Infrared vessel inspection whatever the scale 05 February 2019

Thermal imaging is proving to be a quick and easy way of monitoring the integrity and content level of storage tanks. And now there are several infrared technologies that can be employed to sense particular gases, such as VOCs

Outdoor storage tanks and silos that are unlagged are highly thermally conductive. During the day, the vessel and its contents absorb heat, but at night they respectively lose temperature at different rates. And it’s this difference in temperature that makes the level of their contents visible in infrared wavelengths (7.5-14 micrometres). By the same token, the temperature of the pipe network that feeds it can also provide valuable information. For example, heat losses due to failing insulation will show up very clearly in a thermal image, as will problems with process valves. Also visible is whether a valve is open or closed, if a pipe is blocked, or the drainage efficiency of roofs that are also subject to thermal cycling.

Thermal imaging cameras fall into two categories: uncooled, based on microbolometer detector technology and cooled, which uses a cryogenic Sterling cooler to enable the detector to reach the required temperature. In general terms, cooled offers better sensitivity than uncooled, although the gap between the two is narrowing.

As a result, a good quality uncooled camera can now be used for low-end R&D. New product developments and advances in detection and electronics have brought down prices of uncooled entry-level thermal imaging cameras to £750 and good professional cameras from £7,500 to £10,000, although top-end models still cost £25,000+. Most cooled cameras are used in the world of aerospace and electronics, R&D and particularly in national research centres and universities where much of the high-end research is now conducted.

The larger the detector, the greater amount of detail. For most general inspection tasks requiring accurate temperature measurement, a camera with a 320x240 or 640x480 pixel array is sufficient; but pixel size matters as the target becomes larger or the application more complex. The maximum pixel size of industrial cameras is 1024x768.

Another infrared technology that is increasingly being used for tank storage monitoring is intrinsically safe optical gas imaging (OGI), which comes into its own where tank and pipework leaks are potential environmental or health hazards. OGI cameras use spectral wavelength filtering and Stirling cold-filtering technology to visualise the infrared absorption of gases such as propylene, propane/butane, methane, hydrocarbons/olefins, ethyl alcohol, benzene, sulphur hexafluoride (SF6), R-134a refrigerants, hydrogen/CO2, carbon monoxide CO and ammonia.

These are cooled models, and they display the invisible fugitive gases as plumes of smoke on the camera screen, so their source can be identified. Because they are mainly used in hazardous environments, these cameras are designed to scan large fields of view (hundreds of metres) to get an idea where a leak may occur, but then they need to get quite close to find the exact leak location.

Unlike traditional thermal imaging, OGI is qualitative, not quantitative. Due to the environment variants and background energy differential, an OGI camera alone cannot determine the specific type or amount of gas escaping through a leak. And it should also be noted that no single camera will see all gases, so it’s important to understand the application and its need. For example, a VOC/hydrocarbon OGI camera will not see SF6, and a CO camera will not see refrigerants.

Andrew Baker, sales director North Europe, FLIR Systems

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