) are becoming an increasingly common sight. Their high reliability and low maintenance has made them the gas analyser technology of choice for many companies. However, installation locations and conditions encountered in some processes have limited their application range.
T
he rapid rise of tunable diode laser (TDL) analysers over recent years has led to them becoming established as a core measurement technology within the portfolio of gas analysis techniques available today. The ability of TDLs to interface directly to the process, which eliminates the need for costly and high-maintenance sample handling systems, gives them an inherent rapid speed of response. This makes them ideal for real-time dynamic measurement of process conditions. Conversely, one aspect of TDLs has not advanced at the same pace as the measurement technology itself: the process interface available for cross-stack, in situ units. Installing an optical, cross-stack TDL directly in a process pipe or vessel creates some installation and operational challenges and limitations which need to be considered carefully before considering their deployment. Nowadays, new innovative process adaptions have been developed that allow the operation of TDL analysers in locations and applications previously considered impractical, if not impossible.
Installation point selection and unit size The first consideration when planning a cross-stack/pipe TDL is the installation point. The decision has to be based on executing the measurement at the point in the process where the most pertinent analysis data can be collected. However, this can create the first challenge if there are space constraints or if the diameter of the process pipe is small. Optical restrictions mean that an elongated housing is needed to provide the required focal length between the laser and the
26 WHAT'S NEW IN PROCESS TECHNOLOGY - SEPTEMBER 2017
receiver. These long optical housings not only create installation and measurement limitations on small pipes but, due to their large internal volumes, also exacerbate the stability of alignment, and provision and consumption of purge gas required to keep the analyser’s optics clean.
Optical alignment Even if space limitations are not a concern, alignment of the transmitter and receiver units across the pipe is always a consideration. A cross-stack TDL requires careful alignment, which means mating flanges have to be welded onto the pipe prior to installation, adding cost and complexity to the procedure. To aid alignment of the transmitter and receiver units, various alignment mechanisms are available, ranging from simple, large, compression O-rings to more complex, flexible, metal sealing designs. Care has to be taken to ensure that satisfactory alignment has been achieved, and that the integrity of the process line has not been compromised through introducing leak paths during the alignment process, which could allow the escape of process gases into the atmosphere.
Physical mass An issue that is particularly relevant when considering installing a cross-stack TDL relates to its size. Firstly, the optical alignment can degrade over time as the alignment mechanism sags due to the weight of the transmitter and receiver body together with the long optical housings (Figure 1). Additionally, if the vessel wall is thin it is essential to add braces to provide additional stability.
www.ProcessOnline.com.au