Abstract — Power plant boilers need to be cleaned and

inspected on a regular basis. Boiler chambers can be up to 50m

high requiring scaffolding for all works on the water walls. The

scaffolding itself can take several weeks.

To overcome the disadvantage of the time consuming

scaffolding work, an automated boiler wall crawler for wall

cleaning and inspection is being developed by Alstom in

collaboration with Alstom Inspection Robotics and Waterjet

Technologies.

The automated system will be able to drive up the water wall

with the cleaning or inspection application integrated and no

scaffolding will be required anymore.

The cleaning application consists of a patented suspension

cleaning technology where abrasive and water are mixed and

pumped up to a defined pressure.

The inspection will be done using latest ultrasonic technology

to measure remaining wall thicknesses of the tubes.

The deployment system consists of two magnetic track drives

connected by a frame and two passive magnetic wheels.

Laboratory tests have shown promising results. The system is

able to drive up and down the water wall during the cleaning

with the system running at around 100bar.

Index Terms-- Automation, Maintenance, Power Generation,

Reliability, Boiler, Safety, Robotics, Water Jet Cleaning,

Inspection

I. INTRODUCTION

ower plant boilers are elements of thermal power plant

used to convert the chemical energy of the (fossil) fuel

first into thermal energy by combustion and subsequently

transferring the thermal energy of the combustion gases into

thermal energy of highly pressurized water. The water flows

inside of armed pipes arranged at the inside walls of the boiler

chamber. The boiler chambers can measure up to 35m x 35m

x 50m. As an effect of the continuous combustion process the

pipes are more and more covered by a layer of combustion

residues.

These pipes need to be inspected on a regular basis, mainly

for wall thickness. Before a reliable inspection can take place,

the residue layer has to be removed. As of today, this task is

done manually using dry blasting method. The cleaning and

inspection method require the installation of a scaffold at the

Vera de Vries and Simon Honold are with

ALSTOM Power, 5401 Baden, Switzerland , www.alstom.com/power Philipp Roth is with Waterjet Technologies Ltd,

5210 Windisch, Switzerland ,wwww.waterjet-technologies.ch

Wolfgang Zesch is with Alstom Inspection Robotics Ltd. Technoparkstrasse 1, 8005 Zürich, Switzerland, www.inspection-robotics.com

inside of the boiler in order to allow the personnel to access

the wall.

To reduce the cleaning and inspection time and to make

this service task safer, a system is being developed to execute

the working steps without scaffolding. The aim is to reduce

the process time significantly down to a few hours or days.

Also, the new system allows for recording the inspection data

in order to make it available offline for further analysis.

Fig. 1 Typical boiler of a coal power plant

The inspection and cleaning system consists of a magnetic

deployment device with an integrated cleaning or inspection

application. It is able to crawl up the water wall, carry out the

service task in a certain area and send the data back to the

operator standing on the floor area of the boiler controlling the

system.

The benefit in using the new cleaning and inspection tool is

mainly time saving and improvement of EHS (environmental,

health and safety). In terms of EHS, working at height is not

required anymore. The deployment device will be secured

with a safety device, preventing the system from falling down

in at any moment during service time.

In terms of time saving, the main benefit is to avoid

scaffolding. In many cases, only a partial inspection is

required. For these missions, the impact on the time saving is

even higher. The scaffolding for a partial cleaning and

inspection task takes several shifts and requires up to 10

workers. Compared to the relatively short cleaning and

inspection time for the partial area, this is quite time

consuming. With the new system, the several shifts of

scaffolding can be saved.

Automated Boiler Wall Cleaning and Inspection W. Zesch, S. Honold, Ph. Roth, V. de Vries

P

Considering the high number of boilers installed (e.g. 600

in the USA [2]), and assuming a major outage every 5-10

years, the benefit of having an automated system available is

remarkable.

II. SYSTEM DESCRIPTION

The boiler wall cleaning and inspection system consists of

a magnetic deployment device with an integrated cleaning and

inspection application. The system is highly mobile and the

installation time is short.

A. Integration of cleaning application

The cleaning application integrated in the deployment

device is a suspension cleaning technology. Suspension

cleaning by Waterjet Technologies uses a newly patented

pump system [1], [5] to pump a water/abrasive mixture up to

170bar. The pressurized suspension is conducted to a nozzle,

where an abrasive water jet of a defined geometry is formed.

The process is comparable to sludge cleaning, but works at

higher pressure thus resulting in a higher cleaning

performance.

Fig. 2 Water jet suspension cleaning system

The cleaning system (Fig. 2) fits on a euro pallet (size

800mm x 1200mm) and is easily transportable. The setup time

for the system takes about 1 hour.

The working pressure of 170bar allows using hoses for

tubing as thin as 12mm in diameter. They are extremely

flexible (bend radius < 50mm) to conduct the high-pressure

suspension from the pump to the nozzle system (Fig. 3, (1))

This results in low weight of the supply tube (Fig. 3, (2))

which is important because of the limited payload capacity of

the deployment device.

