RTL produces a range of systems for monitoring corrosion/erosion and thermal behaviour of both high and
low-temperature industrial plant. Primary applications are power generation, incineration and heat
recovery boilers, however the systems are also suited to other plant such as pipelines and storage
vessels. We have around twenty years experience in system design and installation and they can be
tailored to suit individual applications.
Corrosion and Thermal Scanner Systems
Our scanner technology is primarily designed
to directly monitor fireside surface corrosion/erosion and thermal behaviour of boiler walls using
arrays of sensors welded to external (cold-side) surfaces, allowing monitoring and mapping of large
areas of wall. The technology is such, however, that it can be applied to a variety of other industrial
applications, including monitoring of plant where cracking is of concern.
Systems capable of monitoring all four walls of the largest power station boilers are supplied as
multi-enclosure, distributed configurations. More than fifteen such systems have now been installed in
Europe, N. America and Asia.
Smaller single-enclosure systems are also now available. These 'mini-scanners' have on-board computing
power and operate from a single small enclosure. They are simple to install and are designed for
monitoring smaller areas of wall.
Heat Flux and Corrosion (HFC™) Monitors
HFC monitors are small fully-independent units
with on-board computing power. They use a 'focused', single-point monitoring approach. In the case of tube
membrane walls, measurements take place around a small area of tube wall, typically a single tube, as compared
to the 'whole wall' approach, using rectangular arrays of sensors, normally used by the scanners.
For boilers, they can be used in place of intrusive heat flux probes, so avoiding the complexity, expense and risks
associated with tube removal.
HFC monitors can also be used to monitor corrosion on other 'inaccessible' surfaces such as the internal walls of pipes
and storage vessels.
Our corrosion probes are installed through
entry ports in the boiler wall and monitor corrosion/erosion by simulating the conditions of normally
inaccessible internal boiler tubing e.g. superheater and reheater tubing. They can be used as an
investigative tool to identify when corrosion occurs, for examining the effect of changes in boiler
operation on corrosion rates or to evaluate the corrosion performance of different alloys. Air cooling
is used to accurately control the temperature of the probes' corrosion elements to that of the tubing
The schematic below provides an overview of the monitoring options that we can provide for a large power
generation boiler. All three system types (scanners, HFC monitors and probes) can be interconnected and
controlled from a central point.
Corrosion and Thermal Scanners
Our scanner technology is primarily designed to directly monitor fireside surface corrosion/erosion and
thermal behaviour of boiler walls using arrays of sensors welded to external (cold-side) surfaces,
allowing monitoring and mapping of large areas of wall. However, the technology can be applied to a
variety of other industrial applications, including monitoring of plant where cracking is of concern.
Corrosion/erosion scanners, with thermal monitoring capability, are based on
well-established electrical resistance principles where thinning of a metal increases its measured
electrical resistance. However, this measured resistance is also temperature-dependent and the scanners
have the ability to simultaneously measure both temperatures and resistances to a high precision,
allowing this temperature dependency to be effectively nulled out.
Arrays of sensors are welded to external (cold-side) surfaces. Measurements are performed at, and
between adjacent sensor locations in a pre-defined sequence (i.e sensor arrays are scanned) to allow
full surface maps to be produced. Resistance measurements between adjacent sensor locations provide an
average rate of uniform metal loss between those locations. The scanners allow parameters such as metal
loss, corrosion rate, remaining thickness and time-to-replacement to be automatically calculated and
The images below show boiler fireside tube wall corrosion and a corrosion map produced by our very first
scanner in the late 1990s:
The scanner's thermal monitoring capabilities can be used independently of corrosion/erosion
measurements to provide thermal mapping on a real time basis, either as surface temperatures or heat
Dedicated thermal monitoring scanners are also available, where corrosion
monitoring is not required. Depending on system configuration, these systems are able to produce thermal
maps (i.e. heat flux and surface temperatures) from all four wall of a large power station boiler at
around two-minute intervals. This enables, for example, the effects of wall cleaning to be examined and
The images below show typical boiler wall scanner sensor locations and a scanner's thermal map of
fireside tube wall temperatures following a slag 'avalanche' down the wall of a supercritical boiler,
immediately exposing a narrow strip of wall to high radiant heat:
For continuous monitoring, new single-enclosure 'mini-scanner' systems have now
been added to our range of scanner options. Portable systems are also available
and, as well as corrosion/erosion monitoring, our scanners have been employed to monitor, and establish
root causes of, crack growth (see 'Crack Growth' tab).
