Operators of high-temperature pressure vessels can now see in vivid color the advantages of wireless radiometric imaging technology for safety monitoring of shell temperatures. The largest such system, utilizing 14 Mikron infrared cameras and MikroSpec R/T software, has been online for more than a year monitoring a Chevron-Texaco-designed gas separation system in Texas.
The gasification unit consists of two vessels operating at about 1,100 psi with internal firing at approximately 2,600oF (1,427oC) and exterior shell temperatures ranging from 200 to 500oF (93 to 500oC). A lining of AA22 castable refractory insulation 6 to 8" thick protects the integrity of the 1" thick carbon-steel shells, which have a melting temperature around 1,700oF (927oC). Loss or breach of insulation in a monitored area is immediately visible as a temperature spike on the infrared system's monitor graphics, and the system computer generates an alarm.
The gasification plant uses Mikro-Scan 7302 Ethernet-based thermal imaging cameras that provide +/-2 percent or +/-2oC temperature accuracy, 29o H x 22o V field of view, and 30 cm to infinity focus range. The cameras are mounted in totally sealed environmental enclosures with infrared-transparent windows and continuous purging and cooling by instrument air from a UL-certified air-purge system. Positive pressure inside the enclosure prevents dirt or dust from entering, even in the harshest conditions, and protects against explosion hazard in areas where volatile gases may be present.
An antenna in the control room receives the data, which passes over Category 5 cable to a single dedicated process PC. The software simultaneously monitors and analyzes temperature from all the cameras and compares with alarm limits. Output graphics go to individual monitors, or the system can be configured to show multiple screens on a single monitor. Screen choices allow data to be displayed and tracked in multiple, selectable formats.
The wireless visual system replaces a 12" square, thermocouple-grid monitoring system fixed directly to the shell's exterior surface. Failures of thermocouples or problems with fiber optic connecting cables left dangerous holes in the monitoring scheme until replacement or repair could be made -- always under difficult conditions. Grid problems put both the gasification unit and maintenance personnel at risk.
With the infrared system, high resolution monitors provide real-time, color-coded displays of temperature at user-defined regions of interest (ROIs) on the vessels. The software allows each camera's field-of-view to be set up to track up to 32 ROIs, each defined by any of 10 shapes, including freehand. The 14-camera system enables specific temperatures to be monitored at up to 448 discrete locations. More than 80 percent of the total surface of two 60' high vessels, including every critical area, is monitored by the infrared cameras. The plant's engineers knew the weak points in the processing system and concentrated the monitoring resources on those areas. Thermocouples now are limited to noncritical areas where there has never been a problem.
Besides real-time monitoring and alarming, the software allows data to be saved for further analysis. Details can be retrieved on temperature ranges and alarm conditions within each ROI and graphs created by software tools for temperature range analysis. Data can also be exported to Microsoft Excel for saving in a numerical context.
While thermal imaging typically is used for hot spots, vessels also may have piping, manways, nozzles and areas of poor combustion where cooling can be as much of a problem as overheating is in others. If temperature gets too low, condensate can build up between the shell and refractory, which can lead to corrosion and degraded pressure containment capability. Corrosion also can cause the refractory to flake off, allowing sudden burn-through of the shell. PH