Three tungsten carbide nozzles form the jet in the nozzle

system with a jet velocity of approximately 150 m/s. Each

nozzle generates a fanjet with an angle of approximately 50

degrees (Fig. 3), so width of up to 200mm is cleaned in one

pass.

Fig. 3 Integrated water jet nozzle system

For this cleaning application corundum of mesh size 220 is

used. To eliminate the risk of corrosion, an anti-corrosive is

added to the suspension in the mixing tank. Several tests have

shown that the base material removal is negligible (in the

range of microns) [1].

The reasons of using suspension water jet technology

instead of conventional blasting methods are manifold:

Improved surface quality resulting in better inspection

results.

Negligible base material removal

Improved progress speed due to the use of fan jet nozzles

Light weight and flexible supply hoses

Fast system set-up time

Highly mobile system

Recirculation of suspension possible

B. Integration of inspection application

The inspection method used is conventional ultrasound

technology. With the probe and the probe holder mounted on

the deployment system, the focus is to measure the remaining

wall thickness, in a next stage also to find cracks, both from

the hot side only. In order to save inspection time the goal is to

acquire the UT signals simultaneously with cleaning. The

defining factor is the cleaning speed, which is significantly

lower than the possible UT scan speed. The inspection result is

a map (C-scan) of the water wall, presenting the exact

locations of all irregularities, such as depicted in Fig. 4.

The main challenges for integrating the UT system in this

inspection system are:

Large surface roughness of the tubes due to long

exposure to a very aggressive atmosphere. This requires

sufficient couplant feed in order to provide good acoustic

coupling

Couplant supply through >50m tube. An onboard tank

seems not feasible due to large couplant consumption.

Transmission of UT signal to the controller due to the

large distances to the operator (>50m) the digitization

has to be performed on-board

Shielding of inspection part from cleaning part with its

abrasives in the water jet.

(1)

(2)

(3)

Fig. 4 Typical C-scan showing the wall thickness variation using a color code

for the deviation from the nominal thickness

C. Deployment device

The deployment device is a mobile robotic platform

equipped with 2 drive units on both sides (Fig. 5). Different to

the approach presented in [3] [4], each drive unit consists of 5

magnetic wheels and a tooth belt driven by a brushless DC

motor.

Fig. 5: Bottom view of the deployment system, showing the 2 drive units

(blue) with a traction belt and 5 magnetic wheels each

The magnetic wheels are specially designed to overcome

air gaps present in the undulating surface between the water

wall tubes. These wheels press the belt onto the iron tubes thus

creating sufficient friction even if only 2 or 3 wheels are in

contact with the water wall surface. The belt on one side

provides traction but on the other side also prevent the robot

from getting stuck in the gaps between the tubes. Some basic

parameters are given in Tab. 1.

Steering is necessary to both correct for errors in the

robot’s path but also to circumvent obstacles on the water

wall, such as viewing windows, coal and air nozzles or tube

branches. This steering is performed by differential control of

the 2 drive units. Although the tracks tend to move straight,

path radii as small as 750mm can be achieved.

Due to the limited width of the tracks and the magnetic

wheels not all orientation of the robot relative to the directions

of the tubes can be traverse with the same speed and

reliability. To overcome this, the next generation shall

incorporate a combination of wider rollers and belts.

TAB 1 BASIC DATA OF THE ROBOTIC PLATFORM

size (L x W x H) 500 x 350 x 100 mm

weight 5.7 kg

payload capacity ca. 10 kg

speed 150 mm/s

The current prototype system is controlled from a

stationary controller at the operator’s site. A 50m long

umbilical cable provides the power to the 2 motors and feeds

back the position information (motor encoders) to the

controller. In a next step, the controller shall be placed on-

board. The umbilical will then provide DC power and

communication to the robot via an Ethernet connection.

The operator controls the prototype robot with a joystick. In

cleaning mode the speed is set to an optimum given by the

water jet system, i.e. as fast as possible, but still below the

limit where cleaning becomes incomplete. The operator

corrects the straight path of the robot from time to time in

order to cover the water wall completely and not to leave

certain spots unclean. In manual mode the user has full control

over speed and rotation rate of the robot. This is used, to

maneuver the robot to a certain location quickly or to

circumvent obstacles.

In the future, the system shall clean and inspect the water

wall more autonomously relying on several sensors. Contour

sensors (e, g, inductive, laser, camera) will allow the robot to

follow the tube direction or to use this information for position

estimation. An inclination sensor provides the orientation of

the robot on the water wall and thus aids to increase accuracy

of the odometric system. Eventually a global positioning

sensor, such as a laser tracker will provide continuous position

information free of integration errors.

III. VERIFICATION IN THE LABORATORY

First test with the prototype system were executed in the

lab of Alstom Inspection Robotics on a clean wall of tubes

(Fig. 6). The system was run under dry condition at first.