Scanner Systems for up to 500 Sensor Locations
These multi-enclosure systems are capable of monitoring multiple large areas, for example all four wall
of a power generation boiler. They have a central data logger and control unit, which can be positioned
some distance from the monitoring area(s). Instrument enclosures, located near the monitoring areas,
send back sensor information to the data logger. Approximately fifteen such systems are currently
installed on conventional sub-critical, super-critical and FBC boilers in Europe, Asia and the US.
Corrosion/erosion and thermal monitoring systems perform continuous corrosion monitoring
whilst allowing real-time thermal monitoring to be performed between corrosion measurements:
Dedicated thermal monitoring systems designed for continuous monitoring of heat flux
and surface temperatures, are specifically tailored to high heat flux applications such as power plant
and waste incinerator boiler walls:
The videos below are compiled using sequences of maps created using the thermal scanner's data analysis
and presentation software. Maps can be produced and viewed in real time, for example on a control room display:
The video above shows an example of flame impingement on a 800MW supercritical boiler wall in the US.
The scanner electrode matrix is 7 x 13 electrode locations. Brighter colours represent higher fireside
tube wall temperatures (purple (800F) to white (1200F and above)).
Improved heat transfer, as a result of soot-blowing activity on a 800MW supercritical boiler wall, can be
seen in the video above. The scanner electrode matrix is 9 x 9 locations.
An example of natural slag shedding is presented in this next map sequence (7 x 13 electrode matrix)
Mini-TC Scanners™ for up to 50 Sensor Locations
These new, fully-independent systems are designed for smaller monitoring areas of up to 50 or more
sensor locations. Like multi-enclosure systems, mini-scanners are designed for both corrosion and
All electronics are housed in a single enclosure positioned close to the monitoring area, have an
on-board computer and are powered by a single low-voltage DC supply. This compact, self-contained
design helps to keep installation costs to an absolute minimum.
Systems have optional Ethernet and serial links to remote PCs in control room or office and also
incorporate 0-10V, 4-20 mA analogue outputs. Multiple systems can be linked to central computer for
data storage and analysis.
Direct, online monitoring and mapping of corrosion/erosion, remaining thickness etc. in high or low
Direct, online monitoring/mapping of external surface temperatures, estimates of heat flux and
fireside tube temperatures. Provides information on slagging behaviour, effectiveness of wall cleaning
or damaging tube wall conditions such as flame impingement and excessive tube temperatures.
Sensors welded to external surfaces – no requirement for probe entry ports.
Maintenance-free at sensor locations.
Increased confidence in plant operation and efficiency over extended time periods.
Can be integrated with plant information systems.
Dedicated software allows data analysis and presentation in a multitude of ways - historical,
real-time, linear traces and 2-D plots.
Systems adaptable and expandable with designs tailored to individual customer requirements.
Our other monitoring hardware can be inter-linked with the scanners and controlled from a central
Portable Systems for Periodic Monitoring
These have been developed for applications where continuous monitoring, using permanently-installed
systems, may not be the most appropriate or cost-effective approach. An example may be where there
is a requirement for monitoring at a number of areas within a production or processing plant.
The portable scanner has recently been re-designed, making it lighter and more compact than the original.
The power supply, and multiplexing for up to 16 sensor locations, are now incorporated within the scanner case.
There is no limit to the number of monitoring areas that may be monitored with a single set of portable
A specialised version of the scanner technology was developed for use on Electric Power Research Institute
(EPRI) sponsored projects in the USA to help establish the root causes of crack growth on weld-overlaid boilers tubes. These systems have been
installed on two supercritical power generation boilers in Pennsylvania and Texas. Systems may also
be used to monitor crack growth, for example on pipelines at welded joints, and other items of plant.
