Systems Overview

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' approach to monitoring: 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.

They can be used in place of intrusive heat flux probes, so avoiding the complexity, expense and risks associated with tube removal.

We can also supply systems that combine the benefits of scanners and HFC systems, providing 'the best of both worlds' for optimum data quality. These provide flexibility in choosing either point or average (i.e. point-to-point) corrosion measurements.

Corrosion Probes

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 under investigation.

The schematic below provides an overview of the monitoring options that we can provide for a large power generation boiler. All three system types can be interconnected and controlled from a central point.

Monitoring options for a large power generation boiler

Corrosion and Thermal Scanners

Introduction

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 mapped.

The images below show boiler fireside tube wall corrosion and a corrosion map produced by our very first scanner in the late 1990s:

Tube wall corrosion rate map

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 flux.

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 monitored.

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:

Sensors and Slag Avalanche

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:

View Corrosion and Thermal Scanner Brochure

Dedicated thermal monitoring systems designed for continuous monitoring of heat flux and surface tempearatures, are specifically tailored to high heat flux applications such as power plant and waste incinerator boiler walls:

View Thermal Scanner Brochure

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:

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 thermal monitoring.

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.

View Mini-TC Scanner Brochure

Principle Scanner Benefits and Features

  • Direct, online monitoring and mapping of corrosion/erosion, remaining thickness etc. in high or low temperature environments.
  • 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 control unit.

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 chemical complex or oil refinery. Sensors are permanently fixed at each monitoring location with associated cabling running back to a small local site box. This arrangement is suitable for hazardous areas, as power is only supplied when the portable components are connected, when conditions are safe to do so

There is no limit to the number of monitoring areas that may be monitored with a single set of portable equipment.

View portable systems brochure

Crack Growth

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.

HFC™ Monitors

Rowan Technologies' new Heat Flux and Corrosion (HFC) monitors are small dual-purpose systems for monitoring both tube wall heat flux and fireside tube wall corrosion of boiler membrane walls.

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.

To monitor heat flux, a unique dual-measurement approach is used that combines 'active' measurements that pass signals around the whole tube circumference, including the fireside front face, with 'passive' surface measurements.

Alongside heat flux, HFC monitors are able to simultaneously monitor 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.

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:

  • Like the scanners, sensors are welded to external (cold-side) tube wall surfaces.
  • 'Focused' single-point monitoring of heat flux and corrosion/erosion, typically at a single tube.
  • They use a unique dual-measurement approach to monitor heat flux.
  • 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:
HFC Monitors Scanners
Monitoring Configuration Point locations Whole areas using sensor arrays
Thermal Monitoring Active and passive measurements. Passive, sequential measurements across all sensor locations.
Corrosion Monitoring Highly-focused measurements at point locations. Measurements between adjacent sensor locations.
Hardware Configuration 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.
View HFC system brochure

Corrosion Probes

VTER Probe 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 boiler plants.

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 air-cooled probes.

These systems formed the foundations for the development of our electrical resistance scanner technology.
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 conditions.

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.

VTC Probes

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

Crack Growth

Rowan Technologies scanner systems can be used for monitoring and establishing the root causes of crack growth:

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 quantify.

The scanners added ability to monitor thermal behaviour enables it 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.