Fig. 6: Prototype of cleaning system on test mockup: dry run

During these tests the payload of the device was measured

and improved. Additionally the movement was analyzed and

the drive control optimized.

Afterwards a durability test on a mockup in the lab of

Waterjet Technologies has been undertaken.

Fig. 7: Durability test on test mockup

With the running cleaning system the crawler was moved up

and down in a loop on the mock up (Fig. 7). The durability test

lasted for more than 8 hours of constant system run.

The Test showed promising results:

The cleaning rate and quality was sufficient and proven

during 8 hours of constant run.

The crawler was not affected by the abrasive water jet.

Fig. 8: Cleaning test result

They payload was sufficient to carry the complete system

during cleaning, no detachment of the crawler occurred.

IV. OUTLOOK

As a next step, a test under real conditions in a boiler

environment on-site is planned. The deposit on the boiler

walls (slug – Fig. 9) has to be removed manually in order to

prevent heavy and big parts falling down on the operator. The

second cleaning step will be carried out using the new

automated device to prepare the surface for the subsequent

inspection.

Fig. 9: Slug on the boiler wall

The on-site tests will serve to validate the cleaning process as

well as the handling of the tool, the reliability and the safety.

The cleaning time will be measured, recorded and compared

to the conventional process. The surface quality will as well be

measured and recorded. The handling of the tool and the

accessibility (Fig. 10) will be validated and documented.

Fig. 10: Manhole to enter the boiler

V. CONCLUSIONS

The cleaning and inspection system being developed in

collaboration with Alstom Switzerland, Alstom Inspection

Robotics and Waterjet Technologies is a compact and

automated tool for cleaning and inspection of boiler water

walls. The main benefits are the time saving due to omitting of

scaffolding works and the increase of safe operation since no

working at height is required.

The deployment system is a mobile device able to drive

across the boiler wall tubes. The system is controlled from a

stationary controller by one operator. Different speed rates

depending on the application are possible optimizing the

installation, cleaning and inspection process time.

The integrated cleaning system is based on a suspension

cleaning technology resulting optimal cleaning quality and

performance. Followed by the cleaning, the integrated

ultrasound technology allows a reliable wall thickness

measurement.

The lab tests show promising results in terms of system

reliability and cleaning performance. The validation under real

conditions will prove the reliability and improved performance

using the automated cleaning and inspection tool.

VI. REFERENCES

Technical Reports: [1] 2011_01_03_Summary_WJCleaning.doc, Project summary report,

Alstom Switzerland, V. de Vries [2] Business Case scenario 2.xls: Business case for partial cleaning and

inspection, Alstom Switzerland, V. de Vries

Papers Presented at Conferences (Unpublished): [3] Slocum H. Magnebots – A Magnetic Wheels Based Overhead

Transportation Concept International Workshop on Advances in Service Robotics, 2003

[4] Hirose S, Tsutsumitake H, Toyama R, Kobayashi K. Disk Rover: A

Wall-Climbing Robot Using Permanent Magnet Discs IEEE/RSJ Int. Conf. on Intelligent Robots and Systems, 1992

Patents: [5] P. Roth, “Method for working on workpieces by means of a water jet

that contains abrasive and emerges under high pressure from a nozzle,

water jet installation for executing the method, and application of the method”, European Patent EP2308646, Sep. 28. 2010

[6] J. Brignac, R. Moser; “Automated system for waterwall cleaning and

inspection”; U.S: Patent US2008308128, Jun. 12 2007

VII. BIOGRAPHIES

Simon Honold graduated in 2006 from École

Polytechnique Fédérale Lausanne in Microsystem Engineering. Since his graduation he’s working for

Alstom (Switzerland) AG in different positions

related to robotic & diagnostic equipment for power plant inspections.

Vera de Vries graduated in 2001 (Mechanical

Engineering) from the Swiss Federal Institute of

Technology (ETH) in Zürich. After her PhD at the

Department of Mechanical Engineering at the ETH

(2006) she was working as project manager in the

field of Inspection Technologies R&D at ALSTOM Power. Since September 2011 she is the Manager

Department Manager R&D Robotics and Automation

s in ALSTOM Power in Baden

Philipp Roth graduated in 2004 from the University

of Applied Sciences Aargau in Brugg-Windisch. After his studies he was working for two years as a

research assistant at the Institute for Thermal and

Fluid Engineering at the University of Applied Sciences Aargau. In 2007 he co-founded Waterjet

Technologies Ltd and is since then CEO of the

company.

Wolfgang Zesch graduated as a M.Sc. in

mechatronics from the Swiss Federal Institute of Technology (ETH) in Zürich and earned his Ph.D. in

micro robotics at the Institute of Robotics of the

same university. During a post-doctoral stay at the University of California at Berkeley (UCB) he

conducted research in automated handling of micro parts. After 8 years in engineering and project

management in mechatronics, sensors & actuators

and optics he joined ALSTOM Inspection Robotics Ltd as General Manager in 2007, now occupying the

positions of CTO.