Non-intrusive Performance Mapping Provides On-Line Corrosion Data

Issue 6 and Volume 106.

Various non-destructive methods are available for mapping and measuring corrosion and erosion in power plant equipment. Some are non-intrusive as well. One such system is the Performance Mapping Scanner (PMS) system, which monitors thermal characteristics – temperature or heat flux – and corrosion/erosion directly on sections of the plant. The system can map large plant areas or smaller areas where localized corrosion and fouling may be a problem.

The method, produced by Rowan Technologies Ltd., is non-intrusive because the portion of the plant being inspected becomes the sensor. The method is especially preferable over intrusive systems when high-pressure systems are being examined.

AES Drax Power Station, UK, the largest coal-fired station in Western Europe with six 660 MW units, is using the system. Membrane walls at the Drax station are constructed from welded carbon steel tubes through which the heated water/steam passes. The tube walls directly exposed to the internal boiler conditions can, under certain conditions, be subject to considerable corrosion. Heat transfer through the walls is also subject to considerable variation because of the build-up of deposits, changes in the burner configuration and variation of the flame characteristics.

The corrosion/erosion aspect of the system has evolved from the well-established electrical resistance (ER) technique, where the thinning of a metal increases its measured electrical resistance. However, this measured resistance is also temperature-dependant and the scanner has the ability to simultaneously measure temperatures and resistances to a high degree of accuracy, allowing this temperature dependency to be effectively nullified. The thermal monitoring hardware can operate independently of ER measurements to provide two-dimensional thermal maps or local information in real time.

The system provides monitoring and mapping of corrosion/erosion or remaining thickness in high and low temperature environments and real-time thermal mapping with the output capable of interfacing with the plant’s DCS/PLC systems. It measures short and long-term trends and is adaptable and expandable to suit future requirements. The PMS corrosion mapping system collects data in real time. Because of the slow nature of corrosion, the data is post-processed every four weeks to generate the corrosion/metal loss maps.

Dedicated thermal monitoring systems are available if corrosion/erosion monitoring is not required. Under the hostile conditions of a coal-fired boiler, the PMS system can detect metal loss to better than 0.3 percent of start thickness. That equates to a 20-micron loss on a 6-mm thick wall.

The system provides direct on-line monitoring of corrosion/erosion rates and direct on-line, real-time monitoring and mapping of temperatures and heat flux. The system can be installed easily to provide 2-D mapping using a matrix of simple connections. No maintenance is needed at the monitoring locations.

The first scanner system at Drax was installed on a sidewall of boiler No. 4. The system monitors both corrosion and thermal behavior of the tube wall in an area of relatively high corrosion. The scanner electrodes are spot welded to the outside face of the membrane between the tubes and form a square matrix of connections covering some 50 m2 of the wall.

Information provided by the maps had previously only been obtainable during major boiler outages when ultrasonic measurements of remaining wall thickness were made from within the boiler. The scanner system now provides this information on-line within weeks rather than years. The scanner has shown that high periods of corrosion are frequently transient, lasting a matter of only a few weeks before moving to another location. This may be associated with flame chemistry where the proximity of the flame results in reducing conditions on the walls.

The scanner has consistently shown generally higher corrosion rates during the winter period, possibly due to increased water in the feedstock. The system has the added advantage in being able to identify changes in corrosion rate as a function of medium-term changes in operating conditions, something that the station has never previously been able to quantify.

The electrode configuration at the furnace wall also enables wall temperatures to be measured. By rapidly scanning the temperature measurements at each node, contour maps can be produced within a few seconds of fireside wall temperature, water/steam temperature, and heat flux.

Clear pictorial information, available in real time, enables the boiler operators to see the immediate effect of changes in boiler operation on the wall’s thermal performance, information valuable both for boiler efficiency and materials performance reasons.