Rowan Technologies' Heat Flux and Corrosion (HFC) monitors are small dual-purpose systems primarily designed for monitoring both tube wall heat flux and fireside tube wall corrosion of boiler membrane walls. However, they can also be used to monitor general corrosion of pipelines and storage vessels.
Just like our scanner sensors, the monitor's sensors are welded directly to the cold-side wall surfaces - no access through the boiler wall is required.
For membrane tube walls, heat flux is monitored using a dual-measurement approach that combines 'active' measurements that pass signals around the whole tube circumference, including the fireside front face, with 'passive' surface measurements.s.
Alongside heat flux, HFC monitors are able to simultaneously monitor membrane tube wall corrosion using the same method developed for our scanner technology. However, unlike the scanners, HFC monitors use a 'focused' approach: all measurements taking place around a small area of tube wall, typically a single tube, as compared to the 'whole wall' approach, using rectangular arrays of sensors, normally used by the scanners.
The monitor’s techniques used for monitoring of corrosion/erosion of tube membrane walls can equally be applied to other surfaces that might suffer from general corrosion or erosion. These include the internal walls of pipelines and storage vessels. Again, the monitor’s electrodes are welded to the external pipe or metal surfaces.
All electronics is housed in a small fully self-contained enclosure, powered from a low voltage DC supply. This, combined with their external wall sensors, makes these monitors an easy-install, cost-effective choice.
The key features of the HFC monitors are:
'Focused' single-point monitoring of heat flux and corrosion/erosion.
They use a dual-measurement approach to monitor heat flux.
Like the scanners, sensors are welded to external (cold-side) surfaces.
They are fully self-contained, using an on-board computer for system control, data acquisition and data processing. A separate data logger is not necessary.
All hardware is housed in single small enclosure, powered by a low-voltage DC supply.
Easy installation, with no wall intervention other than welding to external surfaces.
A single unit can monitor up to two point locations.
Thermal and corrosion data can be delivered to plant information systems in a variety of ways:
0-10V, 4-20mA, serial communications or Ethernet.
Monitors can be inter-connected to produce a single larger system that communicates with a central
PC (e.g. in a control room) for data analysis and display.
They can be integrated into scanner systems in the same way as our corrosion probes, to form part of
a larger multi-purpose monitoring system.
The table below compares the attributes of HFC and scanner systems:
Whole areas using sensor arrays
Active and passive measurements.
Passive, sequential measurements across all sensor locations.
Highly-focused measurements at point locations.
Measurements between adjacent sensor locations.
Small, fully-independent units.
Multi-enclosure configurations or small fully-independent units.
Installation and Commissioning
Can be self-installed. Minimal installation costs.
Installation by local contractors. RTL oversees commissioning of larger systems.
Variable Temperature Electrical Resistance (VTER) systems monitor the electrical resistance of corroding
elements mounted at the end of temperature-controlled probes, enabling corrosion rates to be calculated.
The technology is suitable for both short or long term monitoring of corrosion rates in industrial and
Air cooling is used to control the temperature of the corroding element mounted at the probe end; the
elements themselves can be a variety of shapes and thicknesses, and includes tubular geometries. As the
elements thin due to corrosion/erosion, the measured electrical resistance increases.
The VTER measurement electronics are extremely sensitive and are able to detect resistance changes as
small as 50ppm (0.005%) under laboratory conditions. A single VTER system is able to control up to six
These systems formed the foundations for the development of our electrical resistance scanner
View VTER brochure
VTEC Probe Systems
Designed for aqueous environments, these Variable Temperature Electrochemical (VTEC) systems use a three
electrode element which may be air cooled to the same temperature as surrounding surfaces.
VTEC systems are suitable for applications where rapid response takes priority over absolute corrosion
rate measurements and monitor the electrochemical signals produced by the corrosion processes in aqueous
A Linear Polarisation Resistance Monitor (LPRM) is used to monitor corrosion rates and can be interfaced
to a dedicated data logger or plant information system as required.
Variable Temperature Coupon (VTC) probe systems provide accurate corrosion rates together with
morphological information at a low cost. These cumulative exposure probes are weighed before and after
exposure to yield total metal loss information. We can supply a range of tubular and flush fitting
probes to suit specific applications.
Probe Temperature Control Systems
All the above probes incorporate proportional temperature control hardware to air-cool the elements or
coupons to better than ±1°C
Rowan Technologies scanner systems can be used for monitoring and establishing the root causes of crack
The scanners use the in-built electrical resistance measurement approach to monitor crack propagation.
Changes in measured signals, as a consequence of crack growth, can be relatively small, particularly in
early stages of crack initiation and so monitoring growth can be challenging in some circumstances.
However, as cracks deepen, changes in measured resistance values increase and so become easier to
Our paper 'Detection and Quantification of Cracks and Pits in Pipes and Other Industrial Plant using DC
Electrical Resistance Techniques' discusses some of the technical issues and the techniques that might be
employed using our systems. See 'Further Reading' below.
The scanners added ability to monitor thermal behaviour also enables them to provide insights into the possible
root causes of cracking.
Please contact Rowan Technologies to discuss possible applications.
Case Study: Circumferential Crack Growth – Supercritical Power Generation Boiler.
This case study describes an EPRI-funded project that took place on a US supercritical power generation
boiler (2006-9) where a scanner system was installed to help establish the root causes of
circumferential cracking on the fireside tube walls:
Brunner Island Unit 3 is an 800MW coal-fired supercritical unit. Like many supercritical units, the
boiler is prone to circumferential cracking of the fireside tube walls under particular operating
regimes. Plant modifications to reduce NOx emissions, together with the introduction of tube weld
overlay to inhibit corrosion, resulted in a greater prevalence of this type of cracking.
Wall cracking is thought to have a number of causal factors including high wall temperatures, large tube
wall temperature differences and corrosion fatigue. Although cracks were initially superficial in
nature, severe cracking of the tube walls at Brunner Island had resulted in a number of tube leaks and
subsequent unscheduled shut downs for repair.
As part of the EPRI project, a scanner system was installed on Unit 3 to monitor areas of both front and
side wall that had previously suffered tube wall cracking. Roughly 170 sensor locations were installed
to cover some 150 sq. meters of tube wall. This unit was one of two supercritical units in the US to
have scanner systems installed as part of this project.
The Brunner Island installation provided the first scanner field trial for circumferential crack growth
monitoring and a combination of small signal changes and some inevitable background 'noise' made this
task significantly more challenging than on-site monitoring of corrosion.
More crucially, the system helped identify the thermally-related root causes of the cracking phenomena
so that appropriate action can be taken to minimise or eliminate it completely. Because cracking is
partly attributable to high and variable tube wall temperatures, monitoring and analysis of the walls'
thermal characteristics formed a critical part of this project.
The scanner provided a wealth of thermal data to help understand the underlying causes. To ensure
adequate data sampling, the scanner typically performed a thermal scan cycle at a rate of roughly 200
sensors every minute, enabling rapid thermal transients to be captured and analysed. Data was sent
directly to the plant historian, allowing immediate data processing and presentation in the form of 2D
maps and real-time traces from each sensor.
The scanner's ability to 'see the whole picture' allowed previously unseen phenomena to be visualised
for the first time. These included the real-time thermal impact of mechanical wall cleaning, the nature
of natural slag shedding and identification of likely flame impingement. All these phenomena are
possible contributory factors to wall cracking. Maps could be compiled into video sequences; as well as
having obvious visual appeal, these help to interpret more subtle time-dependant behaviour. Using
another investigative approach, time-dependant traces from individual scanner sensor locations could be
correlated with boiler operations to help understand the dynamics of the wall's thermal behaviour.
Following the initial findings of the research project, the plant made operational changes to tackle the
cracking problem on this unit. These seemed to have the desired effects, resulting in reduced down time
together with improved performance.